The evidence that the fever is transmitted by this tick is conclusive. Koch found that from five per cent to fifteen per cent, and in some places, fifty per cent of the ticks captured, harbored the spirochæte. The disease is readily transmitted to monkeys, rats, mice and other animals and the earlier experiments along these lines have been confirmed by many workers.

Not only are the ticks which have fed on infected individuals capable of conveying the disease to healthy animals but they transmit the causative organism to their progeny. Thus Möllers (1907), working in Berlin, repeatedly infected monkeys through the bites of nymphs which had been bred in the laboratory from infected ticks. Still more astonishing was his discovery that ticks of the third generation were infective. In other words, if the progeny of infected ticks were fed throughout life on healthy animals, and on maturity deposited eggs, the nymphs which hatched from these eggs would still be capable of carrying the infection.

The developmental cycle of the spirochæte within the tick has not been fully worked out, though the general conclusions of Leishman (1910) have been supported by the recent works of Balfour (1911 and 1912), and Hindle (1912), on the life cycle of spirochætes affecting fowls.

Spirochæta duttoni ingested by Ornithodoros moubata apparently disappear within a few days, but Leishman believed that in reality they break up into minute granules which are to be found in the alimentary canal, the salivary glands and the Malpighian tubes of the tick. These granules, or "coccoid bodies," as Hindle calls them, are supposed to be the form in which the spirochætes infect the new host. We shall see later that Marchoux and Couvy (1913) dissent wholly from this interpretation.

According to Leishman, and Hindle, the coccoid bodies are not injected into the vertebrate host with the saliva of the tick, as are the sporozoites of malaria with that of the mosquito. Instead, they pass out with the excrement and secondarily gain access to the wound inflicted by the tick.

Nuttall (1912) calls attention to the fact that the geographical distribution of Ornithodoros moubata is far wider than our present records show for the distribution of the relapsing fever in man and that there is every reason to fear the extension of the disease. Huts where the ticks occur should be avoided and it should be remembered that in infected localities there is special danger in sleeping on the ground.

European Relapsing Fever—There is widely distributed in Europe a type of relapsing fever which is caused by Spirochæta recurrentis. It has long been supposed that this disease is spread by the bed-bug and there is some experimental evidence to show that it may be conveyed by these insects.

Direct experimental evidence of the agency of lice in transmitting relapsing fever is especially clear in the case of a type of the disease prevalent in parts of North Africa. We shall consider this evidence later.

Other Types of Relapsing Fever of Man—In addition to the three types of human relapsing fever already referred to, several others have been distinguished and have been attributed to distinct species of spirochætes. The various spirochætoses of man are:

North African Relapsing Fever of Man—The type of human relapsing fever to be met with in Algeria, Tunis, and Tripoli, is due to a Spirochæta to which does not differ morphologically from Spirochæta duttoni, but which has been separated on biological grounds as Spirochæta berberi.

Nicolle, Blaizot, and Conseil (1912) showed that the louse did not transmit the parasite by its bite. Two or three hours after it has fed on a patient, the spirochætes begin to break up and finally they disappear, so that after a day, repeated examinations fail to reveal them. They persist, nevertheless, in some unknown form, for if the observations are continued they reappear in eight to twelve days. These new forms are virulent, for a monkey was infected by inoculating a single crushed louse which had fed on infected blood fifteen days before.

Natural infection is indirect. Those attacked by the insect scratch, and in this act they excoriate the skin, crush the lice and contaminate their fingers. The least abrasion of the skin serves for the entrance of the spirochætes. Even the contact of the soiled fingers on the various mucosa, such as the conjunctive of the eye, is sufficient.

As in the case of Spirochæta duttoni, the organism is transmitted hereditarily in the arthropod vector. The progeny of lice which have fed on infected blood may themselves be infective.

Spirochætosis of Fowls—One of the best known of the spirochætes transmitted by arthropods is Spirochæta gallinarum, the cause of a very fatal disease of domestic fowls in widely separated regions of the world. According to Nuttall, it occurs in Southeastern Europe, Asia, Africa, South America and Australia.

In 1903, Marchoux and Salimbeni, working in Brazil, made the first detailed study of the disease, and showed that the causative organism is transmitted from fowl to fowl by the tick Argas persicus. They found that the ticks remained infective for at least five months. Specimens which had fed upon diseased birds in Brazil were sent to Nuttall and he promptly confirmed the experiments. Since that date many investigators, notably Balfour and Hindle, have contributed to the elucidating of the life-cycle of the parasite. Since it has been worked out more fully than has that of any of the human spirochætes, we present Hindle's diagram (fig. 143) and quote the brief summary from his preliminary paper (1911b).

"Commencing with the ordinary parasite in the blood of the fowl, the spirochæte grows until it reaches a certain length (16-19µ) and then divides by transverse division. This process is repeated, and is probably the only method of multiplication of the parasite within the blood. When the spirochætes disappear from the circulation, some of them break up into the coccoid bodies which, however, do not usually develop in the fowl. When the spirochætes are ingested by Argas persicus, some of them pass through the gut wall into the cœlomic fluid. From this medium they bore their way into the cells of the various organs of the tick and there break up into a number of coccoid bodies. These intracellular forms multiply by ordinary fission in the cells of the Malpighian tubules and gonads. Some of the coccoid bodies are formed in the lumen of the gut and Malpighian tubules. The result is that some of the coccoid bodies may be present in the Malpighian secretion and excrement of an infected tick and when mixed with the coxal fluid may gain entry into another fowl by the open wound caused by the tick's bite. They then elongate and redevelop into ordinary spirochætes in the blood of the fowl, and the cycle may be repeated."

Hindle's account is clear cut and circumstantial, and is quite in line with the work of Balfour, and of Leishman. Radically different is the interpretation of Marchoux and Couvy (1913). These investigators maintain that the granules localized in the Malpighian tubules in the larvæ and, in the adult, also in the ovules and the genital ducts of the male and female, are not derived from spirochætes but that they exist normally in many acariens. They interpret the supposed disassociation of the spirochæte into granules as simply the first phase, not of a process of multiplication, but of a degeneration ending in the death of the parasite. The fragmented chromatin has lost its affinity for stains, remaining always paler than that of the normal spirochætes. On the other hand, the granules of Leishman stain energetically with all the basic stains.

Further, according to Marchoux and Couvy, infection takes place without the emission of the coxal fluid and indeed, soiling of the host by the coxal fluid diluting the excrement is exceptional. All of the organs of the Argasid are invaded by the parasites, but they pass from the cœlom into the acini of the salivary glands and collect in its efferent canal. The saliva serves as the vehicle of infection.

Thus, the question of the life cycle of Spirochæta gallinarum, and of spirochætes in general, is an open one.

It should be noted that Argas persicus, the carrier of Spirochæta gallinarum, is a common pest of poultry in the southwestern United States. Though the disease has not been reported from this country, conditions are such that if accidentally introduced, it might do great damage.

Other Spirochæte Diseases of Animals—About a score of other blood inhabiting spirochætes have been reported as occurring in mammals, but little is known concerning their life-histories. One of the most important is Spirochæta theileri which produces a spirochætosis of cattle in the Transvaal. Theiler has determined that it is transmitted by an Ixodid tick, Margaropus decoloratus.

Typhus Fever and Pediculidæ

Typhus fever has not generally been supposed to occur in the United States, but there have been a few outbreaks and sporadic cases recognized. According to Anderson and Goldberger (1912a), it has been a subject of speculation among health authorities why, in spite of the arrival of occasional cases in this country and of many persons from endemic foci of the disease, typhus fever apparently does not gain a foothold in the United States. These same workers showed that the so-called Brill's disease, studied especially in New York City, is identical with the typhus fever of Mexico and of Europe.

The conditions under which the disease occurs and under which it spreads most rapidly are such as to suggest that it is carried by some parasitic insect. On epidemiological grounds the insects most open to suspicion are the lice, bed-bugs and fleas.

In 1909, Nicolle, Comte and Conseil, succeeded in transmitting typhus fever from infected to healthy monkeys by means of the body louse (Pediculus corporis). Independently of this work, Anderson and Goldberger had undertaken work along this line in Mexico, and in 1910 reported two attempts to transmit the disease to monkeys by means of body lice. The first experiment resulted negatively, but the second resulted in a slight rise in temperature, and in view of later results it seems that this was due to infection with typhus.

Shortly after, Ricketts and Wilder (1910) succeeded in transmitting the disease to the monkey by the bite of body lice in two experiments, the lice in one instance deriving their infection from a man and in another from the monkey. Another monkey was infected by typhus through the introduction of the feces and abdominal contents of infested lice into small incisions. Experiments with fleas and bed-bugs resulted negatively.

Subsequently, Goldberger and Anderson (1912b) indicated that the head louse (Pediculus humanus) as well, may become infected with typhus. In an attempt to transmit typhus fever (Mexican virus) from man to monkey by subcutaneous injection of a saline suspension of crushed head lice, the monkeys developed a typical febrile reaction with subsequent resistance to an inoculation of virulent typhus (Mexican) blood. In one of the three experiments to transmit the disease from man to monkey by means of the bite of the head louse, the animal bitten by the presumably infected head lice proved resistant to two successive immunity tests with virulent typhus blood.

In 1910, Ricketts and Wilder reported an experiment undertaken with a view to determining whether the young of infected lice were themselves infected. Young lice were reared to maturity on the bodies of typhus patients, so that if the eggs were susceptible to infection at any stage of their development, they would have every opportunity of being infected within the ovary. The eggs of these infected lice were obtained, they were incubated, and the young lice of the second generation were placed on a normal rhesus monkey. The experimenters were unable to keep the monkey under very close observation during the following three or four weeks, but from the fact that he proved resistant to a subsequent immunity test they concluded that he probably owed this immunity to infection by these lice of the second generation.

Anderson and Goldberger (1912b) object that due consideration was not given to the possibility of a variable susceptibility of the monkey to typhus. Their similar experiment was "frankly negative."

Prophylaxis against typhus fever is, therefore, primarily a question of vermin extermination. A brief article by Dr. Goldberger (1914) so clearly shows the practical application of his work and that of the other investigators of the subject, that we abstract from it the following account:

"In general terms it may be stated that association with a case of typhus fever in the absence of the transmitting insect is no more dangerous than is association with a case of yellow fever in the absence of the yellow fever mosquito. Danger threatens only when the insect appears on the scene."

"We may say, therefore, that to prevent infection of the individual it is necessary for him only to avoid being bitten by the louse. In theory this may readily be done, for we know that the body louse infests and attaches itself almost entirely to the body linen, and that boiling kills this insect and its eggs. Individual prophylaxis is based essentially, therefore, on the avoidance of contact with individuals likely to harbor lice. Practically, however, this is not always as easy as it may seem, especially under the conditions of such intimate association as is imposed by urban life. Particularly is this the case in places such as some of the large Mexican cities, where a large proportion of the population harbors this vermin. Under such circumstances it is well to avoid crowds or crowded places, such as public markets, crowded streets, or public assemblies at which the 'peon' gathers."

"Community prophylaxis efficiently and intelligently carried out is, from a certain point of view, probably easier and more effective in protecting the individual than is the individual's own effort to guard himself. Typhus emphasizes, perhaps better than any other disease, the fact that fundamentally, sanitation and health are economic problems. In proportion as the economic condition of the masses has improved—that is, in proportion as they could afford to keep clean—the notorious filth disease has decreased or disappeared. In localities where it still prevails, its further reduction or complete eradication waits on a further improvement in, or extension of, the improved economic status of those afflicted. Economic evolution is very slow process, and, while doing what we can to hasten it, we must take such precautions as existing conditions permit, looking to a reduction in or complete eradication of the disease."

"When possible, public bath houses and public wash houses, where the poor may bathe and do their washings at a minimum or without cost, should be provided. Similar provision should be made in military and construction camps. Troops in the field should be given the opportunity as frequently as possible to wash and scald or boil their body linen."

"Lodging houses, cheap boarding houses, night shelters, hospitals, jails and prisons, are important factors in the spread and frequently constitute foci of the disease. They should receive rigid sanitary supervision, including the enforcement of measures to free all inmates of such institutions of lice on admission."

"So far as individual foci of the disease are concerned these should be dealt with by segregating and keeping under observation all exposed individuals for 14 days—the period of incubation—from the last exposure, by disinfecting (boiling or steaming) the suspected bedding, body linen, and clothes, for the destruction of any possible vermin that they may harbor, and by fumigating (with sulphur) the quarters that they may have occupied."

"It will be noted that nothing has been said as to the disposition of the patient. So far as the patient is concerned, he should be removed to 'clean' surroundings, making sure that he does not take with him any vermin. This may be done by bathing, treating the hair with an insecticide (coal oil, tincture of larkspur), and a complete change of body linen. Aside from this, the patient may be treated or cared for in a general hospital ward or in a private house, provided the sanitary officer is satisfied that the new surroundings to which the patient has been removed are 'clean,' that is, free from vermin. Indeed, it is reasonably safe to permit a 'clean' patient to remain in his own home if this is 'clean,' for, as has already been emphasized, there can be no spread in the absence of lice. This is a common experience in native families of the better class and of Europeans in Mexico City."

"Similarly the sulphur fumigation above prescribed may be dispensed with as unnecessary in this class of cases."

CHAPTER XI

Infantile Paralysis or Acute Anterior Poliomyelitis

C. W. Howard and Clark (1912) presented the results of studies in this phase of the subject. They dealt especially with the house-fly, bedbug, head, and body lice, and mosquitoes. It was found that the house-fly (Musca domestica) can carry the virus of poliomyelitis in an active state for several days upon the surface of the body and for several hours within the gastro-intestinal tract. Mosquitoes and lice were found not to take up or maintain the virus. On the other hand, the bedbug (Cimex lectularius) was found to take the virus from the infected monkeys and to maintain it in a living state within the body for a period of seven days. This was demonstrated by grinding up in salt solution, insects which had fed on poliomyelitic animals and injecting the filtrate into a healthy monkey. The experimenters doubted that the bedbug is a carrier of the virus in nature.

Earlier in the same year, Brues and Sheppard published the results of an intensive epidemiological study of the outbreak of 1911, in Massachusetts. Special attention had been paid to the possibility of insect transfer and the following conclusion was reached:

"Field work during the past summer together with a consideration of the epidemiology of the disease so far as known, points strongly toward biting flies as possible carriers of the virus. It seems probable that the common stable-fly (Stomoxys calcitrans L.) may be responsible to a certain extent for the spread of acute epidemic poliomyelitis, possibly aided by other biting flies, such as Tabanus lineola. No facts which disprove such a hypothesis have as yet been adduced, and experiments based upon it are now in progress."

As stated by Brues (1913), especial suspicion fell upon the stable-fly because:

1. The blood-sucking habits of the adult fly suit it for the transfer of virus present in the blood.

2. The seasonal abundance of the fly is very closely correlated with the incidence of the disease, rising rapidly during the summer and reaching a maximum in July and August, then slowly declining in September and October.

3. The geographical distribution of the fly is, so far as can be ascertained, wider, or at least co-extensive with that of poliomyelitis.

4. Stomoxys is distinctly more abundant under rural conditions, than in cities and thickly populated areas.

5. While the disease spreads over districts quickly and in a rather erratic way, it often appears to follow along lines of travel, and it is known that Stomoxys flies will often follow horses for long distances along highways.

6. In a surprisingly large number of cases, it appeared probable that the children affected had been in the habit of frequenting places where Stomoxys is particularly abundant, i.e., about stables, barnyards, etc.

The experiments referred to were carried on during the summer of 1912 and in September Dr. Rosenau announced that the disease was transferred by the bite of the stable-fly.

A monkey infected by inoculation was exposed to the bites of upwards of a thousand of the Stomoxys flies daily, by stretching it at full length and rolling it in a piece of chicken wire, and then placing it on the floor of the cage in which the flies were confined. The flies fed freely from the first, as well as later, after paralysis had set in. Alternating with the inoculated monkey, healthy monkeys were similarly introduced into the cage at intervals. New monkeys were inoculated to keep a supply of such infected animals and additional healthy ones were exposed to the flies, which fed willingly and in considerable numbers on each occasion. "Thus the flies were given every opportunity to obtain infection from the monkeys, since the animals were bitten during practically every stage of the disease from the time of the inoculation of the virus till their death following the appearance of paralysis. By the same arrangement the healthy monkeys were likely to be bitten by flies that had previously fed during the various stages of the disease on the infected monkeys. The flies had meanwhile enjoyed the opportunity of incubating the virus for periods varying from the day or two which usually elapses between consecutive feedings, to the two or three-week period for which at least some (although a very small percentage) of the flies lived in the cage."

"In all, twelve apparently healthy monkeys of a small Japan species were exposed to the flies in the manner described for the infected monkeys. Some were placed in the cage only once or twice and others a number of times after varying intervals. These exposures usually lasted for about half an hour, but were sometimes more protracted. No results were apparent until two or three weeks after the experiment was well under way, and then in rather rapid succession six of the animals developed symptoms of poliomyelitis. In three, the disease appeared in a virulent form, resulting in death, while the other three experienced transient tremblings, diarrhœa, partial paralysis and recovery."—Brues, 1913.

Very soon after the announcement of the results of experiments by Rosenau and Brues, they were apparently conclusively confirmed by Anderson and Frost (1912), who repeated the experiments, at Washington. They announced that through the bites of the Stomoxys flies that had previously fed on infected monkeys, they had succeeded in experimentally infecting three healthy monkeys.

The results of these experiments gained much publicity and in spite of the conservative manner in which they had been announced, it was widely proclaimed that infantile paralysis was conveyed in nature by the stable-fly and by it alone.

Serious doubt was cast on this theory by the results of further experiments by Anderson and Frost, reported in May of 1913. Contrary to the expectations justified by their first experience, the results of all the later, and more extended, experiments were wholly negative. Not once were these investigators again able to transmit the infection of poliomyelitis through Stomoxys. They concluded that it was extremely doubtful that the insect was an important factor in the natural transmission of the disease, not only because of their series of negative results, "but also because recent experiments have afforded additional evidence of the direct transmissibility or contagiousness of poliomyelitis, and because epidemiological studies appear to us to indicate that the disease is more likely transmitted largely through passive human virus carriers."

Soon after this, Kling and Levaditi (1913) published their detailed studies on acute anterior poliomyelitis. They considered that the experiments of Flexner and Clark (and Howard and Clark), who fed house-flies on emulsion of infected spinal cord, were under conditions so different from what could occur in nature that one could not draw precise conclusions from them regarding the epidemiology of the disease. They cited the experiments of Josefson (1912), as being under more reasonable conditions. He sought to determine whether the inoculation of monkeys with flies caught in the wards of the Hospital for Contagious Diseases at Stockholm, where they had been in contact with cases of poliomyelitis, would produce the disease. The results were completely negative.

Kling and Lavaditi made four attempts of this kind. The flies were collected in places where poliomyelitics had dwelt, three, four and twenty-four after the beginning of the disease in the family and one, three, and fifteen days after the patient had left the house. These insects were for the greater part living and had certainly been in contact with the infected person. In addition, flies were used which had been caught in the wards of the Hospital for Contagious Diseases at Söderkoping, when numbers of poliomyelitics were confined there. Finally, to make the conditions as favorable as possible, the emulsions prepared from these flies were injected without previous filtering, since filtration often causes a weakening of the virus. In spite of these precautions, all their results were negative, none of the inoculated animals having contracted poliomyelitis. They also experimented with bedbugs which had fed upon infected patients at various stages of the disease, but the results in these cases also were wholly negative.

Kling and Levaditi considered at length the possibility of transmission of the disease by Stomoxys. As a result of their epidemiological studies, they found that infantile paralysis continued to spread in epidemic form in the dead of winter, when these flies were very rare and torpid, or were even completely absent. Numerous cases developed in the northern part of Sweden late in October and November, long after snow had fallen. On account of the rarity of the Stomoxys flies during the period of their investigations they were unable to conduct satisfactory experiments. In one instance, during a severe epidemic, they found a number of the flies in a stable near a house inhabited by an infected family, though none was found in the house itself. These flies were used in preparing an emulsion which, after filtering, was injected into the peritoneal cavity of a monkey. The result was wholly negative.

As for the earlier experiments, Kling and Levaditi believe if the flies were responsible for the transmission of the disease in the cases reported by Rosenau and Brues, and the first experiments of Anderson and Frost, it was because the virus of infantile paralysis is eliminated with the nasal secretions of paralyzed monkeys and the flies, becoming contaminated, had merely acted as accidental carriers.

Still further evidence against the hypothesis of the transmission of acute anterior poliomyelitis by Stomoxys calcitrans was brought forward by Sawyer and Herms (1913). Special precautions were used to prevent the transference of saliva or other possibly infectious material from the surface of one monkey to that of another, and to avoid the possibility of complicating the experiments by introducing other pathogenic organisms from wild flies, only laboratory-bred flies were used. In a series of seven carefully performed experiments, in which the conditions were varied, Sawyer and Herms were unable to transmit poliomyelitis from monkey to monkey through the agency of Stomoxys, or to obtain any indication that the fly is the usual agent for spreading the disease in nature.

The evidence at hand to date indicates that acute anterior poliomyelitis, or infantile paralysis, is transmitted by contact with infected persons. Under certain conditions insects may be agents in spreading the disease, but their rôle is a subordinate one.

Pellagra

Pellagra is an endemic and epidemic disease characterized by a peculiar eruption or erythema of the skin (figs 144 and 145), digestive disturbances and nervous trouble. Insanity is a common result, rather than a precursor of the disease. The manifestations of pellagra are periodic and its duration indeterminate.

The disease is one the very name of which was almost unknown in the United States until within the past decade. It has usually been regarded as tropical, though it occurs commonly in Italy and in various parts of Europe. Now it is known that it not only occurs quite generally in the United States but that it is spreading. Lavinder (1911) says that "There are certainly many thousand cases of the disease in this country, and the present situation must be looked upon with grave concern."

It is not within the scope of this book to undertake a general discussion of pellagra. The subject is of such importance to every medical man that we cannot do better than refer to Lavinder's valuable précis. We can only touch briefly upon the entomological phases of the problems presented.

The most commonly accepted theories regarding the etiology of the disease have attributed it to the use of Indian corn as an article of diet. This supposed relationship was explained either on the basis of, (a) insufficiency of nutriment and inappropriateness of corn as a prime article of food; (b) toxicity of corn or, (c) parasitism of certain organisms—fungi or bacteria—ingested with either sound or deteriorated corn.

In 1905, Sambon proposed the theory of the protozoal origin of pellagra and in 1910 he marshalled an imposing array of objections to the theory that there existed any relationship between corn and the disease. He presented clear evidence that pellagra existed in Europe before the introduction of Indian corn from America, as an article of diet, and that its spread was not pari passu with that of the use of corn. Cases were found in which the patients had apparently never used corn, though that is obviously difficult to establish. He showed that preventive measures based on the theory had been a failure. Finally, he believed that the recurrence of symptoms of the disease for successive springs, in patients who abstained absolutely from the use of corn, militated against the theory.

On the other hand, Sambon believed that the periodicity of the symptoms, peculiarities of distribution and seasonal incidence, and analogies of the symptoms to those of other parasitic diseases indicated that pellagra was of protozoal origin, and that it was insect-borne.

The insect carriers, he believed to be one or more species of Simuliidæ, or black-flies. In support of this he stated that Simulium appears to effect the same topographical conditions as pellagra, that in its imago stage it seems to present the same seasonal incidence, that it has a wide geographical distribution which seems to cover that of pellagra, and that species of the genus are known to cause severe epizootics. Concluding from his studies in Italy, that pellagra was limited almost wholly to agricultural laborers, he pointed out that the Simulium flies are found only in rural districts, and as a rule do not enter towns, villages, or houses.

When Sambon's detailed report was published in 1910, his theory was seized upon everywhere by workers who were anxious to test it and who, in most cases, were favorably disposed towards it because of the wonderful progress which had been made in the understanding of other insect-borne diseases. In this country, the entomological aspects of the subject have been dealt with especially by Forbes (1912), and by King and Jennings, under the direction of W. D. Hunter, of the Bureau of Entomology, and in coöperation with the Thompson-McFadden Pellagra Commission of the Department of Tropical Medicine of the New York Post-Graduate Medical School. An important series of experiments with monkeys has been undertaken by S. J. Hunter, of Kansas, but unfortunately we have as yet no satisfactory evidence that these animals are susceptible to the disease—a fact which renders the whole problem difficult.

The accumulated evidence is increasingly opposed to Sambon's hypothesis of the transmission of pellagra by Simulium. This has been so clearly manifested in the work of the Thompson-McFadden Commission that we quote here from the report by Jennings (1914):

"Our studies in 1912 convinced us that there was little evidence to support the incrimination of any species of Simulium in South Carolina in the transmission of pellagra. Reviewing the group as a whole, we find that its species are essentially "wild" and lack those habits of intimate association with man which would be expected in the vector of such a disease as pellagra. Although these flies are excessively abundant in some parts of their range and are moderately so in Spartanburg County, man is merely an incidental host, and no disposition whatever to seek him out or to invade his domicile seems to be manifested. Critically considered, it is nearer the fact that usually man is attacked only when he invades their habitat."

"As our knowledge of pellagra accumulates, it is more and more evident that its origin is in some way closely associated with the domicile. The possibility that an insect whose association with man and his immediate environment is, at the best, casual and desultory, can be active in the causation of the disease becomes increasingly remote."

"Our knowledge of the biting habits of Simulium is not complete, but it is evident, as regards American species at least, that these are sometimes not constant for the same species in different localities. Certain species will bite man freely when opportunity offers, while others have never been known to attack him. To assume that the proximity of a Simulium-breeding stream necessarily implies that persons in its vicinity must be attacked and bitten is highly fallacious. In Spartanburg County attacks by Simulium seems to be confined to the immediate vicinity of the breeding-places. Our records and observations, exceedingly few in number, refer almost exclusively to such locations. Statements regarding such attacks, secured with much care and discrimination from a large number of persons, including many pellagrins, indicate conclusively that these insects are seldom a pest of man in this county. A certain number of the persons questioned were familiar with the gnats in other localities, but the majority were seemingly ignorant of the existence of such flies with biting habits. This is especially striking, in view of the fact that the average distance of streams from the homes of the pellagra cases studied was about 200 yards, many being at a distance of less than 200 yards, and that 78 per cent of these streams were found to be infested by larval Simulium. Such ignorance in a large number of persons cannot be overlooked and indicates strongly that our belief in the negligible character of local attacks by Simulium is well founded."

"In localities infested by 'sand-flies,' mosquitoes, etc., these pests are always well known and the ignorance described above is very significant."

"Such positive reports as we received nearly always referred to bites received in the open, along streams, etc., and observations made of their attack were of those on field laborers in similar situations. Males engaged in agricultural pursuits are almost exempt from pellagra in Spartanburg County. During the season of 1913, in some two or three instances, observations were made of the biting of Simulium and some additional and entirely creditable reports were received. These observations and reports were under conditions identical with those referred to in the reports of 1912 and confirm the conclusions based on the observations of that year. I would repeat with emphasis that it is inconceivable that a fly of the appearance and habits of the prevalent species of Simulium could be present in such a region, especially about the haunts of man and attack him with sufficient frequency and regularity to satisfactorily account for so active and prevalent a disease as pellagra without being a well-known and recognized pest."

"In connection with the conditions in the Piedmont region of South Carolina, it may be well to cite the results of a study of those in the arid region of western Texas."

"In May, 1913, in company with Capt. J. F. Siler of the Thompson-McFadden Pellagra Commission, I visited the region of which Midland in Midland County is the center. This region is very dry and totally devoid of running water for a long distance in every direction. The only natural source of water-supply, a few water holes and ponds, were visited and found to be of such a nature that the survival of Simulium, far less its propagation in them, is absolutely impossible. The nearest stream affording possibilities as a source of Simulium is 60 miles away, while the average distance of such possibility is not less than 100 miles."

"Artificial sources of water-supply were also investigated carefully and were found to offer no opportunity for the breeding of Simulium."

"At Midland the histories of five cases of pellagra were obtained, which gave clear evidence that this place or its immediate vicinity was the point of origin. Persons of long residence in the country were questioned as to the occurrence of such flies as Simulium and returned negative answers. These included a retired cattle owner, who is a man of education and a keen observer, an expert veterinarian stationed in the country who has the cattle of the country under constant observation, and a practical cattle man, manager of a ranch and of wide experience. The latter had had experience with 'Buffalo gnats' in other localities (in the East) and is well acquainted with them. His close personal supervision of the cattle under his charge, makes it practically certain that he would have discovered these gnats had they been present in the country."

"At the time the study was made, Simulium was breeding and active in the adult state in the vicinity of Dallas, Texas, in the eastern part of the state. We have here a region in which cases of pellagra have originated, yet in which Simulium does not and cannot breed."

Other possible insect vectors of pellagra have been studied in great detail and the available evidence indicates that if any insect plays a rôle in the spread of the disease, Stomoxys calcitrans most nearly fills the conditions. This conclusion was announced by Jennings and King in 1912, and has been supported by their subsequent work.

Yet, after all the studies of the past decade, the old belief that pellagra is essentially of dietary origin is gaining ground. Goldberger, Waring and Willets (1914) of the United States Public Health Service summarize their conclusions in the statement, (1) that it is dependent on some yet undetermined fault in a diet in which the animal or leguminous protein component is disproportionately large and (2) that no pellagra develops in those who consume a mixed, well-balanced, and varied diet, such, for example, as that furnished by the Government to the enlisted men of the Army, Navy, and Marine Corps.

Leprosy

The same worker undertook to determine whether flies are able to transmit leprosy. He experimented with five species found in Honolulu,—Musca domestica, Sarcophaga pallinervis, Sarcophaga barbata, Volucella obesa and an undetermined species of Lucilia. The experiments with Musca domestica were the most detailed. From these experiments he concluded, first, that all of the above-named flies, when given an opportunity to feed upon leprous fluids, will contain the bacilli in their intestinal tracts and feces for several days after such feeding. Second, that considering the habits of these flies, and especially those of Musca domestica, it is certain that, given an exposed leprous ulcer, these insects will frequently convey immense numbers of lepra bacilli, directly or indirectly, to the skins, nasal mucosa, and digestive tracts of healthy persons. Additional evidence along this line has recently been brought forward by Honeij and Parker (1914), who incriminate both Musca domestica and Stomoxys calcitrans. Whether or not such insect-borne bacilli are capable of infecting persons whose skin and mucosa are thus contaminated, Currie was unwilling to maintain, but he concludes that until we have more accurate knowledge on this point, we are justified in regarding these insects with grave suspicion of being one of the means of disseminating leprous infection.

Various students of the subject have suggested that bed-bugs may be the carriers of leprosy and have determined the presence of acid-fast bacilli in the intestines of bed-bugs which had fed on leprous patients. Opposed to this, the careful experiments of Thompson (1913) and of Skelton and Parkham (1913) have been wholly negative.

Borrel has recently suggested that Demodex, may play a rôle in spreading the infection in families. Many other insects and acariens have been suggested as possible vectors, but the experimental data are few and in no wise conclusive. The most that can be said is that it is quite possible that under favorable conditions the infection might be spread by any of the several blood-sucking forms or by house-flies.

Verruga peruviana

Verruga peruviana is defined by Castellani and Chalmers as "a chronic, endemic, specific, general disorder of unknown origin, not contagious, but apparently inoculable, and characterized by an irregular fever associated with rheumatoid pains, anemia, followed by granulomatous swellings in the skin, mucous membranes, and organs of the body." It has been generally believed by medical men interested that the comparatively benign eruptive verruga is identical with the so-called Oroya, or Carrion's fever, a malignant type. This view is not supported by the work of Strong, Tyzzer and Brues, (1913).

The disease is confined to South America and to definitely limited areas of those countries in which it does occur. It is especially prevalent in some parts of Peru.

The causative organism and the method of transfer of verruga are unknown. Castellani and Chalmers pointed out in 1910 that the study of the distribution of the disease in Peru would impress one with the similarity to the distribution of the Rocky Mountain fever and would lead to the conclusion that the ætiological cause must in some way be associated with some blood-sucking animal, perhaps an arachnid, and that this is supported by the fact that the persons most prone to the infection are those who work in the fields.

More recently, Townsend (1913), in a series of papers, has maintained that verruga and Carrion's disease are identical, and that they are transmitted to man by the bites of the Psychodid fly, Phlebotomus verrucarum. He succeeded in producing the eruptive type of the disease in experimental animals by injecting a physiological salt trituration of wild Phlebotomus flies. A cebus monkey was exposed from October so to November 6, by chaining him to a tree in the verruga zone, next to a stone wall from which the flies emerged in large numbers every night. Miliar eruption began to appear on the orbits November 13 and by November 21, there were a number of typical eruptions, with exudation on various parts of the body exactly like miliar eruptive sores commonly seen on legs of human cases.

An assistant in the verruga work, George E. Nicholson, contracted the eruptive type of the disease, apparently as a result of being bitten by the Phlebotomus flies. He had slept in a verruga zone, under a tight net. During the night he evidently put his hands in contact with the net, for in the morning there were fifty-five unmistakable Phlebotomus bites on the backs of his hands and wrists.

Townsend believes that in nature, lizards constitute the reservoir of the disease and that it is from them that the Phlebotomus flies receive the infection.

Cancer

There are not wanting suggestions that this dread disease is carried, or even caused, by arthropods. Borrel (1909) stated that he had found mites of the genus Demodex in carcinoma of the face and of the mammæ. He believed that they acted as carriers of the virus.

Saul (1910) and Dahl (1910) go much further, since they attribute the production of the malignant growth to the presence of mites which Saul had found in cancers. These Dahl described as belonging to a new species, which he designated Tarsonemus hominis. These findings have since been confirmed by several workers. Nevertheless, the presence of the mite is so rare that it cannot be regarded as an important factor in the causation of the disease. The theory that cancer is caused by an external parasite is given little credence by investigators in this field.

In conclusion, it should be noted that the medical and entomological literature of the past few years abounds in suggestions, and in unsupported direct statements that various other diseases are insect-borne. Knab (1912) has well said "Since the discovery that certain blood-sucking insects are the secondary hosts of pathogenic parasites, nearly every insect that sucks blood, whether habitually or occasionally, has been suspected or considered a possible transmitter of disease. No thought seems to have been given to the conditions and the characteristics of the individual species of blood-sucking insects, which make disease transmission possible."

He points out that "in order to be a potential transmitter of human blood-parasites, an insect must be closely associated with man and normally have opportunity to suck his blood repeatedly. It is not sufficient that occasional specimens bite man, as, for example, is the case with forest mosquitoes. Although a person may be bitten by a large number of such mosquitoes, the chances that any of these mosquitoes survive to develop the parasites in question, (assuming such development to be possible), and then find opportunity to bite and infect another person, are altogether too remote. Applying this criterion, not only the majority of mosquitoes but many other blood-sucking insects, such as Tabanidæ and Simuliidæ, may be confidently eliminated. Moreover, these insects are mostly in evidence only during a brief season, so that we have an additional difficulty of a very long interval during which there could be no propagation of the disease in question." He makes an exception of tick-borne diseases, where the parasites are directly transmitted from the tick host to its offspring and where, for this reason, the insect remains a potential transmitter for a very long period. He also cites the trypanosome diseases as possible exceptions, since the causative organisms apparently thrive in a number of different vertebrate hosts and may be transmitted from cattle, or wild animals, to man.

Knab's article should serve a valuable end in checking irresponsible theorizing on the subject of insect transmission of disease. Nevertheless, the principles which he laid down cannot be applied to the cases of accidental carriage of bacterial diseases, or to those of direct inoculation of pyogenic organisms, or of blood parasites such as the bacillus of anthrax, or of bubonic plague. Accumulated evidence has justified the conclusion that certain trypanosomes pathogenic to man are harbored by wild mammals, and so form an exception. Townsend believes that lizards constitute the natural reservoir of verruga; and it seems probable that field mice harbor the organism of tsutsugamushi disease. Such instances are likely to accumulate as our knowledge of the relation of arthropods to disease broadens.

CHAPTER XII

CRUSTACEA

Arthropods having two pairs of antennæ which are sometimes modified for grasping, and usually with more than five pairs of legs. With but few exceptions they are aquatic creatures. Representatives are: Crabs, lobsters, shrimps, crayfish, water-fleas, and woodlice. To this class belongs the Cyclops (fig. 122) a genus of minute aquatic crustaceans of which at least one species harbors Dracunculus medinensis, the Guinea worm (fig. 121).

MYRIAPODA

Elongate, usually vermiform, wingless, terrestrial creatures having one pair of antennæ, legs attached to each of the many intermediate body segments. This group is divided into two sections, now usually given class rank: the Diplopoda or millipedes (fig. 13), commonly known as thousand legs, characterized by having two pairs of legs attached to each intermediate body segment, and the Chilopoda or centipedes (fig. 14) having only one pair of legs to each body segment.

ARACHNIDA

In this class the antennæ are apparently wanting, wings are never present, and the adults are usually provided with four pairs of legs. Scorpions, harvest-men, spiders, mites, etc.

HEXAPODA (Insects)

True insects have a single pair of antennæ, which is rarely vestigial, and usually one or two pairs of wings in the adult stage. Familiar examples are cockroaches, crickets, grasshoppers, bugs, dragon-flies, butterflies, moths, mosquitoes, flies, beetles, ants, bees and wasps.

ORDERS OF THE ARACHNIDA