The Fundamentals of Bacteriology

CHAPTER XXIII.
PATHOGENIC BACTERIA OUTSIDE THE BODY.

Pathogenic bacteria may exist outside the body of the host under a variety of conditions as follows:

1. I. In or on inanimate objects or material.
   1. (a) As true saprophytes.
   2. (b) As facultative saprophytes.
   3. (c) Though obligate parasites, they exist in a latent state.
2. II. In or on other animals, or products from them:
   1. A. Susceptible to the disease.
      1. (a) Sick themselves. (As far as human beings are concerned these are mainly:
         1. 1. Other human beings for most diseases.
         2. 2. Rats for plague.
         3. 3. Dogs for rabies.
         4. 4. Horses for glanders.
         5. 5. Cattle, swine, parrots for tuberculosis).
      2. (b) Recovered from illness.
      3. (c) Never sick but “carriers.”
   2. B. Not susceptible.
      1. (d) Accidental carriers.
      2. (e) Serving as necessary intermediate hosts for certain stages of the parasite—this applies to protozoal diseases only, as yet.

I.

(a) The bacilli of tetanus, malignant edema and the organisms of “gas gangrene” are widely distributed. There is no evidence that their entrance into the body is at all necessary for the continuation of their life processes, or that one case of either of these diseases ever has any connection with any other case; they are true saprophytes. Manifestly it would be futile to attempt to prevent or eradicate such diseases by attacking the organism in its natural habitat. Clostridium botulinum, which causes a type of food poisoning in man, does not even multiply in the body, but the disease symptoms are due to a soluble toxin which is produced during its growth outside the body.

(b) Organisms like the bacterium of anthrax and the bacillus of black-leg from their local occurrence seem to be distributed from animals infected, though capable of a saprophytic existence outside the body for years. These can no more be attacked during their saprophytic existence than those just mentioned. Doubtless in warm seasons of the year and in the tropics other organisms pathogenic to animals may live and multiply in water or in damp soil where conditions are favorable, just as the cholera organism in India, and occasionally the typhoid bacillus in temperate climates do.

(c) Most pathogenic organisms, however, when they are thrown off from the bodies of animals, remain quiescent, do not multiply, in fact always tend to die out from lack of all that is implied in a “favorable environment,” food, moisture, temperature, light, etc. Disinfection is sometimes effective in this class of diseases in preventing new cases.

II. A.

(a) The most common infectious diseases of animals are transmitted more or less directly from other animals of the same species. Human beings get nearly all their diseases from other human beings who are sick; horses, from other horses; cattle, from other cattle; swine, from swine, etc. Occasionally transmission from one species to another occurs. Tuberculosis of swine most frequently results from feeding them milk of tuberculous cattle or from their eating the droppings of such cattle. Human beings occasionally contract anthrax from wool, hair and hides of animals dead of the disease or from postmortems on such animals; glanders from horses; tuberculosis (in children) from tuberculous milk; bubonic plague from rats; rabies practically always from the bites of dogs and other rabid animals, etc. The mode of limiting this class of diseases is evidently to isolate the sick, disinfect their discharges and their immediate surroundings, sterilize such products as must be handled or used, kill lower animals that are dangerous, and disinfect, bury properly, or destroy their carcasses.

Classes of the sick that are especially dangerous for the spread of disease are the mild cases and the undetected cases. These individuals do not come under observation and hence not under control.

(b) This class of carriers offers a difficult problem in the prevention of infectious diseases since they may continue to give off the organisms indefinitely and thus infect others. Typhoid carriers have been known to do so for fifty-five years. Cholera, diphtheria, meningitis and other carriers are well known in human practice. Carriers among animals have not been so frequently demonstrated, but there is every reason for thinking that hog-cholera, distemper, roup, influenza and other carriers are common. Carriers furnish the explanation for many of the so-called “spontaneous” outbreaks of disease among men and animals.

It is the general rule that those who are sick cease to carry the organisms on recovery and it is the occasional ones who do not that are the exceptions. In those diseases in which the organism is known it can be determined by examination of the patient or his discharges how long he continues to give off the causative agent. In those in which the cause is unknown (in human beings, the commonest and most easily transmitted diseases, scarlet fever, measles, German measles, mumps, chicken-pox, small-pox, influenza), no such check is possible. It is not known how long such individuals remain carriers. Hence isolation and quarantine of such convalescents is based partly on experience and partly on theory. It is highly probable that in the diseases just mentioned transmission occurs in the early stages only, except in small-pox and chicken-pox where the organism seems to be in the pustules and transmission by means of material from these is possible, though only by direct contact with it.

The fact that such individuals are known to have had the disease is a guide for control. The methods to be used are essentially the same as for the sick, (a), though obviously such human carriers are much more difficult to deal with since they are well.

(c) Another class of carriers is those who have never had the disease. Such individuals are common and are very dangerous sources of infection. Many of them have associated with the sick or with convalescents and these should always be suspected of harboring the organisms. Their control differs in no way from that of class (b). Unfortunately a history of such association is too often not available. Modern transportation and modern social habits are largely responsible for the nearly universal distribution of this type of carrier. Their detection is probably the largest single problem in the prevention of infectious diseases. A partial solution would be universal bacteriological examination. In our present stage of progress this is impossible and would not detect carriers of diseases of unknown cause.

The various classes of carriers just discussed are in a large part responsible for the continued presence of the commoner diseases throughout the country. The difficulties in control have been mentioned. A complete solution of the problem is not yet obtained. The army experience of the past few years in the control of infectious diseases shows what may be done.

There is another class of carriers which might be called the “universal carrier,” i.e., there are certain organisms which seem to be constantly or almost constantly present in or on the human body. These are micrococci, streptococci and pneumococci, all Gram positive organisms. They are ordinarily harmless parasites, but on occasion may give rise to serious, even fatal, infection. Infected wounds, pimples, boils, “common colds,” most “sore throats,” bronchitis, pneumonia are pathological conditions that come in this class. Such infections are usually autogenous. There is a constant interchange of these organisms among individuals closely associated, so that all of a group usually harbor the same type though no one individual can be called the carrier. Whenever, for any reason, the resistance of an individual (see Chaps. XXV et seq.) is lowered either locally or generally some of these organisms are liable to gain a foothold and cause infection. It sometimes happens that a strain of dangerous organisms may be developed in an individual in this way which is passed around to others with its virulence increased and thus cause an epidemic. Or, since all of the group are living under the same conditions the resistance of all or many of them may be lowered from the same general cause and an epidemic result from the organism common to all (pneumonia after measles, scarlet fever and influenza in camps). Protection of the individual is chiefly a personal question, i.e., by keeping up the “normal healthy tone” in all possible ways: The use of protective vaccines (Chap. XXX) appears to be advisable in such instances (colds, pneumonia after measles and influenza, inflammation of throat and middle ear following scarlet fever and measles). Results obtained in this country during the recent influenza epidemic have been conflicting but on the whole appear to show that preventive vaccination against pneumonia liable to follow should be practiced.

It would seem that among groups of individuals where infection may be expected the proper procedure would be to prepare autogenous vaccines (Chapter XXX) from members of the group and vaccinate all with the object of protecting them.

II. B.

(d) In this class come the “accidental carriers” like flies, fleas, lice, bed-bugs, ticks, and other biting and blood-sucking insects, vultures, buzzards, foxes, rats, and carrion-eating animals generally; pet animals in the household, etc. Here the animals are not susceptible to the given disease but become contaminated with the organisms and then through defilement of the food or drink or contact with individuals or with utensils pass the organisms on to the susceptible. Some biting and blood-sucking insects transmit the organisms through biting infected and non-infected animals successively. The spirilloses and trypanosomiases seem to be transmitted in this way, though there is evidence accumulating which may place these diseases in the next class. Anthrax is considered in some instances to be transmitted by flies and by vultures in the southern United States. Transmission of typhoid, dysentery, cholera and other diseases by flies is well established in man. Why not hog-cholera from farm to farm by flies, English sparrows, pigeons feeding, or by turkey buzzards? Though this would not be easy to prove, it seems reasonable.

Preventing contact of such animals with the discharges or with the carcasses of those dead of the disease, destruction of insect carriers, screening and prevention of fly breeding are obvious protective measures.

(e) In this class come certain diseases for which particular insects are necessary for the parasite in question, so that certain stages in its life history may be passed therein. The surest means for eradicating such diseases is the destruction of the insects concerned. Up to the present no bacterial disease is known in which this condition exists, unless Rocky Mountain spotted fever and typhus fever shall prove to be due to bacteria. Such diseases are all due to protozoa. Among them are Texas fever, due to Piroplasma bigeminum in this country which has been eradicated in entire districts by destruction of the cattle tick (Margaropus annulatus).

Piroplasmoses in South Africa among cattle and horses, and in other countries are transmitted in similar ways. Probably many of the diseases due to spirochetes and trypanosomes are likewise transmitted by necessary insect intermediaries. In human medicine the eradication of yellow fever from Panama and Cuba is due to successful warfare against, a certain mosquito (Stegomyia). So the freeing of large areas in different parts of the world from malaria follows the destruction of the mosquitoes. The prevention of typhus fever and of trench fever by “delousing” methods is familiar from recent army experience though for typhus this method has been practiced in Russia for more than ten years to the author’s personal knowledge. The campaign against disease in animals and man from insect sources must be considered as still in its infancy. The full utilization of tropical lands depends largely on the solution of this problem.