CLASSIFICATION

For our purpose it will not be necessary to try to give a system of classification of all the mosquitoes. Those interested in this phase of the subject will find several books and papers devoted wholly to it. It is quite important, however, that we know something about a few of the more familiar groups and kinds, especially those concerned in the transmission of diseases.

THE ANOPHELES

In pointing out the differences between male and female mosquitoes we noted that in one group, the genus Anopheles, both sexes have long maxillary palpi (Figs. 72, 73). This is the most important character separating this genus from the other common forms and as the Anopheles are the malaria carriers it is important that this difference be remembered. Most of the members of this group have spotted wings (Fig. 74), but as some other common kinds also have spotted wings (Fig. 75) this character will not always be reliable. When an Anopheles mosquito is at rest the head and proboscis are held in one line with the body and the body rests at a considerable angle to the surface on which it is standing. Other kinds rest with the body almost or quite parallel to the surface on which they are standing. So if you find a female mosquito with long mouth-palpi and spotted wings resting at an angle to the surface on which it stands you may be reasonably sure that it is an Anopheles and therefore may be dangerous (Figs. 76, 77, 78, 79).

In the United States there are three species of Anopheles—maculipennis, punctipennis and crucians—which are common in various localities, and one or two other species that so far as known are local or rare.

The Anopheles eggs are not laid in masses as are the eggs of many other mosquitoes, but are deposited singly on the surface of the water where they may be found often floating close together.

The eggs (Figs. 80, 81) are elliptical in outline and are provided with a characteristic membranous expansion near the middle.

The larvæ may be found at the proper season and in the localities where they are abundant in almost any kind of standing water, in clear little pools beside running streams, in the overflow from springs, in swamps and marshy lands, in rain-barrels or any other places or vessels where the water is quiet. They do not breed in brackish water. As they feed largely on the algæ or green scum on the surface of the water they are especially apt to be found where this is present. We have already noted that their positions in the water differ from that assumed by other species (Fig. 82).

As the breathing-tube is very short the larvæ must come close to the surface to breathe, and when they are feeding we find them lying just under and parallel to the surface of the water with their curious round heads turned entirely upside down as they feed on the particles that are floating on the surface (Figs. 83, 84).

The pupæ do not differ very much from the pupæ of other species although the breathing-tubes on the thorax are usually shorter and the creature usually rests with its abdomen closer to the surface, that is, it does not hang down from the surface quite as straight as do other forms (Fig. 85).

The adults may be found out of doors or in houses, barns or other outbuildings. They do not seem to like a draft and consequently will be more apt to frequent rooms or places where there is little circulation of air. Although they are usually supposed to fly and bite only in the evening or at night, they may occasionally bite in the daytime. One hungry female took two short meals from my arm while we were trying to get her to pose for a photograph one warm afternoon.

The female passes the winter in the adult condition, hibernating in any convenient place about old trees or logs, in cracks or crevices in doors or out of doors. In the house they hide in the closets, behind the bureau, behind the head of the bed, or underneath it, or in any place where they are not apt to be disturbed. During a warm spell in the winter or if the room is kept warm they may come out for a meal almost any time.

THE YELLOW FEVER MOSQUITO

Ranking next in importance to Anopheles as a disseminator of disease and in fact solely responsible for a more dreaded scourge, is the species of mosquito now known as Stegomyia calopus. While this species is usually restricted to tropical or semi-tropical regions it sometimes makes its appearance in places farther north, especially in summer time, where it may thrive for a time. The adult mosquito (Fig. 104) is black, conspicuously marked with white. The legs and abdomen are banded with white and on the thorax is a series of white lines which in well-preserved specimens distinctly resembles a lyre. These mosquitoes are essentially domestic insects, for they are very rarely found except in houses or in their immediate vicinity. Once they enter a room they will scarcely leave it except to lay their eggs in a near-by cistern, water-pot, or some other convenient place.

Their habit of biting in the daytime has gained for them the name of "day mosquitoes" to distinguish them from the night feeders. But they will bite at night as well as by day and many other species are not at all adverse to a daylight meal, if the opportunity offers, so this habit is not distinctive. The recognition of these facts has a distinct bearing in the methods adopted to prevent the spread of yellow fever. There are no striking characters or habits in the larval or pupal stages that would enable us to distinguish without careful examination this species from other similar forms with which it might be associated. For some time it was claimed that this species would breed only in clean water, but it has been found that it is not nearly so particular, some even claiming that it prefers foul water. I have seen them breeding in countless thousands in company with Stegomyia scutellaris and Culex fatigans in the sewer drains in Tahiti in the streets of Papeete. As the larvæ feed largely on bacteria one would expect to find them in exactly such places where the bacteria are of course abundant.

The fact that they are able to live in any kind of water and in a very small amount of it well adapts them to their habits of living about dwellings.

So far as known the members of these two genera are the only two that are concerned in the transmission of disease in the United States. In other countries other species are suspected or proven disseminators of certain diseases, but these will be discussed in connection with the particular diseases in later chapters.

OTHER SPECIES

The many other species of mosquitoes that we have may be conveniently divided as to their breeding-habits into the fresh-water and the brackish-water forms. Among the fresh-water kinds some are found principally associated with man and his dwelling places, others live in the woods or other places and so are far less troublesome. Most of these do not fly far. Several of the species that breed in brackish water are great travelers and may fly inland for several miles. Thus the towns situated from one to three or four miles inland from the lower reaches of San Francisco Bay are often annoyed more by the mosquitoes that breed only in the brackish water on the salt marshes than they are by any of the fresh-water forms (Figs. 86, 87). The worst mosquito pest along the coast of the eastern United States and for some distance inland is a species that breeds in the salt marshes.

NATURAL ENEMIES OF MOSQUITOES

In combating noxious insects we learned long ago that often the most efficient, the easiest and cheapest way is to depend on their natural enemies to hold them in check. Under normal or rather natural conditions we find that they are usually kept within reasonable bounds by their natural enemies, but under the artificial conditions brought about by the settling and developing of any district great changes come about. It very often happens that these changes are favorable to the development of the noxious insects and unfavorable to the development of their enemies.

A striking example and one to the point is afforded in the introduction of mosquitoes into Hawaii. Up to 1826 there were no mosquitoes on these islands. It is supposed that they were introduced about that time by some ships that were trading at the islands. Indeed it is claimed that the very ship is known that brought them over from Mexico.

Once introduced they found conditions there very favorable to their development, plenty of standing water and few natural enemies to prey on them, so they increased very rapidly and gradually spread over all the islands of the group. This was the so-called night mosquito, Culex pipiens. Much later another species, Stegomyia calopus, just as annoying and much more dangerous was introduced and has also become very troublesome. We have a few species of top-minnows (Fig. 88) occurring in sluggish streams in the southern part of the United States that are important enemies of the mosquitoes of that region. A few years ago some of these were taken over to Hawaii and liberated in suitable places to see if they would not help solve the mosquito problem there. The fishes seem to be doing well. Already they are destroying many mosquito larvæ, and there are indications that they are going to do an important work, but of course can be depended on only as an aid.

On account of the various habits of both the larvæ and adults it will never be possible for any natural enemy or group of natural enemies effectively to control the mosquitoes of any region, but as certain of them are important as helpers they deserve to be mentioned.

ENEMIES OF THE ADULTS

Birds devour a few mosquitoes, the night-flying forms being particularly serviceable, but the number thus destroyed is probably so small as to be of little practical importance.

The dragon-flies (Figs. 89, 90, 91) or mosquito hawks have long been known as great enemies of mosquitoes, and they certainly do destroy many of them as they are hawking about places where mosquitoes abound. Dr. J.B. Smith of New Jersey very much doubts their efficiency, but observations made by other scientific men would seem to indicate that they often devour large numbers of mosquitoes during the course of the day and evening.

Spiders and toads destroy a few mosquitoes each night. Certain external and internal parasites destroy a few more, but the sum total of all of these agencies is probably not very considerable, for while the adults may have several natural enemies they are not of sufficient importance to have any appreciable effect on the number of mosquitoes in a badly infested region.

ENEMIES OF THE LARVÆ AND PUPÆ

The larvæ and pupæ on the other hand have many important enemies. Indeed under favorable conditions these may keep small ponds or lakes quite free from the pests. The predaceous aquatic larvæ of many insects feed freely on wrigglers. The larvæ of the diving beetles which are known as water-tigers are particularly ferocious and will soon destroy all the wrigglers in ponds where they are present (Fig. 92). Dragon-fly larvæ also feed freely on mosquito larvæ. Whirligig beetles are said to be particularly destructive to Anopheles larvæ and many other insects such as water-boatmen, back-swimmers, etc., feed on the larvæ of various species. A few of these introduced into a breeding-jar with Anopheles larvæ will soon destroy all of them, even the very young bugs attacking larvæ much larger than themselves.

It is interesting to note that the larvæ of some mosquitoes are themselves predaceous and feed freely on the other wrigglers that may chance to be in the same locality.

Various species of fish are, however, the most important enemies of the mosquitoes. Great schools of tide-water minnows (Fig. 93) are often carried over the low salt-marshes by the extreme high-tides and left in the hundreds of tide pools as the tide recedes. No mosquitoes can breed in a pool thus stocked with these fish. In the fresh-water streams and lakes there are several species of the top-minnows, sticklebacks (Fig. 94), etc., that feed voraciously on mosquito larvæ and unless the grass or reeds prevent the fish from getting to all parts of the ponds or lakes very few mosquitoes can breed in places where they are present.

Minute red mites such as attack the house-flies and other insects sometimes attack adult mosquitoes, but they are rarely very abundant. Parasitic roundworms attack certain species. Others suffer more or less from the attacks of various Sporozoan parasites.

FIGHTING MOSQUITOES

When mosquitoes are bothering us we usually begin by trying to kill the individual pests that are nearest to us. We try to crush them if they bite us; we screen the doors and windows to keep them from the house. In warmer countries the people are a little more hospitable and do not screen the mosquitoes out of the house entirely, but screen the beds for protection at night, and if the mosquitoes get too insistent during the day the bed makes a safe and comfortable retreat. All the mosquitoes in a room may be killed by fumigating with sulphur at the rate of two pounds to the thousand cubic feet of air-space. Pyrethrum is also used largely, but it only stupefies the mosquitoes temporarily instead of killing them. While in that condition they may be swept up and destroyed.

Various substances are sometimes used as repellants by those who must be in regions where the mosquitoes are abundant. With many of these, however, "the cure is worse than the disease." Smudges are often built to the windward of a house or barn-yard and the smoke from a good smoldering fire will keep a considerable area quite free from mosquitoes. The man who can keep himself enveloped in a cloud of tobacco smoke will not be bothered by mosquitoes. Oil of pennyroyal, oil of tar or a mixture of these with olive oil, and various other concoctions are sometimes smeared over the face and hands. These will furnish protection as long as they last. Dr. Smith says that he has found oil of citronella quite effective and of course less objectionable than the other things usually used. Care should be taken not to get it in the eyes. An ointment made of cedar oil, one ounce; oil of citronella, two ounces; spirits of camphor, two ounces, is said to make a good repellant and is effective for a long time.

FIGHTING THE LARVÆ

All of the efforts directed against the adult mosquitoes are usually of little avail in decreasing the number in any region. It is comparatively easy, however, to fight them successfully in the larval stage. We have seen that standing water is absolutely necessary for mosquitoes to breed in. This makes the problem much simpler than if they could breed in any moist places such as well-sprinkled lawns, a shady part of the garden, etc. The whole problem of successful campaigns against the mosquitoes resolves itself into the problem of finding and destroying or properly treating their breeding-places. We have seen how certain kinds, such as the yellow fever mosquito, are "domestic" species. They never go far from their breeding-places. If a house is infected by one of these species the immediate premises should be searched for the source. Cisterns, rain-barrels, sewer-traps, cesspools, tubs or buckets of water or old tin cans in out-of-the-way corners, are all suitable places for them to breed in. Cisterns and rain-barrels should be thoroughly screened so that no mosquitoes can get in or out, or the surface should be covered with a film of kerosene which will kill all the larvæ in the water when they come to the surface to breathe, and will also kill the females when they come to deposit their eggs. The vent to open cesspools should be thoroughly screened or the surface of the water kept well covered with oil. Water standing in any vessels in the yards should be emptied every week or ten days and the old tin cans destroyed or hauled away. In fighting these domestic species you need be concerned only with your own yard and that of your near-by neighbors. Other species, while also rather local in their distribution, fly much farther than the really domestic ones. In fighting these the region for a considerable distance around must be taken into consideration. Watering-troughs (Fig. 95) that are left filled from week to week, the overflow from such places, and the tracks made in the mud round about them (Fig. 96), small sluggish streams, irrigating ditches, and small ponds or lakes not supplied with fish are excellent breeding-places for several species of mosquitoes including Anopheles and others. The remedy at once suggests itself. The watering-trough can be emptied and renewed every week during the summer time, the overflow can be taken care of in a ditch that will lead it away from the trough to where it will sink into the ground, the banks of the streams or ponds or lakes can be cleared in such a way that fish can get to all parts of the water; most of the small ponds can be drained or their surface may be covered over with a thin film of kerosene. This is best applied as a spray; one ounce to fifteen square feet will suffice. If the oil is simply poured over the surface more will be required.

The fighting of the species that breed on the extensive salt-marshes in many regions is a larger and more difficult problem, but as it is a matter that usually concerns large communities, sometimes whole states, it can be dealt with on a larger scale. The very excellent results that have been accomplished in New Jersey and on the San Francisco peninsula, and in a smaller way in other places, show what may be done if the community goes about the fight in an intelligent manner. In the fight in New Jersey hundreds of acres of tide-lands have been drained so that they no longer have tide pools standing where the mosquitoes may breed. When it is impracticable to drain them the pools may be sprayed occasionally with kerosene.

The value of the land that is reclaimed by a good system of draining is often enough to pay many times over the cost of draining, thus the mosquitoes are gotten rid of and the land enhanced in value by a single operation.

CHAPTER VII

For instance, we find some of the very early writers emphasizing the point that swampy localities should be avoided for they produce animals that give rise to disease, or that the air is poisoned by the breath of the swamp-inhabiting animals.

These views of the origin of the fever prevailed until about the beginning of the eighteenth century when the recently discovered microscope began to reveal the various kinds of animalculæ to be found in decaying material.

In 1718 Lancisi held that the myriads of insects, particularly gnats or mosquitoes, that arose from such swampy regions might carry some of these poisonous substances and by means of their proboscis introduce them into the bodies of the people, and although he had made no experiments to test the assumption he did not consider it impossible that such insects might also introduce the smallest animalculæ into the blood. It took almost two centuries of study and investigation before this guess was proved to be right.

One reason why the mosquitoes were not earlier associated with these diseases was that all who investigated the matter at all turned their attention to the bad condition of the air in these swampy regions. Malaria means bad air. We all know that we can see the mists arising from such regions, particularly in the evening or at night, and as exposure to these mists very often meant an attack of malaria they were naturally supposed to be the cause of the disease. So for a long time the whole attention of investigators was turned toward studying and analyzing these vapors, and various experiments were made which seemed to show conclusively that the malaria was caused only by these emanations. The investigations even went so far that the exact germs that were supposed to cause the fever were separated and experimented with.

THE PARASITE THAT CAUSES MALARIA

The blood had been studied time and again and the characteristic appearance of the blood of a malarial patient was well known. In 1880 Laveran, a French army surgeon in Algiers, began to study the blood of such patients microscopically and soon was able to demonstrate the parasite that caused the disease. His discoveries were not readily accepted, but other investigations soon confirmed his observations and the fact was gradually firmly established. Not until recently, however, did this distinguished physician receive a full recognition of his work. A few years ago he was awarded the Nobel prize for medicine, perhaps the highest honor that can be bestowed on any physician. It is interesting, too, to note in this connection that it was another French surgeon who in 1840 discovered that sulphate of quinine is a specific for malaria.

The next important step was made in 1885 by Golgi, an Italian, who studied the life-history of the parasite in the blood and distinguished the three forms which cause the three most familiar kinds of malarial fevers, the tertian, the quartan and the remittent types. From this time on this parasite has been studied by physicians of many nationalities and the whole course of its life-history worked out. In order that we may understand how it was that mosquitoes were determined to be the means of disseminating this parasite we will discuss first its life-history in the human blood.

The parasites that cause the malarial fevers are Sporozoans and belong to the genus Plasmodium. Other names such as Hæmamœba and Laverania have been used for them, but the term Plasmodium is the one now most commonly employed. The three most common species are vivax, malariæ and falciparum, causing respectively the tertian, quartan and remittent fevers.

LIFE-HISTORY OF PARASITE

The life-history of all of these is very similar, the principal difference being in the length of time it takes them to sporulate. Let us begin with the parasite after it has been introduced into the blood and trace its development there. At first it is slender and rod-like in shape. It has some power of movement in the blood-plasm. Very soon it attacks one of the red blood-corpuscles and gradually pierces its way through the wall and into the corpuscle substance (Fig. 99); here it becomes more amœboid and continues to move about, feeding all the time on the corpuscle substance, gradually destroying the whole cell. As the parasite feeds and grows there is deposited within its body a blackish or brownish pigment known as melanin.

During the time that the parasite is feeding and growing it is also giving off waste products, as all living forms do in the process of metabolism, but as the parasite is completely inclosed in the corpuscle wall these waste products cannot escape until the wall bursts open. After about forty hours if the parasite is vivax or about sixty-five hours if it is malariæ it becomes immobile, the nucleus divides again and again and the protoplasm collects around these nuclei, forming a number of small cells or spores, as they are called. In about forty-eight or seventy-two hours, depending on whether the parasite is vivax or malariæ the wall of the corpuscle bursts and all these spores with the black pigment and the waste products that have been stored away within the cell are liberated into the blood-plasm.

These spores are round or somewhat amœboid and are carried in the blood for a short time. Very soon, however, each one attacks a new red corpuscle and the process of feeding, growth and spore-formation continues, taking exactly the same time for development as in the first generation, so every forty-eight hours in the case of the vivax, and every seventy-two hours in the case of the malariæ a new lot of these spores and the accompanying waste products are thrown out into the blood. Thus in a very short time many generations of this parasite occur and thousands or hundreds of thousands of the red-blood corpuscles are destroyed, leaving the patient weak and anemic. It will be seen, too, that the recurrence of the chills and fevers is simultaneous with the escaping of the parasites from the blood-corpuscles, together with the waste products of their metabolism.

These waste products are poisonous, and it is believed that this great amount of poison poured into the blood at one time causes the regular recurring crisis. Zoölogists well know that this process of asexual reproduction, i. e., reproduction without any conjugation of two different cells, cannot go on indefinitely, and those who were studying the life-cycle of these parasites were at a loss to know where the sexual stage took place. In the meantime studies of other parasites more or less closely related to Plasmodium showed that the sexual stage occurred outside the vertebrate host. The remarkable work of Dr. Smith on the life-history of the germ that causes the Texas fever of cattle had a strong influence in directing the search for this other stage of the malarial parasite. Another thing that indicated that this sexual generation must take place outside the body of the vertebrate host was the fact that the investigators found that the parasites in certain of the cells did not sporulate as did the others. When these individuals were drawn from the circulation and placed on a slide for study it was found that they would swell up and free themselves from the inclosing corpuscle and some of them would emit long filaments which would dart away among the corpuscles.

Many men have worked on this problem, but perhaps the most credit for its solution will always be given to Sir Patrick Manson, the foremost authority on tropical diseases, and to Ronald Ross, a surgeon in the English army. There is no more interesting and inspiring reading than that which deals with the development of the hypothesis by Manson and the persistent faith of Ross in the correctness of this theory, and his continuous indefatigable labors in trying to demonstrate it. It was an important piece of scientific work, and shows what a man can do even when the obstacles seem insurmountable.

THE PARASITE IN THE MOSQUITO

Briefly stated again, the problem was this: We have here a parasite in the blood which behaves as do many other forms of life. Some of these parasites do not go on with their development until they are removed from the circulation. Now, how are they thus removed from the circulation under normal conditions? This must first be solved before the still greater and more important problem of how the parasite gets from one human host to another can be taken up. In studying this over Manson reasoned that certain suctorial insects were the agencies through which blood was most commonly removed from the circulation and he ventured the guess that this change in the parasite that may be seen taking place on the slide under the microscope, normally takes place in the stomach of some insect that sucks man's blood. Ross was greatly impressed with the theory and began his long and apparently hopeless task of finding these parasites in the stomach of some insect. When we remember that they are so minute that they can only be seen by the use of the highest power of the microscope we can realize something of the magnitude of the task. Ross, who was at that time stationed in India, selected the mosquito as the most likely of the insects to be the host that he was looking for. For over two and one-half years he worked with entirely negative results, for after examining thoroughly many thousands of mosquitoes he found no trace of the parasite.

Practically all his work was done on the most common mosquito of the region, a species of Culex. But one day a friend sent him a different mosquito, one with spotted wings, and in examining it he was interested to note certain oval or round nodules on the outer walls of the stomach. On closer examinations he found that each of these nodules contained a few granules of the coal-black melanin of malarial fever. Further studies and experiments showed that these particular cells could always be found in the walls of the stomach of this particular species of mosquito a few days after it had bitten a malarial patient. This epoch-making discovery was made in 1898. Ross was detailed by the English government to devote his whole time to the further solution of the problem, and after two years more of careful experimentation and study was able to give a complete life-history of this parasite. His experiments have been repeated many times, and the conclusions he arrived at are as undeniable as any of the known facts of science.

The whole life-history as we now know it can be summed up as follows: The parasites develop within the circulation but certain of them seem to wander about and do not go on with their development there. When these particular parasites are taken into the stomach of most mosquitoes they are digested with the rest of the blood. But when they are taken into the stomach of a mosquito belonging to the genus Anopheles or other closely related genera they are not digested but go on with their development, conjugation and fertilization taking place, resulting in a more elongated form which makes its way through the walls of the stomach on the outside of which are formed the little nodules discovered by Ross on his mosquitoes. Within these nodules further division and development takes place until finally the nodule is burst open and many thousand minute rod-like organisms, sporozoites, are turned loose into the body-cavity of the mosquito. Owing to some unknown cause these little organisms are gathered together in the large vacuolated cells of the salivary glands of the mosquito, and when the mosquito bites a man or any other animal they pour down through the ducts with the secretion and are thus again introduced in the circulation.

The nodules or cysts on the walls of the stomach of the mosquito may contain as many as ten thousand sporozoites, and as many as five hundred cysts may occur on a single stomach.

It takes ten, twelve or more days from the time the parasites are taken into the stomach of the mosquito before they can go through their transformations and reach the salivary gland, the time depending on the temperature. So it is ten or twelve days or sometimes as much as eighteen or twenty days from the time an Anopheles bites a malarial patient before it is dangerous or can spread the disease. On the other hand, the sporozoites may lie in the salivary gland alive and virulent for several weeks. It does not give up all the parasites at one time, so that three or four or more people may be affected by a single mosquito.

It is well known that two parasites may often be seen in the same corpuscle. This is often simply a case of multiple infection, but Dr. Craig has very recently shown that under certain conditions two individuals may enter the same corpuscle and conjugate and the resulting individual will be resistant to quinine and may remain latent in the spleen or bone marrow for a long time. Under favorable conditions it may again begin the process of multiplication and the patient will suffer a relapse.

SUMMARY

Now let us sum up some of the reasons why we believe that the malaria fever can be transmitted only through the agency of mosquitoes. First, we know the life-history of the parasite, it has been studied in both of its hosts. Attempts have been made to rear it in other hosts but without avail, and we know from the general relations of the parasite that it must have this sexual as well as the asexual generations. Second, in some regions which would seem to be malarial, that is, where the miasmatic mists arise, no malaria occurs. Why? Usually it can be definitely shown that no Anopheles occur there. Other mosquitoes may be there in abundance, but if no Anopheles, there is no malaria. In certain regions this is well demonstrated. The west coast of Africa is one of the worst pest-holes of malaria and Anopheles. The east coast has no malaria and no Anopheles. In many islands the same condition exists. On the other hand, the Fiji Islands have Anopheles but no malaria. No malaria has ever been introduced there to infect the mosquitoes. In the same way Stegomyia occurs in some of the South Sea islands and yet there is no yellow fever there.

EXPERIMENTS

We may review, too, a few of the classic experiments that have served to show that malaria can be contracted in no other way than through the bite of the mosquito.

For many years Grassi, an Italian, devoted almost his whole time to the study of malaria. In 1900 he received permission from the government to experiment on the employees of a piece of railroad that was being built through a malarial region. This was divided for the purpose of the experiment into three sections, a protected zone in the middle and an unprotected zone at each end.

Those working in the protected zone had their houses completely screened and no one was allowed out of doors after sunset except they were protected with veils and gloves. Early in the season they were all given doses of quinine to prevent auto-infection. In the unprotected zone no screens were used and every one was allowed to go without special protection. The result for the summer was that there were no new cases of fever in the protected zone. In the unprotected zones practically all had the fever as usual.