CHAPTER VI. Arthropods As Direct Inoculators of Disease Germs

ARTHROPODS AS DIRECT INOCULATORS OF DISEASE GERMS

We have seen that any insect which, like the house-fly, has access to disease germs and then comes into contact with the food or drink of man, may serve to disseminate disease. Moreover, it has been clearly established that a contaminated insect, alighting upon wounded or abraded surfaces, may infect them. These are instances of mere accidental, mechanical transfer of pathogenic organisms.

Closely related are the instances of direct inoculation of disease germs by insects and other arthropods. In this type, a blood-sucking species not only takes up the germs but, passing to a healthy individual, it inserts its contaminated mouth-parts and thus directly inoculates its victim. In other words, the disease is transferred just as blood poisoning may be induced by the prick of a contaminated needle, or as the laboratory worker may inoculate an experimental animal.

Formerly, it was supposed that this method of the transfer of disease by arthropods was a very common one and many instances are cited in the earlier literature of the subject. It is, however, difficult to draw a sharp line between such cases and those in which, on the one hand, the arthropod serves as a mere passive carrier or, on the other hand, serves as an essential host of the pathogenic organism. More critical study of the subject has led to the belief that the importance of the role of arthropods as direct inoculators has been much overestimated.

The principal reason for regarding this phase of the subject as relatively unimportant, is derived from a study of the habits of the blood-sucking species. It is found that, in general, they are intermittent feeders, visiting their hosts at intervals and then abstaining from feeding for a more or less extended period, while digesting their meal. In the meantime, most species of bacteria or of protozoan parasites with which they might have contaminated their mouth-parts, would have perished, through inability to withstand drying.

In spite of this, it must be recognized that this method of transfer does occur and must be reckoned with in any consideration of the relations of insects to disease. We shall first cite some general illustrations and shall then discuss the role of fleas in the spreading of bubonic plague, an illustration which cannot be regarded as typical, since it involves more than mere passive carriage.

SOME ILLUSTRATIONS OF DIRECT INOCULATION OF DISEASE GERMS BY ARTHROPODS

In discussing poisonous arthropods, we have already emphasized that species which are of themselves innocuous to man, may occasionally introduce bacteria by their bite or sting and thus cause more or less severe secondary symptoms. That such cases should occur, is no more than is to be expected. The mouth-parts or the sting of the insect are not sterile and the chances of their carrying pyogenic organisms are always present.

More strictly falling in the category of transmission of disease germs by direct inoculation are the instances where the insect, or related form, feeds upon a diseased animal and passes promptly to a healthy individual which it infects. Of such a nature are the following:

Various species of biting flies are factors in the dissemination of anthrax, an infectious and usually fatal disease of animals and, occasionally, of man. That the bacteria with which the blood of diseased animals teem shortly before death might be transmitted by such insects has long been contended, but the evidence in support of the view has been unsatisfactory. Recently, Mitzmain (1914) has reported a series of experiments which show conclusively that the disease may be so conveyed by a horse-fly, Tabanus striatus, and by the stable-fly, Stomoxys calcitrans.

Mitzmain's experiments were tried with an artificially infected guinea pig, which died of the disease upon the third day. The flies were applied two and one-half hours, to a few minutes, before the death of the animal. With both species the infection was successfully transferred to healthy guinea pigs by the direct method, in which the flies were interrupted while feeding on the sick animal. The evidence at hand does not warrant the conclusion that insect transmission is the rule in the case of this disease.

The nagana, or tsetse-fly disease of cattle is the most virulent disease of domestic animals in certain parts of Africa. It is caused by a protozoan blood parasite, Trypanosoma brucei, which is conveyed to healthy animals by the bite of Glossina morsitans and possibly other species of tsetse-flies. The flies remain infective for forty-eight hours after feeding on a diseased animal. The insect also serves as an essential host of the parasite.

Surra, a similar trypanosomiasis affecting especially horses and mules, occurs in southern Asia, Malaysia, and the Philippines where the tsetse-flies are not to be found. It is thought to be spread by various species of blood-sucking flies belonging to the genera Stomoxys, Haematobia, and Tabanus. Mitzmain (1913) demonstrated that in the Philippines it is conveyed mechanically by Tabanus striatus.

The sleeping sickness of man, in Africa, has also been supposed to be directly inoculated by one, or several, species of tsetse-flies. It is now known that the fly may convey the disease for a short time after feeding, but that there is then a latent period of from fourteen to twenty-one days, after which it again becomes infectious. This indicates that in the meantime the parasite has been undergoing some phase of its life-cycle and that the fly serves as an intermediate host. We shall therefore consider it more fully under that grouping.

These are a few of the cases of direct inoculation which may be cited as of the simpler type. We shall next consider the role of the flea in the dissemination of the bubonic plague, an illustration complicated by the fact that the bacillus multiples within the insect and may be indirectly inoculated.

THE ROLE OF FLEAS IN THE TRANSMISSION OF THE PLAGUE

The plague is a specific infectious disease caused by Bacillus pestis. It occurs in several forms, of which the bubonic and the pneumonic are the most common. According to Wyman, 80 per cent of the human cases are of the bubonic type. It is a disease which, under the name of oriental plague, the pest, or the black death, has ravaged almost from time immemorial the countries of Africa, Asia, and Europe. The record of its ravages are almost beyond belief. In 542 A. D. it caused in one day ten thousand deaths in Constantinople. In the 14th century it was introduced from the East and prevailed throughout Armenia, Asia Minor, Egypt and Northern Africa and Europe. Hecker estimates that one-fourth of the population of Europe, or twenty-five million persons, died in the epidemic of that century. From then until the 17th century it was almost constantly present in Europe, the great plague of London, in 1665 killing 68,596 out of a population of 460,000. Such an epidemic would mean for New York City a proportionate loss of over 600,000 in a single year. It is little wonder that in the face of such an appalling disaster suspicion and credulity were rife and the wildest demoralization ensued.

During the 14th century the Jews were regarded as responsible for the disease, through poisoning wells, and were subjected to the most incredible persecution and torture. In Milan the visitation of 1630 was credited to the so-called anointers,--men who were supposed to spread the plague by anointing the walls with magic ointment--and the most horrible tortures that human ingenuity could devise were imposed on scores of victims, regardless of rank or of public service (fig. 112, a). Manzoni's great historical novel, "The Betrothed" has well pictured conditions in Italy during this period.

In modern times the plague is confined primarily to warm climates, a condition which has been brought about largely through general improvement in sanitary conditions.

At present, the hotbed of the disease is India, where there were 1,040,429 deaths in 1904 and where in a period of fifteen years, ending with January 1912, there were over 15,000,000 deaths. The reported deaths in that country for 1913 totaled 198,875.

During the winter of 1910-11 there occurred in Manchuria and North China a virulent epidemic of the pneumonic plague which caused the death of nearly 50,000 people. The question as to its origin and means of spread will be especially referred to later.

Until recent years, the plague had not been known to occur in the New World but there were outbreaks in Brazil and Hawaii in 1899, and in 1900 there occurred the first cases in San Francisco. In California there were 125 cases in the period 1900-04; three cases in the next three years and then from May 1907 to March 1908, during the height of the outbreak, 170 cases. Since that time there have been only sporadic cases, the last case reported being in May 1914. Still more recent were the outbreaks in the Philippine Islands, Porto Rico, and Cuba.

On June 24, 1914, there was recognized a case of human plague in New Orleans. The Federal Health Service immediately took charge, and measures for the eradication of the disease were vigorously enforced. Up to October 10, 1914 there had been reported 30 cases of the disease in man, and 181 cases of plague in rats.

The present-day methods of combating bubonic plague are well illustrated by the fight in San Francisco. Had it not been for the strenuous and radical anti-plague campaign directed by the United States Marine Hospital Service we might have had in our own country an illustration of what the disease can accomplish. On what newly acquired knowledge was this fight based?

The basis was laid in 1894, when the plague bacillus was first discovered. All through the centuries, before and during the Christian era, down to 1894, the subject was enveloped in darkness and there had been a helpless, almost hopeless struggle in ignorance on the part of physicians, sanitarians, and public health officials against the ravages of this dread disease. Now its cause, method of propagation and means to prevent its spread are matters of scientific certainty.

After the discovery of the causative organism, one of the first advances was the establishment of the identity of human plague and that of rodents. It had often been noted that epidemics of the human disease were preceded by great epizootics among rats and mice. So well established was this fact that with the Chinese, unusual mortality among these rodents was regarded as foretelling a visitation of the human disease. That there was more than an accidental connection between the two was obvious when Yersin, the discoverer of Bacillus pestis, announced that during an epidemic the rats found dead in the houses and in the streets almost always contain the bacillus in great abundance in their organs, and that many of them exhibit veritable buboes.

Once it was established that the diseases were identical, the attention of the investigators was directed to a study of the relations between that of rats and of humans, and evidence accumulated to show that the bubonic plague was primarily a disease of rodents and that in some manner it was conveyed from them to man.

There yet remained unexplained the method of transfer from rat to man. As long ago as the 16th century, Mercuralis suggested that house-flies were guilty of disseminating the plague but modern investigation, while blaming the fly for much in the way of spreading disease, show that it is an insignificant factor in this case.

Search for blood-sucking insects which would feed on both rodents and man, and which might therefore be implicated, indicated that the fleas most nearly met the conditions. At first it was urged that rat fleas would not feed upon man and that the fleas ordinarily attacking man would not feed upon rats. More critical study of the habits of fleas soon showed that these objections were not well-founded. Especially important was the evidence that soon after the death of their host, rat fleas deserted its body and might then become a pest in houses where they had not been noticed before.

Attention was directed to the fact that while feeding, fleas are in the habit of squirting blood from the anus and that in the case of those which had fed upon rats and mice dying of the plague, virulent plague bacilli were to be found in such blood. Liston (1905) even found, and subsequent investigations confirmed, that the plague bacilli multiply in the stomach of the insect and that thus the blood ejected was richer in the organisms than was that of the diseased animal. It was found that a film of this infected blood spread out under the body of the flea and that thus the bacilli might be inoculated by the bite of the insect and by scratching.

Very recently, Bacot and Martin (1914) have paid especial attention to the question of the mechanism of the transmission of the plague bacilli by fleas. They believe that plague infested fleas regurgitate blood through the mouth, and that under conditions precluding the possibility of infection by dejecta, the disease may be thus transmitted. The evidence does not seem sufficient to establish that this is the chief method of transmission.

Conclusive experimental proof that fleas transmit the disease is further available from a number of sources. The most extensive series of experiments is that of the English Plague Commission in India, which reported in 1906 that:

On thirty occasions a healthy rat contracted plague in sequence of living in the neighborhood of a plague infected rat under circumstances which prevented the healthy rat coming in contact with either the body or excreta of the diseased animal.

In twenty-one experiments out of thirty-eight, healthy rats living in flea-proof cages contracted plague when exposed to rat fleas (Xenopsylla cheopis), collected from rats dead or dying of septicaemic plague.

Close contact of plague-infected with healthy animals, if fleas are excluded, does not give rise to an epizootic among the latter. As the huts were never cleaned out, close contact included contact with feces and urine of infected animals, and contact with, and eating of food contaminated with feces and urine of infected animals, as well as pus from open plague ulcers. Close contact of young, even when suckled by plague-infected mothers, did not give the disease to the former.

If fleas are present, then the epizootic, once started, spreads from animal to animal, the rate of progress being in direct proportion to the number of fleas.

Aerial infection was excluded. Thus guinea-pigs suspended in a cage two feet above the ground did not contract the disease, while in the same hut those animals allowed to run about and those placed two inches above the floor became infected. It had previously been found that a rat flea could not hop farther than about five inches.

Guinea pigs and monkeys were placed in plague houses in pairs, both protected from soil contact infection and both equally exposed to aerial infection, but one surrounded with a layer of tangle-foot paper and the other surrounded with a layer of sand. The following observations were made:

(a) Many fleas were caught in the tangle-foot, a certain proportion of which were found on dissection to contain in their stomachs abundant bacilli microscopically identical with plague bacilli. Out of eighty-five human fleas dissected only one contained these bacilli, while out of seventy-seven rat fleas twenty-three were found thus infected.

(b) The animals surrounded with tangle-foot in no instance developed plague, while several (24 per cent) of the non-protected animals died of the disease.

Thus, the experimental evidence that fleas transmit the plague from rat to rat, from rats to guinea pigs, and from rats to monkeys is indisputable. There is lacking direct experimental proof of its transfer from rodents to man but the whole chain of indirect evidence is so complete that there can be no doubt that such a transfer does occur so commonly that in the case of bubonic plague it must be regarded as the normal method.

Rats are not the only animals naturally attacked by the plague but as already suggested, it occurs in various other rodents. In California the disease has spread from rats to ground squirrels (Otospermophilus beecheyi), a condition readily arising from the frequency of association of rats with the squirrels in the neighborhood of towns, and from the fact that the two species of fleas found on them are also found on rats. While the danger of the disease being conveyed from squirrels to man is comparatively slight, the menace in the situation is that the squirrels may become a more or less permanent reservoir of the disease and infect rats, which may come into more frequent contact with man.

The tarbagan (Arctomys bobac), is a rodent found in North Manchuria, which is much prized for its fur. It is claimed that this animal is extremely susceptible to the plague and there is evidence to indicate that it was the primary source of the great outbreak of pneumonic plague which occurred in Manchuria and North China during the winter of 1910-11.

Of fleas, any species which attacks both rodents and man may be an agent in the transmission of the plague. We have seen that in India the species most commonly implicated is the rat flea, Xenopsylla cheopis, (= Loemopsylla or Pulex cheopis) (fig. 89). This species has also been found commonly on rats in San Francisco. The cat flea, Ctenocephalus felis, the dog flea, Ctenocephalus canis, the human flea, Pulex irritans, the rat fleas, Ceratophyllus fasciatus and Ctenopsyllus musculi have all been shown to meet the conditions.

But, however clear the evidence that fleas are the most important agent in the transfer of plague, it is a mistake fraught with danger to assume that they are the only factor in the spread of the disease. The causative organism is a bacillus and is not dependent upon any insect for the completion of its development.

Therefore, any blood-sucking insect which feeds upon a plague infected man or animal and then passes to a healthy individual, conceivably might transfer the bacilli. Verjbitski (1908) has shown experimentally that bed-bugs may thus convey the disease. Hertzog found the bacilli in a head-louse, Pediculus humanus, taken from a child which had died from the plague, and McCoy found them in a louse taken from a plague-infected squirrel. On account of their stationary habits, the latter insects could be of little significance in spreading the disease.

Contaminated food may also be a source of danger. While this source, formerly supposed to be the principal one, is now regarded as unimportant, there is abundant experimental evidence to show that it cannot be disregarded. It is believed that infection in this way can occur only when there is some lesion in the alimentary canal.

Still more important is the proof that in pneumonic plague the patient is directly infective and that the disease is spread from man to man without any intermediary. Especially conclusive is the evidence obtained by Drs. Strong and Teague during the Manchurian epidemic of 1910-11. They found that during coughing, in pneumonic plague cases, even when sputum visible to the naked eye is not expelled, plague bacilli in large numbers may become widely disseminated into the surrounding air. By exposing sterile plates before patients who coughed a single time, very numerous colonies of the bacillus were obtained.

But the great advance which has been made rests on the discovery that bubonic plague is in the vast majority of cases transmitted by the flea. The pneumonic type forms a very small percentage of the human cases and even with it, the evidence indicates that the original infection is derived from a rodent through the intermediary of the insect.

So modern prophylactic measures are directed primarily against the rat and fleas. Ships coming from infected ports are no longer disinfected for the purpose of killing the plague germs, but are fumigated to destroy the rats and the fleas which they might harbor. When anchored at infected ports, ships must observe strenuous precautions to prevent the ingress of rats. Cargo must be inspected just before being brought on board, in order to insure its freedom from rats. Even lines and hawsers must be protected by large metal discs or funnels, for rats readily run along a rope to reach the ship. Once infested, the ship must be thoroughly fumigated, not only to avoid carrying the disease to other ports but to obviate an outbreak on board.

When an epidemic begins, rats must be destroyed by trapping and poisoning. Various so-called biological poisons have not proved practicable. Sources of food supply should be cut off by thorough cleaning up, by use of rat-proof garbage cans and similar measures. Hand in hand with these, must go the destruction of breeding places, and the rat-proofing of dwellings, stables, markets, warehouses, docks and sewers. All these measures are expensive, and a few years ago would have been thought wholly impossible to put into practice but now they are being enforced on a large scale in every fight against the disease.

Rats and other rodents are regularly caught in the danger zone and examined for evidence of infection, for the sequence of the epizootic and of the human disease is now understood. In London, rats are regularly trapped and poisoned in the vicinity of the principal docks, to guard against the introduction of infected animals in shipping. During the past six years infected rats have been found yearly, thirteen having been found in 1912. In Seattle, Washington, seven infected rats were found along the water front in October, 1913, and infected ground squirrels are still being found in connection with the anti-plague measures in California.

The procedure during an outbreak of the human plague was well illustrated by the fight in San Francisco. The city was districted, and captured rats, after being dipped in some fluid to destroy the fleas, were carefully tagged to indicate their source, and were sent to the laboratory for examination. If an infected rat was found, the officers in charge of the work in the district involved were immediately notified by telephone, and the infected building was subjected to a thorough fumigation. In addition, special attention was given to all the territory in the four contiguous blocks.

By measures such as these, this dread scourge of the human race is being brought under control. Incidentally, the enormous losses due to the direct ravages of rats are being obviated and this alone would justify the expenditure many times over of the money and labor involved in the anti-rat measures.