O. A. Johannsen

“ Parasitism is an adaptation which has originated very often among living organisms and in widely separated groups. It would seem simple to define what is meant by a "parasite" but, in reality, the term is not easily limited. It is often stated that a parasite is "An organism which lives at the expense of another," but this definition is applicable to a predatory species or, in its broadest sense, to all organisms. ”

“ The experiments of Strickland on copulation and oviposition in the rat flea showed that fleas do not copulate until they are sexually mature and that, at least in the case of Ceratophyllus fasciatus, the reproductive organs are imperfectly developed for some time (more than a week) after emerging from the pupa. When mature, copulation takes place soon after the fleas have fed on their true host—the rat—but not if they have fed on a facultative host only, such as man. Copulation is always followed by oviposition within a very short time. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ As Newstead says, "In consideration of the facts which have so far been brought to light regarding the economy of Phlebotomus, it is clearly evident that the task of suppressing these insects is an almost insurmountable one. Had we to deal with insects as large and as accessible as mosquitoes, the adoption of prophylactic measures would be comparatively easy, but owing to the extremely minute size and almost flea-like habits of the adult insects, and the enormous area over which the breeding-places may occur, we are faced with a problem which is most difficult of solution." ”

“ A number of the parasitic Acarina evidently secrete a specific poison, presumably carried by the saliva, but in most cases its effect on man is insignificant. There is an abundant literature dealing with the poisonous effect of the bite of these forms, especially the ticks, but until recently it has been confused by failure to recognize that various species may transmit diseases of man, rather than produce injury through direct poisoning. We shall therefore discuss the Acarina more especially in subsequent chapters, dealing with parasitism and with disease transmission. ”

“ It is clear from what has been said regarding the life cycle of the malarial parasite that the mosquito is harmless if not itself diseased. Hence malarial-bearing species may abound in the neighborhood where there is no malaria, the disease being absent simply because the mosquitoes are uninfected. Such a locality is potentially malarious and needs only the introduction of a malarial patient who is exposed to the mosquitoes. ”

“ 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. ”

“ 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. ”

“ Just as certain individuals may be poisoned by what, for others, are common articles of food, so some persons may be abnormally susceptible to insect poison. Thus, the poison of a bee sting may be of varying severity, but there are individuals who are made seriously sick by a single sting, regardless of the point of entry. Some individuals scarcely notice a mosquito bite, others find it very painful, and so illustrations of this difference in individuals might be multiplied. ”

“ A species which very commonly attacks man is Conorhinus sanguisugus, the so-called "big bed-bug" of the south and southern United States. It is frequently found in houses and is known to inflict an exceedingly painful bite. As in the case of a number of other predaceous Hemiptera, the salivary glands of these forms are highly developed. The effect of the bite on their prey and, as Marlatt has pointed out, the constant and uniform character of the symptoms in nearly all cases of bites in man, clearly indicate that their saliva contains a specific substance. ”

“ Roughly speaking, insects may be grouped into those which undergo complete metamorphosis and those which have incomplete metamorphosis. They are said to undergo complete metamorphosis when the young form, as it leaves the egg, bears no resemblance to the adult. For example, the maggot changes to a quiescent pupa and from this emerges the winged active fly. They undergo incomplete metamorphosis, when the young insect, as it leaves the egg, resembles the adult to a greater or less extent, and after undergoing a certain number of molts becomes sexually mature. ”

“ Low temperatures have a very great effect not only on development, but on the activity and even life of the adults. Long before the method of transmission of yellow fever was discovered it was well known that the epidemics were brought to a close by heavy frosts, and it is now known that this is due to the killing of the mosquitoes which alone could spread the disease. ”

“ The evidence indicated that the tick suffers from a relatively harmless, generalized infection and the virus proliferates in its body. The disease probably is transferred through the salivary secretion of the tick since inoculation experiments show that the salivary glands of the infected adult contain the virus.
It is probable that in nature the reservoir of the virus of spotted fever is some one or more of the native small animals. Infected ticks have been found in nature, and as various wild animals are susceptible to the disease, it is obvious that it may exist among them unnoticed. ”

“ One of the most important measures for preventing the spread of disease by flies is the abolition of the common box privy. In villages and rural districts this is today almost the only type to be found. It is the chief factor in the spread of typhoid and other intestinal diseases, as well as intestinal parasites. Open and exposed to myriads of flies which not only breed there but which feed upon the excrement, they furnish ideal conditions for spreading contamination. Even where efforts are made to cover the contents with dust, or ashes, or lime, flies may continue to breed unchecked. ”

“ 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." ”

“ While Simulium pictipes does not attack man, there are a number of the species which are blood-sucking and in some regions they are a veritable scourge. In recent years the greatest interest in the group has been aroused by Sambon's hypothesis that they transmit pellagra from man to man. This has not been established, and, indeed, seems very doubtful, but the importance of these insects as pests and the possibility that they may carry disease make it urgent that detailed life-histories of the hominoxious species be worked out. ”

“ The fact that certain arthropods are poisonous, or may affect the health of man as direct parasites has always received attention in the medical literature. We come now to the more modern aspect of our subject,—the consideration of insects and other arthropods as transmitters and disseminators of disease. ”

“ It is well known that mosquitoes, when they bite, inject into the wound a minute quantity of poison. The effect of this varies according to the species of mosquito and also depends very much on the susceptibility of the individual. Soon after the bite a sensation of itching is noticed and often a wheal, or eminence, is produced on the skin, which may increase to a considerable swelling. The scratching which is induced may cause a secondary infection and thus lead to serious results. Some people seem to acquire an immunity against the poison. ”

“ 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 [Pg 240] to keep clean—the notorious filth disease has decreased or disappeared. ”

“ The next important step was taken in 1889 when Smith and Kilbourne conclusively showed that the so-called Texas fever of cattle, in the United States, is due to an intracorpuscular blood parasite transmitted exclusively by a tick. This discovery, antedating by eight years the work on the relation of the mosquito to malaria, had a very great influence on subsequent studies along these lines.
While much of the recent work has dealt with the true insects, or hexapods, it is now known that several of the most serious diseases of animals, and at least two important diseases of man are tick borne. ”

“ 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. ”

“ The House-fly As a Carrier of Disease
Up to the past decade the house-fly has usually been regarded as a mere pest. Repeatedly, however, it had been suggested that it might disseminate disease. We have seen that as far back as the sixteenth century, Mercurialis suggested that it was the agent in the spread of bubonic plague, and in 1658, Kircher reiterated this view. In 1871, Leidy expressed the opinion that flies were probably a means of communicating contagious diseases to a greater degree than was generally suspected. ”

“ Since Glossina palpalis acts as an essential host of the parasite and the chief, if not the only, transmitter, the fight against sleeping sickness, like that against malaria and yellow fever, becomes primarily a problem in economic entomology. The minutest detail of the life-history, biology, and habits of the fly, and of its parasites and other natural enemies becomes of importance in attempts to eradicate the disease. ”

“ 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. ”

“ Other Stinging Forms—Of the five thousand, or more, species of bees, most possess a sting and poison apparatus and some of the larger forms are capable of inflicting a much more painful sting than that of the common honey-bee. In fact, some, like the bumble bees, possess the advantage that they do not lose the sting from once using it, but are capable of driving it in repeatedly. In the tropics there are found many species of stingless bees but these are noted for their united efforts to drive away intruders by biting. ”