Parasitology

“ The relation of bacteriology and parasitology to the infectious avian diseases as mentioned before, is fundamental. If bacteriologists and protozoologists will enter upon this field of avian diseases as a basis for their research in their respective fields, the results of their investigations will lead to an improvement in the conditions of our poultry yards, and give facts which are necessary before any treatment can be found. With men of this character at work upon avian pathology, success is inevitable. ”

“ Besides those from the intestines of animals and birds, cultures were successfully made from the following natural sources: lichen soil, moss soil, morainic mud, guano, ice and snow. The results may open some new problems in bacteriology.
Of recent years much attention has been given to the study of parasites—parasitology. Parasites may be external, on the skin; internal, in the alimentary canal; or resident, in the corpuscles of the blood. In tropical countries, where there is great promiscuity of life, one is led to expect their almost universal presence. ”

“ Other Parasitic Animals cause Disease.—Besides parasitic protozoans other forms of animals have been found that cause disease. Chief among these are certain round and flat worms, which have come to live as parasites on man and other animals. A one-sided relationship has thus come into existence where the worm receives its living from the host, as the animal is called on which the parasite lives. Consequently the parasite frequently becomes fastened to its host during adult life and often is reduced to a mere bag through which the fluid food prepared by its host is absorbed. ”

“ A number of diseases in man and other animals are due to the presence of these Sporozoans, for they are all parasitic. Few if any animals are exempt from their attacks. They even attack other minute Protozoa. One hundred and fifty-seven species have been recorded as attacking insects, one hundred species attack birds, fifty-two reptiles, eighty crustaceans, twenty-two fish, and so through the list. Ten have been recorded as attacking man. In some instances the parasite is always present in the host and some hosts may harbor several different species of Sporozoa. ”

“ One other point in regard to the relation of the parasite to its host and this part of the subject may be dismissed. From our standpoint we look upon the presence of parasites in the body as an abnormal condition. From a biological standpoint their presence there is perfectly normal; it is a necessary part of their life. We think that they have no business there, but from the viewpoint of the parasites their whole business is to be just there. If they are not, they perish. ”

“ Then, too, the parasite often gives up all its independence and becomes wholly dependent on its host or hosts not only for its food but for its dissemination from one animal to another, in order that the species may not perish with the host. But in return for all this it has gained a life of ease, free from most of the dangers that beset the more independent animals, and is thus able to devote its whole time and energy to development and the propagation of the species. ”

“ Species that are unable to live independently of a host and are permanent parasites are usually the most injurious to the animal. This is especially true of parasites that are capable of puncturing the skin or burrowing into it. Temporary parasites may cause fatal forms of disease. This is true of the larva? of the sheep bot-fly, which develop in the sinuses of the head, causing severe inflammation of these parts, nervous symptoms and death. The character of the symptoms of a parasitic disease depends on the habits of the parasite, and the tissue or organ, that it may attack. ”

“ It doubtless has some home outside the blood of animals, and very likely it may pass through other stages of a metamorphosis in the bodies of other animals. Most parasitic animals have two or more hosts upon which they live, alternating from one to the other, and that such is the case with the malarial parasite is at least probable. But as yet bacteriologists have been unable to discover anything very definite in regard to the matter. Until we can learn something in regard to its life outside the blood of man we can do little in the way of devising methods to avoid it. ”

“ Having found a suspected parasite in the blood or flesh of a patient, we can not accuse the parasite with certainty of being the cause of the disease, unless we can separate it entirely from the fluids and cells of the diseased body without depriving it of its virulence. In some cases it is not easy, if possible, to cultivate the parasite outside of the body; in other instances it can be readily accomplished. Of course, all such attempts require scrupulous care to prevent contamination from other germs that might accidentally be introduced into the same soil. ”

“ Early in 1898 (as announced by me at the meeting of the British Medical Association, in Edinburgh, in July, 1898) he showed that if a particular species of mosquito be fed on the blood of plasmodium-infected birds, the parasite, which both in habit and structure closely resembles the malaria parasite of man, enters the stomach wall of the insect, grows and sporulates there, and that the resulting sporozoites subsequently enter the salivary gland of the insect, and that the insect is then capable of infecting by its bite other birds. ”

“ The larval and adult stages of endoparasites become similarly specialized, and although there are many cases in which the parasites that excite a disease in one kind of animal are able to infect animals of different species, the general tendency is in the direction of absolute limitation of one parasite, and indeed one stage of one parasite to one kind of host. The series of events seems to be a gradual progression from temporary or occasional parasitism to obligatory parasitism and to a further restriction of the obligatory parasite to a particular kind of host. ”

“ If any argument were necessary to show how desirable it is to furnish full descriptions of the commoner kinds of parasite, I could adduce numerous instances that have been brought under my notice where professional men and others have been entirely mistaken as to the essential nature of their parasitic finds. Thus, I have known an instance where a great authority on the diseases of dogs has persisted in asserting for the free proglottides of a tapeworm a nematode origin; and, in like manner, human tapeworm-segments have frequently been mistaken for independent fluke parasites. ”

“ But in the midst of this varied kind of food there is always some species which forms the staple of the usual bill of fare, and it is necessary to find out what this is if we wish to ascertain the parentage and the metamorphoses of the parasite, since it is that which conducts the parasite to its new destination. The mouse is destined to the cat, and the rabbit to the dog; in the same manner, each one of the herbivora is intended to be the prey of a carnivorous animal, if not larger and stronger than itself, at least more cunning. ”

“ He discovered and described developmental stages of the parasite within the intestine of the insect. In other words, the tsetse-fly (in nature, Glossina morsitans) , serves as an essential host, within which an important part of the life cycle of the parasite is undergone. These conclusions were quickly verified by Bruce and numerous other workers and are no longer open to question. Klein and Taute are even inclined to think that mechanical transmission plays practically no rôle in nature, unless the fly is interrupted while feeding and passes immediately to a new animal. ”

“ Effect of Parasitism on the Parasite.—The usual effect of parasitism on the parasitic organism is that the various organs necessary for a free life tend to degenerate, whilst there is a multiplication and development of organs of adhesion, by means of which the parasite maintains its hold on its host. There is further an immense increase in the powers of reproduction, which may take the form of an increase in the number of fertilised eggs produced, or the parasite may at some time of its life reproduce asexually, by budding, or fission, or parthenogetically. ”

“ The parasite is without mouth, anus, or excretory pore. The eggs hatch out in the uterus of the mother, and the larvae are male and female; they make their way into the body-cavity of the host, and here they pass an unusually long time, five months, soaking in osmotically the nutriment contained in the blood of the insect. Eventually they burrow through the walls of {151}the intestine, and leaving the body of their host through the anus, find their way to the earth. Here, according to zur Strassen, the females die without playing any part in the perpetuation of the species. ”

“ They are not bacteria, but little animal bodies resembling the amœba—tiny little portions of protoplasm. The parasite in its earliest form is a small, clear, ring-shaped body inside the red blood-corpuscle, upon which it feeds, gradually increasing in size and forming within itself blackish grains out of the coloring matter of the corpuscle. When the little parasite reaches a certain size it begins to divide or multiply, and an enormous number of these breaking up at the same time give off poison in the blood, which causes the paroxysms of fever. ”

“ 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. ”

“ Parasitism is a phenomenon common in all the large groups of animals, though the parasites themselves are mostly invertebrates. There are parasitic Protozoa, worms, crustaceans, insects, and molluscs, and a few vertebrates.
Some parasites, like the fleas and lice, live on the surface of the body of the host. These are called external parasites. Others, as the tapeworms, live exclusively inside the body; such are called internal parasites. Again, some, as the bird-lice, which are external parasites feeding on the feathers of birds, spend their whole lifetime on the host ”

“ Some years since, no one suspected that a parasite could live in any other animal than that in which it was discovered. All helminthologists, with few exceptions, looked upon worms in the interior of the body as formed without parents in the same organs which they occupy. Worms which are parasites of fish, had been seen a long time before this in the intestines of various birds: experiments had even been made to satisfy observers of the possibility of these creatures passing from one body to another ”

“ 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. ”

“ It must be obvious that in all cases where the intermediate host can be captured and destroyed, the life-cycle of the parasite can be broken and interrupted, and if thus broken, there is an end to the further propagation of the species. The knowledge that we have acquired by experimental research in this connection has already enabled us to set a limit upon the prevalence of certain well-known disorders, such as Trichinosis, Cestode-tuberculosis, and so forth. ”

“ While of course most of these microbes are to be regarded as absolutely harmless and some as very useful, many have the power to do much injury if [Pg 11] the proper conditions for their rapid development should at any time exist. While the size of the parasite is always a factor in the damage that it may do to the host the factor of numbers is perhaps of still greater importance because of the power of very rapid multiplication possessed by so many of the smaller forms. ”

“ 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. ”

“ When an animal enters a new habitat, or comes into new associations with other organisms, it may be invaded by a microbe or by some larger parasite to which it is unaccustomed and to which it can offer no resistance. With many parasites a "live-and-let-live" compromise is arrived at, but new parasites are apt to be fatal, as man knows to his cost when he is bitten by a tse-tse fly which infects him with the microscopic animal (a Trypanosome) that causes Sleeping Sickness. ”

“ Rowley-Lawson is of the opinion that the greater red cell destruction than would be represented by percentage of cells showing parasites is explained by parasites migrating from cell to cell so that many red cells may be destroyed by a single parasite.
4. Transmission to larvae.—There has been an idea that sporozoites might enter the ovaries and ova as well as the salivary glands so that a second generation of mosquitoes might transmit malaria. There is no proof that such a method is ever operative. ”