Part 58

“Judging from reports and based upon the observations of Morse upon Grouse disease it would seem that the incubation period of the disease is about eight to ten days. However, one of the third lot of our birds died within three days of its arrival at this Garden, and therefore within three days of its exposure to the second arrivals; if it be correct that this second lot brought the disease and the third lot did not have it, it would seem that the incubation period can be as short as three days; how long it may be is only suggested by the fact that some of the third lot did not die for three weeks after arrival and exposure. All the Gambel’s and scaled quail succumbed to the disease, but two of the twelve bobwhite survived. It would seem that although these last birds probably introduced the disease, they still possessed more resistance than the others, for the second death among them occurred seventeen days after the first death. The epidemic as we have seen it here seems to be the same as Grouse disease of Scotland and as the Grouse disease in this country as reported by Morse (Bureau of Animal Industry Report 109, May 18, 1907).

“The means of transmission of the disease is not exactly known, but is in all probability by a pollution of the food, the water supply or the ground. Since the lesions are so marked in the lower ileum, cecum and colon, a possible transmission by cohabitation must not be entirely overlooked. There does not seem to be any means of limiting the epidemic in a flock by segregation or sacrifice of the infected birds, because symptoms are few and do not appear until shortly before death. Each bird would have to be put into a separate cage until proved infected. Scrupulous cleansing of the enclosure is desirable, but its efficiency is difficult to estimate.”

SECTION XVIII THE ANIMAL PARASITES, THEIR INCIDENCE AND SIGNIFICANCE

FRED D. WEIDMAN, M. D.

It is quite to be expected that animal parasites would be found in the animals of zoological gardens, garnered as these beasts are from all parts of the world, tropical and otherwise. It inevitably follows that many of the forms should be strange and new, enticing one to the fascinating determination of their identity, life history and hygienic importance; and, developing from all this, one can easily imagine how limitless the opportunities are for scientific work in parasitology in a laboratory like ours.

As in other biological fields, the taxonomic range of parasites here is wide. It extends from the lowly protozoa to the insecta, and, dropping to the smaller subdivisions, includes not only most of the genera familiar to human parasitology but many known only among the lower animals. From the standpoint of the host, the biologic state of parasitism extends from the lowest protozoa to homo.

The above will suffice to indicate the wide range of parasitism in animals, but the extent of work actually done thus far in wild animal material is a different story. Collated, consistent studies, so far as I am aware, have been undertaken only at the London Garden, here at Philadelphia, and at Washington, D. C., by Dr. Charles W. Stiles and Albert Hassal. The data, collected by the last mentioned workers are incidental to the Index Catalogue of Veterinary and Medical Zoology, and embrace only the (index) phase indicated by the title, but it is so valuable, and withal so altruistic, that it must be credited. What other work there is is scattered where—not in literature—general biological, medical and veterinary. That at London has been conspicuous through the observations of Plimmer and of Beddard on filariæ and cestodes respectively, while the work of Nicoll must not fail of mention.

That the reader may the better appraise the sections of our own work which are to follow I wish at once to indicate their material basis. Ordinarily only the larger parasites are looked for at the autopsy table and there must be special indications to demand search for the finer ones. Those of microscopic size, or so minute as to be overlooked in the guise of seeds, vegetable fibres, etc., have not, both here and elsewhere, been routinely studied as have macroscopic ones.[107] From our autopsies there have accumulated records of nearly 900 parasites— some determined generically, others but as to order. The parasites have in greatest part been preserved and are available for further study; in the past, special groups have been culled out from time to time and examined. Where conditions have been pressing, as in certain epizoötics, investigations have amounted to more than observations and descriptions, and received detailed laboratory examinations with more or less animal experimentation as the occasion demanded.

The foregoing may suffice to apprise the reader that the subject of wild animal parasites has been but broached so that data are especially incomplete on life histories—a phase most important in relation to hygiene; but in spite of this and although the statistics are only approximate, as is the case in most parasitological work, these data have attained to sufficient proportions to justify at least a beginning in the matter of collating and generalization. At any rate the time has arrived to establish at least a nucleus for the accretion of data, which can be later subjected to confirmation or correction. We draw just a grain of comfort from the knowledge that the more fully worked field of human parasitology is also vulnerable to criticism of very much the same order.

THE VALUE OF PARASITOLOGICAL STUDIES IN ZOOLOGICAL GARDENS.

The foregoing chapters have made clear two fields of practical usefulness of any study in such gardens. These—hygiene in relation to the animals and comparison in relation to human beings—need therefore only to be mentioned at present since it is obvious that both benefit by our parasitological work. But there is yet a third—a scientific phase of parasitology which may be considered purely academic. It consists in morphological and other studies necessary for the identification of the parasite, the determination of its life history, etc. These last studies may still in a restricted sense include a modicum of the practical in so far as they have a bearing on the disease with which they are associated. But on the whole they are a source of danger for us since such things as studies on the finer structures of worms, taxonomic arrangements, descriptions of new species of commensals, etc., being alluring, are likely to lead one so far afield that eventually an attitude of stubborn resistance will have to be assumed in order to conserve that precious, volatile laboratory asset—time—for the more crying, practical problems ever reaching out to us.

However, in parasitological investigations as in other scientific work, immediate abstract information may at some time prove to be of greatest practical value. Thus for example if we can discover the exact facts concerning one phase of the life history of a certain parasite, it may be possible by hygienic measures, to break the cycle of development of the parasite at one point thereby preventing its completion. This information is perhaps obtained most readily in experimentation upon the rôle of lower animal forms in the pathogenesis of disease but where reliable evidence is lacking, help may be had by comparison with others in the same taxonomic group. Undoubtedly systematic classification will go far to help solve many of these riddles.

PATHOGENICITY OF ANIMAL PARASITES IN GENERAL.

The first question which arises in this connection concerns the actual ability of animal parasites to produce disease in wild animals. At once it will be seen that this must be a relative matter, for no one on one side would contend that every symbiont in an animal is harmful—parasites _sensu stricto_—nor on the other that none could possibly be, _i.e._, that all are always commensals. It is evident that the issue boils down to questions as to the extent to which they are harmful. Before attempting the answer let us consider the means by which the parasites may conceivably produce disease.

MODES OF DISEASE PRODUCTION (PATHOGENESIS).

The medical reader is familiar enough with the pathogenic powers of some animal parasites, but may be sufficiently interested to glance over specific wild animal instances illustrating them while they are being listed for those less familiar with this subject.

1. MECHANICAL OBSTRUCTION.

I refer here particularly to simple blockage of normal body passages as the result of bulk or mass. This occurs more commonly in the intestines than elsewhere on account of the greater frequency, greater numbers and larger size, in general, of parasites inhabiting this tract. Thus, we have recorded a liothrix (_Liothrix luteus_)[108] where the combination of a small host and consequently narrow gut and comparatively large parasite induced obstruction. Plimmer[109] records microfilaria clogging the brain capillaries. Shipley[110] mentions two specimens of _Ascaris lumbricoides_ obstructing the nares of a chimpanzee (_Pan niger_). Blockage may also be produced secondarily to the presence of the parasite, even in the absence of notable numbers of them, and quite apart from the element of verminous bulk. This occurs through inflammatory swellings which the worms excite. We saw many serious grades of this in our spiroptera epizoötic, the lumen of the proventricle being narrowed by swelling of the mucosa and more or less occluded by exudate and necrotic mucous membrane.

Yet another direction wherein a mechanical rationale pure and simple obtains is by the production of diverticula. Worms encysted in the gut wall may, by weight alone or by excitation of peristalsis, cause the wall to bulge outwards (or inwards even) like a pocket. Such a diverticulum has been noted in the gut of a Pale Cebus (_Cebus flavescens_)[111] parasitized by acanthocephalus, but in this case there were adhesions to the nearby stomach, and it is possible that in this individual case the diverticulum was a traction one, _i.e._, pulled out by the anchorage of adhesions externally.

2. MECHANICAL IRRITATION.—In those instances where inflammation is the manifestation which reflects the simple mechanical effects of parasites it will be difficult indeed to prove, in the present state of our knowledge, that it is not rather the effect of associated toxic substances or excreta elaborated by the parasite. But instances of a purely mechanical irritation there must be, although one can scarcely put the finger upon them and say that this or that individual inflamed mucosa did not become so from a toxic cause. Omitting these then, the more certain, purer, more unequivocal examples will be those where physiological processes become exalted as the result of the parasitic irritation. An example in point is a case of volvulus in a Screech Owl (_Otus asio asio_).[112] Here it is probable that the parasites excited the gut to undue peristaltic action, and that during this process it became twisted. Worms in such passages as the nose and nasal sinuses (I have seen blood-red filariæ in the frontal sinuses of an opossum) undoubtedly produce nervous effects by their presence and movements. Those in the subcutaneous tissue (filariæ of wild cats) probably also do so. It is difficult to judge those cases where doubtfully sensitive parts are the ones affected. Probably the intestinal and intraperitoneal worms, and less certainly the generally-migrating ones analogous to _Filaria loa_, produce no nervous effects mechanically.

3. PRODUCTION OF HEMORRHAGES.—Hemorrhages large enough to kill suddenly are theoretically possible, since worms occasionally produce aneurysms which may rupture; we have seen such an accident in a Paradoxure (_Paradoxurus leucomystax_). But certainly it is the long continued, wasteful small hemorrhages that are important, inducing an anemia often of severe and fatal grade. The hookworms are the shining offenders here, yet we have seen very much the same effect from Acanthostoma in the intestine of monkeys. Œsophagostomum has also been incriminated at the London Garden in young Rhesus Macaques (_Macacus rhesus_)[113] where the young forms of the parasite did the damage as they burrowed into the wall of the gut.

4. OPENING UP AVENUES OF INFECTION.—This may be accomplished either by passage of parasites from one position normally containing bacteria to another which is susceptible to infection, or by devitalizing a tissue which is ordinarily resistant to infection; _i.e._, creating a _locus resistentiæ minoris_. The intestinal tract is the most common organ concerned, but the illustrations to follow will give variety. Thus, the mature examples of œsophagostoma in young rhesuses just referred to above burrowed into the gut wall and led to both local and general peritonitis. In one of our “spiroptera” parrots the worm had passed through the proventricular wall and a chronic fibrosis resulted around it. At the autopsy on a Rhesus Macaque Doctor Fox found a localized abscess adjacent to the gut wall, and in it a whipworm was imbedded. Passing from these examples of intestinal worms, I can mention the loss of a valuable Philippine Spotted Deer (_Cervus alfredi_) as the result of secondary infection of a cysticercus cyst of the lesser omentum which led to a nearby peritonitis. Lung infections are not uncommon. Murray[114] records that forty-four out of eighty-five young rhesus monkeys dying from pneumonia showed an acarian, and he ascribed the pulmonary irritation to certain crystals in the excreta of the mite. I have studied a case of bronchopneumonia in a prairie dog where great numbers of an arachnid were present. The reports of the London Zoological Society are replete with notes of round worm pneumonias of reptiles. These pulmonary cases must result from decreasing of tissue resistance by the presence of the worms, and are easy to understand, much more so than the intestinal infections when one recalls how sensitive lung tissue is to foreign bodies, and that there seems to be no indication that this tissue becomes accustomed to infestation such as may be argued for the gut. All these citations must convince us that parasites are most important predisposing agents to infection, and that this is one of the most sinister phases of animal parasitism.

5. DESTRUCTION OF TISSUE.—This heading does not refer to the comparatively trivial effects that accompany the more acute inflammations secondary to parasites, albeit certainly the absorption of their disintegrative tissue products has some effect on the economy; but our ideas of such are so vague as to justify their being disregarded here. What I refer to is the more massive destruction such as may occur in the blood, for instance, from the action of protozoa. There is also loss of mucosa in those chronic infestments of the stomach where we find excessive fibrous tissue overgrowth. The most striking example of tissue destruction we have seen was in the cirrhotic livers of prairie dogs affected by _Hepaticola hepatica_, where in extreme cases, the amount of functionating liver substance was reduced to a very small fraction of its normal bulk.[115]

6. TOXINS.—We have no direct evidence to offer that noxious products of parasites are concerned in producing disease in wild animals. The local effects of such toxins are not distinctive enough—individual enough to toxins or to the animal body—to separate them from the effects of such accompanying factors as bacterial inflammations; nor can we separate the general effects of these toxins from what might have been, for instance, the effects of an accompanying anemia of hemorrhagic or other origin. From a knowledge of what happens in human prototypes though, there is scant doubt that some one of the multitudinous species must be capable of producing toxins, but just which varieties are concerned cannot be listed by anyone. By analogy we can at most only suspect the hookworms and the dibothriocephalidæ. Under this same category of the toxins come the worm-products which are reputed to have a destructive effect upon the digestive enzymes in the gastrointestinal tract of the host, and which would thereby interfere with the proper assimilation of pabulum, resulting in malnutrition. For the same reasons as above indicated for the toxins one is unable to speak for or against these “anti-enzymes.”

7. PRECLUSION OF NUTRITION.—This must be a very unimportant phase of the activity of intestinal parasites, when one compares the bulk of food which passes through the bowel and the average number of worms present; and the same holds good for some interstitial parasites like the adult filariæ. Even in amazingly heavy infestments of the intestines one will be constrained to dismiss this idea when he compares the bulk of parasites with that of the host, and recalls what the physiologist terms the “factor of safety” inherent in this tract as elsewhere. But in the case of blood parasites the matter may be different. Here we are concerned with the withdrawal of refined foodstuffs—those which have been worked over and over by subtle internal metabolic processes; and we are not so sure, especially on recalling the enormous numbers of parasites usual to blood infestments, that there is the capacity on the part of these internal processes to meet increased demands that we count upon for the intestinal functions. It is much more serious to be deprived of the finished product than of the crude because it means the undoing of “digestive” work all along the line, from gut to tissue cell. Furthermore, a blood infestment guarantees that the parasite has been feeding upon and depriving the animal of the precise foodstuffs the cells require, and not by any chance upon, even in part, intestinal substances that were wastes or residues. If we except the blood parasites, then, it seems safe to conclude on the whole that the amount of pabulum used by parasites is unimportant to the animal.

Having reviewed the manner in which parasites may conceivably be harmful, it is time to return to the question of the actual exercise of these powers.

The older appraisal of parasites in animals, namely that they were rather innocent of disease production, was suggested by and borrowed from the veterinarian, probably being engendered in him by their frequency in what appeared to be normal domestic specimens. Yet it is only proper to add that one of our former pathologists, and sometime professor of veterinary pathology, Dr. C. Y. White, is a medical man and is of much the same opinion. Older writers regarded worms even as “guardian angels” of children. Very recently Schwartz[116] reviews some work in this connection showing that, _in vitro_, some cestode extracts were inhibitory to certain bacteria (_B. anthracis_, _B. pyocyaneus_ and _B. dysenteriæ Shiga_). This relationship is so different from natural conditions as to need no further comment.

At the London Garden the view appears to be different. In the 1910 report they charge five deaths against perforation by worms of the stomach and intestines; in the 1911 report they record giant toads dead from lung infestment; in 1912 “eighteen cases of enteritis were due to worms”; and in 1917 they mention pneumonia in a toad and perforation of the stomach of a puma. These reports represented evidently the more striking, unequivocal examples of death from parasites which had outspoken anatomical expressions, and omitted those in which the more subtle agencies of parasitic pathogenesis were concerned. Their experience has apparently been much the same as ours.

The ideal approach to a decision in reference to the importance of parasites would appear to be a mathematical one, something as follows: First, to determine what species infest animals and how commonly, then to decide which ones are pathogenic and thirdly to estimate the severity of the disease induced; so that finally, by an analysis and comparison of the three results—a comparison and analysis judicial in the broadest sense—we might hope to come to an opinion. Let us consider the three avenues in order. At the first glance it must be evident that a list of all possible parasitic varieties does not exist and may never be compiled. The most that can be done is to tabulate the findings in scattered laboratories, data usually recorded in terms of the individual observer’s studies and often inadequate to give the compiler all the facts desired. The same remarks apply to the percentage incidence of parasitism. Not to prolong the academic discussion, suffice it to say that very much the same obstacles present in the second avenue—that of pathogenicity of the individual species. Our own data referring to this second heading will be presented later, but after the failure of the first avenue, the second and third lose greatly in value. At best, statistics can be only suggestive. Unless critically and suspiciously interpreted, and with a full appreciation of their limitations from a foreknowledge of the way in which they were compiled, they would only delude the reader and offend science, and so we abandon this line of reasoning.

At present the best results of the study of pathogenesis by animal parasites will probably be reached by a combination of methods, as follows:

1. Direct. How commonly do we see clinical symptoms and morbid anatomical changes that are incontrovertibly due to the parasite? We restrict ourselves here to a narrow group of infestments indeed, and think of such diseases as trichosomiasis in prairie dogs and spiroteriasis in parrots.

2. By comparison with analogous infestments of domestic animals and man— more thoroughly studied and therefore more accurately appraised, in general, as to pathogenicity; a comparison from the standpoint of disease production rather than natural habits of the parasite. Example, coccidiosis and hookworm disease in foxes and dogs.

3. By inference through deduction. This is the most unsatisfactory consideration of all, and should be well checked up and discounted. Here we would evaluate the known propensities of the parasite first, such as its size, motility, anatomic position in the host and the general pathological traits of the genus and family to which it belongs, etc., and then compare these verminous properties with those of the host—its size, temperament, physical stamina, etc. This third consideration must necessarily overlap with or be supplementary to the first two. For example, this consideration would have to be resorted to in many cases of ascaris infestment where anatomical changes are generally not demonstrable.

Acting on these three considerations, and after twelve years of observation on parasites here in the Garden, a fresh review of our records, and a recent review of the accessible relevant parasitological literature I have come to the conclusion that, considering wild animal collections the world over, there is no justification for an unqualified, definite answer to the question of pathogenic parasitism that will meet all conditions. We lack data on too many species that are not sufficiently represented in collections or indeed not represented at all. It is the liability to infestment of each order or family of beasts that will have to be determined, and, depending on the assortment each garden has on exhibition, will the importance of parasites to the garden as a whole vary.