The flukes of the dog are few in number. Perhaps the most important is Distoma conjunctum, originally discovered by myself in an American fox (Canis fulvus) that died at the Zoological Gardens. Lewis, eleven years afterwards, found it in the pariah dogs of India, where it is of frequent occurrence. It infests the bile ducts. As already stated, Prof. McConnell subsequently found this entozoon in man (1875), a second instance being recorded later on (1878). Another species of canine liver fluke has been described by Prof. Ercolani (D. campanulatum), besides which there is the winged species occupying the small intestines (Holostoma alatum). This latter is also found in Canis azaræ.
The tapeworms of the dog are not only numerous, but also particularly injurious, alike to their bearers and to mankind. By experimental research we have ascertained the sources of most of the Tæniæ. The serrated species (T. serrata) is derived from Cysticercus pisiformis infesting hares and rabbits. This is common in sporting animals, owing to the careless practice of allowing gamekeepers and kennel masters to throw the fresh viscera of the intermediate hosts to the dogs. I have witnessed this stupid habit in the field. The cucumerine tapeworm (T. cucumerina) is, by most observers, considered to be identical with the T. elliptica of the cat. I regard it as a variety. This delicate species is excessively common and is now, through Melnikow’s discovery, known to be derived from the louse of the dog (Trichodectes latus). This circumstance affords a curious illustration of the fact that an ultimate host may carry the intermediate host upon its back.
Fig. 57.—Tænia cucumerina. a, Strobile; b, head (enlarged); c, proglottoid, showing the sexual orifices; d, termination of the reproductive organs on one side. Magnified 60 diameters. Original.
As regards the relative prevalence of these tapeworms in England, it may be said that whilst T. serrata occurs almost entirely in our harriers, greyhounds, sheep-dogs, and lurchers (taught to “pick up” hares), the T. cucumerina is liable to infest any variety of dog, and probably infests nearly 70 per cent. According to Krabbe the prevalence of the last-named species is 57 per cent. in Iceland and 48 per cent. in Copenhagen, whereas the T. serrata is almost absent from those countries. The gid tapeworm (T. cœnurus) is derived from the ordinary gid hydatid infesting the brains of sheep and lambs. The polycephalous bladder-worm (Cœnurus cerebralis), so familiar to agriculturists and veterinarians, is often confounded with the ordinary hydatid infesting ruminants. Cœnuri infest the soft parts of rabbits, but it remains to be shown whether they are the same species. Possibly the Cœnurus cuniculi is merely a variety. The gid tapeworm is not very abundant in England. In Denmark it appears to be rare, occurring in 1 per cent. only; but in Iceland Krabbe found it in 18 per cent. In common with other helminthologists, I have frequently reared this and the serrated species by worm feedings administered to dogs. The lettered tapeworm (T. litterata) is very commonly spoken of as the Tænia canis lagopodis. It was so named by Viborg, but I prefer the more distinctive nomenclature of Batsch. We know nothing, for certain, respecting the source of this entozoon. It is rare if not altogether wanting in Denmark, but abundant in Iceland (21 per cent.). I have obtained specimens from a cheetah (Canis jubatus) which died at the Zoological Gardens, and Mr W. H. Jackson, of Oxford, found it in a cat. The worm is certainly not confined to the Arctic fox (C. lagopus.).
A well-known tapeworm infests the fox which has not yet been noticed in the dog. This is the Tænia crassiceps, whose scolices (Cysticercus longicollis) reside in the viscera and soft parts of field mice and voles (Arvicola arvalis, A. terrestris, A. amphibius). This relationship was pointed out by Leuckart. Another tapeworm (T. opuntioides) mentioned by Rudolphi as occurring in the wolf, seems to be of doubtful authenticity. A formidable and not uncommon tapeworm is Tænia marginata. This large species occurs in at least 25 per cent. of English dogs, whilst in Iceland its prevalence reaches 75 per cent. In Denmark about 14 per cent. only.
It is well known that the larval or scolex stage (Cysticercus tenuicollis) of the margined tapeworm resides in the sheep and dog. In a feeding experiment with five examples of this bladder worm I reared five strobiles of ten days’ growth. These immature tapeworms were each one inch long. By far the most important tapeworm of the dog, however, is the hydatid-forming species (T. echinococcus). This remarkable entozoon is the sole cause of the terrible echinococcus disease, so prevalent in Iceland and elsewhere. Experimental research, initiated by von Siebold, has explained its origin; Van Beneden, Zenker, and others have also experimented successfully. Rarely attaining a length of 1/3″, the perfect strobile is made up of only three proglottides in addition to the head, the lowermost segment being sexually mature. As hydatids (Echinococcus veterinorum or E. hominis) are found in a great variety of animals as well as in man, and as these bearers form so many kinds of intermediate hosts, it is easy to understand how readily dogs and wolves may acquire the sexually-mature tapeworm. I am in possession of hydatids from the liver of a clouded tiger (Felis macroscelis). In England the Tænia echinococcus is excessively rare, and has not been seen in any dog which had not previously been subjected to a feeding experiment. Mr Nettleship succeeded in rearing large numbers. In Iceland, Krabbe found dogs to be infested to the extent of 28 per cent., a proportion fully explaining the prevalence of hydatid disease in that country. The remaining tapeworms of the dog belong to the genus Bothriocephalus; of these, the broad tapeworm (B. latus) is best known, because it infests man. Diesing has described a variety found in the Pomeranian dog as a separate species (Dibothrium serratum). The museum of the Royal Veterinary College contains a very perfect specimen of B. latus from an English dog, but the parasite is of rare occurrence in this country. It is generally supposed that this tapeworm is derived from the consumption of fish belonging to the salmon and trout family, but Dr Fock, of Utrecht, thinks that the bleak (Leuciscus alburnus) is the usual intermediate host. I have already discussed this question at some length. Experimental proof is still wanting. In addition to B. latus the dog is liable to harbor B. cordatus, B. fuscus, and also two varieties of the last-named species (B. dubius and B. reticulatus, Krabbe). Taking the pit-headed tapeworms as a whole, their prevalence in Iceland is not considerable, amounting to about 5 per cent. only. Lastly, it may be mentioned that instances are recorded of the occurrence of the hog-measle (Cysticercus cellulosæ) in the dog. Though many have felt sceptical on this point, Gurlt’s authority is not to be lightly set aside, confirmed, as it has been, I believe, by MM. Mégnin and Leblanc.
Passing to the round worms it may be said that Ascaris marginata is, at the best, a mere variety of A. mystax of the cat, with which must also be placed A. leptoptera of the lion and other felines. The lateral appendages not only vary in breadth in these three forms, but also in the specimens obtained from each host. I have encountered examples in a dog, which measured more than six inches in length. The worm is excessively common in England, occurring in probably not less than 75 per cent., whilst in Denmark it occurs in about 24 per cent. According to Krabbe it is rare in Iceland. Its presence is at all times more or less injurious to the bearer, being a frequent cause of sickness, colic, convulsive fits, and paralysis. Occasionally the worms prove fatal to dogs by wandering into the trachea. At the Royal Veterinary College, in 1864, a litter of six puppies, of only three weeks growth, died rather suddenly in consequence of the presence of these worms in the stomach and small intestines. So far back as the year 1684 Redi described round worms from the walls of the œsophagus of a dog. These were afterwards noticed by various observers in tumours of the mucous membrane of the stomach. Owing to their red color, derived from the ingested blood of the host, the species was named Spiroptera sanguinolenta. In 1867 I suggested that the minute Filariæ found by Grube and Delafond in the blood of dogs would probably turn out to be referable to this species. The researches of Lewis have proved that this supposition was correct. To be sure, other nematoid hæmatozoa, of microscopic dimensions, occur in the dog, but those described by Grube and Delafond may be referred to Spiroptera. These authors estimated their number in the canine host to vary from 11,000 to upwards of 200,000. In one instance Messrs Grube and Delafond found six worms lodged in a clot occupying the right ventricle of the heart. Four were females and two males. Although they were described as representing an altogether new species, which they termed Filaria papillosa hæmatica canis domestici, I think there can be little doubt that they were examples of Spiroptera sanguinolenta not fully grown. The writings of Lewis abound with interesting details respecting the structure and development of this worm, and as much may be said of the writings of Manson and Welch concerning the cruel threadworm (Filaria immitis, Leidy) occupying the right cavities of the heart. I was first made acquainted with this entozoon in 1853, by examining specimens in the possession of Prof. Hughes Bennett of Edinburgh; at which time also I was put in possession of a valuable MS. (since lost) describing the ravages of this entozoon in the dogs of China. I have since received numerous verminiferous hearts both from China and Japan, and also some heart-worms from Charleston, U. S., sent by Mr M’Innes. In a recent communication, Dr Manson has spoken of this worm as if it were comparatively harmless, but all the evidence I long ago received through the late Mr Swinhoe, formerly H. B. M. Consul at Amoy, through Mr Dare’s letters enclosing Dr Orton’s valuable observations (addressed to the editor of the ‘Field’), through the lost MS. above alluded to, through Dr Lamprey’s statements, and through many other sources, lead to the very opposite conclusion. No doubt the canine hosts do for a time appear to be little inconvenienced by their nematode guests, but sooner or later the most distressing symptoms set in. As in Hoysted’s case (quoted below) the convulsive spasms may occasion death in a few minutes, but frequently they last for hours or days, with more or less prolonged intervals of relief before the final struggle.
Fig. 58.—Filaria immitis. Tail of male. Enlarged. Original.