Strongylus vulgaris infection in the horse

Duncan, James L (1973) Strongylus vulgaris infection in the horse. PhD thesis, University of Glasgow.

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The work described in this thesis is concerned with studies on various aspects of infection with one of the most common and pathogenic of the equine helminths, Strongylus vulgaris. In order to facilitate these experimental studies, an efficient technique was developed for the production of large numbers of S. vulgaris infective larvae in pure culture; in this, third stage S. vulgaris larvae were obtained by surgically transferring adult worms, collected from horses immediately after slaughter, directly into the caeca of worm-free yearling ponies. Subsequently, various investigations on the life-cycle, pathogenesis, epidemiology and immunity of S. vulgaris infection were carried out. Considerable controversy existed concerning the migratory route taken by developing parasitic larval stages of S. vulgaris in the tissues of the host. In order to elucidate this, worm-free pony foals were infected with a pure culture of infective larvae and killed at intervals over a period of 9 months. The results showed that infective larvae exsheathed and penetrated the intestine within a few days of infection. These larvae then moulted in the sub-mucosa, penetrated small arteries and had migrated within the lumina of the intestinal arteries to the anterior mesenteric site by 14 days after infection; in this predilection site larvae developed from early to late fourth stage. After a period of 3 - 4 months the fourth moult was completed and the young adults returned to the intestine again via the lumina of the arteries. Nodules were formed with the subsequent release of young adults into the intestinal lumen. The pre-patent period was 6-7 months. Studies on the pathogenesis of S. vulgaris infection showed that a distinct clinical syndrome occurred within 3-4 weeks of infection in foals given doses of 750 infective larvae. This syndrome was one of pyrexia, anorexia and colic associated with the presence of fourth stage larvae in the intestine and mesenteric arteries. Examination of tissues taken from foals killed over a period of 9 months demonstrated the sequence of pathological changes. During the course of these studies infected foals showed a marked polymorphonuclear leucocytosis and an eosinophilia together with an increase in the serum globulin levels; the latter appeared to be due mainly to increases in the p -globulin levels. Subsequently using 125I labelled albumin and 51Cr labelled red blood cells it was shown that the rate of albumin catabolism was increased and the red cell survival time reduced in ponies with patent infections. The epidemiology of equine helminthiasis was studied in 2 separate systems of management over 2 years. Basically 2 regimens were employed: one where untreated mares and foals went out in the spring to pastures never previously grazed by horses while in the other, mares, regularly treated with anthelmintics, and their foals grazed contaminated pasture. These systems allowed 2 sources of infection for susceptible foals: where mares were treated, the main source of infection was from overwintered larvae; where infected mares went out to clean pasture infection of the foals could only occur from larvae which developed from eggs passed in the faeces of the mare. The results demonstrated the following points. First, foals going out to grass in the spring can be infected by both overwintered larvae and by eggs passed in the faeces of their dams but the latter is by far the more important source of infection. Secondly, where horses remain untreated throughout the grazing season high levels of third stage strongyle larvae develop on pasture, whereas regular anthelmintic treatment at intervals of 2 - 4 weeks reduces pasture contamination to a minimum. Thirdly, although high levels of infective larvae may occur on pasture grazed by infected animals during the summer and autumn, these levels fall during the winter and by May the following year virtually no third stage larvae can be recovered from these pastures. The results of a series of experiments on the immunity to S. vulgaris infection provided the following Information. First, an age immunity was demonstrated when S-year-old worm-free ponies were infected with S. vulgaris larvae. No clinical signs or gross arterial lesions developed in these animals and no fourth stage larvae were recovered at necropsy. Secondly, a degree of acquired immunity in terms of resistance to reinfection was demonstrated in groups of naturally and experimentally infected animals. Finally, the results of a preliminary vaccination experiment have shown that the arterial lesions are less severe and that there is an 80% reduction in larval numbers in vaccinated foals compared to challenge controls.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: G M Urquhart
Keywords: Veterinary science, Animal diseases, Parasitology
Date of Award: 1973
Depositing User: Enlighten Team
Unique ID: glathesis:1973-73217
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jun 2019 08:56
Last Modified: 14 Jun 2019 08:56

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