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On the Life Cycle of Strongyloides westeri in the Equine

1973; American Society of Parasitologists; Volume: 59; Issue: 5 Linguagem: Inglês

10.2307/3278405

1937-2345

Ε. T. Lyons, J. H. Drudge, S. C. Tolliver,

Parasites and Host Interactions

Adult Strongyloides westeri infections in foals appear to originate mainly from larvae passed in the milk of mares. Larvae were found in milk samples of 32 mares for 4 to 47 days postpartum. No larvae were found in colostrum samples. There was no evidence of prenatal infection of foals. Periodicity was observed in the number of larvae passing in the milk; for 5 mares, sampled over 24-hr periods, 65.5 to 88.9% of the larvae were found in the AM and 11.1 to 34.5.% were found in the PM. The prepatent period of S. westeri in worm-free foals was 10 to 14 days when free-living third-stage larvae were given and 8 to 12 days when parasitic third-stage larvae were given. Most naturally infected foals sampled began passing eggs at about 2 weeks of age and they lost infections of adult worms at about 20 to 25 weeks of age. Under natural conditions, free-living third-stage larvae appear to play a minor role in producing infections of adult worms in foals. Three wormfree foals died after being given several million free-living third-stage larvae. Administering high numbers of freeliving third-stage larvae to preparturient mares did not noticeably increase the numbers of larvae shed in the milk. Slight differences were noted in measurements of free-living and parasitic third-stage larvae. It has long been known that foals pass eggs of Strongyloides westeri at a very young age. Since eggs of this parasite are not found in feces of adult horses and an environmental source of infection appeared unlikely (Russell, 1948), the means of transmission was unclear until recently when a few preliminary observations were reported (Lyons, Drudge, and Tolliver, 1969) on larvae of S. westeri being found in milk of mares. Other reports of passage of larval nematodes through the mammary system include Uncinaria lucasi of fur seals (Olsen and Lyons, 1962, 1965), Strongyloides ransomi of pigs (Moncol and Batte, 1966), and Ancylostoma caninum of dogs (Stone and Girardeau, 1966) found in the colostrum, and Toxocara canis of dogs (Stone and Girardeau, 1967), Neoascaris vitulorum of cattle (Warren, 1969), Strongyloides papillosus of cattle and sheep (Lyons, Drudge, and Tolliver, 1970), and Toxocara cati of cats (Swerczek, Nielsen, and Helmboldt, 1971) found in the milk. The object of the present paper is to provide more details concerning the earlier observations on S. westeri and to present new data on the life cycle of this parasite. Received for publication 6 March 1973. * The investigation reported in this paper (73-4-48) was made in connection with a project of the Kentucky Agricultural Experiment Station and is published with the approval of the Director. Supported in part by a grant from The Grayson Foundation, Inc., Lexington, Kentucky 40501. MATERIALS AND METHODS This investigation included Thoroughbreds, Standardbreds, Shetland Ponies, Hackney Ponies, draft horses, and mixed light horse breeds. Natural infection of foals kept with mothers Feces was examined daily from 83 naturally infected foals from 7 through 16 days of age to establish when S. westeri eggs were first present. Fecal egg counts were determined for 27 other foals at weekly intervals up to 19 to 34 weeks of age and for 24 additional foals, either weekly or biweekly, through 23 to 45 weeks of age. Natural infection of foals taken from mothers soon after birth A total of 157 pony foals were removed from their mothers at 1 to 11 days after birth. Eightythree of the foals were taken from their dams at 1 day of age, 44 at 2 days, 15 at 3 days, 4 at 4 days, and the other 11 at 5 to 11 days. Fecal samples were examined, usually at weekly intervals, for presence of S. westeii eggs in order to determine the incidence of infection. Another pony foal (68-OL-3) was taken from its mother at birth, before it nursed. It was fed colostrum collected fresh from its mother for 19 hr and then fed other mare milk that had been collected previously and frozen for at least 3 weeks. The purpose of this diet was to simulate the natural diet. Daily fecal samples were examined for eggs. At 19 days of age it was killed and the small intestine was examined for S. westeri adults and larvae. Parturition of mares in isolation To evaluate whether foals become infected by their dams, 7 pregnant mares were put in isolation where free-living third-stage larvae were unlikely to be present. Two of the mares were put in stalls previously uninhabited by horses, 2 in concrete-