A diagnostic test that promises to help identify horses infected with the larval stages of the large strongyle, Strongylus vulgaris, has been developed, as concerns grow the parasite may be making a comeback.
The test is the result of collaboration between scientists in Denmark and the United States, the latest issue of Equine Science Update reports.
Strongylus vulgaris, is the most pathogenic of the large strongyles, or large redworms. Damage is caused by the larval stages of the parasite, which migrate in the arteries that supply the large intestines. The larvae cause endarteritis and thrombosis and may result in blockage of the arteries and intestinal infarctions.
By the time eggs are detected in the faeces, the damage has already been done. A significant feature of the S vulgaris life cycle is the long prepatent period – the time taken for the larvae to mature into egg-producing adults. This can take up to six months or more. During this time the migrating larvae are virtually undetectable with current tests.
Colic due to damage caused by S vulgaris used to be common before the advent of effective anthelmintics. The interval worming schemes that became popular were very effective at controlling this parasite.
There is concern that S vulgaris may be making a comeback.
An accurate test that could detect migrating larvae during the pre-patent phase would be a useful tool for controlling the disease. Scientists in Denmark and the United States have been working to develop such a test.
The result of their research is a serum enzyme-linked immunosorbent assay (ELISA) which has been validated for detecting migrating larvae in the bloodstream of horses.
The test detects antibodies against a polypeptide in the excretory/secretory fraction of the migrating larvae, named SvSXP.
Further work using the test showed that foals receive colostral antibodies from the mare which may persist for up to 13 weeks of age. Foals start being able to mount their own immune response to the parasites from about three months of age, depending on the grazing conditions and the stage at which they were exposed to the parasites.
In younger foals, the serum ELISA does not give reliable diagnostic information, despite heavy infections. The researchers found that young foals can be heavily infected without showing positive ELISA test results.
They also found that higher infection levels tended to be associated with higher ELISA levels.
Once they had started producing antibodies, foals continued to maintain high ELISA levels throughout the study. There was a tendency for the ELISA levels to decline gradually as the numbers of migrating larvae declined and adult intestinal worm burdens increased.
The scientists suggest that further research into the time taken for antibody levels to decline after effective anthelmintic treatment would be very useful.
If the test becomes commercially available it will be a useful addition to the armoury of diagnostic tests for investigating parasitic disease in horses.
U.V. Andersen, D.K. Howe, S. Dangoudoubiyam, N. Toft, C.R. Reinemeyer, E.T. Lyons, S.N. Olsen, J. Monrad, P. Nejsum, M.K. Nielsen (2013)
SvSXP: a Strongylus vulgaris antigen with potential for prepatent diagnosis.
Parasit. Vectors, 6 p. 84
Nielsen MK, Vidyashankar AN, Gravatte HS, Bellaw J, Lyons ET, Andersen UV.
Development of Strongylus vulgaris-specific serum antibodies in naturally infected foals.
Vet Parasitol. 2013 Dec 31. pii: S0304-4017(13)00691-2.