Equis ISSN 2398-2977

Strongylus spp

Contributor(s): Susan Dawson, Maggie Fisher, Sheelagh Lloyd, Craig Reinemeyer

Introduction

Classification

Taxonomy

  • Phylum: Nematoda.
  • Superfamily: Strongyloidea.
  • Family: Strongylidae.
  • Genus:Strongylus.
  • Species:vulgaris, edentatusandequinus.

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Clinical Effects

Epidemiology

Lifecycle

  • Adult worms in large intestine.
  • Eggs laid and passed out in feces.
  • L1 hatches from egg - develops to L3.
  • L3 ingested by horse.
  • L3-L5 carry out characteristic migration within horse that varies according to species. The larvae are migrating mainly over the winter months in northern temperate climates.
  • L5 returns to large intestine to mature.

Transmission

  • Via ingestion of pasture containing infective L3 larvae.

Pathological effects

Larvae
  • S. vulgaris:
    • Damage wall of cranial mesenteric artery and its branches during their migration.
    • In heavy infections, colic can occur about 2 weeks after infection when the larvae are migrating to the anterior mesenteric artery.
    • Cause thrombus formation and thickening of arterial wall, particularly ileocecal and ileocolic arteries   Strongylus vulgaris: anterior mesenteric artery 01 - embolus    Strongylus vulgaris: anterior mesenteric artery 02 - embolus  , and embolism   Strongylus vulgaris: emboli  .
    • Previously lesions termed 'verminous aneurism' now 'verminous arteritis'   Cardiovascular: parasitic arteritis    Cardiovascular: parasitic arteritis - pathology  as true aneurisms, ie dilation of the artery with thinning of the wall, are rare.
    • Colic may result from blockage of arterial supply to part of intestine and subsequent infarction but infarction is rare due to raid effective collateral circulation.
    • Youngstock infected with a number of larvae in the anterior mesenteric artery over winter will be tucked up, inappetant, growing poorly, with poor coat with intermittent colic signs.
    • If larvae are removed, healing of the artery occurs with time, which improves its function   Strongylus vulgaris: anterior mesenteric artery - healing  .
    • Repeated infections result in permanent thickening of the arterial wall and at this point few or no larvae are found in the artery.
  • S. edentatus:
    • Lesions may be seen on post-mortem in the liver   Strongylus edentatus: liver - fibrosis  and intestine but are rarely associated with clinical signs in the living horse, probably because horses are normally exposed to a trickle infection from the pasture.
    • Occasionally the inflammatory reaction to larvae in retroperitoneal sites   Strongylus edentatus: peritoneum - inflammation    Strongylus edentatus: hemorrhage  causes colic and even rupture of vessel and hematoma, causing colic.
  • S. equinus:
    • There do not appear to be clinical signs associated with the migrating larvae but it, withS. edentatus, can cause fibrosis in the liver after heavy infections   Strongylus spp: liver - fibrosis  .

Adults

  • The adults are plug feeders   Strongylus spp: adult - feeding  .
  • There is loss of tissue fluid and blood associated with the damage caused by the adult worms to the intestinal mucosa as they feed.
  • Ulcers left where the worms have fed will scar.
  • A small number of worms will be tolerated but large numbers (that are not common) will result in ill-thrift and anemia. However, there are rarely more than 100-200S. vulgarisadults, as their migrating stages are so pathogenic, butS. edentatusnumbers can be a bit higher.
  • Difficult to precisely identify the individual effects of each species as most natural infections include the range of species and other nematodes, particularly the cyathostomes.

Control

Control via chemotherapies


Anthelmintic resistance byStrongylusnot yet demonstrated but care must be taken.

Anthelmintic treatment

  • US:
    • Oxibendazole   Oxibendazole  , oxfendazole, fenbendazole   Fenbendazole  , ivermectin   Ivermectin  , pyrantel pamoate   Pyrantel  , are licensed in the US for the removal of adult large strongyles from the intestine; oxfendazole and levamixole/piperazine are also available.
    • Ivermectin   Ivermectin  is indicated for the removal of tissue and arterial larvae using the normal single dose and fenbendazole   Fenbendazole  is indicated at daily treatments at 10 mg/kg/day for 5 days to remove migrating larvae or as a single dose of 30 mg/kg.
  • UK:
    • Oxibendazole   Oxibendazole  , febantel   Febantel  , mebendazole   Mebendazole  , haloxon   Haloxon  , fenbendazole   Fenbendazole  , ivermectin   Ivermectin  , pyrantel embonate   Pyrantel  are licenced in the UK for the removal of adult large strongyles from the intestine.
    • Ivermectin   Ivermectin  and moxidectin   Moxidectin  are indicated for the removal of tissue and arterial larvae using the normal single dose and fenbendazole   Fenbendazole  is also indicated at daily treatments at 10 mg/kg/day for 5 days to remove migrating larvae or as a single dose of 30 mg/kg.
  • Instigation of an anthelmintic control program with regard to factors such as pasture usage, pre-patent periods of the worms, history of the horses and stables, anthelmintic spectrum and the avoidance of the induction of resistance, will assist in reducing the larval challenge over time.

Rotate doses of anthelmintic on an annual basis - not more frequently.

AddAnoplocephalaorGasterophilustreatments in years when pyrantel or ivermectin are not used. Preferably control them by other means.

Treat strategically - egg production by reactivated larvae causing the spring/early summer rise in FEC are the important source of infection - these are the eggs which perpetuate the lifecycle. Therefore, treat horses in early spring (March). Treat again at a time dependent on the drug used (4-6 weeks - benzidazole and pyrantel; 8 weeks plus - ivermectin; 12 weeks - moxidectin). Determine the time interval by FEC and treat animals individually as FEC increases above 100-200 eggs/g.

  • Worm only the wormy horses when their FEC reaches 100-200 eggs/g feces.

Control via environment

  • Regular dung collection from pasture - frequency will determine the degree of reduction of pasture larval contamination that may be achieved.
  • Even twice weekly (or weekly in a cool temperate climate) will substantially reduce numbers of larvae on the pasture. This is a very effective measure of control provided it is routinely followed and should be used preferentially over the other methods.
  • Possible future biological control using nematophagus fungusDuddingtonia flagransagainst free-living larvae. Spores would be given daily in the feed.
  • Treat newcomers with moxidectin or 5-day fenbendazole prior to introduction to pasture.
  • Allow approximately 3 days for all intestinal contents that may still contain eggs to be passed, prior to introduction to pasture.

Diagnosis

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Molento M B, Antunes J, Bentes R N & Coles G C (2008) Anthelmintic resistant nematodes in Brazilian horses. Vet Rec 162 (12), 384-385 PubMed.
  • Kaplan R M & Little S E (2000) Controlling equine cyathostomes. Comp Cont Educ Pract Vet 22 (4), 391-395.
  • Silvina Fernandez et al (1999) A new isolate of the nematophagus fungus Duddingtonia flagrans as biological control agent against free-living larvae of horse strongyles. Equine Vet J 31 (6), 488-491 PubMed.
  • Proudman C J (1999) The role in parasites in equine colic. Equine Vet Educ 11 (4), 219-224.
  • Bucknell D, Hoste H, Gasser R B & Beveridge I (1996) The structure of the community of strongyloid nematodes of domestic equids. J Helminthol 70 (3), 185-192.
  • Chapman M R et al (1994) In vitro culture of equine strongylid to the fourth larval stage in a cell-free mediumJ Parasitol 80, 225-231 PubMed.

Other sources of information

  • Higgins A J & Wright I M (1995) The Equine Manual. W B Saunders.
  • Ogbourne C P & Duncan J L (1985) Strongylus Vulgaris in the Horse - Its Biology and Veterinary Importance. Commonwealth Agricultural Bureau, UK.

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