This is a clinical term encompassing these superfamilies of bursate nematodes:
Their eggs are very similar and all contribute to parasitic gastroenteritis.
General epidemiology of large and small strongyles
Strongylosis occurs in
- Young horses
- Adult animals (especially if overcrowding, poor hygiene)
- Animals on permanent pasture
Sources of infection
- Overwintered L3 on pasture
- Many adult horses pass significant numbers of strongyle eggs throughout their lives
- "Spring rise" in faecal egg output occurs in both breeding and non-breeding horses
Pattern of infection on pasture
- Pattern of L3 on pasture is similar to gastrointestinal worms in cattle
- Main difference is that the mare makes a major contribution to pasture contamination (c.f. cow)
Hypobiosis of cyathostomin larvae
- Occurs throughout the year, but particularly in late summer/autumn
- EL3 may remain arrested for years
- Resumption of normal development can occur
- seasonally in late winter/early spring
- following removal of adult worm population via anthelmintic treatment
Larval cyathostominosis
- Sudden onset diarrhoea and/or weight-loss
- Diagnosis difficult, prognosis guarded
- Generally in late winter/spring
- Usually <5 years old
- Sporadic, but increasing in incidence
- Hyperglobulinaemia, especially IgG(T)
- Hypoalbuminaemia
- Leukocytosis
- Sometimes peripheral oedema
- Faecal egg-count low (disease caused by emerging larvae)
- Larvae may be found in faeces or on faecal glove
Pathogenesis
Resumed development of massive numbers of larvae → subsequent emergence of bright red L4 → massive eosinophilic infiltration of mucosa → catarrhal and haemorrhagic colitis
Control of cyathostomin infections in horses
Anthelmintics
- Only 3 chemical groups currently available
- Avermectin/milbemycins
- Benzimidazoles
- Pyrantel
- Resistance is an emerging problem (especially to benzimidazoles)
Target life-cycle stages
- These are not all equally susceptible to each anthelmintic
- Pyrantel is affective against
- Adult worms in the lumen
- Ivermectin or a one off administration of Fenbendazole is affective against
- Adult worms and L4 in the lumen
- Moxidectin or a 5 day course of Fenbendazole is affective against
- Adult worms and L4 in the lumen
- Developing and hypobiotic L3 in the mucosa
Egg reappearance period
- This is the time from treatment until eggs reappear in the faeces. It is determined by
- degree of activity against mucosal larval stages
- persistency of anthelmintic treatment
Prevention of pasture contamination
- The objective is to create safe grazing by preventing depostion of strongyle eggs onto pasture
- Treat all grazing horses at intervals determined by
- Egg reappearance time of chosen anthelmintic
- Risk level
- Treat all new arrivals and stable for 48-72 hours so that eggs are not passed onto pasture
- Adopt strategy that will minimise risk of resistance developing (you may need to include tapeworm and stomach bots in your scheme)
- No new eggs passed → no new L3 developing, however it is important to use epidemiological knowledge to predict how long existing L3 will survive as the pasture will not be safe for use before then
- Remove faeces from paddocks at least weekly:
- This markedly reduces dependence on anthelmintics
- Increases available grazing
- But is labour intensive and less effective in rainy weather
- Examine faecal samples twice yearly to monitor effectiveness of your chosen strategy
Pasture management
- Reserve clean grazing for nursing mares and foals
- Rest pastures used the previous year until overwintered L3 have gone
- Mixed or alternate grazing with cattle or sheep
- These are refractory to most horse worms, except T.axei
Chemoprophylaxis of larval cyathostominosis
- Needed if a horse is known to have grazed heavily contaminated pasture and may therefore be harbouring massive numbers of hypobiotic larvae
- Fenbendazole treatment given daily for 5 consecutive days in autumn or winter will reduce the risk of clinical disease developing.