Strongyles

This is a clinical term encompassing these superfamilies of bursate nematodes:

Their eggs are very similar and all contribute to parasitic gastroenteritis.

Strongyles (Red worms)

The strongyles that occur in the horse can be divided on the basis of size into two groups

Large strongyles
- Strongylus species (3 species; used to be widespread prior to the introduction of worm control programmes; now uncommon)
- Triodontophorus species (common)
Small strongyles
- Also known as Cyathostomins (preferred term), cyathostomes, trichonemes or small redworms
- Cyathostomins (widespread, including 4 genera and over 40 species of worms)

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.

Hepatitis

horses
it is common to find fibrous tags incidentally on the liver surface and adjacent diaphragm
these are remnants of fibrous repair following the egress of the parasites from the liver

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