Category:Horse Nematodes

Gastrointestinal Nematodes

Introduction

Parascaris equorum - Joaquim Castellà Veterinary Parasitology Universitat Autònoma de Barcelona

Parascaris equorum - Joaquim Castellà Veterinary Parasitology Universitat Autònoma de Barcelona

Parascaris equorum - Joaquim Castellà Veterinary Parasitology Universitat Autònoma de Barcelona

Parascaris equorum - Joaquim Castellà Veterinary Parasitology Universitat Autònoma de Barcelona

Strongylus - Joaquim Castellà Veterinary Parasitology Universitat Autònoma de Barcelona

Many nematode species occur in the equine gastrointestinal tract, although not all are of equal importance:

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.