Strongyle Disease and Cyathostominosis
Learn what the condition is, how it may be detected early, how it is treated or managed, and which breeds or species are linked to it.
Condition snapshot
A concise guide to the condition’s pattern, detection, management and urgency.
Limit spread while seeking adviceSeparate affected animals when practical and obtain veterinary guidance on contacts, cleaning and normal mixing.
This snapshot is a general guide, not a diagnosis or treatment plan. New, severe or worsening signs require veterinary assessment.
About the condition
Strongyles are pasture-transmitted roundworms of horses. Large strongyles, particularly Strongylus vulgaris, migrate through arteries and can damage intestinal blood supply, although modern control has reduced their prevalence in many managed populations. Small strongyles, or cyathostomins, are now the dominant group. Their larvae encyst within the caecal and colonic wall and may remain inhibited before emerging.
Many horses carry adult cyathostomins without illness. Heavy adult burdens can contribute to weight loss, poor condition and diarrhoea. Larval cyathostominosis is a distinct, potentially severe syndrome in which large numbers of encysted larvae emerge from the bowel wall, causing acute inflammation, watery diarrhoea, rapid weight loss, colic, fever and marked protein loss. Young horses in cool temperate regions are commonly affected during late winter or spring, but patterns vary with climate.
Large-strongyle arterial injury may cause recurrent colic, intestinal infarction or sudden surgical disease. Routine fecal egg counts cannot distinguish large from small strongyle eggs and do not measure encysted cyathostomin larvae. A low egg count therefore does not rule out larval disease, while a high count indicates pasture contamination potential rather than inevitable clinical illness.
Anthelmintic resistance is widespread, especially among cyathostomins. Frequent indiscriminate dosing selects resistance and can leave a property with fewer effective drugs. Modern control aims to limit disease and environmental contamination while preserving susceptible parasites in refugia, rather than attempting to eliminate every worm from every horse.
Small strongyle larvae can encyst within the large-intestinal wall for months and emerge together, producing larval cyathostominosis with diarrhoea, protein loss and oedema. Faecal egg counts measure egg shedding by adult worms but do not quantify encysted larvae. Large strongyles are now less common under modern control, yet Strongylus vulgaris can damage mesenteric arteries and cause serious colic.
Screening & Early Detection
Routine control uses quantitative fecal egg counts to identify horses shedding many strongyle eggs and to guide treatment frequency. Samples should be collected and handled consistently. A fecal egg-count reduction test compares counts before and approximately two weeks after treatment to assess drug effectiveness. Interpretation must consider the product, timing and laboratory variation.
Egg counts do not diagnose larval cyathostominosis. A horse with acute diarrhoea, weight loss and low blood protein needs clinical assessment even if its recent count was low. Blood work may show neutrophilia, hypoalbuminaemia, inflammation and acid-base disturbance. Visible larvae in manure support exposure but do not prove that they caused the intestinal disease. Other infectious, inflammatory and parasitic causes must be excluded.
Testing schedules should reflect age, pasture density, climate, treatment history and previous reduction results. New arrivals need a quarantine parasite plan before joining shared grazing. Where available, specialist testing can investigate large strongyle risk, but routine flotation cannot identify every species. Records should follow the individual horse so repeated high shedding and emerging resistance are recognised over time.
Treatment & Management
Clinically healthy horses receive targeted anthelmintic treatment based on veterinary parasite-control advice, local season and fecal results. Drug choice must account for demonstrated farm resistance; simply rotating brand names does not ensure different effective drug classes. Dose is calculated from an accurate body weight, and administration technique is checked to prevent underdosing.
Larval cyathostominosis requires veterinary treatment, not routine paddock deworming. A larvicidal regimen may be selected, but currently used drugs do not reliably remove every encysted stage. Severe intestinal inflammation may require intravenous fluids, electrolyte and protein support, nutritional care and anti-inflammatory treatment. Rapid parasite killing in a compromised horse can complicate disease, so the plan and monitoring must be individual.
Pasture management reduces reinfection: remove manure frequently, avoid overstocking, rotate or rest grazing appropriately and prevent contaminated feed from being eaten off heavily soiled areas. Treatment success is checked with egg-count reduction testing rather than assumed. Horses with infarcted intestine from large-strongyle damage may require surgery and have a guarded prognosis. Most routine infections are controllable, but severe larval disease can be fatal despite intensive support.
Severe larval disease may need plasma, fluids and nutritional support in addition to parasite control.
Breeds or species affected
Akhal-Teke
Open horse breed description, care requirements and conditions
Anglo-Arabian
Open horse breed description, care requirements and conditions
Arabian
Open horse breed description, care requirements and conditions
Barb
Open horse breed description, care requirements and conditions
Caspian
Open horse breed description, care requirements and conditions
Kathiawari
Open horse breed description, care requirements and conditions
Marwari
Open horse breed description, care requirements and conditions
Shagya Arabian
Open horse breed description, care requirements and conditions
Thoroughbred
Open horse breed description, care requirements and conditions
American Quarter Horse
Open horse breed description, care requirements and conditions
American Paint Horse
Open horse breed description, care requirements and conditions
Appaloosa
Open horse breed description, care requirements and conditions
Australian Stock Horse
Open horse breed description, care requirements and conditions
Brumby
Open horse breed description, care requirements and conditions
Waler
Open horse breed description, care requirements and conditions
Morgan
Open horse breed description, care requirements and conditions
Standardbred
Open horse breed description, care requirements and conditions
American Saddlebred
Open horse breed description, care requirements and conditions
Tennessee Walking Horse
Open horse breed description, care requirements and conditions
Missouri Fox Trotter
Open horse breed description, care requirements and conditions
Rocky Mountain Horse
Open horse breed description, care requirements and conditions
Peruvian Paso
Open horse breed description, care requirements and conditions
Criollo
Open horse breed description, care requirements and conditions
Mustang
Open horse breed description, care requirements and conditions
Canadian Horse
Open horse breed description, care requirements and conditions
Andalusian
Open horse breed description, care requirements and conditions
Lusitano
Open horse breed description, care requirements and conditions
Lipizzaner
Open horse breed description, care requirements and conditions
Friesian
Open horse breed description, care requirements and conditions
Dutch Warmblood
Open horse breed description, care requirements and conditions
Hanoverian
Open horse breed description, care requirements and conditions
Holsteiner
Open horse breed description, care requirements and conditions
Oldenburg
Open horse breed description, care requirements and conditions
Westphalian
Open horse breed description, care requirements and conditions
Trakehner
Open horse breed description, care requirements and conditions
Selle Français
Open horse breed description, care requirements and conditions
Belgian Warmblood
Open horse breed description, care requirements and conditions
Danish Warmblood
Open horse breed description, care requirements and conditions
Swedish Warmblood
Open horse breed description, care requirements and conditions
Swiss Warmblood
Open horse breed description, care requirements and conditions
Australian Warmblood
Open horse breed description, care requirements and conditions
Irish Sport Horse
Open horse breed description, care requirements and conditions
Cleveland Bay
Open horse breed description, care requirements and conditions
Württemberger
Open horse breed description, care requirements and conditions
Zangersheide
Open horse breed description, care requirements and conditions
Knabstrupper
Open horse breed description, care requirements and conditions
Orlov Trotter
Open horse breed description, care requirements and conditions
Camargue
Open horse breed description, care requirements and conditions
Icelandic Horse
Open horse breed description, care requirements and conditions
Pony of the Americas
Open horse breed description, care requirements and conditions
Quarter Pony
Open horse breed description, care requirements and conditions
Miniature Horse
Open horse breed description, care requirements and conditions
Clydesdale
Open horse breed description, care requirements and conditions
Shire
Open horse breed description, care requirements and conditions
Percheron
Open horse breed description, care requirements and conditions
Belgian Draft
Open horse breed description, care requirements and conditions
Suffolk Punch
Open horse breed description, care requirements and conditions
American Cream Draft
Open horse breed description, care requirements and conditions
Ardennes
Open horse breed description, care requirements and conditions
Breton
Open horse breed description, care requirements and conditions
Boulonnais
Open horse breed description, care requirements and conditions
Comtois
Open horse breed description, care requirements and conditions
Dutch Draft
Open horse breed description, care requirements and conditions
North Swedish Horse
Open horse breed description, care requirements and conditions
Norwegian Fjord
Open horse breed description, care requirements and conditions
Haflinger
Open horse breed description, care requirements and conditions
Gypsy Cob
Open horse breed description, care requirements and conditions
Irish Draught
Open horse breed description, care requirements and conditions
Welsh Pony and Cob
Open horse breed description, care requirements and conditions
Connemara Pony
Open horse breed description, care requirements and conditions
New Forest Pony
Open horse breed description, care requirements and conditions
Dartmoor Pony
Open horse breed description, care requirements and conditions
Exmoor Pony
Open horse breed description, care requirements and conditions
Shetland Pony
Open horse breed description, care requirements and conditions
Highland Pony
Open horse breed description, care requirements and conditions
Fell Pony
Open horse breed description, care requirements and conditions
Dales Pony
Open horse breed description, care requirements and conditions
Australian Pony
Open horse breed description, care requirements and conditions
Finnhorse
Open horse breed description, care requirements and conditions
Black Forest Horse
Open horse breed description, care requirements and conditions