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Basics ⬇

Outline

BASICS

Definition!!navigator!!

Cyathostomins, also termed small strongyles or small red worms, are considered the most prevalent and important equine intestinal parasite. The increasing prevalence of cyathostomins, widespread anthelmintic resistance, and difficulty preventing and treating larval cyathostominosis pose significant challenges for veterinarians.

Pathophysiology!!navigator!!

Cyathostomins comprise over 50 species of small strongyles. Their life cycle is direct and does not involve an intermediate host or extraintestinal migration. The minimum prepatent period is 5 weeks. Adult cyathostomins, located predominantly in the colon and cecum, produce large numbers of eggs, which account for almost all of the strongyle eggs in equine feces. Eggs hatch and develop on pasture, through L1 and L2 stages to the infective L3 stage. L3 acquire a protective sheath which facilitates prolonged survival on pasture, even in freezing conditions, although survival is reduced by hot and dry weather. Ingested L3 penetrate the mucosa and submucosa of the cecum and colon. A small proportion of L3 develop into L4 without interruption, while it is thought that, in some individuals, up to 90% undergo prolonged periods (measured up to 3 years) of inhibited development as encysted early L3. In temperate climates most larvae undergo inhibition in winter, while in tropical climates this may occur in summer. Clinical disease, termed acute larval cyathostominosis, results from en masse ingress or emergence of large numbers (millions) of previously encysted larvae as L4, typically in the fall, winter, or early spring. The host inflammatory response to migrating larvae causes colitis and typhlitis. Water, electrolytes, and proteins leak into the intestinal lumen, and luminal endotoxins are absorbed systemically. Hypoproteinemia may cause peripheral and intestinal edema. Dysmotility and inflammation of the large intestine may cause severe colic. Death may result from hypovolemic and endotoxic shock. Luminal adult cyathostomins (6–20 mm long) may cause catarrhal colitis, with resultant lethargy, weight loss, and diarrhea, but typically only in young naive horses that have heavy burdens. Cyathostomins may also cause nonspecific colic, nonstrangulated intestinal infarction, granulomatous colitis, cecal tympany, and cecocolic or cecocecal intussusceptions.

Signalment!!navigator!!

All ages can be infested, although young (<5 years old) and geriatric horses are more susceptible. Larval cyathostominosis is more common in young horses.

Signs!!navigator!!

  • Acute larval cyathostominosis may cause acute-onset diarrhea, anorexia, lethargy, dramatic weight loss, weakness, pyrexia, colic, peripheral edema, dehydration, and signs of endotoxemia
  • High burdens of adult cyathostomins may cause ill thrift

Risk Factors!!navigator!!

  • Risk factors for cyathostomin infestation include age, season, time since last deworming, and high pasture larval burdens. The latter reflect inadequate parasite control measures, including high stocking density, overgrazing of pastures, inappropriate use of anthelmintics, and failure to remove feces from pastures
  • Anthelmintic treatment may precipitate acute larval cyathostominosis

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

Other causes of diarrhea including Clostridium difficile, Clostridium perfringens, salmonellosis, and proliferative enteropathy should be considered.

CBC/Biochemistry/Urinalysis!!navigator!!

While hypoalbuminemia, hyperglobulinemia, particularly involving the β-globulin fraction, increased acute phase proteins, increased serum alkaline phosphatase, neutrophilia, and microcytic anemia are common, none of these findings is specific for cyathostominosis. Systemic eosinophilia is uncommon.

Other Laboratory Tests!!navigator!!

Histologic examination of rectal mucosal biopsies or biopsies of the large intestine and cecum may identify larvae and/or the associated inflammatory response. Abdominocentesis and fecal culture may aid evaluation of other differential diagnoses.

Imaging!!navigator!!

Abdominal ultrasonography may identify mural edema and thickening of the colon and cecum, liquid colonic and cecal contents, and possibly ascites.

Other Diagnostic Procedures!!navigator!!

A high suspicion of larval cyathostominosis may be obtained from typical clinical signs, signalment, and history. Confirmation is made by gross observation of large numbers of small red L4 and L5 larvae in feces or on the sleeve used to perform a per rectum examination. To aid detection of larvae, feces can be diluted (1:10 in water) and examined under a microscope × 10. Fecal egg counting is unreliable for diagnosis of larval cyathostominosis; indeed counts are often negative because the infestation may comprise primarily immature larvae.

Large burdens of adult parasites in horses with ill thrift may be evidenced by a high fecal strongyle egg count. Fecal strongyle egg counts are also an essential component of a targeted worming program.

Pathologic Findings!!navigator!!

  • Acute larval cyathostominosis induces acute typhlitis and colitis with mucosal hyperemia, edema, hemorrhage, congestion, ulceration, and necrosis. Transillumination of the mucosa from the serosal surface may aid visualization of cyathostomin larvae, which appear as small (1–2 mm), gray to red mucosal nodules. Histologic examination reveals large numbers of mucosal and submucosal cyathostomin larvae, edema, and infiltration with lymphocytes, eosinophils, plasma cells, and macrophages. Adult worms may be present within the lumen
  • Chronic cases may have only mucosal thickening

Treatment ⬆ ⬇

TREATMENT

  • For routine control of adult cyathostomins, anthelmintic treatment is generally recommended when fecal strongyle egg cell counts exceed 200 eggs per gram. An effective anthelmintic must be used. In developed regions, benzimidazole resistance is widespread and pyrantel resistance is present on many premises. Suspected moxidectin-resistant cyathostomins have been reported in donkeys, and shortening of the egg reappearance times after ivermectin and moxidectin administration has been documented in horses in several regions. Moxidectin is the treatment of choice for eliminating encysted larvae when dosing horses in fall and winter. Horses should receive the correct dose of anthelmintic, ideally based on an objective assessment of body weight
  • Acute larval cyathostominosis requires intensive treatment. Moxidectin should be administered since this is the only anthelmintic with good efficacy against larvae. Care should be taken when calculating the dose as moxidectin can be toxic at twice the recommended dose, especially in thin horses. Glucocorticoids (prednisolone at 1 mg/kg PO SID or dexamethasone 0.1 mg/kg IV SID) may attenuate intestinal inflammation and reduce the associated morbidities. Supportive therapy includes correction of fluid, electrolyte and acid–base abnormalities, oncotic support (plasma, synthetic colloids), and analgesia.

Medications ⬆ ⬇

MEDICATIONS

Drug(s) of Choice

  • Moxidectin 0.4 mg/kg PO has licensed efficacy against all stages of cyathostomins, and in drug-sensitive populations suppresses fecal egg counts for approximately 3 months following treatment. A single dose of moxidectin 0.4 mg/kg PO is indicated for acute larval cyathostominosis
  • Ivermectin 0.2 mg/kg PO has licensed efficacy against adult stages, luminal larval stages, and developing stages of larvae in mucosa, but variable low efficacy has been observed against inhibited stages
  • Oxfendazole 10 mg/kg PO or fenbendazole 5 mg/kg PO will control benzimidazole-susceptible strains of adults and developing larvae, but should be avoided on premises which have benzimidazole resistance. Administration of fenbendazole for 5 consecutive days is no longer indicated for acute larval cyathostominosis or larvicidal treatments in the fall/winter in temperate regions
  • Pyrantel embonate 19 mg/kg PO kills adults but not inhibited larvae. It can be used as an alternative to moxidectin for routine strategic deworming of horses during spring and summer, but should be avoided where resistance occurs

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Prevention/Avoidance!!navigator!!

  • An integrated parasite control program, ideally designed by a veterinarian, which is tailored to the needs of individual premises, should be implemented. The key objective is to reduce the number of infective larvae on the pasture. All horses on the premises should be included in the program
  • Strategic targeted deworming should be practiced to reduce anthelmintic use and consequently minimize development of anthelmintic resistance. Repeated interval dosing throughout the life of each horse should be avoided. Since most adult horses have some immunity against cyathostomins, only a small portion (~20%) of the herd harbors significant numbers of egg-producing adults, thereby contributing to the bulk of the pasture larval burden. These animals are identified by performing frequent fecal strongyle egg counts on all horses throughout the grazing season. The frequency of testing is dependent on the anthelmintics that are used for treatment and testing after treatment should occur at or soon after the expected egg reappearance period (this information is available on datasheets). An effective anthelmintic is then administered only to those with >200 eggs per gram. Moxidectin is commonly administered to all horses in fall/winter to reduce the cyathostomin burden, including inhibited larvae, and to eliminate large strongyles from horses that had not received an anthelmintic earlier in the grazing season. The efficacy of particular anthelmintics on premises should be assessed annually, ideally in summer, by undertaking a fecal egg count reduction test
  • Anthelmintic treatment is only an adjunct to good pasture management. Pasture parasite burden can be reduced by removing feces from pastures twice weekly, avoidance of overstocking, rotating pastures, and grazing horse pastures with ruminants. Harrowing pasture can reduce pasture larval burden but only in dry weather; harrowing in wet weather is detrimental because it disperses infective larvae away from fecal-contaminated zones. New arrivals should be quarantined and dewormed with moxidectin/praziquantel before being admitted to the herd

Expected Course and Prognosis!!navigator!!

Larval cyathostominosis has a guarded prognosis, with up to 50% of affected horses dying despite appropriate intensive treatment. For survivors, return to normal intestinal function may be slow.

Miscellaneous ⬆ ⬇

MISCELLANEOUS

Suggested Reading ⬆

Lester HE, Matthews JB. Control of equine nematodes: making the most of faecal egg counts. In Pract 2015;37:540–544.

Matthews JB. Anthelmintic resistance in equine nematodes. Int J Parasitol Drugs Drug Resist 2014;4:310–315.

Nielsen MK, von Samson-Himmelstjerna G, Pfister K, et al. The appropriate anti-parasitic treatment; coping with emerging threats from old adversaries. Equine Vet J 2016;48:374–375.