Trichostrongylus calcaratus



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Trichostrongylus parasitic worms are commonly observed to infest wild rabbits and cottontails, in particular the eastern cottontail (Sylvilagus floridanus) in the USA. They are rare in pet rabbits. Trichostrongylus calcaratus is found in the small intestine and the colon of the rabbit. Extra-intestinal migration is unknown. The rabbits become infested by eating food (hay, straw, fresh vegetables) that has been contaminated by the larvae.

This parasite does not represent a public health danger.

The eggs produced by the female will pass in the feces and hatch outside the host. They measure between 80 and 90 mm (micrometer) long. They are already segmented when laid and develop into infective larvae within 6 days. The L1 and L2 stages of the larvae are microbivorous. The L3 stage, reached after 16-18 days, is non-feeding and infective by ingestion. L3, L4 and L5 are immature adults that develop into mature forms in the digestive tract of the host. The life cycle is direct, with no intermediate hosts.

The worms are slim, with small anterior ends and no buccal cavity. Male worms can be recognized by their asymmetrical dorsal ray and two short nearly equal spicules. The female has a vulva of about 1 mm near the tip of the tail. Often eggs can be observed in her body.


Trichostrongylus sp. femelle worm and egg

Several Trichostrongylus species have been observed in rabbits. They can be differentiated by the size and the structures of the spicules.

Trichostrongylus affinis

This strongyle parasitic worm is found throughout the USA, and infests cottontails (Sylvilagus floridanus) and snowshoe hares (Lepus americanus). It is rare in pet rabbits. It infects the cecum and the large intestine.

The eggs, which measure an average of 61 * 37 mm, will pass with the feces. They hatch and develop outside the host and the larvae become infective after about 10 days. The adult worms have an average size of 5-7.5 mm for the males and 8.7-9.3 mm for the females. The male possesses spicules, while the female has a vulva at the posterior end.

Severe infestation can cause a loss of weigh.

Trichostrongylus retortaeformis

This parasite is found only in Europe, the UK, and was introduced to Australia. It lives in the small intestine of rabbits and hares (Lepus europaeus). Its life cycle is probably similar to that of T. affinis.

The eggs measure about 87 * 33 mm. They are found in the portion of herbage that presents the least climatic changes. Their development fails when the maximum temperatures do not exceed 10C. The larvae are able to migrate inside the vegetation to moist conditions. The adult worms measure between 6.8 and 8.4 mm long for the males and 9.6 and 10.4 mm long for the females. They are characterized by thin transverse and longitudinal grooves.

Pathogenesis related to the presence of Trichostrongylus retortaeformis can affect and reduce a population of rabbits. Clinical test include fecal flotation at 25C. Higher temperatures will kill the parasite.

Trichostrongylus ransomi

Trichostrongylus ransomi has been reported to infest cottontails (Sylvilagus floridanus) in Louisiana, USA. It is not frequent in rabbits. It infests the small intestine. The life cycle of this worm is unknown but probably similar to that of T. affinis.

The eggs measure about 65 * 33 mm. The adult worms are small: 2.2 - 3 mm for the males and 3 - 3,5 mm for the females.

Trichostrongylus colubriformis (T. instabilis)

It is a cosmopolitan parasite that infests the small intestine of cattle and other ruminants, but also lagomorphs.

Clinical signs

The intrinsic pathogenic strength of these worms is weak and the rabbit remains asymptomatic in most cases. Severe infestation can, however, lead to a weight loss and/or anemia and aggravate other rabbit disorders, such as diarrhea, cause high eosinophilia and occasionally death. The mucous layer of the intestine is often irritated, which can lead to blood loss, sometimes the presence of nodules is observed.

The presence of Trichostrongylus worms is diagnosed by fecal flotation for the presence of strongyle-type eggs in the feces. The culture of these eggs to the L3 stage is necessary for a specific identification of the species. Adults can be specifically identified by their structure and appendices.




10-20 mg/kg, PO, repeat in 10-14 days


100-200 mg/kg, PO


10 mg/kg



0.2 - 0.4 mg/kg, PO, SC, repeat in 10-14 days

Further Information

Anderson RC (2000) Nematode Parasites of Vertebrates. Their Development and Transmission. 2nd Ed. CABI Publishing, Oxon, UK.

Andrews CL, Davidson WR. Endoparasites of selected populations of cottontail rabbits (Sylvilagus floridanus) in the southeastern United States. J Wildl Dis. 1980; 16(3):395-401.

Audebert F, Cassone J, Hoste H, Durette-Desset MC. Morphogenesis and distribution of Trichostrongylus retortaeformis in the intestine of the rabbit. J Helminthol. 2000; 74(2):95-107.

Audebert F, Hoste H, Durette-Desset MC. Life cycle of Trichostrongylus retortaeformis in its natural host, the rabbit (Oryctolagus cuniculus). J Helminthol. 2002; 76(3):189-92.

Boag B. The incidence of helminth parasites from the wild rabbit Oryctolagus cuniculus (L.) in eastern Scotland. J Helminthol. 1985; 59(1):61-9.

Boag B, Iason G. The occurrence and abundance of helminth parasites of the mountain hare Lepus timidus (L.) and the wild rabbit Oryctolagus cuniculus (L.) in Aberdeenshire, Scotland. J Helminthol. 1986; 60(2):92-8.

Ciordia H BIZZELL WE, Porter DA, Dixon CF. The effect of culture temperature and age on the infectivity of the larvae of Trichostrongylus axei and T. colubriformis in rabbits and guinea pigs. J Parasitol. 1966; 52(5):866-70.

Duwel D, Brech K. Control of oxyuriasis in rabbits by fenbendazole. Lab Anim. 1981; 15(2):101-5.

Hoste H, Reilly M. Scanning electron microscopy of the jejunal and ileal mucosa of rabbits infected with Trichostrongylus colubriformis. Ann Rech Vet. 1988; 19(2):123-8.

Hoste H, Mallet S. Effects of size of Trichostrongylus colubriformis infections on histopathology of the mucosa along the whole small intestine in rabbits.
J Comp Pathol. 1990; 103(4):457-65.

Hoste H, Mallet S, Koch C. Trichostrongylus colubriformis infection in rabbits: persistence of the distal adaptive response to parasitism after anthelmintic treatment. J Comp Pathol. 1995; 113(2):145-53.

Iason GR, Boag B. Do intestinal helminths affect condition and fecundity of adult mountain hares? J Wildl Dis. 1988; 24(4):599-605.

Molina X, Casanova JC, Feliu C. Influence of host weight, sex and reproductive status on helminth parasites of the wild rabbit, Oryctolagus cuniculus, in Navarra, Spain. J Helminthol. 1999; 73(3):221-5.

Prasad D, The effects of temperature and humidity on the free-living stages of Trichostrongylus retortaeformis. Can. J. Zool. 1959; 37: 305-316.

Purvis GM, Sewell MM. The host-parasite relationship between the domestic rabbit and Trichostrongylus colubriformis. Vet Rec. 1971; 89(5):151-2.

Strohlein DA, Christensen BM. Metazoan parasites of the eastern cottontail rabbit in western Kentucky. J Wildl Dis. 1983; 19(1):20-3.

Wiggins JP, Cosgrove M, Rothenbacher H.Gastrointestinal parasites of the eastern cottontail (Sylvilagus floridanus) in central Pennsylvania.
J Wildl Dis. 1980; 16(4):541-4.