Toxocara canis and Trichuris vulpis – Common dog parasites (Proceedings)

These two worms are probably the most common parasites of the dog throughout most of its range around the world. Both are soil transmitted, and both typically do not cause disease. However, when massive infections occur, they can both cause signs in dogs. One is zoonotic, and one is not.

Toxocara canis

Toxocara canis is probably the most common and well known parasite of the dog around the world. The worms are passed in the feces sporadically or after treatment, and the eggs are easily detected using standard fecal flotation methods. The adult worms live in the small intestine, the eggs pass in the feces of the host into the environment. In the soil the eggs become infective in about 9 days at 26-30°C, but it has also bee shown that they can persist under sub-zero temperatures without being killed.

Dogs become infected by eating embryonated eggs or paratenic hosts (e.g., mice, chickens, and lambs). Invertebrate paratenic hosts, like earthworms, probably help the worms make their way to the vertebrate paratenic hosts. The most significant mode of transmission in the dog is transplacental transmission of larvae.

Dogs infected prenatally can develop pulmonary lesions. Light infections cause mild petechial hemorrhaging, while heavy infections may cause pneumonia. Mature in heavy infections can be found in the stomach, bile duct, and peritoneal cavity. Heavily infected puppies can develop mucoid enteritis and intestinal impaction. Often the migrating larvae are arrested in the tissues forming granulomas. Gut penetration by larvae can cause severe anemia as a prelude to death.

Clinical signs occur only in young dogs following the ingestion of eggs: coughing and nasal discharge that usually subsides after about 3 weeks. Heavy infections can cause vomiting, anorexia, abdominal distension, mucoid diarrhea, debilitation, reduced growth rate, allergic pruritus, and a characteristic foul oral odor. Adult dogs typically are without clinical signs.

Toxocara canis causes visceral larva migrans in humans. Eggs in the soil require about two weeks to embryonate, but can then remain infective for several years. Larval toxocariasis manifests in a spectrum of syndromes that includes visceral, neural, ocular, covert, and asymptomatic toxocariasis. Asymptomatic toxocariasis is the most common form, but clinical cases do still occur. The most well-recognized source of infection is ingestion of contaminated soil by toddlers. Anti-Toxocara antibodies among the U.S. population was recently measured to be 13.9% using sera from the Third National Health and Nutrition Examination Survey. Having a positive titer was associated with a low-level of education, being born in a foreign country, living in overcrowded conditions and in conditions of poverty. Pet ownership was not associated with increased seroprevalence.

Trichuris vulpis

Whipworms live in the cecum and colon. The worms live with the anterior end threaded through a cellular syncytium within the mucosa, while the posterior end is free within the lumen. The eggs leave the female from the vulva located at about the level where the body thickens behind the esophagus. The males have a coiled tail and a single spicule.

The eggs will develop in the soil to contain infective first-stage larvae in 9-10 days at 33º-38ºC and 25-26 days at 19º-25ºC. There are three potential life-cycle scenarios after egg ingestion that have been described, and each has some experimental support: (1) larvae enter the mucosa of the small intestine for 8 to 10 days before reentering the lumen and moving to the cecum where they complete development; (2) larvae enter the mucosa of the small intestine and migrate within the mucosa to the cecum; or (3) larvae enter the mucosa of the cecum and remain there to maturity. The prepatent period is 70 to 100 days, and adult worms live months to years.

In light infections, worms are usually found only in the cecum, and a few worms typically cause no noticeable signs. As the number of worms increases towards one or several hundred, the worms also are found in the mucosa of the colon. Disease, bloody diarrhea, dehydration, and anemia, occurs mainly in heavier infections

Trichuris vulpis is on occasion stated to be a zoonotic infection, but the data is not convincing. Human whipworms (Trichuris trichiura) sometimes produce large eggs that are similar in size to Trichuris vulpis, and these abnormal eggs, when alongside the regular-sized and smaller Trichuris trichiura eggs in a human fecal, can resemble the eggs of Trichuris vulpis. Adult worms have been recovered and examined from these patients and have been determined to be Trichuris trichiura. In dogs also, some Trichuris vulpis eggs are enlarged compared to regular eggs.

Whipworms are common nationally in dogs from shelters of all ages. For dogs <6 months of age, the prevalence was 7.84%; 6-12 months, 16.83%; 1-3 years, 15.38%; 3-7 years, 15.48%; and >7 years 11.74%. Adult dogs are about as susceptible to infection as younger dogs. Also, foxes and coyotes are both regular hosts for this parasite and act as reservoirs of canine infections.