Nematodes in Dogs and Cats

Three species of ascarids are commonly found in pet animals: Toxocara canis (infecting dogs), Toxocara cati (infecting cats), and Toxascaris leonina (infecting dogs and cats). The adult females of this worm may reach a length of 18 cm for T. canis and 2.5 inches for the other two species. Transmission in all species may be direct by ingesting eggs or transuterine by larvae from the tissues of infected females to the fetuses. The life cycle of the ascarids comprises eggs produced by the female and five larval stages that develop in various organs and the intestine of the host.

Life Cycle

A female may lay about 200.000 eggs per day, which are excreted in the feces of the host and shed in the environment. An infective larval stage develops within the eggs on the ground in 3-7 days between a wide temperature range. The eggs which posses a thick protective shell may survive in the environment for more than one year.

After ingestion by the host, the eggs hatch to release the second stage larva. Extensive somatic and tracheal types of migrations of the larvae occur before they develop to adults. The larvae migrate from the intestine through the liver to the lungs and then through the trachea and the oesophagus to the intestine.

Prenatal infection by these nematodes is extremely common in Toxocara canis infection, however infection occurs also by ingestion of eggs by adult dogs as well as by young puppies. After the initial infection larvae are able to remain dormant in the female dog in various tissues for nearly all her lifetime and she may transmit infection to several litters. Coprologic examination of bitches infected with tissue larvae may show negative results, however, they are still capable of transmitting infection to their progeny.

Activation of the larvae in the mother, followed by migration to the fetus, occurs usually around the 42nd day of pregnancy and third stage larvae (1.0 mm of length) are found in the lungs of the fetuses before birth. A molt to the fourth larval stage (5-7 mm) occurs in puppies during the first week after birth when they are found in the intestine. Throughout the second and third week after birth a fifth larva stage grow rapidly to become an adult worm.

Rodents and Invertebrates Transmit Parasites

Rodents and some invertebrates are susceptible to Toxocara and Toxascaris infections and second stage larvae that develop in these paratenic hosts may infect dogs and cats predating on infected mice or cockroaches.

Ascarids

Ascarids are particularly injurious to puppies and kittens. The commonest signs are unthriftiness, digestive disturbances, bloated and pot bellied appearance. Puppies with heavy prenatal infection with T. canis suffer from diarrhea, show frequent vomiting and occasionally die from intestinal obstruction or perforation. Anemia and nervous disturbances are observed although no satisfactory explanation for this manifestation exists. Migration through the lungs may trigger pneumonia which is complicated by secondary infections.

Toxocariasis in Humans

Toxocariasis in man (observed mostly in children) is caused mainly by T. canis, but also T. cati and T. leonina have been incriminated. Ocular larva migrans causes granulomatous inflammation, which may result in a variety of other clinical symptoms including keratitis, iridocyclitis, chronic endophthalmitis and detached retina. Treatment with antinematodal drugs (diethyl carbamazine, thiabendazole, mebendazole) has little effect and may worsen the situation by killing the parasite inside the animal body.

>Treatment of Nematode Infection

The efficacy of selamectin against experimentally induced and naturally acquired T. canis and T. leonina in dogs was evaluated in controlled and field studies. T. canis after a single application of selamectin( 93.9 - 98.1 %), after two monthly applications (88.3 - 98.6 %), and 100 % after three monthly applications. It appears that somatic larvae that are usually reactivated during the last three weeks of pregnancy (and migrate through the placenta ) were killed by selamectin before being established in the fetus. The treatment also prevented possible milk infestation of the pups.

References:
1. Recent development in the Control of Ectoparasites and Endoparasites of Dogs and Cats with Selamectin, E. Pipano.