Human toxicariasis and direct contact with dogs

Petting infected dogs may put humans at risk from toxicariasis

source: A.Wolfe and I.P. Wright
Veterinary Record vol 152, no 14, April 5 2003
Starts p419, 4 pages long

Toxicariasis canis is a nematode which lives as an adult in the small intestines of canids such as foxes and the domestic dog. Eggs are shed through canid faeces, and take 2-7 weeks to become infective. The most important way that dogs are infected is through their mothers, and infection rates when dams are not wormed can reach almost 100%. Dogs can also become infected when suckling, and from eating infected small rodents. Dogs tend to develop immunity as they age. Some 2%-31% of the healthy European human population could have antibodies to toxicariasis, though there is a low incidence of clinical toxicariasis in Europe. Humans can become infected in different ways, one of which may be ingesting contaminated soil, though there is little evidence to support this.

This study investigates results found after testing hair clippings from 60 pet, farm, and shelter dogs from the UK and Ireland. The hair was removed from the tail underside, the perianal region, and the caudal area of the dogs' back legs. No sample had visible faecal staining. T canis eggs were found on 15 of the dogs, or 25% of the total sample, with 71 eggs recovered, 24% embryonating and 4% embryonated. Embryonated and embryonating eggs were found at higher densities than those recorded in samples of soil.

There do not appear to be strong links between humans testing seropositive for Tcanis,and soil contamination. Seroprevalence tends to be higher in some rural than urban areas, despite urban densities of dogs being higher. Humans have a three-fold increase in risk of contracting toxicariasis if they have a dog in the household, and a five-fold increase if they have a litter of puppies at home. Owners of more than one dog are also more at risk. A higher proportion of people in rural areas own dogs, so have more direct contact. Ireland has the highest European dog ownership rate, and the highest seroprevalence rate.

Most infected dogs were found to have hair with non-infective T canis eggs, and many humans may have exposure to T canis antigens, but no disease. Direct contact with dogs appears to be more important for transmission of T canis to humans than transmission through contaminated soil.