Traditionally, toxocariasis in humans, which may manifest as larval migrans, visceral larval migrans, and covert toxocariasis, was thought to be contracted through ingestion of eggs from contaminated soil. However, it has been reported that the risk for toxocariasis can increase three-fold if a dog is present in the household and more than five-fold if a litter of puppies is present (puppies are infected transplacentally), depending on deworming history and other factors. This paper reviews the soil-contamination hypothesis literature and proposes that toxocariasis is caused by direct contact with dogs. For this first-ever study, hair was collected from 60 dogs from various locations in Ireland and the United Kingdom and examined for Toxocara canis eggs. Perianal, caudal hindlimb, and tail underside clippings were taken from shelter animals, working farm dogs, and domestic pets; samples averaged between 0.05 g to 2.9 g for each dog. T. canis were found in the hair of 25% of the dogs. A total of 71 eggs were recovered, of which 4.2% had embryonated and 23.9% were currently embryonating. Maximum egg densities were calculated at 20 embryonated and 180 embryonating eggs per gram of hair-much higher than densities reported for soil samples around the world. For example, Ireland has a low level of soil contamination with T. canis, but the highest rate of dog ownership in Europe. In Dublin, 85% of stray dogs were infected with T. canis (1995). In addition, 31% of healthy Irish school-aged children were seropositive for T. canis, the highest rate in Europe (1995). The author suggests that T. canis eggs on the hair coat provide an ample and intimate source of exposure to humans.
COMMENTARY: This article only emphasized the importance of strategic deworming to rid dogs and cats of parasites before they can lay eggs. The Centers for Disease Control and Prevention (CDC) and the American Association of Veterinary Parasitologists have developed a guide for veterinarians that can be found on the CDC Web site: http://www.cdc.gov/ncidod/dpd/parasites/ascaris/prevention.htm. A hard copy of this information, Prevention of Zoonotic Transmission of Ascarids and Hookworms of Dogs and Cats: Guidelines for Veterinarians, is also available from CDC.
Human toxocariasis and direct contact with dogs. Wolfe A, Wright IP. VET REC 152:419-422, 2003.