Epidemiology & Risk Factors
Infected dogs and cats shed Toxocara eggs in their feces into the environment. Once in the environment, it takes 2 to 4 weeks for Toxocara larvae to develop inside the eggs and become infectious. Toxocara eggs have a strong protective layer, which allows the eggs to survive in the environment for months or even years under the right conditions. Humans or other animals (e.g. rabbits, pigs, cattle, or chickens) can be infected by accidentally ingesting Toxocara eggs. For example, humans can become infected if they work with dirt and accidentally ingest dirt containing Toxocara eggs. Although rare, people can be infected by eating undercooked or raw meat from an infected animal such as a lamb or rabbit. Because dogs and cats are frequently found where people live, there may be large numbers of infected eggs in the environment. Once in the body, the Toxocara eggs hatch and the larvae can travel in the bloodstream to different parts of the body, including the liver, heart, lungs, brain, muscles, or eyes. Most infected people do not have any symptoms. However, in some people, the Toxocara larvae can cause damage to these tissues and organs. The symptoms of toxocariasis, the disease caused by these migrating larvae, include fever, coughing, inflammation of the liver, or eye problems.
A U.S. study in 1996 showed that 30% of dogs younger than 6 months deposit Toxocara eggs in their feces; other studies have shown that almost all puppies are born already infected with Toxocara canis. Research also suggests that 25% of all cats are infected with Toxocara cati. Infection rates are higher for dogs and cats that are left outside and allowed to eat other animals. In humans, it has been found that 5% of the U.S. population has been infected with Toxocara. Globally, toxocariasis is found in many countries, and prevalence rates can reach as high as 40% or more in parts of the world. There are several factors that have been associated with higher rates of infection with Toxocara. People are more likely to be infected with Toxocara if they own a dog. Children and adolescents under the age of 20 are more likely to test positive for Toxocara infection than adults. This may be because children are more likely to eat dirt and play in outdoor environments, such as sandboxes, where dog and cat feces can be found. This infection is more common in people living in poverty. Geographic location plays a role as well, because Toxocara is more prevalent in hot, humid regions where eggs are able to survive better in the soil.