Toxocara canis and toxocara cati are roundworms that may cause a condition called toxocarosis in humans. Diseases that are transmitted from animals to humans are described medically as zoonoses.
As its name suggests, toxocara canis (T canis) is generally found in dogs, although it may also be detected in foxes. The dog usually doesn't display any signs of infection unless there are a significant number of worms living in the animal. The roundworms may grow to between 4cm and 12cm in length while living in the dog's intestine. A pregnant bitch may transmit eggs to her unborn foetus via the placenta, and hence a puppy may be born already infected with worms, perhaps unknowingly to the owner. Toxocara cati (T cati) is found in cats.
Eggs from the roundworms are usually excreted in the faeces of the affected animal, and it takes from ten days to three weeks for the eggs to mature and become infectious. Some cases of T cati toxocarosis, however, have been traced back to exposure to cat vomit. As the animal faeces naturally break down, the eggs are released into the environment, such as into the soil, where they may survive for months or even years. Each gram of faeces may contain up to a 100,000 eggs.
One study from 1998 showed that 66 per cent of soil samples taken from parks around the London area were contaminated with eggs. Data suggests that contaminated soil may contain up to 23 eggs per gram.
The toxocara eggs are very sticky and may adhere to toys, food and, of course, fingers. Ingestion of an undamaged infectious egg by a human can lead to toxocarosis. Once ingested, the eggs release larvae into the intestine of the human host; they are unable to develop into adult worms in humans, but they are able to migrate throughout the body. Direct human contact with dogs that are infected with roundworms tends to pose a minimal risk of developing toxocarosis, as the eggs that are shed haven't matured.
SYMPTOMS, DIAGNOSIS AND TREATMENT
Toxocara may affect any age group, although children between one and four years of age are those mostly affected, mainly due to their lack of hygiene awareness.
The disease can manifest in several different ways. In some, the infection may not cause any symptoms at all, whereas in others, toxocarosis may lead to fever, fatigue, irritability, weight loss, swollen glands, hives, cough, breathlessness, wheeze, abdominal pain, enlarged spleen and liver, visual disturbance, headache and fits.
Ocular toxocarosis describes disease of the eyes that may lead to blindness. A number of organs may be affected by the parasite, including the liver, heart and lung, and the larvae may even migrate to the brain. While infestations of these organs are serious, they are also rare and fewer than ten cases are reported to the Health Protection Agency every year.
The diagnosis of toxocarosis is usually based on clinical grounds in combination with specific blood tests. Positive antibodies specific to toxocara species suggest either past or current infection depending on the antibody type. High levels of a white cell in the blood called eosinophils are also found in people with toxocarosis, but these cells are also raised in many other conditions. Doctors may require additional tests such as X-rays, CT or MRI scans.
Treatment of toxocarosis usually results in complete recovery and blindness is now considered a very rare consequence. Anti-worm medications, also known as antihelminthics, are used to treat toxocarosis. Occasionally, steroids, laser treatment to the eye, anti-seizure medication and oxygen support may be required.
PREVENTION
It is crucial to prevent infection by toxocara in the first place, particularly considering that the eggs are very robust and there is no available vaccination. Pet owners should ensure their dogs, cats, puppies and kittens regularly receive worming treatments and faeces should be cleared up according to the Dogs (Fouling of Land) Act 1996 and more recently, the Clean Neighbourhoods and Environment Act 2005. Keeping animals away from where children play, and vice versa, is important.
Appropriate hygiene measures should be adhered to, such as simple handwashing and the washing of garden vegetables. Raking the garden regularly can destroy eggs, so this is recommended.
MORE INFORMATION
It is always important to review reputable medical sources such as www.patient.co.uk, the health protection agency website, NHS Choices (www.nhs.uk), or speak to your doctor.
Dr Raj Thakkar BSc(Hons) MBBS MRCGP MRCP(UK) is a full-time GP in Buckinghamshire.
