Summary Information on Toxocarosis

Introduction

Toxocarosis is a disease transmitted from animals to humans (a zoonosis). It is produced by parasitic roundworms (nematodes) called Toxocara canis and Toxocara cati (T. mystax). It is difficult to differentiate between them, but T.canis seems to be more common, affecting man, dogs and foxes. The worms are common in dogs and cats throughout Europe. Disease is not transmitted by person-to-person spread.

Animal infection

Infection with Toxocara canis is common in dogs, but unless the worms are present in large numbers the animals appear healthy. The life cycle of Toxocara is complex with the greatest number of eggs produced by adult worms in the intestine of the pregnant bitch (other adult dogs/foxes also produce eggs but at much lower levels). Pregnant bitches may infect the foetus through larvae passing across the placenta or from her milk. The infected newborn puppy will start to eliminate eggs at about 2 1/2 weeks.

Environmental contamination

Eggs passed in the faeces become infectious in the environment within 10 - 21 days after they have been shed; faeces disintegrate to release eggs into the surrounding soil. The eggs have thick sticky coats, are highly resistant, and can survive in the environment for many months and possibly years, even in cold weather. They need a temperature of at least 10°C mature and only temperatures greater than 30 - 35°C and desiccation will destroy them. Further information

Routes of infection

Human infection occurs by ingestion of viable eggs from contaminated soil, hands or contact with dogs, especially pups. Children are at greatest risk as they may ingest soil with contaminated eggs in playing areas (geophagia pica). The eggs have a sticky consistency that may allow them to adhere to surfaces such as fingers, toys and foodstuffs. For this reason vegetables or undercooked meat may be also contaminated, although the importance of the foodborne route has never been ascertained.

The disease

There are four syndromes caused by toxocara: visceral, ocular, asymptomatic and the recently described covert (mild) toxocarosis. Symptoms in humans are due to the migration of larval forms of the parasite into various tissues. Diagnosis is by blood test. A positive blood test result does not necessarily mean that the person has clinical toxocarosis. They may have been infected previously without showing any symptoms. The test may also be negative in the early stages of infection. The tests routinely used do not differentiate between T.canis and T.cati.

Epidemiology

Toxocarosis is not a notifiable disease, and the only routine source of national data is based on infections reported by laboratories to the Health Protection Agency (HPA). The true incidence of human toxocarosis in England and Wales is unknown. This is because the infection may result in mild, non-specific symptoms. In severe cases, infection may lead to eye problems, lung, liver, heart, or brain lesions. The proportion of infections that lead to serious disease is unknown. Due to the low number of reports received annually, it is difficult to interpret the regional distribution of reports and we are often unaware when or where infections may have been acquired.

Since 2000, between 1 and 8 laboratory confirmed reports have been received annually by the HPA. Infections have been reported in all age groups.

Prevention and control

Eggs are hard to eliminate because their thick shell confers considerable resistance. On dry pavements exposed to sunlight, eggs will desiccate, but those in cracks will be protected. Aqueous iodine solutions may be effective in kennels and hypochlorite (bleach) solution will decoat the sticky surface of eggs so they can be washed away. Only dry heat (flame gun) is lethal. Soil in gardens can be broken up and flamed repeatedly or about 20 cm of topsoil can be removed and replaced with fresh. Sand in sandboxes can be steam sterilised or ideally, replaced. Toxocara eggs can survive sewage processing although the subsequent risks from them are undetermined; flushing the faeces down the toilet is considered inappropriate. Pet owners should be advised to collect their pets' faeces every day and burn them or place them in the garbage.

The primary control measure in dogs is by periodic worming to control intestinal infection. Even when treated, dogs become re-infected within a short time. In the United Kingdom the recommended treatment interval has been decreased from 6 to 3 months, but even this may be insufficient. Publicity and education for this are important, especially for dog owners.

An appropriate regimen of wearing gloves when gardening and thorough hand washing is recommended. Children at play should be supervised to prevent geophagia. Vegetables to be eaten raw should be washed and gardens should be fenced to exclude all canines.