Survival and Destruction of Toxocara Canis Ova in the Environment

Human infection with the roundworm Toxocara canis is acquired by ingestion of fully embryonated ova. Transmission to humans occurs by ingestion of viable eggs from contaminated soil, hands or other objects. Direct contact with infected dogs where the adult stage of the worm lives in the small intestine may be less important because of the time from embryonation of the ova to infectivity (see below). Humans are a dead-end host, able to allow the survival but not the further development of larval parasites.

The frequency of positive faecal specimens in dogs has been reported in the past to range from 33% in London amongst dogs under one year old, to 66% in rural Germany. In older dogs, prevalence ranges from 6 - 22%. In a study of parks and gardens in London (Gillespie et al., 1991), 6.3% of soil samples were found to contain ova of Toxocara canis (range 0 - 20%) although in some cases sample sizes were small. Another survey found egg-positive soil samples in 66% of London parks (Lloyd, 1998).

Conditions for development of the ova
Once passed in canine faeces, T. canis ova require a period of development in the soil before they become infective. They require temperatures of 12 to 32°C, at least 85% humidity, shade and oxygen to develop the infective larvae. Because of variation and interaction between these factors, the eggs probably need at least 10-14 days before becoming infective in nature.

The eggs of T. canis are very resistant to both chemical agents and climatic influences. Survival in an appropriate external environment has been reported to range from 2 years up to 6 years. In studies of soil contamination about 68% of recovered ova remained viable, able to embryonate and presumably therefore to infect.

PREVENTION AND CONTROL

Disposal of canine faeces
Dog owners should collect their pets' faeces every day and burn them or place them in the garbage. Flushing the faeces down the toilet may not always be appropriate because a high proportion of eggs will survive sewage treatment.

Treatment of areas contaminated by canine faeces
In soil protected by vegetation, the eggs can survive both the winter and summer conditions found in continental USA and similar levels of survivability can be expected in the UK and most European countries. They are resistant to all common disinfectants at conventional (domestic) strengths but are killed by aqueous solutions containing 120 ppm of free iodine (which would be sufficient to scorch the ground!). When not protected by faeces, soil, vegetation or debris, the eggs are susceptible to desiccation, direct sunlight or temperatures above 37°C. Clean eggs die in less than a day in dry gravel, soil or on concrete floors. Regular raking of the ground to a depth of 1 to 2 inches to loosen the soil and promote desiccation and sunlight penetration should kill the eggs rapidly. Flaming or burning the ground are effective but rather extreme measures.

Control in dogs
Control measures in dogs depend on the route of transmission. Transplacental infection occurs when larvae resident in bitch tissues pass around day 42 of pregnancy to foetuses in utero. Infection of pups can also occur via larvae reactivated in late pregnancy being ingested with the mother's milk. The primary control measure consists of the periodic worming of dogs to control intestinal infection. Chemical inhibition is also used to control tissue larvae, which become metabolically active during pregnancy and lactation. Chemical control is reported to be variably effective and dependent on the nematocide and dosage used .

References
Bignall J (1993). Toxocara: a reminder. Lancet 342; 800.

Gillespie S H, Pereira M, Ramsay A (1991). The prevalence of Toxocara canis ova in soil samples from parks and gardens in the London area. Public Health, 105: 335-359.

Holland C, O'Connor P, Taylor M R H, Hughes G, Girdwood RWA, Smith H (1991). Families, Parks, Gardens and Toxocariasis. Scand J Infect Dis 23 (2): 225-231.

Lloyd S (1998). Toxocarosis. In: Zoonoses. Biology, Clinical Practice and Public Health Control. (Ed: S R Palmer, Lord Soulsby, and D I H Simpson); pp. 841-854. Oxford University Press, Oxford.

Last reviewed: 25 May 2011