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In villages, towns, and cities throughout the country, land is set aside for human recreation. Public and private playing fields enable the population to indulge in sport and recreation, but these areas are often convenient for another form of human activity: dog walking. Sportsmen may be concerned that toxocaral infection may be acquired from canine defaecation on public playing fields, and the fastidious groundsmen may remove offending articles before matches are played. However, in doing so, they do little to reduce the risk of toxocaral infection.
Toxocara canis is the round worm (ascarid) parasite of canids: dogs and foxes. It has a complex and unusual life cycle. Eggs, when ingested, hatch in the small intestine and invade the intestinal wall. They are taken up in the portal system and distributed through the liver-lung migration cycle, going through larval moults, and being coughed up and swallowed when they develop into adult worms. This characteristic ascarid life cycle is confined mainly to dogs under the age of six months; above this age, larval development is arrested at the second larval stage (L2). This apparent dead end is important for the survival of the parasite as these L2 larvae are re-activated during pregnancy and migrate across the placenta to invade the puppies, and larvae are also excreted in the milk. Consequently, almost all puppies develop an active toxocaral infection. Eggs are also infective for a wide range of mammalian hosts, but larvae are unable to develop beyond the L2 stage and continue to migrate through the body.1Toxocara larvae excrete a complex mixture of glycoproteins that are potent stimulators of the host immune system, and it is this that is responsible for the characteristic symptoms and signs of toxocaral infection.2
Toxocariasis takes three main forms: an occult form characterised by failure to thrive and abdominal pains in children and visceral larva migrans characterised by fever, wheeze, cough, and eosinophilia.1, 3 In addition, there is an ocular form when trapped larvae stimulate a potent immune response leading to a spectrum of problems including endophthalmitis, uveitis, pars planitis, and subsequent granuloma formation.4 Ocular toxocariasis is relatively uncommon: some estimates suggest between 50 and 100 new cases of ocular disease a year in the British Isles. In contrast, asymptomatic toxocariasis is common, with 5–7% of an adult population in industrialised countries having evidence of previous infection.1
So what are the risks to sportsmen on the playing fields? Firstly, fresh Toxocara infected faeces pose no threat to human health because eggs must embryonate, a process that takes up to one month, and only 6–15% of dogs excrete eggs.1 The consequences of this are detected in surveys of parks within cities in the United Kingdom, which indicate that between 5 and 10% of samples will have infective Toxocara eggs.5 Visceral larva migrans peaks in children aged three years and is more common in boys.3 Ocular disease presents slightly later in life, between 7 and 10 years.4 Adults appear to be relatively resistant to symptomatic infection—for example, surveys of hydatid control in New Zealanders who have intense contact with dogs show that more than 25% have antibodies to Toxocara without any evidence of clinical disease.6 Thus one would expect the risk for adult sportsmen from contaminated fields to be relatively low, but the risk for children is higher but hard to quantify.
In summary, promiscuous canine defaecation poses a significant aesthetic hazard in playing fields. For children, there is a risk of serious toxocaral infection that may lead to visceral larva migrans or visual loss. For adults, the risk is much lower. Therefore everyone involved in the management of playing fields, whether public or private, should encourage dog owners to be responsible, clear up after their pets, and take steps to enforce this behaviour.