Toxocariasis in Africa and Malta: The frequency of infection in host animals and its incidence and distribution in humans as revealed by skin sensitivity tests

Abstract

Many of the clinical features of toxocariasis in man are commonly found in the tropics but they can also be caused by infection with other helminths; the prevalence of infection is therefore difficult to determine clinically. It had been shown previously that a toxocara skin sensitivity test was a useful epidemiological tool in the diagnosis of past or present toxocaral infection in humans and that the antigen did not give rise to significant numbers of cross positive or false positive reactions. Using this test, surveys were undertaken of the prevalence and distribution of toxocariasis in selected parts of Africa and the Mediterranean. Since infection can only occur in man if there is a considerable reservoir of infection in dogs and cats, surveys of the prevalence in dogs were performed simultaneously in the same regions as the surveys in humans. It was found that T. canis infection of dogs was prevalent in all the regions studied, and it was thought that the variations in the prevalence rates—from 36·5% and 28% infected in the humid areas of Ibadan, Nigeria and Dar-es-Salaam, Tanzania respectively, to 12% in the dry area of Masailand and only 6% in the cool of Nairobi, Kenya—were probably attributable to differences in the humidity and temperature in the different regions. In only a few cases could the age of the dog be established; in these cases the prevalence rate was higher in dogs of under one year compared to dogs of over that age. It was further found that in East Africa T. canis infection was less prevalent among owned than among stray dogs, and that this applied particularly to developed regions. Elsewhere such differences were not considerable, doubtless reflecting the similarity of the environmental conditions of owned and stray dogs in less developed regions. In human populations, analysis of the results of the toxocara skin test indicated that over the age of 10 years the prevalence rate of toxocariasis remains relatively stable. Comparisons between different regions were therefore made in respect of those aged 10 years and over. It was found that in Malta the prevalence of toxocaral infection (in man and dogs) was similar to that found previously in London, England. However, in Ibadan, Nigeria, higher rates of infection were encountered, and the hygienic conditions of the community together with the larger reservoir in host animals were considered to account for this. In Nairobi prevalence of toxocaral infection in dogs and man was low but in the Masai in the surrounding region although prevalence in dogs was low in comparison with Britain there was evidence of much transmission occurring from a smaller proportion of infected dogs than in Britain: in Dar-es-Salaam the proportions both of infected humans and dogs were greater than in Britain. The survey suggests that in all the regions studied, and therefore probably throughout the tropics, toxocariasis is a considerable health problem in man.