The areas chosen had similar-sized populations, with similar sex ratios and age dis-
tributions. The surveyors often encountered persons with multiple infections of closely
related species. This condition can lead to difficulties in both treatment and diagnosis.
For example in Zaire they found that five different species of filarial worms are coendemic
and are often found within the same individuals.
A cross-sectional study of stools, urine, skin biopsies and blood samples showed that
only a few persons at any age were entirely free of parasites. The majority had only one
or two parasites, although a decreasing number were shown to be more heavily
infected. In Laka tribesmen from central Africa, two or more parasites were detected in
38% of the population but only 3% (of the males) had at least four parasites.
The most frequent parasites were found in the following order: Onchocerca,
Schistosoma, Dipetalonema persistansand malarial parasites.
Loa loacauses loaiasis; and its prevalence in Chaillu (central Africa) was found to be
related to the ecology of the area. The humidity within the forests encouraged the
breeding of the arthropod intermediate host. The infection rate in one village was found
to be 35% and in another village, closer to the forest, it was found to be over 40%. Women
and men were equally exposed, yet males over 30 years were more infected than
females. Pygmy groups who were equally exposed to the infection in the same environ-
ment had significantly lower rates of infection.
Ascaris lumbricoides and Trichurisspp surveyed in children from slum areas in Malaysia
showed that the prevalence was related directly to sanitation and population densities.
For a detailed study of an infection, such as visceral leishmaniasis in a forest area, to
provide a complete picture of the spread and distribution of the infection it is important
to obtain the following type of data:n Social status, age, sex, ethnic group, occupation and reason for entering the forest.
n Clinical data: number of, localisation of and type of lesions.
n Biological data: mean diameters of the amastigotes on the smears.
n Date of appearance of the first lesion and the geographical site where the infection
occurred.Results from such surveys have shown that in all cases infections resulted from tra-
velling into the forest either for work or leisure. No infections were observed in people
who had never entered the forest. The legs and forearms, the most exposed parts of the
body, were the most infected. The infection appeared seasonally, corresponding to
periods of low rainfall — the peak time for the vector.
An important aspect of any epidemiological study is to ascertain whether or not the
parasites found are zoonotic and if the animal hosts can be identified.n 8.1.2 RURAL–URBAN MOVEMENT
Studies of people who travel from an urban into a rural environment show an interest-
ing distribution of parasites transmitted by arthropod vectors such as Plasmodiumspecies.
Blood samples taken from children in Lusaka, the capital city of Zambia, who regularly
travelled into the surrounding rural areas produced some interesting results (Watts
et al. 1990). Out of 423 urban children examined for blood parasites and serum anti-
bodies, 10.3% were found to have live parasites and 62% had anti-malarial antibodies but
no parasites. The presence of the antibodies was shown to be associated with journeys
outside the towns, which suggests that they had suffered some form of malaria. Enlarged
spleen (splenomegaly) is caused by an infection and the rate of splenomegaly in childrenPARASITOLOGY