SUMMARYSera from an age-stratified sample of 1810 people from the Caribbean island of St Lucia were tested for antibodies against varicella-zoster virus. The results indicate that very few infections occur in childhood, which agrees with clinical survey data from other tropical countries, but contrasts with the observed high case rate in children in temperate countries. The alternative hypotheses which may explain these results are discussed, and it is suggested that high ambient temperatures interfere with the transmission of the virus. Irrespective of the cause, the pattern of varicella incidence observed has important implications for any vaccination policy adopted in tropical countries. INTROD)UCTIONVaricella (chickenpox), primary infection with varicella zoster virus (VZV), is a very common childhood infection in temperate countries, although the mean age of infection is significantly higher in developing countries [1][2][3][4][5][6][7][8]. This is the reverse of the age pattern observed with many other directly transmitted infections, such as measles, where the mean age of infection is typically low in developing or tropical countries because of a mixture of high population densities, which provide more opportunities for disease transmission, and high birth rates, which provide a large pool of young susceptibles [9]. The reversed age pattern of varicella infection raises questions about the nature of varicella transmission.In order to address these questions this paper discusses the evidence that the force of infection (the per capita rate at which susceptibles acquire infection) of VZV is less in tropical than in temperate countries. The paper examines published reports, describes a serological investigation of the pattern of infection in the population of the Caribbean island of St Lucia and uses the survey data to estimate the age-specific force of infection.The discussion of varicella transmission in this paper centres on differences between tropical and temperate countries. However, tropical countries are usually
Age-related changes in the average worm burden and the prevalence of Trichuris trichiura infection, in a village community in St. Lucia, were examined by field studies based on worm expulsion techniques. Horizontal age-intensity profiles were convex in form with peak parasite loads occurring in the 2 to 15-year-old children. Prevalence is shown to be a poor indicator of changes in average worm load with age. Faecal egg counts (epg and epd) provide a qualitative measure of worm burdens since fecundity is shown to be approximately independent of worm load. The parasites were highly aggregated within the study community, with most people harbouring low burdens while a few individuals harboured very heavy burdens. Of the total parasite populations in the study sample, 84% were harboured by the 2 to 15-year-old children. Of those individuals harbouring 100 worms or more, 87% were in the 2 to 10-year-old age range. Crude estimates of population parameters (basic reproductive rate, 4-5; rate of reinfection, 90 year-1) suggest that the rate of reinfection is higher than for other helminth parasites of man. The control of morbidity and parasite transmission is discussed in the context of targeting drug treatment at the child segment of the study population.
The gastrointestinal helminth infection status of an age-stratified sample from a single Caribbean community was assessed using anthelmintic expulsion techniques. The same sample was re-assessed in a similar manner after a 17 month period of re-infection. The age-prevalence profile of Ascaris lumbricoides was convex while that of Trichuris trichiura was asymptotic. The age-intensity profiles of both species were convex. These differing patterns are attributed to differences in the absolute worm burdens of the 2 species. The frequency distributions of infection intensity were similar for both species, and largely independent of host age. The basic reproductive rate of A. lumbricoides (Ro = 1-1.8) was similar to that recorded elsewhere and much lower than that of T. trichiura (Ro = 4-6), implying that the latter is intrinsically more resistant to control. Individual hosts were predisposed to high (or low) intensity infection with either species, although predisposition to both species simultaneously was not conclusively demonstrated. Further studies are required to determine the cause of these observations.
This study examines the age-dependency of the relationships between human infection with whipworm (Trichuris trichiura) and parasite-specific antibody level measured by ELISA against an extract of adult worms after preincubation of the sera with Ascaris lumbricoides adult worm extract. The convex age-profile of parasite infection intensity is shown to be mirrored by an age-dependent change in age-class mean levels of IgG (all subclasses except IgG3), IgA, IgM and IgE. Mean antibody levels rise with increasing acquisition of infection in childhood and decline as the intensity of infection falls in adulthood. Immunoblot analysis of selected sera from different age-classes indicates that antigen recognition is similarly dependent on infection intensity. In individual children, antibody levels correlate positively with acquisition of infection, consistent with a simple model of antigen dosage specifying the magnitude of the humoral immune response. In adults, IgG4 correlates positively and IgA negatively with intensity of infection, suggesting involvement of these isotypes in functional roles of immune blockade or effector mechanisms, respectively.
The study examines the distribution of Trichuris trichiura infection in a village community in St Lucia, West Indies. The infection intensity of the same age-stratified population was assessed (by drug expelled worm burden and faecal egg count) at the initiation of the study, and after 17 months of reinfection following treatment. The frequency distribution of worm numbers per person was similar at both periods of sampling. There was a significant correlation between the initial infection intensity of an individual, and the intensity acquired by the same individual following the 17 month period of reinfection. This relationship was observed in a broad range of host age classes. The study provides firm evidence that individuals are predisposed to heavy (or light) T. trichiura infection.
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