title: Transmission of Schistosoma haematobium in seasonal pools in the Gambia, with particular reference to the role of human contact
Creators: name: J. Blumenthal, U.
Publisher: name: University of Liverpool Repository
Year: 2023

description: The transmission of S. haematobium in seasonal pools in The Gambia was investigated during the 1982 and 1983 transmission seasons. Studies were carried out on (a) infections in the snail intermediate host, Bulinus senegalensis, (b) cercarial densities in the pools, (c) water contact behaviour at the pools, and (d) the pattern of human infection. It was found that seasonal changes in the pools influenced both the snail populatons and the water contact patterns. Snail infection rates were low overall but were high in particular months and pools. Most infections occurred in one pool (maximum monthly mean infection rate 5.9%). Field and laboratory studies indicated that high mortality of B. senegalensis was counteracted by a high intrinsic rate of natural increase, and transmission of S. haematobium was maintained by a short prepatent period of the parasite. The cercariometric technique used had a 74% recovery accuracy in the laboratory. Cercarial densities in the pools were generally below 1 cercaria/litre, and the maximum density was 5.8 c/1. Cercarial densities were highest around midday, close to the vegetation in the middle of the pool as the pools were drying out. Transmission potential, as measured by cercarial density, varied between contact sites, months, times of day and position within a site. Human water contact was monitored by direct observation using a new "time-point" method and by recording frequency and duration of water contact of individuals. The observed activities were mainly for domestic and recreational purposes. Water contact was focal and seasonal, and the exposure potential (product of cercarial density and duration of contact at each pool) was highest at two pools and in the latter half of the season. The amount of water contact varied markedly between sexes and age groups. Contact increased progressively between ages 2-9 years, remaining at very high levels in females over the age of 10 years but declining to low levels in adult males. Substantial variation occurred in the contact of individuals in a narrow age group. An index of individual exposure to infection was calculated for two cohorts, aged 8-13 years and 2-70 years, by modifying the duration of contact by each individual by factors for the cercarial density at the pool used and for the activity performed. The resultant pattern of exposure to infection was markedly different from the pattern of duration of water contact: in particular, the mean exposure of adult women was reduced to less than that of children, although individual adult women had higher exposure levels than some children. The intensity of S. haematobium infection was highest in children aged 5-14 years and declined to low levels in adults. In 2-14 year olds increased exposure to infection was associated with an increase in reinfection after treatment. Adults had low levels of both infection and reinfection, although some had high levels of exposure to infection. It is concluded that both exposure and age influenced the pattern of infection in the community. The combined results of these studies indicates the complexity of transmission of schistosomiasis from snail to man, and shows that contact to very low cercarial densities can result in high levels of human infection.