A study of the transmission of Schistosoma haematobium in Volta Lake, Ghana

A field study on the ecology and epidemiology of S. haematobium was carried out in 8 different parts of the Volta Lake from November 1978 to June 1980. Snail sampling for B. rohlfsi was conducted monthly in 39 lakeside villages and data on human prevalence rates and egg counts were obtained in 30 of the villages. A detailed, integrated study of S. haematobium was conducted at the large lake- side village of Agbenoxoe. The snail sampling technique was an efficient version of the man- time method. For screening urine samples for S. haematobium eggs, the "Nuclepore" filtration method was used - its first large-scale, field application. At Agbenoxoe, a new method of recording water contact data was initiated. From snail sampling, it was learned that S. haematobium trans- mission was distinctly seasonal, and intensity varied according to the type and amount of vegetation in water contact points (WCPs), the shape of the WCPs, and their geographical location. S. haematobium infection rates in B. rohlfsi around the lake were among the highest in the world. The snail had a high intrinsic rate of natural increase, but could not maintain an equilibrium population in the unstable habi- tat of the lake. An original mathematical model was developed to describe the dynamics of S. haematobium transmission to B. rohlfsi. S. haematobium prevalence rates and egg counts were exceptionally high in 2 lake sections - the Afram and Obosum branches - mainly because of past and present growth of Ceratophyllum which led to high transmission during most months of the year. However transmission became interrupted when Ceratophyllum density became too great and decayed in WCPs. Levels of infection were lower in the other lake sections surveyed, mainly because of less Ceratophyllum growth which confined high transmission to December to March each year. Analysis of all the human data revealed that the 5- 19 year-old age span was responsible for 93% of the potential contamination of S. haematobium eggs in the Volta Lake. At Agbenoxoe, snail sampling, prevalence, egg count, incidence, and water contact data fit together to paint a uniform picture of very focal and seasonal transmission.