Re-Infection and Epidemiology Studies on Schistosoma mansoni in Kenya
In our group schistosomiasis has been studied in a number of field sites around the world, including Kenya, Uganda, Senegal, Brazil and the Philippines. Most of our work has however concentrated of studies carried out in Kenya. Schistosomiasis is endemic in Kenya, with more than 3 million people infected. Two species of schistosome are present, S. mansoni, and S. haematobium . The data given below however only refers to S. mansoni infection. Field work in Kenya has been carried out for the last 17 years in collaboration with the Kenya Ministry of Health and the Kenyan Medical Research Institute. This work including examining the effects of various chemotherapy control programs on the levels of infection in different areas, the nature of the human immune response to schistosome infection, and importantly, the dynamics of the infection of human individuals with the disease, i.e. 'who catches schistosomiasis and why'.
Collecting water at Kambu in Kenya (a typical water contact site)
In an attempt to answer these questions the following extensive operations have been carried out:
Detailed demography of entire study populations, involving many Kenyan villages and thousands of individuals in a number of field sites
Identification and monitoring sites of infection in terms of:
- numbers of snail intermediate hosts
- numbers of parasites present, as detected by examining faecal samples for parasite eggs
- the level of contact between the population living in the study areas, and the contaminated bodies of water used by these peopleMonitoring individual patients in terms of:
- blood samples
- demographic details and water contact observations
- multiple faecal samples
- clinical examination
- treatment
From the data obtained from these studies a number of observations and conclusions may be drawn.
It is commonly observed that the intensity of schistosomiasis infection (measured, in the case of S. mansoni, by the number of eggs found in a small sample of faeces: eggs per gram = epg) reaches a peak in the teenage years and declines steady in adulthood. The five communities plotted below are typical of untreated populations.
The worms are quite long-lived (it has been estimated that the average life span is 6 to 7 years, although worms of 30 year or more have been reported) so many of the worms an adult harbours may have been acquired a decade or more ago. It is, however, possible to look more directly at how rapidly individuals acquire infection. To do this everyone in the community is treated with the drug Praziquantel which kills all the adults worms. Since the infection is almost impossible to eradicate from an area the population will quite rapidly become reinfected. As can be seen below, children are much more rapidly reinfected than adults, a difference that appears quite constant and is much more marked than in populations before treatment.
Might the slower rate of infection among adults provide clues as to how to prevent the spread of this parasite? Do adults acquire immunity to the infection? Or are they innately more resistant to the parasites, perhaps because they have thicker skins which hinder penetration of the cercariae (the infective larvae)? Or are adults simply less exposed to cercariae?
Water Contact Observations
In an attempt to answer this last question we have made extensive observations of water related behaviour in these communities. A local observer, who is familiar with everyone in the community, is stationed on the river bank and records every visit to the water: who made it, what they did, when they did it and how long they took.
Differences between the sexes are not large but generally it is noted that males are more heavily infected yet females have more contact.
But exposure is more complicated than this. The risk of infection is much greater for some types of behaviour than others. As illustrated below at the two sites at Kavilinguni and on the Kambu river, the number of cercariae in the rivers vary both with the time of day and from month to month: cercariae tend to emerge from their snails in the heat of the afternoon while the numbers of snails in the river varies dramatically, largely because seasonal rains wash most of them away twice a year.
This matters because, adults and children tend to visit the water at different times, as, indeed, do males and females. For example both young boys and girls have relatively high levels of water contact in the afternoon (from about 2 pm to 5 pm), in contrast in the older age groups men have comparatively little water contact, whilst the older women have high levels of water contact in the morning ( from about 10 am to 11.30am). Therefore by comparing these water contact graphs with the graphs showing cercarial densities in the rivers it can clearly be seen that different age groups face different levels of risk of infection. For example the children, as well as generally having the greatest amount of total water contact, also as a group have the greatest amount of water contact at the time of day that cercarial densities in the water is greatest. These differing times of water contact are probably related to different types of activities which may also have associated differing risks.
When we investigated this we did indeed find that the type of contact with contaminated water did vary, for example children play in the water while adults work, and these different activities may carry very different risks of infection.
An example of this is where children tend more frequently to immerse themselves completely in the water than do adults, and may therefore be more at risk of infection as more of their body surface area is available for cercarial penetration. Not only does water contact behaviour vary with age and sex, but these pattern of this variation also differs between communities and from year to year.
Thus, when the relative risk of exposure to cercariae is taken into account, the patterns of exposure with age appear to be even more variable, in stark contrast to the constant re infection patterns. This leads us to believe that there is more to infection rates than simple exposure.
However, there is much we still do not know about the risk of infection associated with different types of activity. It is still possible that in the communities we have been studying certain ubiquitous forms of contact dominate this risk and hence might completely explain why adults fail to become as heavily reinfected as children. Further studies underway in Kenya and Uganda should help to remove this doubt. In one we are studying an immigrant community at Masongaleni in Kenya only recently exposed to the infection and who should therefore not yet be immune to it. In a second we are studying a fishing village on Lake Albert in Uganda where adults are almost certainly heavily exposed.