EPIDEMIOLOGYfrom Lusaka was 3% in the urban areas, but 27% in the rural areas; and over 97% of
these children were found to have anti-malarial antibodies.
Such surveys with regard to infection rates in a community indicate who and which
people are likely to be susceptible and who will require prophylactic treatment. These
values suggest that there was or is mesendemic malaria among rural children most of
whom developed resistance to malaria. The investigations also indicated that although
there is a low level of transmission in the urban areas, the urban children are more
susceptible when they enter the rural areas because they have not developed any
resistance.
Many of the parasitic helminth infections rise steadily to a peak in early teenage years
and then fall off to relatively low levels in adulthood. A recent survey of Schisotosoma
mansoniinfections in Senegal (Fulford et al. 1998) has demonstrated the existence of
such an age-intensity profile. Other studies in endemic areas have confirmed this fall-off
of the infection rate during puberty (see Box 8.2).
n 8.2 PROBLEMS AND DIFFICULTIES
It is important to understand and appreciate the problems and difficulties involved in any
epidemiological study. In 1991 a survey of a Schistosoma japonicuminfection was carried
out among migrant fisherman in Dongting Lake in China (Li and Yu 1991). The popu-
lation and geographical details are shown in Box 8.3.
For purposes of the investigations five sites were chosen; two each in the eastern
and southern lakes and one in the western lake; and 652 boats with 1,842 persons were
registered (details of each family were recorded). The boats were numbered and those
used in the study were chosen at random. Eventually 122 boats with 316 persons over
3 years old formed the study group. This constituted 18.7% of the boats and 17.2% of
the population.
The detection and treatment of patients, snail control, provision of water and sanita-
tion and health education were carried out by the personnel at each field station. A ques-
tionnaire was used to determine the following data for each individual in the survey: sex,
n BOX 8.2
The last statement implies that host hormones might play an important role in development
of immunity to S. mansoni:nThe hormones might have a direct influence on the parasite’s metabolism.
nInduction in the host of physiological and anatomical changes such as increase in skin
thickness, fat deposition etc, which affect the innate resistance.
nChanges to the immune system.With regard to the immune system the following observations have been made:nA steady increase in serum lgE antibody responses into adulthood.
nSerum levels of the antibody lgG 4 against adult worms is greatest at peak levels of
infection.
nAnti-soluble egg antigen lgG 1 antibody levels in the serum are highest before the infec-
tion begins to decline.