Improved quantification of Plasmodium exoerythrocytic forms in rodents.

The result of a Plasmodium sporozoite challenge is currently evaluated either by detecting the emergence or not of parasites in the blood, or by estimating the "prepatent period", which is the time between sporozoite inoculation and the appearance of parasites in the blood. This type of measurement is relatively rough and has given way to another method of measuring sporozoite infectivity, which is to enumerate the exoerythrocytic forms (EEF) by microscopic examination of liver sections. Up until now, two different methods have been proposed to calculate and estimate the number of Plasmodium EEF forms in the livers of infected rodents, both of which are unfortunately biased to some extent. Here, we propose a different method of calculation, which more faithfully reflects the EEF number in the liver. This method is based on the calculated mean number of consecutive liver sections in which a schizont appears, and is host related.

Rodent malaria in the natural host--irradiated sporozoites of Plasmodium berghei induce liver-stage specific immune responses in the natural host Grammomys surdaster and protect immunized Grammomys against P. berghei sporozoite challenge.

The choice of the host in studying host-parasite interactions is of crucial importance, and the use of a natural host is most appropriate in answering pertinent questions related to human malaria. The Grammomys surdaster is the natural host and reservoir of the rodent malaria parasite Plasmodium berghei. This natural host is difficult to protect by irradiated sporozoite immunization, a situation comparable to what has been observed in humans with P. falciparum. This is in contrast to the complete protection that can be induced in artificial hosts like inbred mice strains. The natural host is highly susceptible to P. berghei hepatic stage infections. Immunization with irradiated sporozoites in Grammomys generates blocked hepatic stage parasites and immunized Grammomys protected upon live sporozoite challenge generate antibody and T cell proliferative responses to these hepatic stages. Associated with proliferation, cytokines are secreted into culture supernatants constituted mainly of Interferon gamma, negligible amounts of TNF-alpha, and no IL-4. Natural host-parasite interactions of Grammomys surdaster-P. berghei can help define the effector mechanism(s) in the Plasmodium falciparum-human interaction.

Plasmodium berghei development in irradiated sporozoite-immunized C57BL6 mice.

The C57BL6 strain of mice is highly susceptible to Plasmodium berghei sporozoite infections and consequently requires repeated immunizations with irradiated sporozoites to obtain protective immunity. After a live sporozoite challenge in the immunized hosts, hepatic-stage parasites found in the liver after 48 h are of different sizes--small schizonts corresponding to blocked forms (derived from irradiated sporozoites), and schizonts of intermediate size (derived from live sporozoites). Large schizonts corresponding to mature hepatic forms are found only in unimmunized but challenged C57BL6 mice. Using monoclonal and polyclonal antibodies directed to liver-stage parasites, different patterns of binding reactivity to the above forms are observed. More than 20% of the irradiated sporozoites transform into blocked forms after immunization and persist in the liver. Upon sporozoite challenge in such immunized animals the rate of transformation of sporozoites into hepatic parasites is less than 2%. These observations shed light on the fate of live sporozoite development in irradiated sporozoite-immunized C57BL6 mice.

Blocked hepatic-stage parasites and decreased susceptibility to Plasmodium berghei infections in BALB/c mice.

The BALB/c strain of mice is comparatively more resistant to sporozoite infections of Plasmodium berghei than the C57BL6 strain. Infection with live sporozoites results in the formation of small hepatic forms in the BALB/c liver that persist for as long as 6 days. Upon infection with small numbers of sporozoites, some of the parasites are destroyed in the liver whereas the rest persist as blocked forms. When larger numbers of sporozoites are injected the same process occurs but, in addition, a fraction of the liver-stage parasites complete full development and give rise to blood forms. Although blocked liver forms persist until day 6 post-infection they actually develop to only 24 h of maturity. The nature of these persistent forms is similar to those obtained from irradiated sporozoite immunization. There is a stronger cell proliferation to liver-stage antigens by spleen lymphocytes of irradiated sporozoite-immunized BALB/c mice in comparison to that of immunized C57BL6 mice suggesting that a stronger priming to liver-stage antigens, probably due to the presence of blocked hepatic forms in the liver for a longer period of time (as compared to C57BL6), occurs in the BALB/c mice. This could be a reason for the long-lasting protective memory observed in BALB/c mice.

[Population dynamics and infestation of Bulinus truncatus and Bulinus forskalii by schistosomal larvae in the Sudan-Sahelian zone of Cameroon]. / Dynamique des populations et infestation de Bulinus truncatus et Bulinus forskalii par les larves de schistosomes en zone soudano-sahélienne au Cameroun.

The evolution of the population densities of Bulinus truncatus and Bulinus forskalii has been studied during 18 months in a number of sites of the Sudan-Sahelian zone of Cameroon, in relation to certain environmental factors. Only the altitude of the water in the sites has an influence on the density of the snails. The populations of B. truncatus present a greater proportion of young molluscs during the entire year, with the exception of the last months of the dry season. The population dynamics of B. forskalii varies depending on whether the environment is temporarily stagnant, temporarily running or stable. The proportions of B. truncatus and B. forskalii that emit cercariae of schistosomes are 1.2% and 0.8% respectively. Study of the adult worms obtained after infestation of the laboratory mammals show that B. truncatus emits cercariae of Schistosoma haematobium at the end of the dry season and at the beginning of the rainy season. We have not been able to identify precisely the cercariae emitted by B. forskalii, which raises the necessity of further systematic studies being carried out on the schistosomes of Cameroon.

[Population dynamics and infestation with Bulinus globosus in a Sudan-Sahelien zone of Cameroon]. / Dynamique des populations et infestation de Bulinus globosus en zone soudano-sahelienne du Cameroun.

The population dynamics of Bulinus globosus was followed at seasonal and perennial habitats in three villages in the Extreme North Province, Cameroon, known to have high prevalences of Schistosoma haematobium. The study covered 2 rainy and 1 dry season between June 1987 to November 1988. Of several environmental factors monitored, only rainfall and the resulting changes in water level were strongly correlated with changes in snail density, and even then only when values were at their extremes. In seasonal pools, adult snails emerge from estivation and begin laying eggs shortly after the first rains. A decline in the number of larger snails 3 to 6 weeks later suggests a die off of the founding population. In general, the population density increases modestly during the rainy season and then rapidly declines after torrential rains have ended in September or October. Adults are again dominate just before the habitats dry up. In seasonal streams, population densities fluctuate dramatically in association with rainfall and subsequent flooding, and populations stabilize only after the torrential rains have ceased. Unlike populations in temporary ponds, young snails constituted a large percentage of the population very late in the season. In perennial habitats, seasonal fluctuations in population density were related to manipulation of water levels in a rice irrigation scheme. The lowest densities occurred when the water levels were at their most extreme levels, between November and April. But declines in the population densities often preceded significant changes in the water level, indicating that this factor alone is not responsible for these declines. Of the B. globosus shedding schistosome cercariae, more than 90% were infected with S. haematobium and the remainder with S. bovis. Of 153 infected B. globosus, 87% were found in seasonal habitats. In those sites the first infected snails were found in July but the large numbers were not present until late August and September. Infected snails declined rapidly before the final decline in the general population of B. globosus in November and December. In perennial habitats, infected snails were most common in May and June, just before and early in the rainy season. These findings may prove useful in developing more effective schistosomiasis control measures in sahelian climates.

Human schistosomiasis in Cameroon. II. Distribution of the snail hosts.

A nationwide survey for snail hosts of human schistosomes was carried out in Cameroon between 1985 and 1988. In total, 668 sites at 432 locations were sampled. In the arid, northern half of the country (tropical climatic zone), where both intestinal and urinary schistosomiasis are hyperendemic, Biomphalaria pfeifferi was the only Schistosoma mansoni host and Bulinus globusus and B. senegalensis the most common S. haematobium hosts. In that region, these snails occurred almost exclusively in temporary bodies of water. Taking into account results from a companion study on the distribution of schistosomiasis in humans, our results clearly show that temporary water bodies in the tropical zone are the principal foci of transmission. These findings disagree with commonly held views about schistosome transmission in Cameroon. B. truncatus, a S. haematobium host, was also present in the tropical zone but was found principally in perennial habitats. Although some perennial habitats were important transmission sites, they represent only a small portion of the overall problem. B. truncatus is the principal S. haematobium host in the wetter southern half of the country where schistosomiasis haematobium is highly focal. Biom. camerunensis was far more common than Biom. pfeifferi in the South but did not occur where S. mansoni prevalence rates were high; thus it appears to be a poor host. B. forskalii, the sole host of S. intercalatum in Cameroon, occurs widely throughout the country; however, the schistosome is restricted to a small region in the South.