Attenuation of Eimeria caviae by selection for precocious development

GENETIC RELATIONSHIPS BETWEEN PARASITE VIRULENCE AND TRANSMISSION IN THE RODENT MALARIA PLASMODIUM CHABAUDI

Many parasites evolve to become virulent rather than benign mutualists. One of the major theoretical models of parasite virulence postulates that this is because rapid within-host replication rates are necessary for successful transmission (parasite fitness) and that virulence (damage to the host) is an unavoidable consequence of this rapid replication. Two fundamental assumptions underlying this so-called evolutionary trade-off model have rarely been tested empirically: (1) that higher replication rates lead to higher levels of virulence; and (2) that higher replication rates lead to higher transmission. Both of these relationships must have a genetic basis for this evolutionary hypothesis to be relevant. These assumptions were tested in the rodent malaria parasite, Plasmodium chabaudi, by examining genetic relationships between virulence and transmission traits across a population of eight parasite clones isolated from the wild. Each clone was injected into groups of inbred mice in a controlled laboratory environment, and replication rate (measured by maximum asexual parasitemia), virulence (measured by live-weight loss and degree of anemia in the mouse), and transmission (measured by density of sexual forms, gametocytes, in the blood and proportion of mosquitoes infected after taking a blood-meal from the mouse) were assessed. It was found that clones differed widely in these traits and these clone differences were repeatable over successive blood passages. Virulence traits were strongly phenotypically and genetically (i.e., across clones) correlated to maximum parasitemia thus supporting the first assumption that rapid replication causes higher virulence. Transmission traits were also positively phenotypically and genetically correlated to parasitemia, which supports the second assumption that rapid replication leads to higher transmission. Thus, two assumptions of the parasite-centered trade-off model of the evolution of virulence were shown to be justified in malaria parasites.

Infectivity and oocyst excretion patterns of Cryptosporidium muris in slightly infected mice

To determine the infectivity of Cryptosporidium to hosts in slight infections, we examined the infectivity and oocyst output patterns of Cryptosporidium muris in mice inoculated with small numbers of oocysts. One of the 25 ICR mice inoculated with 2.4 x 10(1) oocysts and 19 of the 25 mice inoculated with 2.4 x 10(2) oocysts shed oocysts in the feces after inoculation. Four of the 50 mice inoculated with 2.4 x 10(1) oocysts for 10 consecutive days also shed oocysts and their OPG values were similar to that of the mice which received 2.4 x 10(2) oocysts. Consequently, it is clear that less than 10% of the mice which received 2.4 x 10(1) C. muris oocysts for 10 consecutive days.

Infectivity to experimental rodents of Cryptosporidium parvum oocysts from Siberian chipmunks (Tamias sibiricus) originated in the People's Republic of China

We isolated Cryptosporidium parvum-type oocysts from naturally infected siberian chipmunks which originated in the People's Republic of China and examined the infectivity to rodents as experimental animals. The naturally infected chipmunks did not show any clinical symptoms. The oocysts were 4.8 x 4.2 microm on average in size. They were ovoid and morphologically similar to the C. parvum oocysts isolated from human and cattle. Experimental rodents were inoculated with 1.6 x 10(6) original oocysts each. SCID mice began to shed oocysts on day 7 and the OPG value was 10(5) from 50 days. The oocysts were found from ICR mice on days 13 and 16 by only sugar flotation method, however, any oocysts were not detected from the rats, guinea pigs and rabbits until 30 days. Two infected SCID mice were necropsied on days 100 and 102 and examined for coccidian organisms. Merozoites and oocysts were found in the low part of jejunum and ileum, however, no parasites were detected in the stomach. Consequently, it was considered that the present species was C. parvum and was probably genotype 2 from result of infectivity to rodents.