Differential drivers of intraspecific and interspecific competition during malaria-helminth co-infection.

Various host and parasite factors interact to determine the outcome of infection. We investigated the effects of two factors on the within-host dynamics of malaria in mice: initial infectious dose and co-infection with a helminth that limits the availability of red blood cells (RBCs). Using a statistical, time-series approach to model the within-host 'epidemiology' of malaria, we found that increasing initial dose reduced the time to peak cell-to-cell parasite propagation, but also reduced its magnitude, while helminth co-infection delayed peak cell-to-cell propagation, except at the highest malaria doses. Using a mechanistic model of within-host infection dynamics, we identified dose-dependence in parameters describing host responses to malaria infection and uncovered a plausible explanation of the observed differences in single vs co-infections. Specifically, in co-infections, our model predicted a higher background death rate of RBCs. However, at the highest dose, when intraspecific competition between malaria parasites would be highest, these effects of co-infection were not observed. Such interactions between initial dose and co-infection, although difficult to predict a priori, are key to understanding variation in the severity of disease experienced by hosts and could inform studies of malaria transmission dynamics in nature, where co-infection and low doses are the norm.

A Comparison of Antiserum and Protein A as Secondary Reagents to Assess Toxoplasma gondii Antibody Titers in Cats and Spotted Hyenas.

Toxoplasma gondii is a protozoal parasite with worldwide distribution that is able to infect a wide variety of mammals and birds. Our main goal was to screen for T. gondii antibody titers in a previously untested species, the spotted hyena ( Crocuta crocuta); however, this goal first required us to investigate serological procedures that could be suitable for hyenas. Cats are the closest domestic relations of hyenas, so T. gondii antibody titers were first compared in 26 feral cats with specific or nonspecific fluorophore-labeled secondary reagents, i.e., anti-cat IgG or protein A. Substitution of anti-cat IgG with protein A caused a statistically significant drop in titer measurements in cats (P = 0.01) with a reduction of the geometric mean titer equivalent to 1 doubling-dilution. The same procedures were then applied to captive spotted hyenas. Titers measured in 9 of 10 hyenas were identical whether anti-cat IgG or protein A was used as the secondary reagent: 5 had titers <1:16, 2 had titers of 1:16, and 2 had titers of 1:32. One hyena had maximum titers of 1:64 or 1:32 when anti-cat IgG or protein A was used, respectively. The use of protein A as the secondary reagent in serologic assays can be applied to a range of mammalian species and seems unlikely to affect test specificity; however, the use of protein A may reduce test sensitivity, as suggested in the present study using cats. Despite a control program, some exposure to T. gondii had occurred in the Zoo's spotted hyenas.