Jolles, A.E. , Ezenwa, V.O., Etienne, R.S., Turner, W.C., & Olff, H. Ecology. (2008).

"Interactions between macroparasites and microparasites drive infection patterns in free-ranging African buffalo".

Reviewed: 11/04/11

Infection by both macro- and micro- parasites in wild populations occurs at an astonishingly high rate. Among the buffalo herds described in this study the prevalence of Bovine tuberculosis and helminth infection reached as high as 72.5% for TB, and 87.5% for the worms. With such high prevalence of disease it is only natural to assume that some hosts face the double pressure of coinfection; it is also possible to imagine the concept of a herd that contains a higher proportion of coinfected individuals. This study hoped to identify the primary drivers of disease dynamics in buffalo at both the host and host population level.
Worm prevalence in buffalo was found to be highly negatively correlated with TB prevalence. Demographic factors such as sex and age, as well as proportionate amounts of certain individuals within the population, were found to be partial drivers of this pattern. Age plays a role in infection patterns at young ages fro both worms and TB, but males were only slightly less likely to be infected with worms than females.
Researchers also tested the role of a mortality hypothesis in driving disease dynamics using both empirical data and a simple theoretical model to qualitatively assess expected outcomes from different initial conditions. They found that coinfected individuals did indeed have a poorer body condition than those hosts who were singly infected. They also found that individuals with a higher concentration of fecal eggs, a metric of worm pressure, occurred less frequently in TB positive individuals, indicating an increased mortality rate. This pattern was true at the herd level as well based on proportionate amounts of infected individuals. Theoretical work supported a higher mortality rate alone being a partial driver of the negative correlation between worms and TB at the herd level, but only the addition of immunological heterogeneity into the model compared with the disease patterns at the individual level. Individuals infected with worms have a higher immunity to TB infection because of the self-regulation of the immune system in mammals to microparasitic and macroparasitic pathways. The immunity of worm infected individuals to TB reduced the infection rate of TB differentially in the worm-hosts than in regular susceptibles, reducing the total number of coinfections, which have a higher mortality anyways.
This study is interesting because it shows how different kinds of parasites interact and coexist with one another via multiple mechanisms. Differential infection in a shared host population based on sex and age shows niche differentiation, but antagonism between the two parasites was still present in this study. The two immunological pathways within the host reduced the secondary infection of TB when worms were present, though interestingly this was not true in the opposite direction. The result has evolutionary implcations as well, implying that parasites that come in and can reduce their host's risk of infection to a potentially more dangerous potential parasite would have an evolutionary advantage. These same immunological pathways are conserved across mammal species indicating a long occurring pattern of coinfection in the natural world.