Modeling Strongyle Parasite Infection Rates and Extinction Forces in African Equid Hosts

Abstract

This study used the theory of island biogeography – which states that the number of species present on an island is a balance between species colonization and extinction rates - to conceptualize the intensity of parasitic infection in donkeys, plains zebras, and endangered Grevy’s zebras. The parasite of interest was the strongyle, a gastrointestinal nematode transmitted to equids through fecal-oral contact. By collecting dung samples from all 3 equid species at the Mpala Research Centre, Kenya in June-July 2017 and June-July 2018, the study quantified both strongyle loads and gut IgA antibody production. The study first showed that strongyle infection rates are higher in wet years than in dry years. Next, results showed that a zebra’s strongyle infection risk depends on a combination of the proximity of its home range to water points, how much time the zebra spends on glades, and how heavily used by other zebras those glades are. This model of infection rates was designed to capture a zebra’s exposure to zebra dung, which is densest around the water points and glades that attract high zebra activity. An infection risk score was calculated for 11 well-sampled adult Grevy’s zebras and was a significant predictor of average strongyle burden. Meanwhile, high IgA levels were shown to be induced by high parasite egg outputs in well-sampled donkeys and territorial male Grevy’s zebras. Additionally, IgA levels influenced parasite egg outputs in subsequent days; peaks in IgA resulted in decreases in egg loads while troughs in IgA resulted in increases in egg loads. Although these aspects of the IgA and parasite load relationship were only observed in donkeys and adult male Grevy’s and not in other types of equid, the results suggest that IgA is an inducible resistance mechanism in equids; IgA is activated in response to high parasite fecundity and is used to lower the parasites’ fitness. Finally, Grevy’s zebras displayed higher average IgA levels than plains zebras and donkeys. In Grevy’s, high parasite “extinction” rates from high IgA levels, coupled with lower infection rates from generally living far from water points, cause Grevy’s zebras to have less intense strongyle infections than plains zebras on average. Meanwhile, with small extinction rates from low IgA production and high infection rates from grazing in constrained areas, donkeys suffer from much more intense infections than Grevy’s zebras, making them a serious health risk to Grevy’s should the 2 species overlap on a landscape and transmit infection to each other.