THEME 1: CO-INFECTION
Interactions between parasites co-occurring within a single host have profound effects on infection outcomes ranging from disease severity and progression within individual hosts to patterns of disease spread across populations. Since most hosts are routinely infected by more than one type of parasite, concurrent infection (or co-infection) has emerged as a key challenge for wildlife, veterinary, and human health. Two central questions that arise about co-infection are: (i) what are the causes of interactions between parasites? and (ii) what are the consequences of these interactions for patterns of infection in vulnerable hosts? Our work combines field studies, immunological and molecular tools, and ecological theory to address these questions.
Helminth-TB co-infection: Helminth (i.e., parasitic worm) co-infection is fascinating because these parasites can influence the mammalian immune system in ways that alter how hosts interact with intracellular parasites like viruses and many bacteria. Our work on co-infection between gastrointestinal helminths and bovine tuberculosis (TB) in wild African buffalo has used experimental and longitudinal studies, coupled with mathematical models, to documented helminth-mediated immune suppression in the wild; explore the conditions under which immune suppression can alter TB dynamics in buffalo; and identify opposing effects of anthelmintic treatment on TB dynamics at different biological scales. More recently, our work has shown that both active helminth infection and natural resistance to helminths result in immune suppression. However, whereas anthelmintic treatment and associated reduction in worm burden reduces helminth-mediated immune suppression, the presence of more potent constitutive defenses against helminths increases immune suppression. This result implies that host adaptation to common helminth parasites as well as ecological exposure to helminths both cause differential TB progression and disease-induced mortality. Currently, we are investigating the mechanisms linking helminth resistance to variable TB outcomes and quantifying the ecological and evolutionary consequences for hosts and both parasites. Related publications: Jolles et al 2008, Ezenwa et al 2010, Ezenwa & Jolles 2015, Ezenwa et al 2021.
THEME 2: ANIMAL BEHAVIOR AND PARASITISM
Animal behavior and parasitism are inextricably linked. Behavior influences host exposure and susceptibility to infection, while parasitism generates variation in behavior on ecological and evolutionary timescales. Our work focuses on understanding complex links between behavior and parasitism.
Social behavior: Increased parasite transmission is considered a nearly universal cost of group living. However, living in groups can increase infection risk on one hand, but ameliorate costs of infection on the other. Our work on the African antelope, Grant’s gazelle has shown that individuals who live in larger groups are more likely to acquire gastrointestinal worm infections, and these parasites induce anorexia, which is a major cost of infection. However, the magnitude of anorexia is reduced among individuals living in larger social groups, suggesting that group living can mitigate a key cost of worm infection. Theory on parasites and the evolution of social behavior typically focuses on the negative effects of group living on infection rates. However, if our findings apply generally, the net effect of group living on the fitness costs of infection may be positive in some situations, despite social hosts having higher parasite burdens. Our ongoing work on this theme investigates the general role social behavior may play as a form of ‘tolerance’ against parasites. Related publications: Worsley-Tonks & Ezenwa 2015, Ezenwa et al. 2016, Williams et al. 2017, Ezenwa & Worsley-Tonks 2018.
Movement: Animal movement shapes most ecological interactions, including parasite and pathogen transmission. On the one hand, the movement of animals from one location to another can facilitate the spread of parasites. On the other, movement modifies ecological interactions (e.g., consumer-resource interactions) in ways that may constrain parasite dissemination. In collaboration with researchers at the University of Georgia, Wake Forest University, and the University of Glasgow, our lab is studying how mobile hosts affect parasite dynamics in resident species encountered along their routes. Focusing on migrating wildebeest in Serengeti National Park we are quantifying the dual role of mobile wildebeest in transporting fecal-oral transmitted parasites into the home ranges of resident species (e.g., buffalo, Grant’s gazelles, topi) and in modifying the physical structure of vegetation to disentangle the relative roles of transport vs. trophic effects, as well as the intensity vs. duration of movement, in shaping the impact of animal migration on parasite transmission.