Demographic and reproductive associations with nematode infection in a long-lived mammal

Abstract

infection by macroparasites, such as nematodes, varies within vertebrate host systems; elevated infection is commonly observed in juveniles and males, and, for females, with different reproductive states. However, while such patterns are widely recognized in short-lived model systems, how they apply to long-lived hosts is comparatively understudied. Here, we investigated how infection varies with host age, sex, and female reproduction in a semi-captive population of individually marked Asian elephants Elephas maximus. We carried out 1,977 faecal egg counts (FECs) across five years to estimate nematode loads for 324 hosts. Infection patterns followed an established age-infection curve, whereby calves (5 years) exhibited the highest FECs and adults (45 years) the lowest. However, males and females had similar FECs across their long lifespan, despite distinct differences in life-history strategy and clear sexual dimorphism. Additionally, although mothers invest two years in pregnancy and a further three to five years into lactation, nematode load did not vary with four different measures of female reproduction. our results provide a much-needed insight into the host-parasite dynamics of a long-lived host; determining host-specific associations with infection in such systems is important for broadening our knowledge of parasite ecology and provides practical applications for wildlife medicine and management.

Investigating associations between nematode infection and three measures of sociality in Asian elephants

“…Finally, 11 FECs were collected over 5 weeks after social data collection (at exactly 66 days, ~ 8% of the total sample). For each sample, we carried out a faecal egg count (FEC) following the special modification of the McMaster method (MAFF 1986 ), as in Lynsdale et al ( 2020 ), using compound microscopes with × 10 optical zoom and × 10 magnification. We identified ova microscopically to the lowest taxonomic unit via identification of size, morphology, and developmental stage (Taylor et al 2007 ; Bowman 2014 ).…”

“…When investigating host–parasite interactions, it is important to consider the three main components of disease, called the disease triangle: the host, the environment, and the pathogen/parasite (Scholthof 2007 ). Individual host characteristics such as age (Lynsdale et al 2020 ) and sex (Hillegass et al 2008 ), as well as behaviour and social status (Hawley et al 2011 ; Keiser et al 2016 ), can relate to transmission and infection risk. In addition, external factors, such as season or weather conditions, influence sickness behaviour and infection dynamics for environmentally transmitted parasites (Owen-Ashley and Wingfield 2006 ; Rödel and Starkloff 2014 ).…”

“…In conditions where the social setting of large long-lived mammals is artificially modulated by humans, the link between sociality and infection is less well understood. Gastro-intestinal nematodes are among the most abundant internal parasites found in Asian elephants (Fowler and Mikota 2006 ; Lynsdale et al 2020 ), are an important driver of elephant mortality (Lynsdale et al 2017 ), and are linked to reduced elephant health and immunity (Santos et al 2020 ). However, the results regarding the link between parasite load and sociality in elephants are inconsistent (Vanitha et al 2011 ; Abhijith et al 2018 ) and warrant further investigation.…”

“…In addition, the Myanma Timber Enterprise (MTE) has maintained extensive logbooks on each individual, which allow tracking individual elephants’ life events, such as illness and health treatments, and provide detailed data on group compositions and friendship networks. In addition, we capitalize on the longitudinal data on parasite infection already existing in this system (Lynsdale et al 2017 , 2020 ), which is highly important for gaining a reliable quantification of infection dynamics as opposed to cross-sectional studies and opportunistic sampling. In these semi-captive elephants, nematode infections happen via faecal–oral horizontal transmission, which is the same route found for wild elephant populations.…”

Musth and sexual selection in elephants: a review of signalling properties and potential fitness consequences

“…For instance, although free-ranging male African elephants tend to have lower parasite loads than females, parasite load is not affected by musth status in males (Thurber et al, 2011;Baines et al, 2015;Parker et al, 2020). Similarly, studies in Asian elephants show no intersexual differences in parasite infection rates, although parasite prevalence is relatively low in reproductively aged male elephants (Lynsdale et al, 2017(Lynsdale et al, , 2020. However, females may use traits other than musth (e.g., tusk length) to evaluate parasite resistance in potential mates (Watve & Sukumar, 1997).…”