The physiological cost to bats of controlling body temperature during winter in southern Queensland appears indirectly linked to higher rates of Hendra infection and elevated excretion of the virus during the cooler months, according to researchers.
Pteropid bats, known locally as flying-foxes, are the natural reservoir of the dangerous Hendra virus. The first known cases were recorded in 1994, when horses and a racehorse trainer were infected. Winter was quickly identified as a higher-risk time for spillover cases in horses.
The virus is capable of infecting humans following contact with infected bodily fluids from infected horses. Of the seven known human cases, four have proved fatal.
Pteropus alecto (the black flying-fox) and the closely related P. conspicillatus (the spectacled flying-fox) appear to be the main reservoir hosts.
Previous studies have suggested that physiological and ecological factors may underpin infection dynamics in flying-foxes, and subsequent spillover to horses and in turn humans.
Australian researchers set out to study concentrations over time of the stress hormone cortisol in the urine of the wild bats to see if there was any relationship between Hendra virus infection and physiological stress.
Urine samples were non-invasively collected from under roosting flying-foxes in two regions of Queensland. One was in the warmer far north of Queensland, from roosts in Yungaburra and the urban area of Cairns, and the other in the cooler southeast corner of the state, under roosts in Boonah and Toowoomba.
A total of 2208 pooled samples were collected between all the sites. A total of 464 individual samples were additionally collected at the Boonah and Toowoomba roosts.
“We found no direct correlation between increased urinary cortisol and Hendra virus excretion, but our findings do suggest a biologically plausible association between low winter temperatures and elevated cortisol levels in P. alecto in the lower latitude Southeast Queensland roosts,” Lee McMichael and her colleagues reported in the open-access peer-reviewed journal PLOS ONE.
However, they said they believed there was an indirect association between low winter temperatures and increased Hendra virus infection and excretion, affected by the physiological cost of controlling body temperature.
Further work was required to test the hypothesis, they said.
The pooled urine Hendra virus data used in their research were part of a larger study which showed that Hendra virus excretion was higher in Southeast Queensland than in Far North Queensland, with virus excretion peaking in June, July and August (the southern hemisphere winter) in lower latitude Southeast Queensland.
In contrast, it remained more or less constant in Far North Queensland.
Pooled urinary cortisol concentration was consistently elevated in March in the Southeast Queensland Boonah roost, and in February/March in both Far North Queensland roosts, aligning with the February to April breeding season for the bat species in question.
Pooled samples from the Boonah roost also showed elevations in urinary cortisol in June, July and August, corresponding with southern hemisphere winter, and with mid-late pregnancy in P. alecto and the related P. poliocephalus.
No similar elevations were evident in either far north roost.
The individual animal urine samples showed that the winter elevations in urinary cortisol at the southern roosts were equally evident in both males and females, discounting an association with pregnancy.
“We identified a strong correlation between urinary cortisol concentration and minimum daily temperature in P. alecto populations in South East Queensland, and given the absence of evidence of an association with any other measured variable, we suggest that low ambient temperature is a biologically plausible stressor.”
They said their findings indicated that urinary cortisol excretion was modulated by both life cycle and ecological factors.
The study team comprised McMichael, Daniel Edson, Craig Smith, David Mayer, Ina Smith, Steven Kopp, Joanne Meers, and Hume Field.
McMichael L, Edson D, Smith C, Mayer D, Smith I, Kopp S, et al. (2017) Physiological stress and Hendra virus in flying-foxes (Pteropus spp.), Australia. PLoS ONE 12(8): e0182171. https://doi.org/10.1371/journal.pone.0182171