INFECTION DYNAMICS IN FLYING-FOXES
Longitudinal studies, primarily based on HeV PCR detection in urine collected from underneath roosts, have provided key insights into broad scale HeV transmission dynamics in flying-foxes (Field et al.
2011, 2015; Burroughs et al. 2016; Paez et al. 2017; Peel et al. 2019, 2022; Becker et al. 2023; Lunn et al. 2023). First, Field et al. (2011) identified that virus excretion occurred intermittently, concurrently in geographically disparate populations; at any time of year; and that regional variation in excretion prevalence may reflect species composition. A second, major longitudinal study analysed 14 000 underroost urine samples collected from roosts spanning from Cairns in northern Qld to Bateman’s Bay in southern NSW (Field et al. 2015; with further analyses of the same data in Paez et al. 2017; Becker et al. 2023). These studies demonstrated HeV-g1 excretion prevalence varied latitudinally, with some weak synchrony across sites. Excretion was highest in southern Qld/northern NSW, moderate in the Qld tropics and in central NSW, and negligible in southern NSW. While HeV-g1 excretion pulses could occur at any time, the largest amplitude peaks occurred over winter in subtropical regions. Winter peaks are amplified in roost sites outside of the historic winter range and in years following a period of food shortage for flying-foxes. These conditions parallel the winter and interannual clustering of equine cases in this region and are consistent with climatic and ecological drivers of HeV excretion and spillover (Eby et al. 2023).The highest HeV-g1 prevalences have been recorded when black flying-foxes (P. alecto) or spectacled flyingfoxes (P. conspicillatus) are present; with nil or very low positivity rates in roosts thought to exclusively contain grey-headed flying-foxes (P. poliocephalus) and no effect of influxes of little red flying-fox on RNA detection rates (Field et al.
2015). This is consistent with other studies showing no evidence of HeV-g1 RNA in urine from exclusive grey headed flying-fox roosts (Burroughs et al. 2016), and a study that detected viral RNA in individual black flying-foxes, but not in grey-headed or little red flying-foxes (Edson et al. 2015b). Similarly, HeV-g1 RNA detection was significantly higher in black flying-foxes and spectacled flying-foxes in archived bat tissues, supporting previous studies and the contention that these species are primary reservoir hosts of HeV-g1 (Goldspink et al. 2015). Investigations into the differences between HeV-g1 and HeV-g2 infection dynamics are in their infancy, and further dedicated studies across the host species’ ranges are required.Finally, a recent study analysing > 6000 under-roost samples collected from five roosts in south-east Qld and north-east NSW over > 3 years identified that excretion in flying-fox populations is skewed towards higher HeV-g1 viral loads during winter high-prevalence periods (Lunn et al. 2023). HeV spillover risk was highest when both prevalence and viral loads were high, arguing for both factors to be taken into account in assessing spillover risk. By contrast, periods of prolonged low (but non-zero) prevalence were skewed towards lower viral load and appeared to have little influence on spillover risk, perhaps representing prolonged excretion of non- infectious RNA (Lunn et al. 2023).
3.