Discussion
Here we describe interaction of Plasmodium and a Flavivirus in the recurrence of mortality little less than a decade after emergence of BAGV in red-legged partridges in Southern Spain. Flaviviruses andPlasmodium share the same mosquito vectors and coinfections of mosquitoes with Plasmodium and WNV have been described (Medeiros et al., 2016). Coinfections with Plasmodium have been detected in passerines from USUV related bird mortalities in the Netherlands and Belgium and USUV related lesions have recently been described to be more severe and mortality higher in passerines carrying malaria parasites (Giglia et al., 2020; Rijks et al., 2016; Rouffaer et al., 2018).This suggests that interactions between malaria parasites and Flaviviruses may influence the transmission dynamics and host pathogen interaction of these emerging pathogens (Hughes et al., 2010; Medeiros et al., 2014), but the relation and type of interactions between both pathogens in bird-feeding mosquitoes and in their avian hosts are unclear and information available to date is conflicting. In a study involving adult North American passerines coinfection withPlasmodium had a negative effect on WNV serostatus (Medeiros et al., 2014), while in a recent study in Spain WNV serostatus was not affected by concurrent Plasmodium infection (Ferraguti et al., 2021). A recent extensive study in Germany documented WNV and USUV coinfections but did not test for Plasmodium or other avian malaria parasites (Santos et al., 2021). As for vectors, mosquitoes infected with BAGV had reduced Japanese encephalitis (JEV) and WNV replication (Sudeep et al., 2015), but we are not aware of studies on concurrent infections with Plasmodium .
The BAGV sequences detected in this case differ from the BAGV detected in the first outbreak caused by this virus (Figure 2), while thePlasmodium sequences detected in the affected birds are identical and homologous to a sequence detected in biting midges and birds in the nearby Doñana National Park (Ferraguti et al., 2013). The detection of this particular genotype in midges, may be of interest, although we do not at this point have information on the importance of this fact. Also, previous studies on Plasmodium in red-legged partridges suggest that in a given population a number of Plasmodium lineages co-circulate and that Plasmodium prevalence as well as presence of generalist Plasmodium lineages may be influenced by releases of farm-reared partridges (García et al., 2021). Avian malaria parasites are strictly vector bound and suspected to manipulate the biting behavior of their vectors in order to enhance transmission (Cornet et al., 2013), which could in the case of co-circulation in the same vector have an impact on transmission dynamics of locally circulating Flaviviruses such as BAGV. However, for BAGV in addition to mosquito-vector transmission, in red-legged partridges direct transmission has been demonstrated experimentally (Llorente et al., 2015), which suggests some degree of concurrent direct transmission at feeders in the case of highly managed populations in hunting estates.
While BAGV is pathogenic in red-legged partridges, avian malaria parasites are widespread and in Europe generally considered of little pathogenicity to avian hosts. However, especially Plasmodium spp. has led to local extinction of island endemic species when introduced with Eurasian birds to naïve populations and has also recently been suggested as one of the causes of the decline of abundant Eurasian birds such as the house sparrow (Passer domesticus ) (Dadam et al., 2019) and found associated to passerine mortalities in Austria (Dinhopl et al., 2015). Here mortality seemed to be mediated by the interaction of both pathogens. Potentially infection of previously Plasmodiuminfected partridges with BAGV and enhanced direct transmission due to aggregation at feeders and watering points could have led to the observed mortality. The ecoepidemiology and pathogenesis of this interaction in red-legged partridges and why this species seems to be especially susceptible, is unclear and needs further investigation.
Red-legged partridges are both a key species in Mediterranean ecosystems and, through the gamebird industry, an economic motor for depopulated regions in southern Spain (Caro et al., 2014; Casas et al., 2016). Mortality due to BAGV in the 2010 outbreak had a serious impact on the abundance of natural populations of the species and for the small game hunting sector (García-Bocanegra et al., 2013), thus reappearance of BAGV mediated mortality is of concern. More so as zoonotic potential of this virus has been discussed based upon detection of neutralizing antibodies against BAGV in patients with Meningoencephalitis in India (Bondre et al., 2009). Finally, this outbreak evidences the potential of interaction of co-circulating Plasmodium spp. and Flaviviruses, and as both groups harbor human pathogens the potential public health threat that such interaction could encompass should not be neglected.