Abstract
Despite coinfections being recognized as the rule in animal populations, most studies focus on single pathogen systems. Pathogen interaction networks and the drivers of such associations are lacking in disease ecology studies. Common cockle Cerastoderma edule populations are exposed to a great diversity of pathogens, thus making them a good model system to investigate. This study examined the diversity and prevalence of pathogens from different taxonomic levels in wild and fished C. edule on the Irish coast. Potential interactions were tested focussing on abiotic (seawater temperature and salinity) and biotic (cockle size and age, and epiflora on shells) factors. No Microsporidia nor OsHV-1μVar were detected. Single infections with Haplosporidia (37.7%) or Vibrio (25.3%) were more common than two-pathogen coinfected individuals (9.5%), which may more easily succumb to infection. Fished C. edule populations with high cockle densities were more exposed to infections. Higher temperature and presence of epiflora on cockle shells promoted coinfection in warmer months. Low seawater salinity, host condition and proximity to other infected host species influenced coinfection distribution. A positive association between two Minchinia spp. was observed, most likely due to their different pathogenic effect. Findings highlight the major influence that ecological factors have on pathogen interactions and host–pathogen interplay.
Highlights
Animal hosts are exposed to complex pathogen assemblages that form a dynamic community within them (Johnson and Buller, 2011)
Coinfections have been previously reported in shellfish hosts: including Vibrio tapetis, Perkinsus sp. and digenetic trematodes in cockles (Lassalle et al, 2007); ostreid herpesvirus-1 (OsHV-1) and Vibrio spp. in oysters (Petton et al, 2015; Alfaro et al, 2019); and protozoans and bacteria in clams (Arzul et al, 2012; Carella et al, 2020); and in some of the cases with a dramatic effect on disease susceptibility (Cox, 2001; Lassalle et al, 2007; Arzul et al, 2012; Alfaro et al, 2019)
171 (61.7%) of the tested samples that were positive in the generic haplosporidian Polymerase chain reaction (PCR) did not amplify using the more specific PCR for Minchinia spp., which could indicate the presence of other haplosporidian species occurring in the samples
Summary
Animal hosts are exposed to complex pathogen assemblages that form a dynamic community within them (Johnson and Buller, 2011). Interactions among coinfecting infectious agents can, alter host pathology, parasite transmission and virulence evolution, influencing the spread of infections at a population level resulting in disease outbreaks, as described previously in bivalves (Lassalle et al, 2007; Arzul et al, 2012; Alfaro et al, 2019). These effects can be the opposite, i.e. coinfections within the host may reduce infection success yet still enhance pathology or, in contrast, co-occurring parasites/pathogens may weaken host pathology increasing the infection success (Johnson and Hoverman, 2012). Mortality induced by one parasite can eliminate the availability of hosts for other parasites (Jolles et al, 2008); likewise, morbidity induced by one parasite can increase exposure to a second, even if within-host interactions are antagonistic (Karvonen et al, 2009)
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