Factors Influencing Infection
Nutrients - Nitrogen and phosphorus enrichment are widespread forms of anthropogenic environmental change that may influence parasite infection (Johnson and Carpenter 2008). Nutrient enrichment leads to eutrophication, which has been linked with emergence of Ribeiroia ondatrae through direct and indirect effects on aquatic food webs (Johnson and Chase 2004). The underlying mechanism through which eutrophication can increase infection involves increasing the density of infected first-intermediate snail hosts and by increasing the production of parasites by infected snails (Johnson et al. 2007).
Pesticides - Pesticides are still considered a potential factor influencing amphibian malformations (Johnson et al. 2010). However, recent investigations have focused on the interaction of pesticides with parasite infection (Kiesecker 2002; Rohr et al. 2008). Field experiments indicated a link between agricultural run-off and increased infection (Kiesecker 2002). Laboratory studies further demonstrated that pesticide exposure reduced host immunocompetence against parasites as the biological mechanism (Kiesecker 2002). Furthermore, there may be a link between nutrient addition and pesticide contamination jointly leading to increased infections by simultaneously increasing exposure and suppressing host immune systems (Rohr et al. 2008a, b).
Biodiversity - Host species diversity and community structure have been suggested to influence disease dynamics through a hypothesis termed the ‘dilution effect’ (Ostfeld and Keesing 2000). For Ribeiroia ondatrae, host species differences in susceptibility can influence infection patterns in multi-species communities (Johnson et al. 2008, Johnson and Hartson 2009). As a result, more diverse larval amphibian communities that include species with differing susceptibilities can alter parasite infection success and the resulting pathology in sensitive host species. For instance, larval toads (a sensitive species) raised experimentally alongside larval gray treefrogs (a resistant species) exhibited 37% less infection and fewer malformations (Johnson et al. 2008).
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