Genetic diversity, environmental stress and emerging infectious disease in conservation

Emerging infectious diseases have important impacts on agriculture, human health and conservation. Among these, conservation presents unique challenges because it is more difficult to detect, diagnose and treat disease outbreaks in natural populations. Effective disease management for conservation may therefore depend on understanding those factors that can inhibit the colonization or spread of diseases in naive host populations. This study examined the effect of host genetic diversity and environmental stress on the susceptibility of Gila topminnow (Poeciliopsis occidentalis) populations to the bacterial pathogen Listonella (formerly Vibrio ) anguillarum. The Gila topminnow is an endangered fish endemic to parts of Mexico, Arizona and New Mexico, and L. anguillarum is a causative agent of vibriosis, a well-studied septicaemic disease of fish. Controlled, pathogen-exposure experiments and modeling were used to evaluate the role of genetic variation, and controlled, pathogen-exposure experiments were used to assess the vulnerability of Gila topminnows to vibriosis under fluctuating salinity and temperature. Empirical findings failed to support the idea that Gila topminnow populations with greater genetic diversity would suffer less severe outbreaks of vibriosis, and this was consistent whether genetic diversity was assessed using microsatellites or a major histocompatibility complex locus. Model analysis indicated that the effect of genetic diversity on the expected proportion of susceptible hosts was small, and that the direction of the effect depended on assumptions regarding the mechanism of phenotypic expression. Model analysis also indicated that as genetic diversity increased, variation in the expected proportion of susceptible hosts decreased. Thus, natural populations with low levels of genetic variation may be expected to be either entirely resistant or entirely susceptible to a novel pathogen, while populations with greater genetic diversity may have a more consistent, intermediate level of susceptibility to each new pathogen. This result leads to the counterintuitive prediction that genetically diverse populations may be expected to suffer from a greater number of disease outbreaks, and harbor a greater number of endemic pathogens. Empirical results regarding the effects of variation in salinity and temperature on the susceptibility of Gila topminnows to vibriosis indicated that (1) salinity elevation enhanced the infectious efficiency of L. anguillarum on the Gila topminnow, (2) salinity stress increased Gila topminnow susceptibility to vibriosis, and (3) thermal stress did not increase Gila topminnow susceptibility to vibriosis. While the apparent requirement of L. anguillarum for saline conditions suggests that natural populations of the Gila topminnow may be unlikely to be threatened by this particular pathogen, events leading to cumulative, stress-related depression of immunity may leave Gila topminnow populations at an enhanced risk for outbreaks of other diseases, at least temporarily.