Biotic and Abiotic Factors that Impact Host Life History, Parasite Dynamics, and Virulence

The intensity and expression of disease is influenced by interactions that occur between hosts, parasites, and their environment. Host organisms are often infected with multiple parasite genotypes or species, and the severity of the disease that they experience can depend on the intraspecific and interspecific competition that occurs among the parasites within them. In addition to the impact of infection, hosts must also deal with stressors in their environment such as increased temperature or drought. Hosts can respond to parasite infection or environmental stress by altering their life history, but little is known about how hosts respond to infection and stress at the same time. The primary objective of my dissertation research was to explore and describe the impacts that different stressors, including environmental stress and competition, have on host – parasite dynamics. Specifically, this dissertation explores the impact of stressors on host and parasite life history, and the expression of life history adaptations. To do this I used trematode parasite systems, which have complex life cycles that include vertebrate definitive hosts and aquatic snail intermediate hosts. I first reviewed our current understanding of parasite competition and postulated that the amount of movement and resistance in host populations could increase or decrease the relatedness of intraspecific infections, and thus influence the amount of damage, or virulence, that those infections inflict on their hosts. To test the influence of intraspecific relatedness on virulence I then compared related and unrelated infections of the human parasite Schistosoma mansoni in snail intermediate hosts and found support for kin selection theory predictions, in which related parasite infections have lower virulence than unrelated parasite infections. Intermediate hosts infected with unrelated parasites were more similar in their life history to those infected with the less damaging of the two parasites in the infection. To determine the impact of intraspecific parasite relatedness on infections in the definitive host, I infected mice with related and unrelated S. mansoni infections and determined that, as with the intermediate host infections, unrelated infections were more similar in their life history and virulence to one of the two parasites in the composition. In the definitive hosts the infection with unrelated parasites more closely resembled that of the more damaging parasite. These results suggest that there are trade-offs in virulence and infectivity between host stages, and that there could be selection for lower virulence at the intermediate host level while there is selection for higher virulence at the definitive host level. Next, I used a local trematode system to examine the patterns of interspecific competition and their influence on parasite community structure. I hypothesized that weaker competitors (parasites with sporocysts) should avoid co-infection with stronger competitors (parasites with rediae) by avoiding them in time, to prevent exclusion via predation. Specifically, I expected that sporocyst stage parasites would exhibit high prevalence early in the infection season, while redial stage parasites would predominate at the end of the season. While co-infections were rare, I did not document this predicted pattern, suggesting that parasites may have another method of competition avoidance. To examine the ability of hosts and parasites to respond adaptively to stressful environments, I subjected S. mansoni infected snail intermediate hosts and uninfected snail intermediate hosts to drought stress and recorded their life history responses compared them to unstressed hosts. The intermediate host of S. mansoni has life history adaptations in response to infection (fecundity compensation, in which snails increase their reproductive output in response to infection) and to drought (aestivation, in which snails enter a dormant state to survive desiccation). Interestingly, snails that were infected and also experiencing drought stressed did not express the fecundity compensation life history response, suggesting that the presence of two stressors eliminated the ability of the host to respond to infection. However, parasites in infected hosts did exhibit a fecundity compensation response, in which drought-stressed infected hosts produced significantly more parasites than those that were not drought-stressed. This was the first documentation of a parasite life history adaptation to host environmental stress. These findings enhance our knowledge about host-parasite dynamics and life history adaptations, and shed light on the impacts of stressors on the intensity of disease

Statistics Information for each Dependent variable

This is a table containing specific statistical output for each dependent variable

Data from: One stimulus - two responses: host and parasite life history variation in response to environmental stress

Climate change stressors will place different selective pressures on both parasites and their hosts, forcing individuals to modify their life history strategies and altering the distribution and prevalence of disease. Few studies have investigated whether parasites are able to respond to host stress and respond by varying their reproductive schedules. Additionally, multiple environmental stressors can limit the ability of a host to respond adaptively to parasite infection. This study compared both host and parasite life history parameters in unstressed and drought-stressed environments using the human parasite, Schistosoma mansoni in its freshwater snail intermediate host. Snail hosts infected with the parasite demonstrated a significant reproductive burst during the pre-patent period (fecundity compensation), but that response was absent in a drought-stressed environment. This is the first report of the elimination of host fecundity compensation to parasitism when exposed to additional environmental stress. More surprisingly, we found that infections in drought-stressed snails had significantly higher parasite reproductive outputs than infections in unstressed snails. The finding suggests that climate change may alter the infection dynamics of this human parasite

Host and parasite life history Raw Data

This data file contains snail growth, prepatent and patent reproduction, and parasite reproduction data collected from snails exposed to drought stress and/or infection as well as control unstressed and uninfected snails

Aestivation Data R code

This file contains the R code used to run our analyses