Browsing by Subject "Disease"

Disease is an important driver of coral loss regionally and is projected to become more severe as temperatures increase around the world. Although there has been substantial research into the abiotic factors (e.g. temperature, nutrients) controlling coral diseases, we know significantly less about the biotic factors (i.e. species interactions) influencing disease dynamics. We examined how the species living on and within corals affect coral tissue loss from a white syndrome-like condition on Heron Island in the southern section of the Great Barrier Reef. We exposed Acropora aspera fragments in flow-through tanks to a fully crossed factorial experiment with three factors: the presence of a common symbiotic crab (Cyclodius ungulatus), contact with a common macroalgal complex, and simulated wounding mimicking fish predation. We found that crab presence increased coral survival from a white syndrome-type disease by over 25%, likely by removing macroalgae if present and by cleaning infected tissue. Conversely, contact with macroalgae dramatically increased coral mortality, with the chance of survival dropping to nearly 0 by the end of 25 days for corals that were in contact with algae. Wounding had no direct effect on coral health, but wounded corals with crabs did significantly better than corals with no wounding and crabs, which may be the result of coral-crab signaling. We suggest that A. aspera may produce nutrient-rich mucus when wounded, which attracts crab symbionts that help slow disease progression. These results suggest that incorporating biotic interactions into restoration designs may dramatically improve restoration outcomes and that adding beneficial symbionts may improve disease resilience at a local level.

This dissertation focuses on the relationship of industrial shrimp aquaculture and shrimp diseases, with an emphasis on the agency of disease in shaping the history of shrimp production. Shrimp aquaculture is concentrated in developing tropical economies, with the significant majority of shrimp exported to consumers in the Global North. The rise of shrimp aquaculture has been accompanied by the development of new technologies and practices, designed to facilitate and govern the growth of the industry. While successful in making aquaculture the single largest production method for shrimp, these innovations also created ideal environments for the emergence and spread of shrimp diseases, which have caused significant and persistent production losses. Disease has brought volatility and risk to producer livelihoods, while also necessitating further technological modernization and development interventions to curb disease outbreaks.

This research draws on qualitative interviews and contextual economic analyses to explore the role of disease at multiple scales. Chapter 2 examines how disease has shaped industry discourses and he practice of shrimp aquaculture across contexts. The role of the concept of biosecurity is examined to highlight the territorial nature of disease prevention. Chapter 3 explores the context of shrimp aquaculture development in Aceh, Indonesia. This chapter applies the general ideas explored in Chapter 2, to a real-world case, highlighting how the pairing of shrimp and disease is managed as a single commodity. Chapter 4 explores the reach of disease globally, and across methods of production. The economic effects of disease on U.S. wild shrimping are explored, along with the role of disease as a narrative element in resisting global aquaculture.

It is argued that shrimp disease shapes commodity relationships, influencing production decisions, and development priorities at multiple scales. The unsympathetic quality of disease makes disease prevention an ideal project for enrolling broad coalitions of human and non-human actors, and negating the politics embedded in the relationship of disease prevention with commodification more broadly.

Disease risk in wild animal populations is driven by multiple factors, including host, parasite, and environmental traits, that facilitate the transmission of parasites and infection of hosts. Parasites inflict costs on their hosts that affect host fitness with downstream consequences on population structures and disease emergence patterns. Most disease risk-related studies are conducted in captive animals, while few have focused on free-ranging populations because of the logistical challenges associated with long-term monitoring of the hosts and sample collection. Hence, data regarding disease dynamics in natural populations are scarce, which limits our understanding of the ecological and evolutionary context of disease dynamics. In this thesis, we investigate the forces driving disease risk in wild primates and the possible consequences of infection on these hosts.

We used longitudinal and cross-sectional data sets from wild primate populations in Kenya, Eastern Africa, to examine the following aims: 1) the effect of host behavior on hormones associated with disease risk, 2) environmental and host factors that predispose individuals to helminth infections, and 3) the immune responses and fitness costs associated with helminth infections. First, we investigated how two maturational milestones in wild male baboons—natal dispersal and rank attainment—were associated with variation in fecal hormone metabolites (glucocorticoids and testosterone). These two hormones are generally considered to be immunosuppressive and are often associated with high parasite loads. Within this analysis, we also investigated whether changes in the frequencies of behaviors (mating and agonistic encounters) were associated with adult dominance rank attainment. Second, we investigated multiple sources of variance in helminth burdens in a well-studied population of wild female baboons, including factors that contribute to both exposure and susceptibility (group size, social status, rainfall, temperature, age, and reproductive status). Third, we investigated how hematological indices and body mass index were associated with helminth burden.

In the first study, our results revealed that rank attainment is associated with an increase in fecal glucocorticoids (fGC) levels but not fecal testosterone (fT) levels: males that have achieved an adult rank have higher fGC than males that have not yet attained an adult rank. We also found that males win more agonistic encounters and acquire more reproductive opportunities after they have attained adult rank than before they have done so. The second study revealed that female baboons in Amboseli were infected with diverse helminth taxa, including both directly transmitted and indirectly transmitted helminths. In general, high parasite risk was linked to large group sizes, low rainfall conditions, old age, and pregnancy, although these predictors varied somewhat across helminth species. Fecal GC levels were not associated with any measures of helminth burden. The third study found that helminth burdens were positively associated with circulating lymphocyte counts and negatively associated with neutrophil-lymphocyte ratios (NLR). We did not find any associations between helminth burdens and total WBC or eosinophil counts. Red blood cell indices were not predicted by our measures of helminth burden but instead varied with age class and sex. Helminth burdens were also negatively correlated with body mass index (BMI).

Overall, the findings of this thesis are consistent with the hypothesis that host and environmental traits are important predictors of disease risk and infection in wild primate populations. In addition, our results suggest that wild primates mount immune responses to helminth burden and that helminth infections may have detrimental consequences on host body condition. Our work enhances the limited data on sources of disease variation and associated costs in wild populations. It also emphasizes the continued need for disease surveillance and health monitoring in wild populations.

Recent experiments and comparative surveys in Southern US salt marshes revealed that a common larval trematode parasite, Parorchis acanthus, generated a trophic cascade that protected foundational marsh plants (Spartina alterniflora) from drought-associated overgrazing by suppressing the per capita grazing impacts of its host, the marsh periwinkle (Littoraria irrorata). While it is clear that parasites can play a positive role in mediating marsh ecosystem response to disturbance, there is still little known about the context dependency of this interaction, the role of definitive avian hosts in regulating parasite prevalence, and whether other commonly-occurring parasites may also modify processes that underpin ecosystem stability. The purpose of this project was to extend the current understanding of the roles played by parasites and their hosts in mediating marsh ecosystem stability. A field manipulation of Littoraria density in which infection prevalence with Parorchis acanthus was held at a constant value revealed that these parasites yielded positive impacts on Spartina aboveground biomass at middling densities of snails, but the positive effects of parasites were negligible at both low, and high snail densities. Surveys of drought-impacted marshes revealed that birds – the definitive hosts for trematode that infect Littoraria – congregated within die-off areas and that increased bird usage of die-off areas was associated with increased trematode parasitism in snails within grazer fronts, decreased per capita grazing rates of snails, and proportionate decreases in ecosystem die-off rate. Multi-site bird exclusion and mechanistic field studies experimentally confirmed that birds increased ecosystem resistance to drought-driven die-off by acting as the dispersive vectors for parasites that suppress Littoraria grazing. Finally, we explored how the trematode Cercaria opaca in ribbed mussels (Geukensia demissa) influenced the facultative mutualism between Guekensia and Spartina – an interaction that underlies marsh ecosystem resilience to drought-associated die-off. A field manipulation using experimentally infected mussels revealed that mutualistic benefits to Spartina decreased with increasing infection intensity in mussels. Subsequent mechanistic experiments demonstrated that increasing infection with C. opaca decreased mussel biodeposit production, the functional trait underlying mutualistic benefits to Spartina. Additionally, increasing parasite load was associated with decreased strength of both shells and byssal attachments, potentially explaining the relatively higher predation on heavily infected mussels in our field study. A survey of five North Carolina salt marshes revealed that infection intensity in mussels increased with proximity to die-off areas, indicating that C. opaca could influence marsh recovery following die-off events. Taken together, these results underscore the importance of parasitism’s influence on Southern salt marsh ecosystem stability and more generally show that parasites can be major arbiters of community structure and functioning.