Differential host mortality explains the effect of high temperature on the prevalence of a marine pathogen

Abstract

Infectious diseases threaten marine populations, and the extent of their impacts is often assessed by prevalence of infection (the proportion of infected individuals). Changes in prevalence are often attributed to altered rates of transmission, although the rates of birth, recovery, and mortality also determine prevalence. The parasitic dinoflagellate Hematodinium perezi causes a severe, often fatal disease in blue crabs. It has been speculated that decreases in prevalence associated with high temperatures result from lower rates of infection. We used field collections, environmental sensor data, and high-temperature exposure experiments to investigate the factors that change prevalence of infections in blue crab megalopae (post-larvae). These megalopae migrate from offshore waters, where temperatures are moderate, to marshes where temperatures may be extremely high. Within a few days of arriving in the marsh, the megalopae metamorphose into juvenile crabs. We found a strong negative association between prevalence of Hematodinium infection in megalopae and the cumulative time water temperatures in the marsh exceeded 34°C over the preceding two days. Temperatures this high are known to be lethal for blue crabs, suggesting that higher mortality of infected megalopae could be the cause of reduced prevalence. Experimental exposure of megalopae from the marsh to a temperature of 34°C resulted in higher mortality for infected than uninfected individuals, and decreased the prevalence of infection among survivors from 18% to 3%.

Parasitic dinoflagellate Hematodinium perezi prevalence in larval and juvenile blue crabs Callinectes sapidus from coastal bays of Virginia

“…This is in contrast with recent studies from Louisiana, in Fig. 3 which H. perezi DNA was detected in blue crab megalopae (Sullivan et al 2016, Sullivan & Neigel 2017. Although there are ecosystem differences between the Delmarva Peninsula and the Gulf of Mexico, there is a potential confounding factor in the study by Sullivan et al (2016) and their later analyses.…”

“…Little is known about whether the parasite can infect larval stages. Sullivan et al (2016) and Sullivan & Neigel (2017) detected Hematodinium DNA associated with blue crab megalopae collected from inshore locations in Louisiana, but their studies did not demonstrate whether the positive PCR assays were associated with internal infections.…”

Climate and season are associated with prevalence and distribution of trans-hemispheric blue crab reovirus (Callinectes sapidus reovirus 1)

“…This observation is similar to the Hematodinium perezi−C. sapidus pathosystem, in which a negative relationship was found between high water temperature and the prevalence of H. perezi infections in C. sapidus megalopae because higher temperature caused faster mortality of infected crabs (Sullivan & Neigel 2017). We found that the load of CsRV1 RNA was consistently lower in warmer latitudes (Table S3), which could indicate that crabs with higher CsRV1 loads do not live long enough to be sampled.…”

“…To better understand the various influences on this complex pathosystem, it is necessary to combine field observations with experimental data as suggested by Sullivan & Neigel (2017). Laboratory experiments are needed to test the effect of temperature on the progression of CsRV1 infections and resulting mortality rates across a wide temperature gradient representing temperate to tropical environments.…”