COS 243-3 - How does parasite transmission stage concentration change before, during, and after disease outbreaks?

Abstract: Hosts frequently encounter transmission stages of parasites while moving through their environment, and this exposure to parasites can lead to disease outbreaks. Host-parasite contact is necessary for the emergence of disease outbreaks, yet tracking transmission stages in the environment has traditionally been very challenging, leaving a significant gap in our understanding of host-parasite interactions. Now, with improved molecular methods, we can quantify transmission stages and evaluate how the concentration of transmission stages changes over time and in relation to outbreaks.

We used digital PCR to quantify transmission stages of five zooplankton parasites in water samples collected from six lakes in southeastern Michigan every two weeks between June and November 2021. To determine infection prevalence and host density, we collected zooplankton at the same time points and visually diagnosed infections in the lab. Four of the five parasite species studied had significant outbreaks, infecting at least 1% of the host population for a minimum of two consecutive weeks.

For all parasites initiating outbreaks, mean transmission stage concentration increased at least two weeks before infections were observed in hosts. For Pasteuria ramosa, a parasite with strong genotype-genotype interactions with hosts, an increase in transmission stage concentration did not always initiate an outbreak, possibly due to a mismatch with host genotypes. While we observed a consistent trend of transmission stages increasing prior to the emergence of outbreaks, we found that the relationship between peak transmission stage concentration and peak infection prevalence differed for two major types of parasites. For parasites that are obligate killers (e.g., P. ramosa and Metschnikowia bicuspidata), which require host death for the release of more transmission stages, transmission stage concentration peaked before the outbreak peak. For parasites that continuously shed transmission stages from hosts (e.g., a microsporidium of the gut “MicG” and the oomycete Blastulidium paedophthorum), transmission stages reached a maximum concentration at the same time as or shortly after the outbreak peak.

There is a dynamic, two-way relationship between transmission stage concentration and an outbreak because an initial exposure to transmission stages is necessary for the emergence of an outbreak but the outbreak itself fuels the production of more transmission stages, which are released into the environment once again. Our results show that transmission stage concentration increases prior to outbreaks for all four parasites studied but parasite life-history traits may impact the timing of the transmission stage peak relative to the outbreak peak.