There may have been a century between the last two pandemics, but that doesn’t mean we are in the clear for the next 100 years. In case there’s another pandemic soon, scientists at the University of Colorado at Boulder are exploiting the secrets of plankton in an effort to be proactive rather than reactive.
In a paper published in The American Naturalist, the research team suggests the initial immune response of an infected creature is a vital component of determining how a pathogen will spread.
"One of the biggest patterns that we're seeing in disease ecology and epidemiology is the fact that not all hosts are equal," said Tara Stewart Merrill, lead author of the paper and a postdoctoral fellow in ecology.
From parasitic flatworms and mosquitoes to pigs and bats, dangerous pathogens can pass fairly easily from the animal kingdom to humans. It’s the same process every time—an animal becomes infected with a pathogen, then transmits it either directory to humans or through a third-party host.
But, why does it have to happen that way? What if it’s possible, Stewart Merrill says, for the host to fight off the diseases and break the link in the chain that passes them on to humans?
“In infectious disease research, we want to build host immunity into our understanding of how disease spreads,” Stewart Merrill explains.
The ecologist has done that by working with Daphnia dentifera, a small species of zooplankton, to better understand and characterize how the invertebrates respond to pathogen infection. First, the researchers exposed the zooplankton to Metschnikowia bicuspidate, a fungal parasite. The parasite kills by attacking the animal’s gut, filling its body and growing until it is finally released when the host dies.
According to the study, some of the plankton were good at stopping fungal spores from entering their bodies, while others cleared the infection within a limited window after ingesting the spores. The most interesting reaction Stewart Merrill recorded, however, showed that some of the doomed plankton were able to recover.
In subsequent experiments, Stewart Merrill proved the success or failure of pathogen infection depends on the strength of the plankton’s internal defenses at the limited window comprising the onset of exposure.
"Our results show that there are several defenses that invertebrates can use to reduce the likelihood of infection, and that we really need to understand those immune defenses to understand infection patterns," said Stewart Merrill.
Based on the study results, Stewart Merrill and her team developed a simple probabilistic model for measuring host immunity that can be applied across wildlife systems.
While the immediate application of the model is ideal for invertebrates, like malaria-spreading mosquitoes who account for almost 20% of infectious diseases worldwide; the researchers say the model can be easily adapted. Similar models, for example, can be leveraged to help predict the occurrence and spread of future zoonotic spillover events, such as the one that started the COVID-19 pandemic.
Stewart Merrill said she hopes a better understanding of infections in a simple animal like plankton can be applied more broadly to animals that matter for human health.
“We really need to work on understanding prevention of infection, rather than just cures for infection," she concluded.
Photo: A zooplankton infected by the fungal parasite Metschnikowia bicuspidate. The microscopic fungal spores filling the body as visible as black fuzzy spots. Credit: Tara Stewart Merrill