How plankton holds secrets to prevent pandemics
Whether it’s plankton exposed to parasites or people exposed to pathogens, a host’s initial immune response plays a critical role in determining whether infection occurs and how well it spreads within. population, suggests new research from the University of Colorado at Boulder.
The results, published on May 13 in The American naturalist, provide valuable information for understanding and preventing disease transmission within and between animal species. From parasitic flatworms transmitted by snails to humans in developing countries, to zoonotic contagion events from mammals and insects to humans – which have triggered global pandemics like COVID-19 and West Nile virus – the immune response of an infected creature is a vital variable to consider in calculating what happens next.
“One of the biggest patterns we see in the ecology and epidemiology of disease is the fact that all hosts are not equal,” said Tara Stewart Merrill, lead author of the paper and postdoctoral researcher in ecology. “In infectious disease research, we want to integrate host immunity into our understanding of how disease spreads.”
Invertebrates are common vectors of disease, which means they can transmit infectious pathogens between humans or from animals to humans. Vector-borne diseases, such as malaria, account for nearly 20% of all infectious diseases in the world and are responsible for more than 700,000 deaths each year.
Yet epidemiological studies have rarely considered the immunity of invertebrates and the recovery of human disease-carrying creatures. They assume that once exposed to a pathogen, the invertebrate host will be infected.
But what if it were possible for invertebrates to fight these diseases and break the link in the chain that transmits them to humans?
By observing a tiny species of zooplankton (Daphnia dentifera) throughout its life cycle and being exposed to a fungal parasite (Metschnikowia bicuspidata), the researchers saw this potential in action. Some plankton were effective in preventing fungal spores from entering their bodies, and others cleared the infection within a limited time after ingestion of the spores.
“Our results show that there are several defenses that invertebrates can use to reduce the risk of infection, and that we really need to understand these immune defenses to understand the patterns of infection,” said Stewart Merrill.
Stewart Merrill began this work during her first year as a doctoral student at the University of Illinois, studying this small plankton and its collection of tusks. It’s a horrible process if the plankton fails to ward off the parasite: its fungal spores attack the plankton’s gut, fill its body, and grow until released when the host finally dies.
But she noticed something that hadn’t been recorded before: some of the doomed plankton recovered. Several years later, she discovered that in the face of identical exposure levels, the success or failure of these infections depends on the strength of the host’s internal defenses during this limited first window of opportunity.
Based on their observations of these individual results, the researchers developed a simple probabilistic model for measuring host immunity that can be applied to all wildlife systems, with important applications for diseases transmitted to humans from them. invertebrates.
“When immune responses are good, they act as a filter that reduces transmission,” said Stewart Merrill. “But any environmental change that degrades immunity can actually amplify the transmission, because it will let all that exposure go through and eventually become infectious.”
This is a model that can be applied to COVID-19 as well, as research by CU Boulder has shown that not all hosts transmit the coronavirus in the same way and that exposure does not directly determine infection.
COVID-19 is also believed to be the result of zoonotic spillover, an infection that has passed from animals to humans, and similar probabilistic models could be beneficial in predicting the occurrence and spread of future fallout events, Stewart Merrill said.
Understanding infection prevention
Stewart Merrill hopes that a better understanding of infections in a simple animal like plankton can be applied more broadly to invertebrates important to human health.
In Africa, Southeast Asia, as well as South and Central America, 200 million people suffer from infections caused by schistosomes – invertebrates better known as parasitic flatworms. They cause disease and death, with significant economic and public health consequences, so much so that the World Health Organization considers them the second most socio-economically devastating parasitic disease after the malaria.
These are just one of many neglected tropical diseases transmitted to humans by invertebrate hosts such as snails, mosquitoes and biting flies. These diseases infect a large portion of the population, but occur in areas with poor sanitation that do not have the economic resources to fight these diseases, said Stewart Merrill.
Schistosomes live in freshwater environments that people use for their drinking water, laundry, and bathing. So, even if there are treatments, the next day a person can easily re-infect themselves just by accessing the water they need. By better understanding how tapeworms themselves succumb or fight infection, scientists like Stewart Merrill are helping us move closer to stopping the chain of transmission to humans.
“We really need to work on understanding how to prevent infection, and what that risk is in these aquatic systems, rather than just curing the infection,” she said.
The good news is that we can learn from the same invertebrates that infect us. In invertebrate hosts that suffer or die from their infections, there is a good incentive to learn to develop and fight an immune response. Some snails have even shown the ability to retain immunological memory: if they are infected once and survive, they may never be infected again.
“If we can better understand how the environment shapes these defenses, we could predict in the future how environmental changes might amplify or remove the risk of transmission to people,” said Stewart Merrill.
Other authors of this article include Zoi Rapti and Carla Cáceres of the University of Illinois.