Study gives significant knowledge to forestalling worldwide pandemics

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• Source: University of Colorado at Boulder


• Date: 17 May,2021

Whether it’s plankton exposed to parasites or people exposed to pathogens, a host’s initial immune response plays an integral role in determining whether infection occurs and to what level it spreads within a population, new University of Colorado Boulder research indicates.

From parasitic flatworms transmitted by snails into individuals in developing nations, to zoonotic spillover events from mammals and insects to humans–that have caused global pandemics such as COVID-19 and West Nile virus–an infected creature’s immune response is a vital variable to consider in calculating what happens next.

Vector-borne diseases, like malaria, accounts for almost 20 percent of all infectious diseases worldwide and are responsible for over 700,000 deaths every year.

Yet epidemiological studies have seldom considered invertebrate immunity and recovery in animals which are vectors for human disease. They assume that once exposed to a pathogen, the invertebrate host will become infected.

But what if it was possible for invertebrates to fight these diseases, and break the connection in the chain that moves them on to humans?

While observing a tiny species of zooplankton (Daphnia dentifera) throughout its lifecycle and vulnerability to a fungal parasite (Metschnikowia bicuspidata), the researchers saw this potential in action. Some of the plankton were good at stopping fungal spores from entering their bodies, and others cleared the infection within a limited window of time after eating the spores.

“Our results show that there are numerous defenses that invertebrates can use to reduce the likelihood of disease, and that we actually have to know those immune defenses to understand infection patterns,” explained Stewart Merrill.

Unexpected recovery
Stewart Merrill started this job in her first year as a doctoral student at the University of Illinois, studying this tiny plankton and its assortment of defenses. It’s a grisly process if the plankton fails to ward off the parasite: Its fungal spores attack the plankton’s gut, fill its entire body and grow until they are released when the host eventually dies.

But she noticed something which had not been listed before: Some of the doomed plankton recovered. Several decades later, she has found that when confronted with identical levels of exposure, the success or failure of the illnesses depends upon the strength of the host’s inner defenses in this ancient limited window of opportunity.

Based on their observations of these individual results, the investigators developed a simple probabilistic model for measuring host resistance that may be applied across wildlife systems, with important applications for diseases transmitted to humans by invertebrates.

“When immune responses are great, they act as a filter which reduces transmission,” said Stewart Merrill. “However, any environmental change that degrades immunity can actually amplify transmission, because it is going to let all that exposure go through and finally become infectious.”

It is a model that can also apply to COVID-19, as research from CU Boulder shows that not all hosts are the same in transmitting the coronavirus, and vulnerability does not directly determine infection.

COVID-19 is also believed to be caused by a zoonotic spillover, a disease that transferred from animals into individuals, and similar probabilistic models could be advantageous in predicting the incidence and spread of future spillover events,” said Stewart Merrill.

Understanding prevention of infection
Stewart Merrill expects that a better comprehension of infections in a simple animal like plankton can be applied more broadly to invertebrates that matter for human health.

In Africa, Southeast Asia, as well as South and Central America, 200 million people suffer from illnesses caused by schistosomes–invertebrates more commonly known as parasitic flatworms. They cause illness and death, and important economic and public health effects, so much so that the World Health Organization considers them the second-most socioeconomically devastating parasitic disease after malaria.

These diseases infect a large part of a population but occur in areas with low levels of sanitation which don’t have the economic resources to deal with those diseases, said Stewart Merrill.

Schistosomes reside in freshwater environments that people use for their drinking water, laundry and bathing. Therefore, even though there are treatments, the next day a person can easily get reinfected just by accessing the water they require.

“We really need to work on understanding prevention of disease, and what that risk is in those aquatic systems, instead of just cures for disease,” she said.

The great news is we can learn from the very same invertebrates which infect us. In invertebrate hosts that die or suffer from their infections, there is a good incentive to learn how to build an immune response and fight it off. Some snails have shown the ability to keep an immunological memory: If they get infected once and survive, then they may never get infected again.

“If we can better understand how the environment shapes those defenses, we can predict into the future environmental changes could amplify or suppress risk of transmission to people,” said Stewart Merrill.