Snail competition leads to fewer parasites that cause schistosomiasis
Study shows that schistosome transmission can actually be highest when
snail populations are low

Date:
February 28, 2022
Source:
Emory University
Summary:
A new study shows that schistosome transmission can actually be
highest when freshwater snail populations are low. This study
demonstrates how the size of a freshwater snail population relates
to its parasitic infection rate.



FULL STORY ========================================================================== Schistosomiasis is a debilitating disease caused by a parasitic worm
that develops in freshwater snails before infecting people. Knocking
back snail populations with pesticides is one method to control the
spread of the disease, also known as "snail fever."

==========================================================================
A new study led by Emory University, however, shows that schistosome transmission can actually be highest when freshwater snail populations
are low.

The Proceedings of the National Academy of Sciences published the study,
the first to demonstrate how the size of a freshwater snail population
relates to its parasitic infection rate.

"We've shown that the more snails you have in a freshwater source,
the less dangerous each individual snail is, in terms of the number of parasites they're releasing," says David Civitello, an Emory assistant professor of biology and lead author of the study. "The incredible
strength of our finding is that we've demonstrated the effect both in
the field, using natural transmission sites, and in an experimental
context, through outdoor laboratory experiments." The research carries important implications for policies aimed at reducing the transmission of schistosomiasis. Considered one of the most significant of the neglected tropical diseases, the parasites that cause schistosomiasis currently
infect more than 200 million people.

"Our results suggest that if you apply a heavy dose of pesticides to
reduce a snail population, the infectivity of the remaining snails might actually skyrocket," Civitello says. "It's basically impossible to kill
every snail and so you set the stage for a rebound in infection risk. As
the snail population begins to recover, our data tells us that this is
a time with extremely high potential for transmission of the parasites
to humans." Previous laboratory experiments had found that when an
individual freshwater snail infected with the parasite is well fed,
it can generate as many as thousands more parasites per day compared to
an underfed snail. In fact, an underfed infected snail may generate as
few as a single parasite per day.



==========================================================================
"In general, when an animal needs to fight off an infection of some
kind, it helps to have good nutrition to support the immune system,"
Civitello says. "It appears to be the opposite case in these freshwater
snails. When the snails are full of energy, it provides more nutrients for
the parasites to steal from them and to reproduce." Chronic infections
of schistosomiasis cause considerable morbidity in sub- Saharan Africa
and parts of the Middle East, South America and Southeast Asia.

The disease cycles between humans and freshwater snails that live in
water sources where people may bathe, wash their clothes and dishes and
collect water for household use. Children, who like to play in water,
are at especially high risk for infection.

When eggs of the parasitic worms hatch in water, the larvae burrow
into snails.

Once the larvae develop into free-swimming worms, they burrow back out
of their snail hosts and return to the water. These swimming worms can
then burrow into the skin of people who come into contact with the water.

Inside their human hosts, the worms enter blood vessels where they
eat red blood cells for fuel as they mature into adults, pair up and
mate. The female lays hundreds of thousands of eggs per day. Many of
the eggs are excreted through feces and urine that re-enters water
sources, continuing the cycle of infection. Some of the eggs, however,
become lodged in the tissues and organs of their human hosts, leading
to immune reactions and progressive damage to organs, such as the liver,
the bladder, kidneys and the urogenital tract. One of the classic symptoms
of a chronic infection is blood in the urine.

The prescription medication Praziquantel treats schistosomiasis but
has limitations. "One problem is that the drug kills the mature adult schistosomes in humans, but not schistosomes that are only five or six
weeks old and still maturing," Civitello says.



==========================================================================
And a follow-up drug treatment does not eliminate the infection in
the environment.

"There is growing recognition in recent years that effective control of freshwater snails is needed, along with treatment of people, in order
to disrupt transmission of schistosomiasis," Civitello says. "In many
cases, however, snail control policies have not been updated for decades."
For the PNAS paper, the researchers wanted to test whether the effect of
food intake seen on the infection rate of individual freshwater snails in
a laboratory would scale up to a population in the wild. Their hypothesis
was that the larger the snail population, the more the snails would have
to compete for food resources, lowering their energy levels along with
their infectivity rate.

They conducted field research in the Mwanza region of Tanzania where schistosomiasis is endemic, in collaboration with Tanzania's National
Institute for Medical Research Mwanza Center. Running water is not
accessible in villages in the area and many people use surface water
ponds and hand-dug open wells that dot the clay-soil landscape.

The researchers found that snails collected from these water sources where
the snail populations were dense were poorly infectious. In contrast,
in the water sources where the snail population was low, their parasitic infection rate was high.

The outdoor laboratory experiments, conducted in collaboration with the University of South Florida, further showed how the growth of a snail population from low to high density creates a burst of infectivity among
the population before competition once again forces the infectivity
to subside.

"Our results suggest that, if you treat water bodies infrequently with a pesticide to control snails, you are likely to soon get a rebound of the
snail population with a higher infectivity rate, potentially creating a
surge of transmission to people," Civitello. "It may be better either
to not apply a pesticide at all, or else to apply the pesticide more
frequently to prevent the snails rebounding." The Civitello lab plans
to continue to collaborate with colleagues in Tanzania to gather more
detailed data to help develop the most effective methods of freshwater
snail control, for use in combination with other preventative methods
for schistosomiasis.

"It's important to unite the ecology of a pathogen with human disease interventions and control measures," Civitello says.

Co-authors of the PNAS include: Safair Kinung'hi, Teckla Angelo, Moses
Mahalila and Jenitha Charles (National Institute for Medical Research
Mwanza Center); Jason Rohr (University of Notre Dame); Karena Nguyen,
Rachel Hartman, Naima Starkloff and Lynda Bradley (Emory Department of Biology); Andres Manrique (University of Florida); Bryan Delius (Duquesne University); and Roger Nisbet (University of California, Santa Barbara).

The work was supported by the U.S. National Institute of Allergy and
Infectious Diseases, the National Science Foundation, the National
Institutes of Health, and the Indiana Clinical and Translational Sciences Institute.

========================================================================== Story Source: Materials provided by Emory_University. Original written
by Carol Clark. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. David J. Civitello, Teckla Angelo, Karena H. Nguyen, Rachel
B. Hartman,
Naima C. Starkloff, Moses P. Mahalila, Jenitha Charles, Andres
Manrique, Bryan K. Delius, L. M. Bradley, Roger M. Nisbet,
Safari Kinung'hi, Jason R. Rohr. Transmission potential of human
schistosomes can be driven by resource competition among snail
intermediate hosts. Proceedings of the National Academy of Sciences,
2022; 119 (6): e2116512119 DOI: 10.1073/ pnas.2116512119 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220228114346.htm

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