The dangerous parasite Schistosoma mansoni that causes snail
fever in humans could become significantly less common in the future a new
international study led by researchers from the University of Copenhagen
Effects of climate change
The results are surprising because they contradict the
general assumption that climate change leads to greater geographical spread of
diseases. The explanation is that the parasite’s host snails stand to lose
suitable habitat due to climate change.
“Our research shows that the expected effects of climate
change will lead to a reduction in suitable habitats for four out of five
species of host snails for the parasite.
According to our models, several areas will become too hot
for the snails in the future and new precipitation patterns will affect the
freshwater areas where they live”, says postdoc Anna-Sofie Stensgaard from the
Danish National Research Foundation Centre for Macroecology, Evolution and
Climate at the University of Copenhagen.
Several of the freshwater snails acting as intermediate host
for the schistosomiasis parasite, are predicted to have fewer climatically
suitable habitat areas in the future.
Schistosomiasis is an infectious disease caused by parasitic
flatworms of the genus Schistosoma.
They infect humans by penetrating the skin when in contact
with water. They spread in freshwater areas such as rivers and lakes where
fresh water snails act as intermediate host for the parasite's larvae.
Therefore, the snails’ habitats are of great importance for
the spread of the disease.
Up to 19 % reduction
The researchers modelled the changes in snail habitat from
today to 2080 under various climate change scenarios, and what that will mean
for the spread of the parasite. The forecasts show up to 19 % reduction in the
total geographical area of infection risk in Africa, as the geographical
distribution of the main host snail will be reduced significantly.
Predicted scenario for the infection risk of Schistosoma
mansoni in 2080. Red indicates new areas of considerable infection risk
compared to today, green indicates areas with no change, and blue shows areas
with no considerable infection risk in 2080. Overall, a reduction of 19 %.
“Our results are consistent with the scientific view that
climate change leads to lower biodiversity, but not that climate change
necessarily leads to a greater spread of diseases”, Anna-Sofie Stensgaard
explains about the study that has just been published in the scientific journal
Acta Tropica.New areas at risk
Even though the overall infection is predicted to decline in
Africa, the study also identifies some areas where the disease could spread.
Senior researcher Thomas Kristensen from the Department of Veterinary Disease
“Our models are not designed to pinpoint changes on a local
scale but they provide an overall picture of a decline in areas suitable for
the parasite in West and Central Africa, while it may be able to establish
itself in new areas especially in Africa's southern regions.”
In addition, climate change will affect the host snails
differently and one of the studied species actually stands to benefit from the
changes. The study underlines that it is essential to include biological
knowledge of different host species in the models to gain robust future
scenarios for the spread of diseases.
Climate is not
The research also shows, however, that climate is not
necessarily the most important factor for the spread of diseases such as snail
fever. Natural and human-induced changes of the snails’ habitats, which are
difficult to predict, may also play a very important role.
Humans are infected with schistosomiasis in freshwater areas
like these, where freshwater snails act as intermediate hosts for the parasite.
“Over results highlights that especially anthropogenic
environmental change – in combination with climatic factors – is crucial for
the present distribution of host snails in Africa”, concludes Anna-Sofie
This is consistent with other studies showing that man-made
changes in the environment such as the damming of rivers, irrigation of fields
and construction of large water reservoirs can create new habitats for the
snails, which could in turn increase the risk of infection.
The research was conducted in collaboration with researchers
from Switzerland, Zambia, Uganda and Cameroon.