Climate change to put 77 million more people at risk of fly disease

Up to 77million more people could be at an increased risk of contracting sleeping sickness as the tsetse fly, which transmits the disease, spreads due to global warming.

A picture of the tsetse fly which carries the parasites that cause sleeping sicknessA study looked at common strains of tsetse and the parasite it carries, which currently affects 36 sub-Saharan African countries. Results showed the sleeping sickness-carrying fly will spread into Southern Africa and surrounding areas as temperatures rise.

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According to the World Health Organisation (WHO), more than 30,000 cases of sleeping sickness occur each year in eastern, central and western Africa. Sleeping sickness can cause fever, joint pain and if left untreated, brain swelling.

In 2008, the US Wildlife Conservation Society identified sleeping sickness, or African trypanosomiasis, as one of a ‘deadly dozen’ of infectious diseases likely to spread as a result of increasing temperature.

Vector-borne diseases such as trypanosomiasis are particularly sensitive to global warming because temperature changes can alter the carriers’ growth rates and shift their geographical distribution.

Climatologists for the United Nations have previously warned that carbon emissions could lead to an increase in temperature of 1.1 to 5.4 degrees Celsius, raising concerns over global warming.

Based on these findings, scientists at the U.S-based Centres for Disease Control and Prevention devised a computer-stimulated transmission model in order to predict the incidence and distribution of disease. The model looks at the effect of of climate change on the spread of the tsetse fly and the Trypanosoma brucei parasite it carries.

The model predicts that outbreaks can occur when average temperatures are between 20.7°C and 26.1°C and suggests that there will be a large shift in the location of afflicted people.

The new study also suggests that some parts of eastern Africa will become too hot for tsetse larvae to survive whilst other areas like southern Africa, that were previously too cool, will become optimal for tsetse survival and growth.

Recent WHO estimates suggest 70 million people live in tsetse fly-infested areas. The model predicts that by 2090, a further 46–77 million people may be at risk of exposure to the fatal infection they carry, and the convulsions and coma that it induces.

The conclusions, however, are incomplete, as Trypanosomiases specialist Gérard Cuny recognizes. Speaking Le Monde he said: “Moisture is essential for the survival of the flies”, as he commented on key omissions of the study to take into account demographics and humidity which also play a key role in disease spread.

The team, led by Dr Sean Moore, recognise the pitfalls of the model. They plan to expand it to incorporate parameters such as the influence of humidity or rainfall on the distribution and abundance of tsetse fly. “There may be other parameters, like the probability of transmission from flies to humans, that are sensitive to temperature in subtler ways,” Dr Moore said.

“In addition, the parasite may also respond to changes in minimum and maximum temperatures in different ways than the response to mean temperature modeled in this paper”.

Published in the Journal of the Royal Society Interface, the results of this study reinforce messages from previous research of a link between climate change and disease spread.

Global warming over the next century is expected to have a large impact on the interactions between pathogens and their animal or human hosts. Models like this provide a basis for using climate-sensitive aspects of parasite biology in order to predict changes in disease prevalence and risk.

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