Chasing the Tsetse

Tsetse fly specimens. Image from WHO/Andy Craggs

As harmful insects adapt to a changing world, how people are affected by the tropical diseases they carry will also change. To get ahead of one such insect – the tsetse fly – scientists are using new methods to understand how the flies adapt to environmental stress in Sub-Saharan Africa. The goal is to better understand what factors influence how tsetse flies choose their habitat. Another important question could also be addressed through this work: Where might the flies be in the future?

The problem with tsetse flies is that certain species spread parasites which kill thousands of people and millions of domestic animals each year. These trypanosoma parasites have affected humans in Africa for millennia. They may have even influenced the success of certain early humans over others millions of years ago, favouring those that were more resistant to sleeping sickness, as the infection is called in people (Human African Trypanosomiasis or HAT). Later, the disease may have had several effects on the slave trade in Africa, but it wasn’t until late in the colonial period that tsetse flies were identified as the carrier. To this day there is no vaccine, and the disease is usually fatal if untreated.

In the twentieth century, several sleeping sickness epidemics occurred. During these outbreaks hundreds of thousands of people died in Uganda, DRC, South Sudan and other countries. Today the disease is better controlled, with a few thousand new cases being reported each year. Those infected are usually people living rurally near natural habitat for the fly and the animals it feeds on. Although the majority of new cases occur in the D.R.C., as many as 70 million people live in fly-infested areas throughout Sub-Saharan Africa and remain at varying levels of risk of contracting the disease. Over twenty of the 31 species of tsetse fly can infect humans.

Communities living in these ‘tsetse belts’ are further affected by the effect on their animals. Nagana in livestock (African Animal Trypanosomiasis or AAT) is caused by different species of parasite and can be found where sleeping sickness isn’t, and vice versa. The disease can infect pigs, ox and cattle and although not all strains kill the animals outright, they usually have serious implications if left untreated. Nagana is thought to cause billions of damage each year to the households of rural communities through its effects on livestock. The people affected are often extremely dependent on the health of their livestock for their livelihood.

They have their differences, but the future of nagana and sleeping sickness are connected, because both depend on the tsetse flies to incubate the parasites in their bodies, and then inject them into new hosts when taking a blood meal.

Over the last century measures to control the flies, screen populations of people and animals for parasites and develop less-toxic medicine have been helpful, but there are many challenges to success. For example, the life cycle of the parasite in humans, the flies and animals is different in west and central Africa than it is in east and southern Africa and different strategies are required. Importantly, even in areas that manage to control cases in humans or livestock, if conditions change or efforts relax the flies can return with a vengeance. In the 1960s the WHO was confident that sleeping sickness could be effectively controlled, but by the end of the 20th century cases were once again estimated in the hundreds of thousands. Today, the WHO ventures that the disease could be eliminated as a public health issue by 2020, but continues to list it as a neglected tropical disease, alongside Dengue Fever and rabies.

Fighting the problem effectively requires not only an understanding of tsetse fly behaviour, but also how humans and animals interact with their environments. This holistic understanding becomes even more important in a world affected by climate change. One element of climate change – warming temperatures – affects the tsetse flies, but how this warming also affects humans in the area has to also be understood.

Focusing On The Fly

Tsetse flies are sensitive to many environmental factors, including changes in the temperature range they’ve evolved to bear. They can’t regulate their body temperature internally and rely on their environment to do so for them. Inevitably, they have to tolerate some daily swings in temperature. Tsetse flies are usually active in the daytime because the night is too cold for them. During the day they hunt for blood meals during periods that are warm enough for them to be active, but not hot enough to cause harmful stress.

Temperatures beyond what the flies can handle poses several challenges – for example, it kickstarts their metabolism requiring more feeding to keep ahead of starvation. Being exposed to temperatures above a certain level may force the flies into heat shock, or increase the activity of their predators. It may even influence their behaviour in other ways; this study observed tsetse flies in Zimbabwe more frequently entering a house in warmer temperatures, despite the aversion these particular flies usually have to the scent and presence of humans.

Tsetse flies can partially moderate the effects of excessive heat by resting in shade provided by vegetation – another requirement they have of their habitat. Despite this, temperatures above a certain level seem to reduce fly survival. Evidence from field studies suggest that mortality rates increase in wild populations of flies when the average temperature rises above 27°C.

As temperatures increase in Sub-Saharan Africa throughout the century, tsetse flies are expected to be affected. In 2008 the Wildlife Conservation Society listed sleeping sickness as one of twelve diseases expected to worsen because of climate change. Studies have shown that temperature can be effectively used to explain present-day tsetse fly distributions. Using temperature alone isn’t perfect – only around 80% of the flies’ habitat choices can be explained in this way, because vegetation, humidity and animal availability also play a role. However, temperature is unique. Future temperatures over large areas can be predicted in ways that other environmental changes cannot.

Based on how temperature influences both tsetse flies and the trypanosoma parasites, this research predicted that different regions of southern and eastern Africa may be at risk of sleeping sickness outbreaks later this century. A mathematical model was developed which predicted that for an outbreak to occur, mean annual temperatures must fall within 20.7°C and 26.1°C. To predict what areas would experience this temperature range in the future, the model used climate change scenarios from the IPCC over two time periods: 2046-2065 and 2080-2099. Some areas were predicted to become too hot for outbreaks of the disease. Other areas would warm up sufficiently to provide suitable temperatures for outbreaks, being previously too cold. The study cautioned that just because new areas would develop ideal temperatures, the tsetse flies might not actually relocate to them.

Several media outlets highlighted the negative implications of the study. Although the model predicted some areas might finally be rid of sleeping sickness, the regions predicted to become at risk are more densely populated, particularly in the East African Highlands. The model predicted that up to 46-77 million additional people may be at risk by 2090.

However, despite the fly’s sensitivities to temperature, whether or not it would be able to migrate successfully to new habitats also depends on human activity.

Humans Driving Fly’s Future

The effect of warming temperatures, epidemiologist John Hargrove suggests, may be overshadowed by human influences on fly habitat. Certainly, temperature changes affect tsetse flies and in some areas this is already being observed. He mentions that populations of one species of tsetse fly has been reduced by over 90% in the Zambezi Valley in Zimbabwe. This particular reduction is attributed to climate warming. However, he uses a historic example to illustrate the complexity of tsetse-human interactions.

Records of tsetse populations in Zimbabwe have been kept since the 19th century and tsetse fly populations are still studied here today. This continuity of information allows researchers to understand how tsetse flies have been influenced by history. A pandemic of the rinderpest virus killed nearly all of the animals here at the end of the 19th century, removing the tsetse fly’s blood meal source. The tsetse fly population collapsed. Previous to the epidemic, tsetse flies had infested all areas of Zimbabwe below a certain elevation, and therefore a certain temperature range. Animal populations eventually recovered and the flies re-infested most areas, but not all. Hargrove explains they were unable to reenter some areas that had been cleared for agricultural crops. Despite these areas still offering suitable temperatures for the flies, the human removal of brush and vegetation cover was a limiting factor.

A similar scenario could occur in areas that historically have been fly-free because of cooler temperatures. These areas have been able to develop differently. There have been fewer obstacles to raising healthy livestock and using animal-assisted agricultural techniques. Land that’s already been used for livestock grazing or agricultural crops may be difficult for the tsetse flies to move into, even if there’s amenable temperatures and plenty of livestock for them to feast on.

However, despite these challenges, no-one expects the tsetse flies to disappear completely because of climate change. Although some populations of flies may decrease or die out, others will hang on, or find new areas where suitable habitat persists. Monitoring these ‘ideal’ tsetse habitats through measurements and photography from satellites is a new approach to protect communities. This monitoring is important, Hargrove explains, in areas where roles are reversed. There are some regions where instead of tsetse moving into human habitat, humans are moving into tsetse habitat as communities respond to the challenges of environmental change.

In the Serengeti of northern Tanzania, interactions between people, their livestock and tsetse flies are changing each day. Communities of Maasai people living in the Serengeti rely on animal husbandry for their livelihood and raise cattle for milk, blood and meat. These pastoralists have long grazed their animals over large areas of land. However, recent changes in weather patterns, warmer temperatures and longer droughts have forced them to look in new places for suitable pastureland. As herders and cattle roam ever-larger regions of the Serengeti and even enter remote woodlands, they come into contact with different populations of tsetse flies. These fly populations feed on wild animals, who may be reservoirs of parasite species unfamiliar to the livestock and the communities. Sleeping sickness has been dormant in these Maasai communities for years, but threatens to return because of these new types of interactions.

In these communities where movement is part of life, treating cattle with insecticide isn’t effective by itself. The treatments would kill a few flies on one area, others in another, but wouldn’t significantly reduce any one tsetse population, Hargrove points out. It’s in these situations that habitat monitoring is essential, assisting communities to better avoid transmission risk areas in the first place.

A World Without Tsetse Flies

In many areas of Sub-Saharan Africa, tsetse flies live in equilibrium with populations of wild animals who are highly tolerant of the diseases they carry. The flies only become a hazard when infected species come into contact with livestock and humans. Although sleeping sickness and nagana continue to be significant problems through the continent, an Africa without tsetse flies may have different sustainability challenges. The flies have preserved many natural habitats, such as the famous Okavango Delta in Botswana, from being transformed into additional grazing area for cattle. Tourists dislike the risks associated with tsetse flies when they visit wild game reserves; the flies are known to be attracted to safari vehicles. However, without the flies these reserves may be more susceptible to the settlement and degradation that already threatens many of them.

Climate change presents both challenges and opportunities for controlling sleeping sickness and nagana. Keeping up with the dynamic nature of how tsetse flies and humans interact is becoming more complex in a warming world. However, the opportunity is that control measures will better combine climate science with public health, animal biology and community engagement. These integrated approaches may just allow for better targeting of harmful fly populations, without causing unnecessary environmental degradation in the process. Historically, populations slaughtered wild animals and livestock, cleared large areas of natural vegetation and sprayed large areas with DDT in order to stop the disease. Today, these options are not only unpopular but may even damage the already vulnerable communities they aim to help.

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