Horse and donkey sleeping sickness: investigating equine central nervous system trypanosomosis in The Gambia

It is hard to envisage the world without technology, one in which we are still reliant on working animals for transport and agriculture. Yet, in those parts of the Global South where mechanisation is largely absent, millions of horses and donkeys remain heavily relied upon. However, across Sub-Saharan Africa an unrelenting parasitic disease—trypanosomosis—besets these working animals, with a particularly severe neurological variety recently having been identified in West Africa. Here, PhD student Demelza Kingston explores this situation before focusing in on the small African nation of The Gambia, where she has been working with The Donkey Sanctuary and the Gambia Horse And Donkey Trust to research this deadly new form.

Watering the "help". ©Demelza Kingston
Watering the “help”. ©Demelza Kingston

Horse and donkey sleeping sickness:

Investigating equine central nervous system trypanosomosis in The Gambia

As AA Milne’s Eeyore testifies, it can be tough being a donkey. Despite his gloom, Eeyore had it easy compared to most of the world’s estimated 43 million donkeys and 58 million horses (or equids) that still provide transport and traction to much of our global population. These animals increase incomes and free people  from heavy manual work, particularly women. However, disease and poor management conditions are constant challenges to the health and welfare of working equids, issues that carry over to reduce the income and wellbeing of families across the Global South.

Trouble in the tsetse belt

One of the most important infectious diseases encountered by working horses and donkeys is the parasitic disease trypanosomosis. The disease occurs throughout the Global South in various guises: surra, nagana, dourine, mal de caderas and sleeping sickness all describe trypanosome infection in different regions and host species. Across Africa, Asia, South America and the Pacific, animal trypanosomosis continues to inflict huge losses through its impact on the health, welfare and productivity of millions of ruminant livestock and working equids. In addition to veterinary disease, sub-Saharan Africa is also affected by trypanosomes that cause deadly sleeping sickness in people.

South of the Sahara lies the tsetse belt, named for the aggressive blood-feeding fly that calls the region home. The tsetse belt stretches across the breadth of sub-Saharan Africa, encompassing 37 countries. The tsetse plays a vital role in the development and transmission of the trypanosome species that cause human sleeping sickness, as well as two that infect livestock and working equids: Trypanosoma brucei brucei and Trypanosoma congolense. Other trypanosome species simply hitch a ride to the next animal in the mouthparts of other biting flies, and one unusual sub-species, which only affects equines, is transmitted during mating.

Tsetse fly close up. ©Demelza Kingston
Tsetse fly close up. Bernskbarn [CC BY-SA 4.0], via Wikimedia Commons.

University of Glasgow scholars have a long history of involvement in African trypanosomosis research. Muriel Robertson, for example, described the lifecycle of Trypanosoma brucei gambiense—the parasite responsible for the majority of human sleeping sickness cases—back in 1911. Over 100 years later, my focus as a PhD student is on a form of trypanosomosis that causes severe neurological disease in working horses and donkeys in The Gambia.

The Gambia: The smiling coast

The Gambia is a small west African state known in the UK as a popular package holiday destination. The Gambia’s glossy brochure images hide the reality that over half of the population live on less than US $2 per day and that the country has one of the lowest Human Development Index values (derived from life expectancy, education and income) in the world—number 172 out of 187 in 2013. While European tourists bake on the beach, The Gambia’s rural population work the land and transport their produce with the help of an estimated 60,000 donkeys and 20,000 horses.

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The Gambia does not produce enough foals to keep its equine population steady, but its long history of trading with horse-rich Senegal ensures a constant supply of animals into Gambian livestock markets. Living in Africa’s tsetse belt, Gambian horses and donkeys are at high risk of trypanosomosis. The movement of animals from Senegal that lack previous exposure to the parasite, and the whole country’s proximity to the River Gambia, may contribute to the particularly high occurrence of the disease.

Unravelling the mysteries of a deadly disease

When I’m working in the field, helping at mobile veterinary clinics run by The Gambia Horse and Donkey Trust (GHDT), it seems that every other patient I treat is showing signs of trypanosome infection: weight loss, anaemia, fever, swelling of the limbs and lethargy. This is not my primary reason to be in the country though; I’m there to investigate a recently identified form that manifests with severe dysfunction of the brain and spinal cord. The disease, “equine central nervous system trypanosomosis”, is usually fatal and does not respond to the medications currently available.

WARNING: potentially distressing images.

The transmission of the parasite causing this deadly disease is not fully understood. So far, we know that the disease is due to brain and spinal cord infection with one of the members of the Trypanosoma brucei group. This group includes three sub-species that are known to cause disease in equines, but each is very different in the way they are transmitted. Unfortunately we don’t have an easy way of telling them apart in a clinical sample.  To get around this, I’m using a method of DNA fingerprinting, similar to that used in crime investigation and paternity testing, to study the parasite populations.

Over the past three years, with the support of The Donkey Sanctuary, I have visited the Central River Region of The Gambia to investigate suspected cases and collect samples from patients for analysis. The environment is challenging and the facilities are basic. Lack of mains water and power are no obstacle to research though; a couple of photovoltaic panels provide energy to run a camping freezer and microscopes, and a hand-powered centrifuge is used to spin blood samples. Dissecting the spinal column of a 250kg horse is no mean feat! Back in my pathology labs at vet school this was done with a bandsaw, but the team I work with in The Gambia skilfully extract perfect tissue samples with a cutlass.

From the samples I have collected and analysed so far, it appears that the T. brucei populations causing the neurological disease are quite a varied bunch rather than a single population of genetically identical parasites. Although the disease progresses differently in horses and donkeys, it seems that they share parasite populations. This suggests that the two species have a very different way of responding to T. brucei infection. I haven’t found any evidence that the parasites causing neurological disease are a different genetic population from those that cause the more common systemic disease, although the numbers I have so far are small. As with most research, I am coming up with more questions than answers. Why do horses and donkeys respond so differently to infection with the same populations of T. brucei? Why do some infections progress to the severe neurological form and others don’t?

These questions may have to wait for another project, but what about the question of transmission? The diverse nature of the parasite populations I’ve sampled may support our theory that the tsetse fly is involved. To test this theory, we have collaborated with local tsetse experts to collect hundreds of flies caught in villages around the fieldwork site. The trypanosome populations infecting these flies will be analysed for the same genetic markers as those in equid patients. If the populations appear to be shared between tsetse, horses and donkeys, then tsetse transmission is an important factor and should be a focus of disease prevention in the future.

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