The Screwworm menace affecting animals and threatening humans in Senegal

Senegal’s farmers and vets had never seen anything like it before. An infestation tore across the country, inflicting myiasis, a parasitic invasion of a wound by maggots. All mammals, including humans, can fall victim to them. So it is little wonder that the suddenness and severity of the screwworm outbreak set off alarm bells.

“The novelty and the violence of the cases were the trigger” for the country to request a mission by the Food and Agriculture Organization of the United Nations (FAO), says FAO Senior Veterinarian, Frederic Poudevigne, who was part of the Emergency Management Centre (EMC) mission team.

The Screwworm fly lays its eggs in pre-existing wounds of live animals or humans. In Senegal, thousands of animals were affected during the rainy season of July to December 2023 and while some were treated with larvicides or more homemade cures, others were so seriously affected that they died. Adding to the impact on farmers’ livelihoods was the time spent inspecting animals and the cost of veterinary consultations and products.

As part of their joint mission, FAO, the Joint FAO/International Atomic Energy Agency (IAEA) Centre and the Senegalese authorities visited seven of the country’s 14 departments, collecting a total of nearly 2 000 reports of cases from farmers and vets. Only 16 cases were previously known by the animal health authorities. Though Screwworm can also affect humans, no confirmed cases were found in Senegal.

Besides supporting the country in planning its response, one of the key tasks for the experts was to establish which species of Screwworm was behind the outbreak – a question which would have important implications for the response and for future outbreaks.

A preliminary diagnosis of two samples sent to the national veterinary laboratory in Dakar pointed to the New World Screwworm (Cochlyomyia hominivorax) as the culprit. This would have meant the first introduction of a highly aggressive and invasive American species in Sub-Saharan Africa.

Back in the early 1990s, a serious New World Screwworm outbreak was successfully halted in Libya in less than three years by an emergency programme led by FAO and the IAEA. The response focused on the effective Sterile Insect Technique (SIT). In SIT, insects are bred in captivity and irradiated at the pupa stage with gamma radiation to sterilize them. The sterile flies are then dispersed and their mating with fertile wild flies produces no offspring, helping to reduce and eventually eradicate this parasitic population.

Confirming the DNA tests that ruled out New World Screwworm, the Senegal mission together with a senior expert from the IAEA analysed the available samples and confirmed the involvement of the Old World Screwworm (Chrysomya bezziana) instead.

The Old World Screwworm species is endemic to a great portion of the African continent, meaning the ecological impact is far less than that of an invasive species. Unfortunately, the SIT cannot be used for the Old World Screwworm, so authorities must instead rely on preventive measures including insecticides and larvicides.

The Screwworm’s sudden appearance in Senegal remains a mystery given that there have been no reports of it in the country for over 60 years. Moreover, as mysteriously as it appeared, the wave of Screwworm myiasis decreased in Senegal in January 2024, at the end of the rainy season. But the possibility of a higher number of cases cannot be ruled out when the next rainy season begins; the moisture could cause the flies to reemerge from the soil, having completed their life cycle and remained dormant in pupa form.

Amid these concerns and the lack of evidence demonstrating how the infestation evolved, the FAO One Health team conducted extensive interviews with farmers, vets and hospital staff. Based on these field assessments, together with laboratory analyses and data collection, the experts came up with a series of recommendations to mitigate the impact of a potential recurrence.

These include measures at national level to step up surveillance of livestock imports from countries where Screwworm is endemic; make the best treatment available to all and put in place a stockpile if needed; ensure awareness of the population, including farmers, vets and public health workers on Screwworm diagnosis and treatment systematically in both animals and humans.

Luckily, field practitioners, vets and farmers have a well-established national messaging network that offers strong potential for surveillance and reporting, allowing for early warning should new outbreaks occur. Senegal’s official One Health Platform, under Presidential oversight, is also keeping the various actors in contact with each other, coordinating messages that cut across animal and public health sectors.

Senegal is also one of the countries where FAO’s Emergency Centre for Transboundary Animal Diseases (ECTAD) has long been active in supporting the management of animal health emergencies and would be able to support to in the management of samples from affected animals, if needed.

As Poudevigne sums it up, “It’s not as dramatic as an infectious disease, but in the absence of SIT, it’s all about prevention and communication.”

The FAO-IAEA mission has underlined the crucial importance of taking early action and ensuring that samples are brought immediately to laboratories for analysis. These measures will help ensure that Senegal will be in a better position to confront any future outbreak of Screwworm and can doubtless also hold lessons in other countries where a similar threat might emerge.