Rift Valley fever has killed 42 people in Mauritania and Senegal since September
WHO reports 404 confirmed human cases across the two countries in six weeks. The case fatality ratio is 30% in Mauritania and 7.8% in Senegal — a gap that says more about detection than about the virus.
Between 20 September and 30 October, national health authorities in Mauritania and Senegal reported 404 confirmed human cases of Rift Valley fever, including 42 deaths, according to a Disease Outbreak News notice the World Health Organization published on 5 November [s1]. Both countries are endemic for the virus and both have seen it before. What is unusual here is the size of the human case count, its concentration along a single river valley, and the very different fatality ratios the two countries are reporting.
Two outbreaks, one border
Senegal accounts for most of the cases. Between 20 September and 30 October the country confirmed 358 human cases and 28 deaths, a case fatality ratio of 7.8%, spread across 22 health districts in eight administrative regions [s1]. The vast majority — 78% — came from the Saint-Louis region in the north, where the affected districts of Dagana (32 cases), Pete (9), Podor (32), Richard-Toll (133) and Saint-Louis (73) all sit along the Senegal River, on the border with Mauritania [s1]. Smaller numbers were reported from Dakar (nine), Fatick (12), Kaolack (13), Louga (18), Matam (23), Thiès (two) and Tambacounda (two) [s1].
Mauritania, on the other side of that river, confirmed 46 human cases and 14 deaths between 27 September and 30 October — a case fatality ratio of 30% [s1]. Cases came from eleven districts, in regions including Assaba, which borders Mali, and Brakna and Trarza, which both border Senegal along the river [s1]. Of 190 samples tested in Mauritania, 46 were positive, a positivity rate of 24.2% [s1].
The four-fold difference in case fatality ratio between the two countries is almost certainly not a difference in the virus. A ratio calculated from confirmed cases depends heavily on how many mild infections a surveillance system catches. Most Rift Valley fever infections in people are asymptomatic or produce only mild flu-like illness — fever, weakness, lower back pain, dizziness — and only about 2% of patients progress to a severe form involving eye inflammation, encephalitis or haemorrhage [s1]. A country testing 190 samples and confirming 46 is seeing a different slice of the epidemic than one confirming 358. WHO's notice specifically flags delayed detection, underreporting and reliance on central PCR confirmation as problems it is working with both countries to fix [s1].
The animal outbreak underneath
Rift Valley fever is a livestock disease that spills into people. Most human infections come from direct contact with the blood, tissues or organs of infected animals rather than from mosquito bites, which puts herders, farmers, abattoir workers and veterinarians at the highest risk [s1]. No human-to-human transmission has ever been documented [s1].
The animal picture in both countries is correspondingly large. Mauritania's Ministry of Animal Resources reported 62 animal outbreaks as of 30 October, with 235 positive samples out of 1,106 collected, and 235 animal cases including 71 deaths across Aioun, Timbedra and Maghta Lahjar, affecting sheep, goats, dromedaries and cattle [s1]. The first animal cases, in goats and dromedaries, appeared in August [s1]. Senegal has confirmed 160 animal cases across seven regions as of 29 October and reported 640 animal abortions in three regions — abortion storms in small ruminants being one of the classic early signals of a Rift Valley fever epizootic [s1].
Neither country is new to this. Mauritania's last major outbreak, in 2022, produced 47 confirmed human cases and 23 deaths, a case fatality ratio of 49%, across nine of its fifteen regions [s1]. Senegal's last confirmed human case before this outbreak was in January 2025 in Touba, and the last case in the Saint-Louis region specifically was in 2022 [s1].
Why now
A modelling study published in PLOS Neglected Tropical Diseases at the end of September — before this outbreak was reported — offers a partial answer for the Mauritanian side. Using geolocated human, animal and mosquito data from 2019 to 2023, the authors found the strongest predictors of Rift Valley fever virus infection were rainfall in the current and preceding month, followed by average daily temperature in the current month [s2]. August, September and October were the most ecologically favourable months, with risk beginning in the country's southeast and expanding across the whole south by September and October [s2]. The same study notes that outbreaks in Mauritania have been recurring with increasingly short gaps between them [s2].
That is close to a description of what happened. WHO's assessment names heavy rains and increased mosquito activity, along with livestock movement within the countries and toward Mali and the Gambia for grazing and trade, as reasons the risk of further spread remains high [s1].
What the response looks like
Both countries have deployed multidisciplinary rapid response teams drawn from health ministries and veterinary services [s1]. Senegal has set up regional coordination teams with designated incident managers in each affected region and a national incident management system, both built on a One Health framework that puts human health, animal health and environmental sectors under shared command [s1]. WHO is working with the Food and Agriculture Organization and the World Organisation for Animal Health on surveillance, district-level diagnostic capacity, clinician training and risk communication aimed specifically at herders and slaughterhouse workers [s1].
There is no licensed human vaccine for Rift Valley fever, and the outbreak response rests on detection, supportive clinical care and interrupting the animal-to-human exposure route.
WHO currently rates the risk as high at national level in both countries, moderate regionally and low globally [s1]. The regional rating is the one to watch: the affected districts sit on a shared river and an active livestock corridor, and the ecological window that favours transmission does not close sharply.
Sources
- [s1] Rift Valley fever - Mauritania and Senegal, World Health Organization, 5 November 2025. https://www.who.int/emergencies/disease-outbreak-news/item/2025-DON584
- [s2] Local drivers of Rift Valley fever outbreaks in Mauritania: A one health approach combining ecological, vector, host and livestock movement data, PLOS Neglected Tropical Diseases, 30 September 2025. https://doi.org/10.1371/journal.pntd.0013553
Sources
- Rift Valley fever - Mauritania and Senegal — World Health Organization , November 5, 2025
- Local drivers of Rift Valley fever outbreaks in Mauritania: A one health approach combining ecological, vector, host and livestock movement data — PLOS Neglected Tropical Diseases , September 30, 2025
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