The first case of RVF in Mayotte was diagnosed by RT-PCR on November 22, 2018, for a patient living in Mamoudzou. Two weeks later, 4 new cases were diagnosed. No case-patient had traveled during the 2 weeks before symptom onset, and the cases were defined as autochthonous. During the same period, analyses conducted by the agricultural cooperative of Mayotte and confirmed by the Centre for International Cooperation in Agronomic Research for Development showed that RVF seroprevalence among cattle had increased from 3.6% (95% CI 2.3%–5.6%) in July 2017–June 2018 to 10.1% (95% CI 6.5%–15.3%) in July–September 2018.

Figure

Figure. Sources of exposure for 142 patients with Rift Valley fever, by week of laboratory request, in Mayotte, France, 2018–2019.

From November 22, 2018, through July 31, 2019, RT-PCR at the Centre Hospitalier de Mayotte confirmed 142 cases of RVF in humans. The epidemic curve peaked in week 7, when 18 cases were confirmed. The last confirmed case of RVF was in week 28 (Figure).

Most case-patients were male (3 male:1 female), and median age was 41 years (range 4–75 years). Clusters of cases in humans and animals were located mainly in the central and western areas of the island.

Among the 142 human cases, 126 (88.7%) were investigated; of these, 67.5% of case-patients reported having direct contact with livestock (care, treatment, slaughter) or their biological fluids (including consumption of raw or curdled milk), 15.1% reported living or working near livestock, and 22 (17.4%) reported having none of these exposures. Multiple possible exposures (having direct contact with livestock or their biological fluids and living or working near livestock) were declared by 50 case-patients. However, all case-patients had a connection with the affected areas (living, working, or walking in the central or western part of the island), and 4 lived in environments favorable for mosquitoes and mosquito breeding. Furthermore, most case-patients did not use any type of mosquito control in their houses.

Mosquitoes captured by the Agence Régionale de Santé Océan Indien were predominantly of the genus Culex. These mosquitoes were not tested for RVFV. In 100 herds of cattle and 26 herds of small ruminants, PCR detected 165 animals (141 cattle, 17 goats, and 7 sheep) positive for RVFV.

Information about clinical signs and symptoms was available for only 97 human case-patients (Table). Two case-patients had meningitis, and 1 was positive for RVFV by RT-PCR of cerebrospinal fluid. This patient had a stiff neck a week after the onset of signs and exhibited the neurologic sign of initial loss of consciousness. Twelve case-patients, including a pregnant woman in the sixth month of gestation, required hospitalization for at least 48 hours.

Doctors reported 2 severe cases. The first, with a complication of meningoencephalitis, occurred 3 weeks after the first confirmed case. This case-patient was hospitalized for gait disorder, dizziness, confusion, and rapid onset of hemiplegia; the date of symptom onset was March 18. RT-PCR performed on cerebrospinal fluid was negative. The second case-patient experienced an ocular disease 3 weeks after symptom onset. No hemorrhagic fever or deaths related to RVF have been reported since the outbreak onset.

Among livestock, the first confirmed case was reported on December 4, 2018. Clinical signs were reported by the veterinarian at the time of blood sampling. Among the 165 cases confirmed, 121 (100 cattle and 21 small ruminants) had aborted and 44 showed signs such as hyperthermia, nasal discharge, or digestive disorders.

After the first case in a human was diagnosed, the monitoring and managing protocol of the outbreak was shared by the regional unit of the Santé Publique France, the Agence de Santé Océan Indien, and the Centre Hospitalier de Mayotte laboratory. The objective of this protocol was to implement outbreak control actions (e.g., vector control), actively search for symptomatic humans, and conduct clinical and environmental investigations. Two weeks later, the first case in an animal was confirmed and veterinary services conducted surveys.

Public health authorities informed the general population about the situation through media and social networks to encourage persons to take preventive measures against mosquito bites. Farmers and others involved with slaughtering animals are trained to protect themselves from infectious disease agents. In addition, since February 27, 2019, selling raw and curdled milk has been prohibited.

An outbreak of RVF in Mayotte resulted in 142 confirmed cases in humans as of July 31, 2019, and several clusters among livestock were confirmed. The mode of transmission is not well identified for all cases, but most commonly reported were exposure to animals or consumption of raw milk (a common practice in Mayotte). Some evidence indicates that humans may become infected with RVFV by ingesting the unpasteurized or uncooked milk of infected animals (5).

Our surveillance system for dengue-like syndrome is particularly sensitive and has enabled detection of the first RVF case-patients requiring hospitalization. However, given the high number of asymptomatic and paucisymptomatic forms of illness reported in the literature, the epidemiologic situation may be underestimated. The extent of the epidemic could be assessed by a seroprevalence survey at the end of this outbreak.

This epizootic occurred in the context of increased illegal imports of animals (goats, sheep, and cows) over several months from potential disease-endemic/epidemic countries. Infected animals, especially sheep, were identified among intercepted animals.

RVFV, which was identified in Kenya, is still present in eastern Africa (14), but little information is available about its seroprevalence on the Indian Ocean islands. Mayotte is at risk for introduction and circulation of infectious agents involved in outbreaks in neighboring countries, such as recent infections and circulation of RVFV in the Comoros Islands (15).