This week Outbreak Observatory takes us to Mayotte, an overseas department of France in the Indian Ocean where an outbreak of Rift Valley Fever (RVF) is ongoing. While predominantly a disease that affects animals, this outbreak is unique in that a larger number of humans have been infected. In addition, the tropical climate of Mayotte, and its favorable environment for mosquitoes, could lead to the outbreak continuing for the foreseeable future. Below we provide a brief history of RVF in Mayotte and discuss the outbreak the island is currently experiencing.

The Disease

Rift Valley Fever (RVF) is a febrile, vector-borne viral disease that infects ruminants, primarily livestock; however, the RVF virus can also cause infections in humans. Among animals, the disease is often spread through the bite of an infected mosquito, mainly from the Aedes spp, though certain Culex, Mansonia and Anopheles species are also known to act as vectors during outbreaks. While there are documented reports of vertical transmission in ruminants, there appears to be little evidence to show direct horizontal transmission between infected and healthy ruminants. In some cases, RVF can cause severe infections in animals resulting in high abortion rates and high mortality rates for young animals.

Spillover to humans is often the result of direct or indirect contact with blood, bodily fluids, tissues, or organs of infected animals. In addition, the disease can spread through inoculation or inhalation of aerosolized bodily fluids. Consequently, risk factors for RVF comprise of activities including slaughtering, skinning or butchering infected animals, farming, herding and veterinary practices, consuming unpasteurized products, and mosquito bites. As with animals, there are records of vertical transmission but no direct human to human transmission. Human cases usually present as mild illnesses, however severe forms include ocular disease, meningoencephalitis or hemorrhagic fever. Fatality rates range between different outbreaks, but generally are less than 1% with the majority occurring in cases that develop hemorrhagic fever.  WHO’s R&D Blueprint, includes RVF as a priority pathogen for public health research and development efforts given its epidemic potential and absence of a suitable vaccine or drug.

The Island

Comprised of two small islands (Grande Terre and Petite Terre) in the Mozambique Channel of the Indian Ocean, Mayotte is estimated to have a population of 264,900 people. Reported disease outbreaks in Mayotte have included Chikungunya (2006), Dengue (2010), malaria (endemic) and Rift Valley Fever (2007, 2018).

For Mayotte, the first reported human cases of RVF in humans occurred following an outbreak that occurred throughout several African countries from 2006-2007. That outbreak, which was first detected in Kenya in December of 2006, eventually spread to Somalia, Tanzania, Sudan, Mozambique, and the Union of Comoros. In all affected countries, both animals and human cases occurred and there were  high economic consequences due to agricultural losses. Mayotte, which is in the Comoros archipelago, reported its first ever cases of RVF in humans in September 2007. Sequencing of the virus confirmed its connection to the Kenyan outbreak, most likely attributed to the trading of cattle from the African continent through the Union of Comoros. However, surveillance and further sequencing activities showed RVF was likely present in livestock from at least 2004 with the re-emergence of the virus in 2008 attributed to the Kenyan outbreak.

The Current RVF Outbreak

Since November 2018 regional authorities have reported several cases of RVF in Mayotte. In February, the European CDC (ECDC) noted that the detection of RVF cases in humans is not unexpected, due to RVF seroprevalence among ruminants increasing in 2017, but the number of cases in a short period of time is of concern as the current rainy season would be favorable to vectors and may lead to a larger outbreak. As of April 20, 2019, the number of RVF cases identified totaled 104 animal cases and 122 human cases. As RVF has previously been associated with high economic consequences, specifically for the agricultural sector, the low number of animal cases in comparison to the human cases may be due to underreporting because farmers often fear for the destruction of, or import and export bans on, livestock. Of the identified human cases, no fatalities or severe forms of the disease have been recorded. While there appeared to be a decline of cases during March of 2019, the influx of cases in early April 2019 demonstrated the ongoing risk of RVF and resulted in authorities reminding the public of the importance of prevention measures to protect against the infection.  A rapid risk assessment of Mayotte’s RVF outbreak, published by the ECDC on March 7, 2019, reported that the human cases were concentrated to the confined agricultural areas of Mayotte, in the Centre-West and North part of the main island. Over half of the identified human cases have a possible connection to the Ourovéni region, an agricultural area known for its hiking.

According to WHO, while there is a RVF vaccination for animals, the vaccine developed for human use has not been licensed, though it has been used experimentally in the past to protect veterinarians and lab workers. Public health authorities have implemented various control measures in Mayotte in response to the outbreak. Farmers must report any sick animals or abortions to veterinarians so that samples can be taken and investigated for RVF. In addition, protective equipment such as masks, gloves and glasses, is recommended for slaughtering practices. Other recommendations include hand washing, cooking meat, boiling milk products and protection against mosquitoes. On February 27, 2019 selling raw milk was temporarily banned in Mayotte for three months.


The current outbreak shows the importance of surveillance measures. Through serosurveys and animal case reports, the ECDC noted that after the previous human and animal outbreak of RVF there was a steady decline of RVF virus circulating in livestock between 2008-2010. More recently, preceding the reports of human cases in November 2018, the French Agricultural Research Centre for International Development reported an increase in the circulation of the virus in livestock for both 2017 and 2018 in Mayotte. While no reason has been identified for this increase in circulation, the resulting outbreak shows the continued presence of susceptible ruminants and the ideal environment for mosquito vectors allow the virus to persist. Illegal animal trading with neighboring countries also amplifies the risk of RVF outbreaks in Mayotte.

Finally, RVF outbreaks highlight the importance of developing a strong One Health Approach due to its effects on both the animal and human health sectors. Without agricultural monitoring and a robust animal health infrastructure, the virus may circulate undetected for a long period of time, as was seen in Mayotte, in 2007, when serological studies were carried out after human cases of RVF were first reported.  

Photo: Mayotte

Photo courtesy of Pixabay

Outbreak Observatory aims to collect information on challenges and solutions associated with outbreak response and share it broadly to allow others to learn from these experiences in order to improve global outbreak response capabilities.