Plague explained: why Madagascar sees cases nearly every year, and what Kenya did in 2017

Madagascar's Ministry of Public Health announced four confirmed plague cases and five suspected ones on 13 October, Xinhua reported on 14 October. The country records plague cases nearly every year. Plague is a bacterial disease carried by small mammals and their fleas, and antibiotics can cure it if they are given early.

The Ministry of Public Health put the four confirmed cases in three districts, including Ankazobe and Anjozorobe in the Analamanga region, north of the capital, Antananarivo, and the five suspected cases in Midongy Atsimo District in the south-east. About 200 people who had contact with patients are under medical monitoring and have received preventive treatment. Teams are tracing contacts and spraying insecticide against fleas in affected areas and their surroundings. Xinhua said Madagascar has entered a period of increased plague activity that began in late July.

The French vaccination and travel-health site MesVaccins, citing L'Express de Madagascar, reported on 14 October that no deaths had been recorded in the areas with confirmed cases and that samples from the five suspected cases were still being analysed at the reference laboratory.

The cause is a bacterium, Yersinia pestis, which the World Health Organization (WHO) says is usually found in small mammals and their fleas. The WHO fact sheet, dated 29 September 2026, lists three ways people can be infected: the bite of an infected flea, unprotected contact with infectious body fluids or contaminated materials, and breathing in respiratory particles from a patient with pneumonic plague. Symptoms often appear one to seven days after infection.

There are three main clinical forms: bubonic, pneumonic and septicaemic. Bubonic plague, the most common, is marked by painful swollen lymph nodes called buboes. Pneumonic plague is lung-based, and WHO calls it the most virulent form: symptoms sometimes appear in under 24 hours after infection, and left untreated it can be fatal within 18 to 24 hours of onset. The case-fatality ratio is 30% to 60% for the bubonic type and 30% to 100% if plague is left untreated.

Pneumonic plague can also spread from person to person. It is especially contagious and can set off severe epidemics that way. WHO's guidance is to isolate patients and to give close contacts of pneumonic patients a seven-day course of preventive antibiotics.

People are at risk wherever a natural focus of plague, meaning the bacteria, an animal reservoir and a flea vector, exists alongside human populations. In Madagascar, bubonic cases are recorded nearly every year in an epidemic season between September and April. During 2019–2025, most cases reported to WHO came from the Democratic Republic of Congo (DRC) and Madagascar. The Institut Pasteur in Paris says hundreds of cases are reported in Madagascar each year, especially in the rural highlands.

A study by researchers from the Institut Pasteur and the Institut Pasteur de Madagascar, published in the Proceedings of the National Academy of Sciences (PNAS, 2025, vol. 122, no. 24), explains the seasonal pattern. The researchers built a mathematical model on data collected over several years, including field surveys that trapped rats and fleas together. Rat numbers fall in the rainy season because there is less food, wet conditions raise flea numbers inside homes, and the number of fleas per rat rises as rats become fewer, so the risk of transmission to people rises in the rainy season. The team, led by Antoine Brault and Simon Cauchemez in Paris with Fanohinjanaharinirina Rasoamalala of the Pasteur plague unit in Madagascar, says targeting both rats and fleas between July and September, before the peak, could significantly cut outbreak risk. The institute says the aim is to move from reacting to outbreaks to an early-warning system.

The 2017 outbreak in Madagascar was mostly pneumonic. WHO's update of 27 November 2017 counted 2,348 confirmed, probable and suspected cases between 1 August and 22 November that year, 1,791 of them pneumonic, and 202 deaths, a case-fatality rate of 8.6%.

In that update WHO named nine African countries and territories, Kenya among them, as priorities for plague preparedness because of their trade and travel links with Madagascar: Comoros, Ethiopia, Kenya, Mauritius, Mozambique, La Réunion (France), Seychelles, South Africa and Tanzania. It said the risk at regional and global levels was low and advised against any restriction on travel or trade on Madagascar.

Jackson Kioko, then Kenya's Director of Medical Services, said in a statement quoted by Capital FM, a Kenyan news site, on 24 October 2017 that WHO had notified the Ministry of Health of a confirmed ongoing plague epidemic in Madagascar. The ministry raised surveillance of visitors from Madagascar, required aircraft from there to submit health declaration forms and sanitation certificates at points of entry, and told health staff to watch for acute fever, chills, headache, severe malaise, chest pain or difficulty in breathing. Suspected cases were to be reported to the Disease Surveillance and Response Unit.

The WHO fact sheet says WHO does not recommend vaccination except for high-risk groups. The US Centers for Disease Control and Prevention, in a page updated on 24 August 2026, says: "Plague can be cured with antibiotics, but these must be given promptly to prevent serious illness or death."