An outbreak of plague in Madagascar began in August 2017 and expanded rapidly, with about two-thirds of cases transmitted person-to-person as pneumonic plague, the most dangerous form of the disease. The death toll of 124 by 20 October exceeded that of previous outbreaks. More than half of cases have been recorded in the capital of Antananarivo and the main port of Toamasina, the largest cities in Madagascar. Nine nearby countries were considered at high risk of a similar outbreak. The outbreak appeared to peak in Mid-October with the number of new cases declining. Typically the annual plague outbreak peaks in December and runs until April.
On 2 November, a ProMED-mail moderator expressed surprise at the considerable variation reported in numbers of cases and deaths, especially with the relatively low case-fatality rate (CFR) considering that pneumomic plague is reported to account for over 60 percent of deaths. An article from the World Health Organization reported more than 1800 cases as of late October, while nearly 500 fewer had been reported in the week previously.
In January 2018 the experts declared the outbreak over as no new cases had been reported since November 2017, although the World helath organization stated that there was a "moderate" chance or re-occurrence. Malagasy Prime Minister Olivier Mahafaly Solonandrasana had declared the crisis over on 23 November 2017.
Plague is endemic on the central high plateau of Madagascar, usually occurring every year as a seasonal upsurge during the rainy season between September and April. This pattern has occurred since about 1990 with the annual number of cases typically between 800 and 1500. The resurrgence in 1990 was probably related to the breakdown of control measures in place since the 1950s, measures that included vaccination campaigns, improved housing and public hygiene, use of insecticides for control, and streptomycin for treatment. Plague was first brought to the island from India in 1898.
The outbreak began in August 2017 with the death from pneumonic plague of a 31-year-old man who had been traveling in a crowded minibus toward the capital city of Antananarivo in the central highlands. The outbreak expanded rapidly, transmitted person-to-person in the pneumonic form of the disease, which accounted for more than 60 percent of cases. The outbreak was initially recognized on 11 September by local authorities and confirmed by the Institut Pasteur de Madagascar. Authorities called the outbreak "quite worrisome" because the number of cases per day was growing rapidly, and many cases were in urban areas where there are more opportunities for contact between people. Panic was reported in the capital, with the main hospital overcrowded with cases.
The death toll in this outbreak had by mid-October exceeded an outbreak in 2014. Most cases were of the pneumonic form. The bubonic form, transmitted by the bites of fleas from rodents, is more usual in the annual outbreaks in Madagascar. The government announced they had "temporarily suspended gatherings to the general public in places where the traceability of the participants is difficult if not impossible (stadiums, sports palaces, gymnasiums …)".
By 8 November, deaths had risen to 165 with infections totalling over 2000, however the rate of spread had slowed, raising hope that the outbreak was starting to come under control. Concerns continued to be raised that plague might still spread to neighboring countries, or mutate to a form that could be more difficult to treat. By 15 November, there had been 171 deaths and 2119 total cases of plague, however no new infections had been reported since 28 October.
Twelve more cases of suspected plague appeared in the Seychelles days after the death of a 34-year-old male who had recently traveled to Madagascar and who was confirmed as having pneumonic plague on 10 October. Air Seychelles suspended all flights to Madagascar. More sophisticated tests later showed that the infection was not plague.
A South African basketball player who contracted plague while attending a tournament in Madagascar was successfully treated and returned home. The type of plague the player had was not reported, but one of the cases in the Seychelles who died of pneumonic plague was thought to have attended the same tournament.
The World Health Organization warned that there was a high risk the disease could spread to nine other countries in Africa and the Indian Ocean (Ethiopia, Kenya, Tanzania, Mozambique, South Africa, Seychelles, Comoros, Reunion, and Mauritius) because of frequent trade and travel with Madagascar.