Council workers spray disinfectant as they clean-up the market of Anosibe in the Anosibe district, one of the most unsalubrious district of Antananarivo on October 10, 2017.

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A plague outbreak has infected hundreds of people in Madagascar and could spread further.

The form of the disease that’s spreading, pneumonic plague, is the deadliest.

This outbreak is on a larger scale and is spreading far more rapidly than plague normally does.

The man who came down with first known case died on August 27.

He’d been ill for a few days with what was thought to be malaria, according to the World Health Organisation. It turned out to be pneumonic plague, which is the most contagious and deadliest form of the disease, since it can spread through the air when people cough.

The man, who is unnamed in WHO documents but described as 31 years old, died while travelling by bush taxi, a form of public transport in Madagascar. He was en route to the coastal city of Tamatave from the Ankazobe District in the central highlands, and had passed through the capital city of Antananarivo.

Plague cases started to appear throughout Madagascar shortly after that, with a number traced to people that man came into contact with, and then to people those individuals encountered.

As of October 7, at least 343 cases and 42 deaths from the outbreak had been confirmed, with numbers expected to continue rising.

Plague can be treated effectively with antibiotics if it’s caught early enough. On October 6, the WHO announced that they’d delivered 1.2 million doses of antibiotics to the island nation and authorised $US1.5 million in funding to help fight the disease. But the rapid spread of this outbreak is causing international alarm.

An unusual and disturbing outbreak

In Madagascar, Yersinia pestis, the bacteria that causes the three forms of the plague, is endemic in certain locations — as it is in other parts of the world, like the US Southwest. The presence of Y. pestis results in regular outbreaks. The US sees a few plague cases every year. Madagascar usually sees about 400 annually.

“The thing that is unusual is that this is the beginning of the season,” Daniel Bausch of the UK Public Health Rapid Support Team told CNN. “Usually, after six months, you have around 200 cases.”

Plague season runs from September through April, but in the first six weeks, there have already been more than 300 documented cases in Madagascar.

Two other related factors make this outbreak disturbing: the form of disease and locations it has reached.

Normally, plague outbreaks in Madagascar (and elsewhere) are comprised of the bubonic plague, the “Black Death” notorious for devastation in Europe in the 14th century.

Bites from fleas (often carried by rodents) carrying Y. pestis spread bubonic plague, which causes fever, weakness, and buboes — painfully swollen lymph nodes. The third form of the plague, septicemic, happens when the infection gets in the blood and causes skin and tissue to turn black and die. It’s also spread via flea bites.

Untreated, those infections can spread to the lungs, causing pneumonic plague. Without antibiotics, this form of the disease is 100% fatal. And unlike the other two forms, pneumonic plague spreads from person to person.

Because of that, the disease has reached crowded areas in Madagascar that aren’t normally exposed to plague. It’s much harder to slow the disease’s spread once cases start popping up in densely populated places. Schools have been closed and public gatherings prohibited in affected cities.

When the WHO first assessed the outbreak, they calculated that there was a high risk that plague would spread through Madagascar, a moderate risk it would spread to nearby countries, and a low risk for global spread. The WHO is currently revising that assessment to determine whether those risk levels, especially for spread to nearby countries, have changed.

The plague bacteria remain essentially unchanged from their deadly medieval form, though the disease can be stopped with rapid diagnosis and quick medical treatment.

The medicine sent to Madagascar so far, along with another 244,000 doses on the way, will be enough to treat 5,000 patients and could protect up to 100,000 people who may have been exposed, according to the WHO. But the health organisation is still asking for another $US5.5 million to train local health workers to diagnose the disease, treat those infected, and trace the potential contacts of infected individuals. Those actions are necessary to contain the disease.

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