A stubborn and deadly outbreak that can cause flesh-eating disease and toxic shock has stumped local and international experts and left a trail of victims in London.

The Middlesex-London Health Unit alerted health-care providers and hospitals Monday about an outbreak of invasive Group A streptococcus (iGAS).

The bacterium has killed nine people and sent nearly 30 to an intensive-care unit in an outbreak that has continued for 18 months with no signs of abating.

About half of the more than 132 confirmed cases in the iGAS outbreak have been people whose movements are difficult to pin down because they’re injection drug users, homeless or in shelters, the Middlesex-London Health Unit says.

As to other half, public health officials have been unable to figure out what has driven the outbreak.

“We need a better understanding of what’s happening, which is why we’ve issued this alert,” Gayane Hovhannisyan, associate medical officer of health, said Monday.

Health officials understand half the challenge: The key to containing an outbreak is isolating those infected, and testing and possibly treating those with whom they came into close contact. Those tasks are exceedingly difficult when the infected are living on the streets or in shelters, and perhaps sharing needles for drug use.

That’s been evident since March 2016, when local health officials called in a field epidemiologist from the Public Health Agency of Canada, who made a visit to the Salvation Army Centre of Hope, where her team found open wounds among most residents.

“We’ve found about half of the cases are among injection drug users and/or those without access to stable housing,” Hovhannisyan said.

While health staff have struggled to protect those whose living conditions place them at heightened risk, staff simply don’t understand why so many other Londoners have fallen ill.

Local health officials have sought help from experts at the Centers for Disease Control in the United States and those who have combatted outbreaks in Toronto and Alaska.

But the outbreaks in those places don’t match what is happening in London.

“This (outbreak) is very unusual,” Hovhannisyan said.

Group A streptococcus are common bacteria that can cause skin and throat infections, including strep throat, but though most who develop an infection get mild illnesses such as strep throat, the bug can become deadly when the infection invades muscles, blood and other organs.

Symptoms of a more serious iGAS infection can include fever, chills, sore throat, dizziness, confusion, severe pain, and redness or swelling around a wound or injured area. The bacteria are spread by direct contact with nose and throat secretions from an infected person, or by direct contact with infected wounds or sores on the skin.

Flesh-eating disease, or necrotizing fasciitis, is most often caused by group A Streptococcus that spread from the skin into the body. Those infected with the fast-spreading disease can lose limbs or die.

Toxic shock syndrome is caused by toxins produced by group A streptococcus. Toxic shock syndrome can spread rapidly and lead to shock, kidney failure and death.

Though the risk of the more serious iGAS infection is low, health officials recommend the following protective measures:

Wash hands regularly, especially after coughing or sneezing and before handling or eating food

Cover the mouth and nose when you cough or sneeze; using a tissue or your sleeve.

Avoid sharing drinking glasses, cups, water bottles and eating utensils.

Do not share drugs, needles, filters and other drug paraphernalia.

Keep wounds clean and watch for signs of infection.

If you’ve been in close contact with a person who has recently had iGAS disease, see your doctor immediately, as you may need preventative antibiotics.

jsher@postmedia.com

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