Typhoid Mary’s secret was in her cells

Her name is pretty much a synonym for “disease carrier”, and now it seems that Typhoid Mary’s own immune system was the key to her long-lasting infectiousness.

You’ve probably heard of typhoid fever, which was the condition she carried – although it’s unlikely you’ve encountered it yourself. It’s a bacterial disease that is spread from human to human by contact with urine or faeces.

It still affects an estimated 33 million people per year in the developing world, and up to 200,000 people die as a result. But in developed countries it has been almost eliminated through simple hygiene such as washing hands and food, and engineering sewerage systems that remove potential sources of infection.

Up until antibiotics like penicillin were developed in the mid-20th century, preventative measures like these were definitely the main option for avoiding the disease. And in New York in 1907, that meant keeping away from Mary Mallon.

Mary Mallon lying in a hospital bed, in a ward with other women
Mary Mallon (front), aka Typhoid Mary, unhappily in hospital after her first admission in 1907

Better known as ‘Typhoid Mary’, Mallon was a cook in New York.  In 1907, a government sanitation worker traced cases of typhoid fever back to her cooking in the seaside resort town of Oyster Bay.

The mystery was, if she was infected with Typhoid fever – which is caused by the bacteria Salmonella typhi – why was she exhibiting no delirium, or gastric symptoms?

It was known form other cases in Europe that some individuals could carry the bacteria internally, without developing symptoms, and as an asymptomatic carrier, Mallon was quarantined – much to her frustration.

She was allowed to return to the outside world after three years if she promised to never work in a kitchen. But she broke her promise and infected further patients, and was again incarcerated for the protection of the public, this time until her death in 1938.

Her autopsy revealed Salmonella bacteria still living in her gall bladder, yet she never exhibited any symptoms of typhoid fever.

Pathologist Denise Monack and her team working at Stanford University and University of California San Francisco recently published a paper which may offer a potential explanation for Mary Mallon’s relative longevity (Eisele NA, Ruby T, Jacobson A, Manzanillo PS, Cox JS, Lam L, Mukundan L, Chawla A & Monack DM 2013, “Salmonella Require the Fatty Acid Regulator PPARδ for the Establishment of a Metabolic Environment Essential for Long-Term Persistence”, Cell Host & Microbe, vol. 4, no. 2, pp. 171-182, doi:10.1016/j.chom.2013.07.010).

They observed typhoid fever-causing bacteria hijacking immune system cells in mice – the Salmonella bacteria actually invade the macrophage cells sent to destroy them.

They found that bacteria could hide out inside the cells and proliferate there by actually hacking into part of the cell metabolism that allows them to break down fatty acids for energy.

There are not many bacteria that can use this Trojan horse method of survival; typhoid fever is caused by one, while another relatively common pathogen that can is Myobacterium tuberculosis.

Even though typhoid fever isn’t as big a problem today, the discovery will help pathologists understand the life-cycles of hard-to-kill bacterial infections, and potentially aid in the treatment of antibiotic-resistant bacteria.

Just a final point on the elimination of typhoid in developed countries: it declined massively whenever chlorine was introduced to the water supply. This is a friendly reminder to those who are concerned their precious bodily fluids are under attack, that they can attack others in return.

(This story first aired on 29 August 2013 – you can listen to the podcast.)


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