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Thursday, 12 July 2018 13:26

Toxoplasma gondii and neuro-invasive disease with Anita Koshy - MTM 86

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How is Toxoplasma gondii, a protozoan that causes neuro-invasive disease, transmitted as a foodborne pathogen? Why are cats important in transmitting Toxoplasma infection? Anita Koshy answer these questions and talks about her research on the latest Meet the Microbiologist.

Host: Julie Wolf A Koshy 7

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Julie’s Biggest Takeaways:

The primary host for T. gondii is cats, in which the protozoan can undergo sexual reproduction. Why cats? No one knows, in part because there isn’t a good in vitro system to study cat epithelial cell interactions with T. gondii.

Most warm-blooded animals, including birds, can be infected with Toxoplasma. Intermediate hosts can pass Toxoplasma from one to another if one eat these tissue cysts, explaining why Toxoplasma can be a foodborne pathogen.

In healthy individuals, the immune response clears most fast-growing cells (tachyzoites) but some protozoans convert to a slow-growing cell form (bradyzoites). In people, these bradyzoites form cysts predominantly in the brain, the heart and the skeletal muscle.

Some serological studies suggest a tie between Toxoplasma infection and brain disorders, but these are less definitive than causative studies in mice. Populations with high Toxoplasma or low Toxoplasma prevalence don’t see a correlative incidence of disorders such as schizophrenia or Alzheimer’s disease.

Featured Quotes:

“When we talk about neuro-infectious diseases, we're talking about the diseases that cause symptoms. Those that can get into the central nervous system and those that cause symptomatic disease are the same.”

“A parasite is sitting there dormant, or maybe reactivating every so often, and the immune system comes in and deals with that reactivation. But when you lack an immune system, all of a sudden when that parasite reactivates, there is no longer this immune system that will come in and clear it out.”

“What we don't know is whether reactivation occurs preferentially in the brain. There is evidence from HIV patients of inflammation of the heart or inflammation of the skeletal tissue - but those were the symptoms that presented, which were ofthe brain. Did reactivation happen in the brain, or did it occur elsewhere and the parasite was able to travel to the brain and there's no longer an immune system to clear it out?”

Links for This Episode:


History of Microbiology (HOM) Tidbit

It turns out to have been a long road from discovery of the protozoan in 1908 to identification of Toxoplama as an agent of infectious disease in 1932, when it was first discovered in congenitally infected infants. In later studies looking for antibodies to Toxoplasma in people, scientists realized that as high as 80% of a population could have been exposed to the protozoans, and because congenital transmission alone couldn’t account for this high of an infection rate, scientists went in search of a transmission route that could.

Before a role for cat feces was found, infection through contaminated meat was observed. An epidemiological study involving children in a Paris sanitorum showed that they developed antibodies against toxoplasma when eating undercooked beef or uncooked lamb - whether this undercooked meat was deliberately fed to the patients is unclear to me, and may make a good follow up History of Microbiology Tidbit. But primarily vegetarian communities in India also had high rates of people with anti-toxoplasma antibodies, so researchers knew there must be a non-carnivorous infection route.

In 1965, Bill Hutchinson demonstrated that cat feces contained infectious Toxoplasma that could survive for up to one year in the resistant oocyst form. Hutchinson went on to show that other vectors like cockroaches and flies could transmit these oocysts by landing on infected cat feces and then carrying fomites to other places. This foundational work was critical to identifying sexual reproduction of the protozoan and developing public health measures such as warning expectant mothers to avoid kitty litter in case of contracting and passing the infection to the developing fetus. Hutchinson continued to study Toxoplasma transmission and biology, and was quoted as saying ‘any parasite that shares our brain with us is worthy of study.’ Hutchinson died in 1999, and we’ll link to his obituary, as well as a summary of 100 years of Toxoplasma research, on our show notes page.

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Last modified on Friday, 13 July 2018 10:19
Julie Wolf

Julie Wolf is the ASM Science Communications Specialist. She contributes to the ASM social media and blog network and hosts the Meet the Microbiologist podcast. She also runs workshops at ASM conferences to help scientists improve their own communication skills. Follow Julie on Twitter for more ASM and microbiology highlights at @JulieMarieWolf.

Julie earned her Ph.D. from the University of Minnesota, focusing on medical mycology and infectious disease. Outside of her work at ASM, she maintains a strong commitment to scientific education and teaches molecular biology at the community biolab, Genspace. She lives in beautiful New York City.