A parasite called Toxoplasma gondii may infect up to one-third of people worldwide.
Most infected people never know they carry it because the infection often causes no clear symptoms. However, the parasite can remain inside the body for life.
Toxoplasma causes an infection called toxoplasmosis. People can become infected by eating undercooked meat containing the parasite or by swallowing material from contaminated soil, food or water. Cats play an important role in the parasite’s life cycle and can release an infectious form of it in their feces.
After Toxoplasma enters the body, the immune system usually brings the infection under control but does not completely remove it. The parasite can form tiny cysts, particularly in the brain and muscles. These protective structures allow it to survive for many years without causing obvious illness.
For most healthy people, these hidden cysts cause few problems. But if the immune system becomes severely weakened, parasites can become active again and cause dangerous disease, including inflammation in the brain. Infection acquired during pregnancy can also sometimes pass to the developing baby and cause serious complications.
Scientists have long known about these cysts, but they once viewed them mainly as simple resting places filled with similar slow-growing parasites. New research from the University of California, Riverside suggests that the reality is much more complicated. The findings were published in Nature Communications.
The researchers examined individual parasites inside cysts rather than studying thousands of them together. This approach allowed the team to detect differences that would normally be hidden when many cells are combined into one sample. They discovered several distinct types of parasites living within the same cyst.
The team identified at least five subtypes of the slow-growing parasite form known as bradyzoites. These cells do not all appear to behave in exactly the same way. Instead, different groups may be preparing for different jobs during the parasite’s long-term infection.
This changes the traditional picture of a Toxoplasma cyst as a quiet container. Professor Emma Wilson of the UC Riverside School of Medicine and colleagues suggest that a cyst is a much more active and organized environment. Some parasites inside it may be better prepared than others to reactivate or move into another stage of the life cycle.
During an active infection, Toxoplasma can exist as fast-growing cells called tachyzoites. These multiply quickly and spread through the body. When the immune system gains control, the parasite changes into slower-growing bradyzoites that can remain protected inside tissue cysts.
A single cyst can contain hundreds of parasites and may grow surprisingly large compared with many structures found inside cells. Cysts are often found in brain cells, but they can also develop in skeletal and heart muscle. Their presence in animal muscle is one reason eating insufficiently cooked infected meat can transmit toxoplasmosis.
Understanding these cysts is important because current medicines have a major limitation. Treatments can attack the rapidly growing stage of the parasite, but they generally cannot eliminate the long-lasting tissue cysts. As a result, treatment may control active disease without completely clearing the infection from the body.
Studying the cyst stage has been difficult because cysts develop slowly and are buried deep inside tissues. They are also harder to reproduce under laboratory conditions than the rapidly growing form of the parasite. In the new research, scientists used infected mice, which can naturally develop large numbers of brain cysts, to study individual parasites in greater detail.
The discovery could eventually help researchers design better treatments. If particular bradyzoite subtypes are responsible for reactivation or other important stages of infection, scientists may be able to develop medicines that specifically target those cells. A treatment capable of destroying persistent cysts could move medicine closer to completely clearing Toxoplasma infection.
However, the findings are still an early step toward that goal. Much of the research involved mice, and scientists need to determine how closely these different parasite states match those found in humans. Any future medicine would also need to safely reach cysts hidden in places such as the brain.
The study, published in Nature Communications, reveals that one of the world’s most widespread parasites has a surprisingly complex life inside the body.
Rather than simply sleeping inside cysts, Toxoplasma appears to maintain different groups of cells with different biological roles. Understanding that hidden community may eventually reveal new ways to attack an infection that has been extremely difficult to eliminate.
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