October 3, 2013

A parasite that causes infected mice to lose their hard-wired fear of cats may do so by permanently changing the brains of the mice, according to research reported by scientists at the University of California, Berkeley. The parasite, called Toxoplasmosis gondii, causes toxoplasmosis, a widespread infection in cats as well as humans. The researchers reported that the mice in their study remained unafraid of cats even after having only a mild infection or having been cured.

Cats infected with T. gondii shed the parasite in their feces. Rodents usually become infected by eating cat feces. (People usually pick up the parasite by eating food contaminated by cat feces or eating the uncooked meat of infected animals, such as pigs and sheep.) Once the parasite enters the body, it works its way into nearly every organ, including the brain. There it may form cysts (fluid-filled sacs).

A parasite that causes infections in cats can permanently change a mouse's inborn fear of cats. (© Shutterstock)

Mice have an inborn fear of cats. The smell of cat urine, which outdoor cats use to mark their territory, helps mice avoid the predator felines. Researchers had previously discovered that mice with fullblown toxoplasmosis were not frightened by the smell of cat urine. However, the Berkeley scientists found that the mice remained fearless even if they had only a short or mild infection. In other words, just being exposed to the mind-controlling parasite permanently changed the mice’s behavior, making them more vulnerable to the cats.

The scientists don’t know how the parasite changes mice brains. They speculated that it may damage cells in the brain that are involved in smell, or it may “rewire” cells involved in learning and memory.

The findings may have an important effect on the way physicians treat people with infectious diseases. People normally expect that being cured of an infection means the symptoms of the infection will also disappear, team leader Wendy Ingram told BBC news. “Now we have an example where there is no obvious damage done by the parasite, yet major changes in the neurobiology of the mouse remain after the parasite is gone. Parasites and bacteria may affect human health long after the microorganisms have gone from the body.”

Additional World Book articles:

• Infections Spread by Pets (a special report)

September 12, 2013

Tuberculosis (TB), a major cause of death in developing countries, may be so hard to defeat because it likely originated among our ancestors in Africa more than 70,000 years ago. In fact, it may have originated even before modern people migrated from the continent, according to a new study by an international team of scientists led by Sebastien Gagneus, an expert on infectious diseases from the Tropical and Public Health Institute in Basel, Switzerland. For their study, the scientists examined DNA sequences of tuberculosis bacteria from 259 subjects throughout the world. These genetic analyses enabled the scientists to construct a “family tree” of the bacterium, tracing its history through time.

A little more than a century ago, TB ranked among the most common causes of death in the world. Rod-shaped bacteria called tubercle bacilli cause the disease. Tubercle bacilli belong to a genus (group) of bacteria called Mycobacterium. Today, improved methods of prevention, detection, diagnosis, and treatment have greatly reduced the number of people who get the disease and those who die from it. However, even today tuberculosis claims many lives in countries where these improved methods are not widely available.

Evolutionary scientists believe that anatomically modern humans first appeared in Africa during the past 100,000 years. Sometime before about 50,000 years ago, they migrated out of Africa to other parts of the world. The new research suggests that early modern humans suffered from tuberculosis and carried the bacterium with them on their journeys. The scientists believe that the TB bacterium’s ability to go dormant (inactive) for long periods enabled the deadly disease to spread with human populations across the globe.

A volunteer physician with Doctors Without Borders, an international relief organization, treats a patient in Kenya for tuberculosis. (AP/Wide World)

Scholars had long thought that human tuberculosis, which cannot survive or infect any other animal, was a relatively recent disease. They believed that human TB originated from tuberculosis varieties that affect animals, such as cattle. Over the past 10,000 years, as humans developed agriculture and began domesticating animals, scientists believed the tuberculosis bacterium was passed to humans from livestock. Bovine tuberculosis, which affects cattle and can sometimes infect humans, seemed to support this view. The new research, however, indicates that tuberculosis was established in human populations long before people began keeping livestock.

Additional World Book articles:

• BCG
• Calmette, Albert
• Trudeau, Edward Livingston
• Tuberculosis: An Old Foe Gains Ground (a special report)