Deadly amoeba thrives in warmer lakes

"Sapronoses do not follow the rules of infectious diseases that are transmitted from host to host," says Armand Kuris. "They are categorically distinct from the way we think infectious diseases should operate." (Credit: iStockphoto)

A potentially deadly amoeba often found in lakes and rivers is likely benefiting from a widespread drought in the United States.

The drought is making water warmer than usual, allowing the heat-loving amoeba to proliferate.

A nine-year-old Kansas girl recently died of an infection caused by this parasite after swimming in lakes. The amoeba enters the body through the nose and travels to the brain. Nose plugs can lower the odds of the rare but fatal pathogen entering the body.

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The amoeba, Naegleria fowleri, is classified as a sapronosis, an infectious disease caused by pathogenic microorganisms that inhabit aquatic ecosystems and soil rather than a living host.

To quantify the differences between sapronoses and conventional infectious diseases, researchers developed a mathematical model using population growth rates. Of the 150 randomly selected human pathogens examined in this research, one-third turned out to be sapronotic—specifically 28.6 percent of the bacteria, 96.8 percent of the fungi, and 12.5 percent of the protozoa.

The team reports their findings in the journal Trends in Parasitology.

“Sapronoses do not follow the rules of infectious diseases that are transmitted from host to host,” says lead author Armand Kuris, a professor at University of California, Santa Barbara. “They are categorically distinct from the way we think infectious diseases should operate.

“The paper tries to bring this group of diseases into sharp focus and get people to think more clearly about them.”

No virulence trade-off

A well-known example of a sapronosis is Legionnaires’ disease, caused by the bacteria Legionella pneumophila, which can be transmitted by aerosolized water and/or contaminated soil. The bacteria can even live in windshield-wiper fluid. Legionnaires’ disease acquired its name in July 1976, when an outbreak of pneumonia occurred among people attending an American Legion convention at the Bellevue-Stratford Hotel in Philadelphia. Of the 182 reported cases, mostly men, 29 died.

A major group of emerging diseases, sapronotic pathogens can exist independently in an environmental reservoir like the cooling tower of the Philadelphia hotel’s air conditioning system. Some, like the cholera protozoa, rely on mosquitoes to find disease hosts for them. Zoonoses, by contrast, require a human host.

According to Kuris, diseases borne by a vector—a person, animal, or microorganism that carries and transmits an infectious pathogen into another living organism—are more or less virulent depending on how efficiently they are transmitted.

As a result, virulence evolves to a level where it is balanced with transmission in order to maximize the spread of the virus. However, Kuris notes that there is no virulence trade-off for sapronotic disease agents.

Transmission of a sapronosis pathogen is able to persist regardless of any changes in host abundance or transmission rates.

“You can’t model a sapronosis like valley fever with classic models for infectious diseases,” says coauthor Kevin Lafferty, adjunct faculty at UC Santa Barbara and a marine ecologist with the Western Ecological Research Center of the US Geological Survey.

“To combat sapronoses, we need new theories and approaches. Our paper is a start in that direction.”

Source: UC Santa Barbara