Deworming programs in animal, human populations may have unwanted impacts

A study of the effects of worming medications on infectious disease in wildlife herds showed an unexpected and alarming result - it helped reduce individual deaths from a bovine tuberculosis infection, but hugely increased the potential for spread of the disease to other animals.

The findings, from one of the first field studies ever done on this issue, will be published in the journal Science.

They were contrary to expectations based on laboratory studies, and suggest the possibility that broad use of medical treatments such as this can backfire. They may increase the problem with diseases they were meant to reduce.

Both in animals and possibly human disease, treatments that aid an individual could come at the expense of a wider spread of disease in the larger community, the research suggested.

"This study indicates that we need to understand better how some medical treatments affect other health issues, in particular infectious disease," said Anna Jolles, an epidemiologist at Oregon State University and co-author of the study, along with Vanessa Ezenwa at the University of Georgia.

The research, supported by the National Science Foundation, was done with more than 200 animals in two herds of free-ranging African buffalo in Kruger National Park in South Africa. Half were given deworming medication and the others not.

It was known that infection with parasitic helminth worms can decrease the effective immune response against some infectious diseases, in this case bovine tuberculosis which is common among these animals. Scientists expected the worming medications to save lives while reducing the risk of infection and disease progression.

They found that deworming treatments did improve the survival of animals infected with bovine tuberculosis - in fact, dewormed animals with tuberculosis survived just as well as TB-free animals. However, deworming did not reduce the risk of new infections, and there was a dramatic 8-fold increase in the number of buffalos that an infected animal could potentially infect - a reference to the "R-nought," or reproductive multiplier that epidemiologists use to predict the potential for spread of infection in a community.

A buffalo with bovine tuberculosis but no worm treatments has, on average, the potential to infect about one other buffalo. This study found that after worm treatment, a buffalo with this disease had the theoretical potential to infect nine other buffalos. This difference was based on the finding that dewormed buffalo with TB can survive for years, whereas the life expectancy of untreated TB-infected buffalo was much shorter.

These issues are of significant concern not just for animal, but also human health, researchers say.

Helminth worm infections are among the most ubiquitous parasites on Earth, infecting 1 billion people and causing significant losses among both livestock and wildlife. Other studies have linked co-infection with these worms to increased risk of death from both tuberculosis and HIV/AIDS in human patients, largely due to their ability to reduce and otherwise skew the natural immune response to both viral and bacterial infection.

This is a larger problem in the developing world, and some major deworming programs in human populations are already in place due to the range of health concerns posed by the parasites. It's believed that mass deworming programs may reduce overall deaths from some of the major killers in such areas, such as malaria, tuberculosis and HIV infection.

"These results are pretty alarming," said Jolles, who is a researcher in both the OSU College of Science and College of Veterinary Medicine.

"We expected deworming effects to be all positive, both for individual buffalo, and in terms of reducing disease spread," Jolles said. "But what we found is positive effects for individual animals, but potentially much faster disease spread at the population level."

From these results in buffalo, Jolles said, one should not to jump to conclusions about changing deworming treatments in people. But they do raise questions about large, broad-based public deworming programs.

"We must pay attention to health problems that may increase as a result of the program, as well as problems that we are solving," she said.

The findings also raise questions about aspects of animal agriculture, Jolles said, especially in developing countries. It may be important to match deworming programs with vaccines for infectious disease and other treatments to ensure that the overall health of the herd is protected.

The potential to actually increase spread of a disease following a health treatment such as deworming may vary widely, Jolles said, with different animal species and different infectious diseases.

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