Women who have regularly exposed to insecticides — especially women whose jobs require them to prepare insecticide solutions — may have a higher risk of developing autoimmune diseases such as lupus or rheumatoid arthritis.

Wednesday, Feb. 2, 2011 - The greater a woman’s exposure to insecticides, the greater her risk of developing lupus or arthritis, or at least that is how it looks to researchers studying epidemiologic data.

The possible link comes from an observational study by researchers at the National Institute of Environmental Health who study data collected from almost 77,000 women.

After adjusting for other known risk factors, exposure to insecticide increased the risk of lupus or rheumatoid arthritis by almost 60% compared with women who never came in contact with the bug-killers. The findings were reported in the February Arthritis Care & Research.

And the risk almost doubled for women who spent 20 years or more working with the toxic chemicals, according to Dr. Christine G. Parks, of the National Institute for Environmental Health Science in Durham, N.C., and colleagues.

Some studies have suggested possible links between farming and agricultural-related pesticide exposures and autoimmune disease. However, an association with home or workplace exposures had no previously been investigated.

So the researchers analyzed data from 76,648 enrollees in the Women's Health Initiative (WHI) observational study. All of the women were postmenopausal (ages 50 to 79) and free of autoimmune rheumatic disease at baseline.

At the start of the WHI, the women were given questionnaires asking about their medical histories and lifestyle factors, and a follow-up questionnaire a year later inquired about adult pesticide exposure through living on a farm or the use of the chemicals in their immediate surroundings — in the home, workplace, or leisure (such as a garden).

After three years, there were 186 incident cases of rheumatoid arthritis and 35 cases of lupus in the cohort. The majority of women in the cohort were white, as were those with autoimmune disease.

More women with autoimmune disease than those without the diseases reported personal use of insecticides 45 percent versus 38 percent. Additionally women who developed the disease were more likely to live on a farm, 34 percent versus 26 percent.

Interestingly, about two-thirds of the women who developed the diseases said they were exposed to insecticides in their homes.

The three-year risk of developing rheumatoid arthritis or lupus was 0.22 percent among women reporting no personal use of insecticides.

The risk among women whose exposure occurred at home was 0.30 percent and 0.31 percent among those reporting workplace exposures.

The three-year risk of autoimmune disease was 0.50 percent in women reporting personal use of insecticides six times or more per year.

Among women who had not lived or worked on a farm, pesticide exposure through application by someone else (such as a lawn service) was not associated with a significant risk of autoimmune disease.

Various mechanisms could explain the link between pesticides and autoimmune diseases, such as changes in both adaptive and innate responses resulting in immune suppression, and altered responses to infections and inflammation, according to the researchers.

In an editorial accompanying the study, Dr. Karen Costenbader, and Dr. Francine Laden, of Brigham and Women's Hospital in Boston, called for further research into environmental exposures and autoimmune disease.

Limitations of the study included its observational design and reliance on self-reported data, according to Parks and her colleagues.

In addition, the researchers had no information on the types of pests and insects targeted by the chemicals, the specific pesticide products used, or specific application techniques.

Nonetheless, Parks and co-authors concluded, "Our findings are consistent with a hypothesis that personal insecticide use, primarily residential, may increase the use of developing [autoimmune rheumatic disease] in postmenopausal women."

The study was supported by the National Institutes of Health.

The Women's Health Initiative is funded by the National Institutes of Health and the U.S. Department of Health and Human Services.

One co-author reported receiving fees and research support from Merck/Schering-Plough.