The journal Blood recently published a report showing a higher than normal incidence of monoclonal gammopathy of undetermined significance (MGUS), a blood disorder that can progress to multiple myeloma, among men regularly exposed to pesticides.

This study, which assessed the risk of MGUS among male pesticide applicators, supports the hypothesis that certain pesticides may be a cause of multiple myeloma.

MGUS is a non-cancerous condition that affects blood plasma cells. Plasma cells are responsible for producing antibodies, which are an important component of the immune system. Normally the body produces a variety of antibodies, but when one is affected by MGUS (or multiple myeloma), the body overproduces one particular antibody. The levels of overproduced antibody are much lower in those with MGUS than in myeloma patients, and those with MGUS do not experience myeloma symptoms like bone lesions or anemia. MGUS can progress to myeloma at a rate of 1 percent per year.

The authors of the study state that though the causes of MGUS and multiple myeloma remain largely unclear, previous studies have shown an increased incidence of myeloma in agricultural workers. It is suspected that the use of pesticides in the profession elevates the risk of myeloma.

The United States Agricultural Health Study found a 1.34-fold excess risk of multiple myeloma in pesticide applicators as compared to general population rates in Iowa and North Carolina.

The report states that it is “unclear whether the observed increased risk of multiple myeloma among individuals exposed to pesticides might reflect a higher prevalence of MGUS, or an increase in the rate of progression from MGUS to multiple myeloma.”

The main focus of the study in the report was to uncover more about MGUS and its connection to pesticide exposure. It was the first population-based study of MGUS in relation to pesticide exposure.

The report stated that the study aimed to estimate the age-specific prevalence of MGUS among pesticide applicators, to compare the prevalence to that in the general population as determined in a population-based screening study, and to assess the prevalence of MGUS in relation to specific pesticides reportedly used by the farmers in the study.

A sample of 678 male pesticide applicators between age 30 and 94 were studied and compared to a random sample of 9,469 men from Olmsted County, Minnesota. Researchers tested the blood and obtained serum samples to determine which study participants had MGUS.

Thirty-eight of the 678 pesticide applicators (5.6 percent) were determined to have MGUS. They examined the prevalence rate of MGUS among pesticide applicators in different age groups. Out of the 38 men, 2.3 percent were in the age range of 50 to 59 years, 5.5 percent in 60 to 69 years, 14 percent in 70 to 79 years, and 16.7 percent in 80 years or older.

Though MGUS was not detected in any subject younger than 50 years old, 6.8 percent of subjects 50 years of age or older were affected. Researchers took this age-adjusted figure and compared it to that of the Olmsted County men. They found that the rate of MGUS in the pesticide applicators was much higher than in the random population sample, as only 3.7 percent were determined to have MGUS. So, the prevalence of MGUS in pesticide users was twice that of the population-based sample of men in Minnesota at a comparable age.

The researchers determined that three pesticides were especially associated with the condition – chlorinated insecticide dieldrin, fumigant mixture carbontetrachloride/carbon disulfide, and fungicide chlorthalonil. According to the study, there was a 5.6 fold, 3.9 fold, and 2.6 fold increased risk of MGUS, respectively.

The report concludes by stating that the researchers’ observation of increased MGUS risk in pesticide users might be an underlying explanation of previous observations of multiple myeloma associated with agricultural work. They call for future studies to improve knowledge on how pesticides contribute to MGUS and how MGUS progresses to myeloma.

For more information, visit the Blood journal article (article abstract viewable for free; full article available for a fee).