Individuals who apply pesticides to crops are twice as likely as the general population to develop a precancerous blood disorder called MGUS (monoclonal gammopathy of undetermined significance), which can lead to multiple myeloma, according to the results of a study published in Blood.
Multiple myeloma is a cancer of the blood that affects the plasma cells, which are an important part of the immune system that produce antibodies to help fight infection and disease. Multiple myeloma is characterized by an excess production of abnormal plasma cells. Symptoms include increased risk of bacterial infections and impaired immune responses. Almost all patients with multiple myeloma experience the plasma disorder MGUS before developing myeloma.
Previous studies have indicated that pesticides may be associated with an increased risk of multiple myeloma. This more recent study included a cohort of 678 men ages 30-94 from the Agricultural Health Study (which included over 57,000 subjects). The men work as pesticide applicators in North Carolina and Iowa and were enrolled in the study between 1993 and 1997. They provided blood samples that were analyzed for MGUS and then compared with samples from 9,469 men from the general population in Minnesota.
The incidence of MGUS among pesticide workers over age 50 was 6.8%. (There were no cases of MGUS among pesticide workers under age 50.) The rate of MGUS was 1.9 times higher among the pesticide workers than among the control subjects from Minnesota. Furthermore, the researchers identified an increased risk of MGUS associated with different types of pesticides: the risk of MGUS was 5.6 times higher for those who worked with the insecticide dieldrin, 3.9 times higher for the fumigant mixture carbon-tetrachloride/carbon disulfide, and 2.4 times higher for the fungicide chlorothalonil.
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The researchers concluded that “specific pesticides are causatively linked to myelomagenesis” and that further study is warranted in order to understand the link between pesticide use and the subsequent development of cancer.
 Landgren O, Kyle RA, Hoppin JA, et al. Pesticide exposure and risk of monoclonal gammopathy of undetermined significance in the Agricultural Health Study. Blood. 2009; 113:6386-6391.