Researchers from the National Cancer Institute have reported that individuals who apply pesticides to crops are twice as likely as the general population to develop monoclonal gammopathy of undetermined significance (MGUS), which can lead to multiple myeloma. The details of this study appeared in the June 18, 2009 issue of Blood.[1]

Monoclonal gammopathy of undetermined significance is very common and occurs in up to 2% of persons 50 years of age or older and 3% of those older than age 70 years. MGUS has been defined by the presence of serum monoclonal protein at a concentration of 3 g per deciliter or less; no monoclonal protein or only moderate amounts of monoclonal light chains in the urine; the absence of lytic bone lesions, anemia, hypercalcemia, and renal insufficiency related to the monoclonal protein and a proportion of plasma cells in the bone marrow of 10% or less. It has been speculated that MGUS inevitably leads to malignancy, but reliable estimates on rates of progression have not been identified with any accuracy.

In a previous study, researchers from the Mayo Clinic concluded that the risk of progression of MGUS to multiple myeloma or related disorders is about 1% per year. The authors also pointed out that patients with MGUS are more likely to die of an unrelated disease than to have progression to a malignant plasma-cell disorder. Indeed, the actual risk of death from multiple myeloma and related disorders is overstated when one ignores the greater risk of death from other causes in these mostly elderly patients. They suggest that patients with MGUS should be monitored annually with serum protein electrophoresis to detect multiple myeloma before complications such as renal failure or pathologic fractures occur. Such monitoring could avert these events and would likely improve the patient's quality of life and reduce the cost of long-term dialysis or surgical intervention for skeletal complications.

Previous studies have indicated that pesticides may be associated with an increased risk of multiple myeloma. The current 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.

Comments: The 6% incidence rate of MGUS in pesticide applicators is clearly higher than the 2% reported for the general population in the previous Mayo Clinic Study. The authors of the current study 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. 

Reference:

[1] 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.