Link found between Agent Orange and chronic lymphocytic leukaemia

Reviewing recently published scientific evidence, a committee of the US Institute of Medicine has concluded that a positive association exists between exposures to herbicides used as defoliants from 1962 to 1971during the Vietnam war and the risk of developing chronic lymphocytic leukaemia.

The report is the fourth in a series of reviews that the institute has conducted at Congress's request since 1996 of the health effects of these defoliants. Earlier reports had linked Hodgkin's disease and non-Hodgkin's lymphoma with exposure to these defoliants. Chronic lymphocytic leukaemia shares many traits with these disorders.

As both chronic lymphocytic leukaemia and lymphomas originate from malignant B cells and the former can transform into Richter's syndrome—a particularly aggressive form of non-Hodgkin's lymphoma—the committee wondered whether chronic lymphocytic leukaemia should be considered separately from other forms of leukaemia.

Irva Hertz-Picciotto of the School of Public Health, University of North Carolina, Chapel Hill, and chairwoman of the committee, said: “We looked into the matter, and our reassessment indicates that CLL [chronic lymphocytic leukaemia] is indeed a special case. The data are sufficient to support a link between herbicide exposure and this type of cancer.”

The committee's assessment is based on evidence from six studies that looked at cancer rates and other health effects among agricultural workers and farm community residents exposed to herbicides. The panel found that the incidence of chronic lymphocytic leukaemia was raised in those whose occupations involved handling of or exposure to the types of herbicides that were used during the Vietnam War.

“There is a positive association between herbicides and the outcome in which chance, bias and confounding could be ruled out with reasonable confidence,” the committee wrote.

The principal defoliant used by the US military forces in Vietnam was a mixture of 2,4-dichlorophenoxyacetic acid (2,4-D), 2,4,5-trichlorophenoxyacetic acid (2,4,5,-T), and one form of dioxin, TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin), present as contaminant. The defoliant is popularly known as Agent Orange, from the identifying colour of the drums in which it was stored.

The committee conclusions are important to those who served in Vietnam as, if they develop chronic lymphocytic leukaemia, they will be eligible for disability benefits. According to the US Veterans Administration, which provides these benefits, there are about 500 new cases annually.