Livestock and poultry density and childhood cancer incidence in nine states in the USA.

BACKGROUND: Parental occupational and childhood exposures to farm animals have been positively associated with childhood brain tumors, whereas associations with childhood leukemia are equivocal. The developing immune system may be influenced by allergen, virus, or other exposures from animal sources, which may contribute to childhood cancer incidence. METHODS: Incident cancers (acute lymphoblastic leukemia [ALL], acute myeloid leukemia [AML], central nervous system [CNS], peripheral nervous system [PNS]) for children aged 0-4 diagnosed between 2003 and 2008 were obtained from nine National Cancer Institute Surveillance, Epidemiology and End Results (SEER) registries and were linked to U.S. Census of Agriculture data from 2002 and 2007 by county of diagnosis. Animal densities (animal units [AU]/km2; one animal unit is 1000 pounds of animal weight) were estimated for hogs, cattle, chickens (layers and broilers, separately), equine (horses, ponies, mules, burros, donkeys), goats, sheep, turkeys, and total animals. Animal density was examined in models as both continuous (AU per km2) and categorical variables (quartiles). Animal operation densities (per km2) by size of operation (cattle, hogs, chickens, sheep) were modeled continuously. Rate ratios and 95% confidence intervals were estimated using Poisson regression. RESULTS: We found positive associations between AML and broiler chicken densities (RRper 10AU/km2 = 1.14, 95% CI = 1.02-1.26). ALL rates increased with densities of hog operations (RRper operation/100km2 = 1.06, 95% CI = 1.02-1.11). PNS cancer rates were inversely associated with layer chicken density (RRper log of AU/km2 = 0.94, 95% CI = 0.89-0.99). No association was found between any cancer type and densities of cattle, equine, or goats. CONCLUSIONS: Although limited by the ecologic study design, some of our findings are novel and should be examined in epidemiological studies with individual level data.