Long-Term Field Studies of a Rodent Population Continuously Exposed to TCDD

Abstract

Field investigations were conducted during 1973–1978 on populations of the beachmouse, Peromyscus polionotus, from a unique 3.0 km² military test area (Test Area C-52A, Eglin AFB, Florida) that was sprayed with 73,000 kg 2,4,5-trichloro- phenoxyacetic acid (2,4,5-T) herbicide during the period 1962–1970. No residues of 2,4,5-T were detected at a lower detection limit of 10 parts per billion in any soil sample collected during 1971–1972. Residues of 2,3,7,8-tetrachloro- dibenzo-p-dioxin (TCDD) were still present in 1978. During 1974–1978, 54 soil samples were collected to a depth of 15 cm on the test area. TCDD levels ranged from 10 to 1,500 parts per trillion (ppt). Liver tissue from 36 individual beachmice inhabiting the test site contained 300 to 2,900 ppt TCDD. A close relationship between soil and liver levels of TCDD was observed, i.e., high liver levels of TCDD were consistent with high soil levels of TCDD, bioconcentration factors (mean liver concentrations divided by mean soil concentrations) ranged from 6 for females to 18 for males. Whole body analysis of fetuses from test area females indicated apparent placental transport of TCDD. Histopathological examinations were performed on 255 adult or fetal beachmice from the test area and a control area. Examinations were performed on the heart, lungs, trachea, salivary glands, thymus, liver, kidneys, stomach, pancreas, adrenals, large and small intestine, spleen, genital organs, bone, bone marrow, skin and brain. Initially the tissues were examined on a blind study basis. All microscopic changes were recorded including those interpreted as minor or insignificant. The tissues were then reexamined on a control versus test basis, which demonstrated that the test and control mice could not be distinguished histo-pathologically. The mean number of fetuses per observed pregnancy was 3.1 and 3.4 for the test area and a control area, respectively. A single female beachmouse is capable of producing a litter every 26 days. At this frequency, the animals collected in 1978 could have been 50 generations removed from the population studied in 1973. A two-factor (treatment and year) dispropor- tional analysis of covariance of organ weights revealed that liver weights for pregnant beachmice from the test area were significantly heavier (P<.01) than liver weights of pregnant females from the control area, and these differences were consistent over the five years of observation. These studies suggest that long-term, low level exposure to TCDD under field conditions has had minimal effect upon the health and reproduction of the beachmouse.