Acute and Subacute Toxicity of Pesticides in Microtine Rodents

Acute oral median lethal dose (LD50) and 30-day dietary subacute median lethal concentration (LC50) studies of ten selected pesticides were evaluated in microtine rodents. Four species of microtine rodents, including Microtus orchrogaster (MO), Microtus canicaudus (MC), Microtus pennsylvanicus (MP), and Microtus montanus (MM) voles, were used as a means of developing new animal model systems. Data from both the acute and the 30-day subacute studies revealed that MC voles were approximately twice as sensitive to these pesticides as MO voles. Based on the acute studies, the overall order of pesticide toxicity was parathion > methyl parathion > dieldrin > 2,4-dichlorophenoxyacetic acid (2,4-D) > 2,4,5,-trichlorophenoxyacetic acid iso-octyl esters (2,4,5-T) > simazine > propanil = pentachloronitrobenzene (PCNB) = hexachlorobenzene (HCB) = trifluralin. The general order of species sensitivity was as follows: MC > MP ≥ MO ≥ MM. No apparent sex differences were observed in the MP, MO, or MM voles. In the MC voles, the female appeared to be two to three times more sensitive to methyl parathion than the male. Based on the 30-day subacute LC50 studies, the overall order of pesticide toxicity was dieldrin > parathion > methyl parathion > HCB > 2,4,5-T > PCNB > propanil. The toxicological signs were similar to those of general pesticide intoxication observed in laboratory animals such as rats and mice. No gross pathology attributable to pesticide treatment was observed. Based on LD50 values, the laboratory rodents appeared to be more susceptible to 2,4-D, dieldrin, methyl parathion, parathion, propanil, and 2,4,5-T; equally susceptible to HCB, PCNB, and trifluralin; and less susceptible to simazine than the MC voles.