Immunotoxicological insignificance of fenitrothion in mice and rats

Fenitrothion was administered orally to mice or rats in daily doses of up to 1/25 of the LD50 for 14 days, and numbers of splenic plaque-forming cells against sheep red blood cells (SRBC-PFC), one of the most common immune parameters, were measured. Splenic SRBC-PFC number was suppressed by fenitrothion only in rats which received 30 mg/kg body weight (bw) of the compound. Other immune parameters, including the arthus reaction, delayed-type hypersensitivity, and activities of macrophages and natural killer cells in rats, were not influenced by fenitrothion. Adrenal hyperfunction manifesting as increased organ weight and elevated plasma corticosterone level was noted along with strong cholinergic signs in rats which received 30 mg/kg bw of fenitrothion. At lower doses such as 3 or 0.3 mg/kg bw of fenitrothion, rats had no strong cholinergic signs, adrenal hyperfunction, or evidence of immunosuppression despite significant suppression of systemic cholinesterase (ChE) activities. In mice, no suppression of SRBC-PFC number or mixed lymphocyte reaction was noted even at the highest dose (40 mg/kg bw) of fenitrothion, at which significant suppression of systemic ChE activities but no cholinergic signs were noted. These findings strongly suggest that the immunosuppressive effect of fenitrothion noted in rats was due to systemic, potent cholinergic stress and that fenitrothion has no immunotoxicity in mice and rats.