Combined exposure to endocrine disrupting pesticides impairs parturition and causes pup mortality in rats

Risk assessment is currently based on the no observed adverse effect level (NOAELs) for single compounds. Humans are exposed to a mixture of chemicals and epidemiological studies have reported some associations between endocrine disrupting effects and combined exposure to certain pesticides. Although laboratory animal studies have shown that some endocrine disrupting pesticides can affect reproduction and sexual differentiation, individual pesticides may appear to be present in human tissues at too low levels to cause concern for adverse reproductive effects. However, recent studies in our laboratory have shown that combined exposure to endocrine disrupters can cause adverse effects on male sexual development, even though the doses of the single compounds are below their individual NOAELs for anti-androgenic effects. Here, we present results from range finding studies with combined exposure to five pesticides, i.e. procymidone, mancozeb, tebuconazole, epoxiconazole and prochloraz. Common features for the three azole fungicides are that they increase gestational length possibly because of an increase in progesterone levels in dams. Groups of 8 time-mated Wistar rats (HanTac:WH) were gavaged from gestational day 7 to postnatal day (PND)13 with either vehicle (control) or a mixture of the five pesticides at 25%, 50%, 75% or 100% of their individual NOAELs for causing major effects on pregnancy length and pup survival in our earlier studies. The pregnancy length was dose-dependently increased in all exposed groups. Most of the animals in the two highest mixture groups (75% and 100%) were unable to give birth and consequently had to be sacrificed. Examinations of the uterine content showed a high frequency of fully developed foetuses that were either dead or dying. The frequency of perinatal mortality of the offspring was markedly increased and the number of pups born alive was decreased in the groups exposed to 50%, 75% and 100% of their individual NOAELs. In addition, effects on the sexual development of the pups were observed in the groups exposed to 25% and higher. Overall, the findings from these range-finding studies showed clearly that combined exposure to the five pesticides induced severe effects manifested as impairment of parturition and high perinatal mortality at doses below the individual NOAELs of the pesticides for such effects. The magnitude of these effects appeared higher than a crude estimate of the combination effect based on dose-addition, but clear conclusion cannot be drawn due to the limited number of animals. This as well as effects on sexual development of the pups will be explored further in a large dose-response developmental toxicity mixture study.