Is the 90-day dog study necessary for pesticide toxicity testing?

When registering a new pesticide, 90-day oral toxicity studies performed with both rodent and non-rodent species, typically rats and dogs, are part of a standard battery of animal tests required in most countries for human health risk assessment (RA). This analysis set out to determine the need for the 90-day dog study in RA by reviewing data from 195 pesticides evaluated by the US Environmental Protection Agency (USEPA) from 1998 through 2021. The dog study was used in RA for only 42 pesticides, mostly to set the point of departure (POD) for shorter-term non-dietary pesticide exposures. Dog no-observed-adverse-effect-levels (NOAELs) were lower than rat NOAELs in 90-day studies for 36 of the above 42 pesticides, suggesting that the dog was the more sensitive species. However, lower NOAELs may not necessarily correspond to greater sensitivity as factors such as dose spacing and/or allometric scaling need to be considered. Normalizing doses between rats and dogs explained the lower NOAELs in 22/36 pesticides, indicating that in those cases the dog was not more sensitive, and the comparable rat study could have been used instead for RA. For five of the remaining pesticides, other studies of appropriate duration besides the 90-day rat study were available that would have offered a similar level of protection if used to set PODs. In only nine cases could no alternative be found in the pesticide's database to use in place of the 90-day dog study for setting safe exposure levels or to identify unique hazards. The present analysis demonstrates that for most pesticide risk determinations the 90-day dog study provided no benefit beyond the rat or other available data.