Variable responses of species and strains to white mineral oils and paraffin waxes

Recent dietary studies on mineral hydrocarbon (MHC) white oils and waxes have shown inflammatory effects in Fischer 344 (F-344) rats, but not in other rat strains or dogs. Histopathologic effects seen in F-344 rats include mesenteric lymph node histiocytosis, liver granulomas, and inflammation of the mitral valve (only seen with paraffin waxes). Human ingestion of MHC can result in noninflammatory lipogranulomas (oil droplets) in tissues which are regarded as clinically unimportant. It is speculated that inherent interspecies differences in pharmacokinetics and/or immune function may contribute to the differential response to MHC seen in F-344 rats. The F-344 rat retains greater amounts of MHC in target tissues compared to other rat strains and dogs and appears to be more sensitive immunologically to MHC than other species, including humans. This strain may be predisposed to these effects as indicated by a high background incidence of inflammatory granulomatous lesions in control female F-344 rats. Because of its apparently unique sensitivity, relevance of effects seen in F-344 rat to human health is questionable and requires further investigation.