Technical note: Quantitative and qualitative analysis of diatom testing in methomyl poisoning case

Diatom testing is conducted in forensic medicine primarily to support the diagnosis of drowning. In our previous study, a large number of diatoms, potentially originating from methomyl, were detected in the lungs of a cat found dead under circumstances suggestive of methomyl poisoning, but not involving drowning or submersion. In the present study, we collected human and animal cases suspected of methomyl poisoning, where the bodies showed no evidence of drowning or submersion. Diatom tests were performed both quantitatively and qualitatively on the gastric contents, lungs, liver, kidney, and fur samples. Quantitative analysis using light microscopy revealed a large number of Stephanodiscus spp., the same genus of diatoms found in methomyl-containing insecticides, in the gastric contents and in some of the lung samples. Although qualitative analysis with electron microscopy did not enable species-level identification, the observed Stephanodiscus spp. were presumed to be fossil species derived from diatomaceous earth. Since diatomaceous earth is sometimes added as a carrier or diluent to concentrate the active ingredient in insecticides, it is possible that diatoms may be present in the bodies of individuals that died from methomyl poisoning. Therefore, relying solely on the quantity of diatoms may result in false-positive diagnoses of drowning. To avoid such misinterpretation, it is essential to perform qualitative analyses to assess the species composition of the diatoms in addition to quantitative assessment.