Suspected organophosphate delayed neurotoxicity in sheep associated with widespread lobed needlegrass ( Nassella charruana )-induced skin perforations

Abstract

Organophosphate-induced delayed neuropathy (OPIDN) is a rare, chronic manifestation of organophosphate (OP) poisoning that typically emerges days after exposure and affects humans and other animals. Here, we describe the epidemiologic, clinical, and pathologic features of 2 suspected outbreaks of OPIDN in Australian Merino sheep in Uruguay following cutaneous exposure to OP. The outbreaks occurred in yearling sheep at 21 and 44 d after a single exposure to pirimiphos-methyl and diazinon, respectively, involving several hundred animals in each case. Affected animals had massive skin perforations caused by Nassella charruana (i.e., flechilla or lobed needlegrass)—a native grass of the Pampa biome that is common in Uruguayan natural grasslands—which likely facilitated enhanced dermal absorption of OP. Histologic examination revealed axonal spheroids, vacuolation, and variable numbers of digestion chambers in the cervical, lumbar, and sacrococcygeal spinal cord segments, predominantly involving the ventral horn, propriospinal tract, and dorsal and lateral funiculi. We suggest that the cutaneous lesions were a critical predisposing factor for OP absorption and the subsequent development of OPIDN. Management of native Nassella spp. in grazing areas may serve as a prophylactic strategy to mitigate the risk of OP toxicosis on sheep farms where dermal routes are used for antiparasitic treatments.