Identification and characterization of Listeria monocytogenes and Listeria innocua in bovine listeriosis cases in Uruguay

In Uruguay and other countries of southern South America, listeriosis in cattle has been described, often relying solely on histological lesions for diagnosis. This study aimed to: i) analyze a case-series of bovine listeriosis in Uruguay diagnosed through combined pathological and bacteriological methods and, ii) characterize the isolated Listeria strains. From 2014 to 2022, twelve cases were diagnosed. Eleven presented as neurolisteriosis, while one involved a third-trimester abortion. These events primarily occurred in spring (9 cases) and winter (3), affecting individual adult cattle (> 2 years old) on both beef and dairy farms. Interestingly, 5/12 cases (42%) had no prior history of silage feeding. Histopathological examination of neurolisteriosis cases revealed suppurative meningoencephalitis with brainstem microabscesses. The aborted fetus had hepatitis and necrosuppurative nephritis. Listeria was isolated from 11/12 cases: 8 L. monocytogenes and 3 non-hemolytic L. innocua. The identified L. monocytogenes serotypes were 4b (n = 6) and 1/2a (n = 2), which are commonly reported in humans and animals worldwide. Importantly, all L. monocytogenes isolates possessed the inlA gene, a key virulence factor, while none of the L. innocua isolates did. This study highlights the importance of Listeria isolation for characterizing circulating strains and gaining deeper insights into the agent and epidemiology of listeriosis in ruminants of South America.