Serological Investigation for Brucella ceti in Cetaceans from the Northwestern Mediterranean Sea

Neurobrucellosis in cetaceans, caused by <i>Brucella ceti</i>, is a relevant cause of death in striped dolphins (<i>Stenella coeruleoalba</i>) from the Mediterranean Sea. Serological tests are not used as a routinary technique for the diagnosis of this infection. We briefly describe the pathological findings of nine free-ranging stranded cetaceans diagnosed with <i>Brucella</i> disease or infection in our veterinary necropsy service from 2012 to 2022. The findings included focal diskospondylitis and non-suppurative meningitis, choroiditis and radiculitis. Additionally, an exploratory serological study was conducted in sixty-six frozen sera collected in the period 2012-2022 from fifty-seven striped dolphins, five Risso's dolphins (<i>Grampus griseus</i>), two common bottlenose dolphins (<i>Tursiops truncatus</i>), one common dolphin (<i>Delphinus delphis</i>) and one pilot whale (<i>Globicephala melas</i>) to compare antibody levels in <i>Brucella</i>-infected (n = 8) and non-infected (n = 58) animals, classified by the cause of death, sex, age class and cetacean morbillivirus (CeMV) infection status. The authors hypothesized that active infection in cases of neurobrucellosis would elicit a stronger, detectable humoral response compared to subclinical infections. We performed a commercial competition ELISA (cELISA) using serial serum dilutions for each sample, considering a percentage of inhibition (PI) of ≥40% as positive. A titer of 1:160 was arbitrarily determined as the seropositivity threshold. Seropositive species included striped dolphins and Risso's dolphins. Seroprevalence was higher in animals with neurobrucellosis (87.5%) compared to the overall seroprevalence (31.8%) and to other causes of death, indicating, likely, a high sensitivity but low specificity for neurobrucellosis. Animals with chronic CeMV seemed to have higher seroprevalences, as well as juveniles, which also had a higher disease prevalence. These results indicate, as in other studies, that antibodies are not decisive against clinical brucellosis, although they may indicate a carrier state, and that CeMV may influence <i>Brucella</i> epidemiology. More research is required to elucidate the epidemiology and pathogenesis and to resolve the complicated host-pathogen interaction in <i>Brucella</i> species.