Intersecting threats: Brucella spp. and nematodes in small cetaceans of the California coast.

Brucella is a genus of zoonotic, intracellular alphaproteobacteria known to cause disease in marine mammals as well as livestock and companion animals. Specifically, Brucella ceti may be associated with severe neurological, respiratory, and reproductive disease in small cetaceans. Nematodes, particularly members of the Pseudaliidae, are common in small cetaceans and may harbor Brucella ceti, although the nature of this relationship remains unclear. This study investigated the prevalence of Brucella spp. in nematodes collected at necropsy from 64 small cetaceans that stranded on the central California coast between 2017 and 2023, to better document this host-parasite-pathogen relationship and its potential implications for cetacean population health. A total of 2703 nematodes were analyzed using real-time PCR to identify Brucella at both the genus and marine mammal specific species level. Five individual nematodes of three separate species collected from five individual animals were found to have marine brucellae and from one of these, sequencing confirmed the identity as B. ceti. The pooled prevalence of marine mammal Brucella spp. in all parasites was estimated at 0.18%. No significant associations were found between marine mammal Brucella-positive parasites and host demographics, Brucella-clinical hosts, parasite species, or tissue location. While no associations existed between the presence of Brucella within host parasites and host Brucella infection status, demographic features, or parasite characteristics, this is the first time marine Brucella has been detected within parasites from cetaceans in this geographic region. This, along with the novel finding of a Brucella-positive parasite within cetacean mammary tissue, suggests that marine mammal brucellosis and parasitism may coexist within hosts and brucellosis may be more prevalent than anticipated, highlighting the need for additional research to clarify the dynamics between Brucella, parasites, and vulnerable small cetaceans.