PhD Thesis defense by Ignacio Vargas Castro at the VISAVET Centre of the Complutense University of Madrid

June 12^nd, 2024

The importance of cetacean conservation in scientific research is increasingly prominent, giving rise to growing global concern due to the extinction threat faced by several cetacean species. The protection and preservation of these animals benefit the entire marine ecosystem, as they are considered umbrella species. Furthermore, their charisma and popularity make them flagship species, playing a significant role in raising awareness and promoting conservation among the general public. Additionally, they serve a crucial function as sentinels of ocean health.

On the other hand, the "One Health" approach has emerged as an essential perspective to enhance public health by recognizing the complex interactions among humans, animals, and the ecosystems they coexist in. This approach must be extended to the marine ecosystem, making it essential to continually monitor the health and diseases of the species inhabiting it due to the ever-evolving interactions among cetaceans, pathogens, and environmental scenarios.
All of the aforementioned, together with the fact that infectious diseases represent the primary cause of natural death in cetaceans, underscores the importance of investigating the pathogens affecting these species. Valuable information has been demonstrated to be derived from the study of stranded animals, as it proves to be an effective and economical methodology. In fact, long-term monitoring of strandings is considered to provide critical data about local marine health and contributes to the preservation of marine mammals. Hence, this doctoral thesis titled "New perspectives in the study of major emerging diseases in cetaceans: advances in the understanding of Herpesvirus, Morbillivirus, and Brucella" has focused on conducting health surveillance, making use of tissue banks and databases of stranded cetaceans. In this context, three of the four major emerging pathogens have been investigated: Herpesvirus, Cetacean Morbillivirus, and Brucella.

Since the inception of this thesis project, its primary goal has been to delve deeper into the study of major emerging diseases affecting cetaceans. To this end, three specific objectives have been established, each associated with one of the studied pathogens. Different sub-objectives encompassed prevalence studies, phylogenetic analysis, tissue tropism, pathology, description of new host species, and/or diagnosis. The results derived from these objectives and sub-objectives are compiled in five publications, one article currently undergoing peer review, and another article in the preparation phase.

This doctoral thesis provides highly relevant information in the field of cetacean health. Regarding Herpesvirus, significant findings have been unveiled, including the highest prevalences recorded to date in stranded cetaceans, along with the identification of a variety of herpesvirus sequences belonging to the Alphaherpesvirinae and Gammaherpesvirinae subfamilies. New lesions related to herpesvirus infection have been identified, and a strong tissue tropism towards the reproductive and central nervous systems has been documented. Furthermore, the presence of Gammaherpesvirus in the central nervous system of 12 stranded cetaceans has been detected for the first time. Additionally, the presence of herpesvirus in a humpback whale has been described for the first time, expanding the range of cetacean species acting as hosts for this type of virus.

Regarding Cetacean Morbillivirus, one of the highest prevalences reported worldwide was recorded, without an association with an unusual mortality event or disease outbreak. Furthermore, all analyzed sequences belonged to the Atlantic variant of dolphin Morbillivirus, suggesting that this variant might be adapting to an endemic scenario in Mediterranean cetacean populations.
In the study of Brucella, the presence of Brucella ceti was detected in 14 striped dolphins stranded in the Valencian Community. This represents the most comprehensive analysis conducted to date regarding the presence of Brucella in cetaceans on the Spanish coast and is also the second-largest recorded occurrence in the Mediterranean Sea. Strains of B. ceti ST49 have been identified on the Spanish coast, confirming the relationship of this new ST with the Mediterranean Sea. Furthermore, systemic lesions associated with Brucella pinnipedialis have been identified in common dolphins, with indications of transmission through lactation. The lesions were much more severe in the youngest individual.

These results offer valuable insights into cetacean health, while also raising questions that will serve as a starting point for future research. This will allow scientists to confirm or refute the hypotheses put forth in this work, thus advancing our understanding of the health of these animals.