Abstract
Hector's dolphins (Cephalorhynchys hectori) are a small coastal species endemic to New Zealand. The species as a whole is considered endangered, while a genetically and geographically distinct sub-species, known as Maui's dolphin (Cephalorhynchys hectori mauii) is found only in the North Island and is classified as critically endangered,1 with less than 70 individuals estimated to remain2. Accidental capture in fishing nets is believed to be the most significant threat to survival of this species. Until recently, mortality due to infectious disease was not considered in species management plans or in population models.3,4 We conducted investigations on 49 Hector's dolphins found stranded between 2007 and 2012 and found infectious disease to be the cause of death for 13 (27%) of these. Toxoplasma gondii was the single most important infectious agent, causing the death of 7 individuals (2/4 Maui's and 5/45 Hector's). Affected dolphins had severe disseminated disease predominantly affecting the lungs, liver, lymph nodes, and adrenal glands. PCR analysis showed that an additional 10 dolphins had T. gondii DNA present in at least one tissue. A mixed T. gondii genotype containing both type I and type II alleles, not previously described in New Zealand, was recovered from each of these fatal cases.5 Aspergillus fumigatus was diagnosed as the causal agent in two dolphins that had died due to fungal encephalitis and pneumonia and was present as a contributing factor in two other dolphins. Bacterial septicaemia was diagnosed as the cause of death for three dolphins (one case of A. pyogenes infection and two cases with unidentified gram-negative bacilli), and one dolphin had severe pneumonia with intralesional trematode eggs. Six dolphins had at least one Brucella sp. positive tissue on PCR, including one case of metritis. Morbillivirus infection was not detected in any dolphins. The presence of diseases such as disseminated toxoplasmosis and aspergillosis, which are often associated with immunosuppression, suggests that immune function may be compromised in these animals. Possible causes of decreased immune function, which include environmental or physiological stressors, environmental pollutants, concurrent disease, and genetic factors, warrant further investigation. Our results indicate that infectious disease could play a role in population decline of Hector's dolphins.
Acknowledgements
The authors wish to thank the Department of Conservation (DOC) field staff who collected and submitted dolphins as part of this study; Dr Laura Boren of DOC for coordinating the stranding responses; and Evelyn Lupton, Eugene Ndeke, Nicola Wallace, and Saritha Gils of Massey University for technical assistance. We would also like to thank DOC for their financial assistance with this project.
* Presenting author
Literature Cited
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