Off South Africa, nearshore Indian Ocean humpback dolphins (Sousa plumbea), an IUCN listed endangered species with a declining population off the South African east coast, face several anthropogenic threats, including fisheries, pollution and shark nets deployed off the coast of KwaZulu-Natal (KZN).1,2 Documented diseases in this population include pneumonia, enteritis and lobomycosis.3 However, with the exception of Crassicauda spp.-associated bone lysis, pathology of the skull is mostly unknown in this species.4 To explore the diseases that may impact the health and welfare of S. plumbea, we carefully examined the skulls of 28 (14 immature, 14 mature) females and 46 (29 immature, 17 mature) males caught in shark nets along the coast of KZN in 1972–2016. Erosion and lysis of the mandibular fossa (squamomandibular joint - SMJ) were observed in nine (4 immature, 5 mature) or 32% of females and in 23 (14 immature, 9 mature) or 50% of males, aged between 2.5 and 21.5 years. Severity ranged from mild to severe, in which case numerous osteolytic lesions led to marked excavation on over 50% of the articular surface. Severe lesions were seen only in males and were already present in a 2.5 year-old individual. Other findings included lysis and osteitis of the mandibular ramus and maxillaries in four mature males, one immature male and an immature female, aged between 6 and 21.5 years. Dental abscesses are the suspected origin of these lesions. All affected specimens had also SMJ lesions. Altogether these results indicate that lesions of the jaws occur regularly in S. plumbea. Pathology of the SMJ has rarely been reported previously in cetaceans, although it is quite common in other marine mammal species.5,6,7 Together with the lytic lesions and osteitis, it is likely to cause morbidity and considerable pain, may compromise feeding and thus affect physical condition, especially when severe.
We kindly thank Dr. Greg Hofmeyr, Port Elizabeth Museum, for his help with logistics and facilitating access to specimens.
* Presenting author
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2. Plön S, Cockcroft VG, Froneman WP. 2015. The natural history and conservation of Indian Ocean humpback dolphins (Sousa plumbea) in South African waters. In: Jefferson TA, Curry BE, eds. Advances in Marine Biology. 72:143–162, Oxford: Academic Press.
3. Lane EP, de Wet M, Thompson P, Siebert U, Wohlsein P, Plön S. 2014. A systematic health assessment of Indian Ocean bottlenose (Tursiops aduncus) and Indo-Pacific humpback (Sousa plumbea) dolphins incidentally caught in shark nets off the KwaZulu-Natal Coast, South Africa. PLoS One. 9(9):e107038. doi: 10.1371/journal.pone.0107038.
4. Van Bressem M-F, Duignan PJ, Van Waerebeek K, Raga JA, Plön S. 2018. Crassicauda spp. skull lesions in Indian Ocean humpback dolphins (Sousa plumbea) and Indo-Pacific bottlenose dolphins (Tursiops aduncus): implications for conservation. Poster presented to the African Marine Mammal Colloquium, Port Elisabeth, August 2018.
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6. Arzi B, Murphy MK, Leale DM, Vapniarsky-Arzi N, Verstraete FJ. 2015. The temporomandibular joint of California sea lions (Zalophus californianus): part 1 - characterisation in health and disease. Arch Oral Biol. 60:208–215.
7. Ludolphy C, Kahle P, Kierdorf H, Kierdorf U. 2016. Osteoarthritis of the temporomandibular joint in the Eastern Atlantic harbour seal (Phoca vitulina vitulina) from the German North Sea: a study of the lesions seen in dry bone. BMC Vet Res. 14:150–164. doi: 10.1186/s12917-018-1473-5