A study of brain injury in New Zealand sea lion pups : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Massey University, Palmerston North, New Zealand
The New Zealand sea lion (Phocarctos hookeri) is a threatened species endemic to New Zealand. The majority of breeding in this species occurs on the Auckland Islands in the sub-Antarctic, and recent population estimates indicate that pup production is declining. Trauma is a significant cause of mortality in New Zealand sea lion pups, and much of this is believed to be caused by adult and subadult males, that bite, crush, shake and throw young pups.
In this thesis, a number of techniques are used to determine the role played by traumatic brain injury in the mortality of NZ sea lion pups. The findings of gross necropsy examinations show that pups have numerous lesions indicative of traumatic brain injury, including skull fractures and subdural haemorrhages, and that pups die due to crushing and impact injuries. Although some pups have gross lesions considered in human paediatric medicine to be indicative of shaking injury, detailed histological and microbiological studies of sea lion pups show that most of these are associated with meningitis due to Klebsiella pneumoniae. This bacterium is a common cause of pup mortality.
Immunohistochemical techniques are used to demonstrate that axonal injury is common in sea lion pups, but show that shaking is not a common mechanism of this pathological process. Instead, most axonal injury is found to be due to hypoxia-ischaemia, and evidence that raised intracranial pressure has occurred is comparatively common in dead pups. The combined findings of histological and immunohistochemical studies suggest that lesions such as optic sheath haemorrhage, intracranial subdural haemorrhage, spinal sub-meningeal haemorrhage, and optic nerve axonal injury could be caused by pertubations to vascular, intra-ocular, intracranial and subarachnoid pressure rather than being a direct result of trauma as is proposed in shaken baby syndrome.