Australian Fur Seal (Arctocephalus pusillus doriferus) with a shark bite wound targeted because of illness?

We would like to thank ORRCA for their continued vigilance in observing and assisting marine mammals on the NSW coastline and for often being the first line of enquiry for the public regarding these animals.  We would also like to thank the NSW National Parks and Wildlife Service, the RSPCA, and veterinarians, who are often called upon to make the difficult decisions regarding injured marine life.  Thanks also to the NSW Office of Environment and Heritage for their continued support so that we can all continue to work together for the management and care of these species.

HISTORY

The adult, male Australian Fur Seal was found to be repeatedly hauled out around Shelly Beach, Cronulla, NSW since 8th February 2015.  The animal had a severe semicircular injury along the right rump region, which appeared to be infected.  The seal was hardly using its rear flippers, but would re-enter the water and appeared to be catching some fish.  A veterinarian assessed photos of the animal and advised that euthanasia was warranted due to the severity of the wounds and low likelihood of rehabilitation and release.

Image: Emaciated adult, male fur seal hauled out on rocks showing extensive injuries to rear (Image by Leesa Pratt, ORRCA)

Attempts were made to catch the animal, but it repeatedly fled to the water.  RSPCA inspectors attended to the animal and agreed with the veterinary assessment.  A highly trained and experienced RSPCA marksman was enlisted to shoot the animal.  Euthanasia was carried out 16 February 2015.

The carcase was transported to the Registry for post mortem examination.

GROSS EXAMINATION

External examination:  There is marked crepitus of the entire skull composing the calvarium.  A single spot of blood is present on the pelage caudal to the left eye.  A cataract is visible in the lens of the left eye.

There is a 22 cm long x 18 cm diameter, semicircular, full thickness, foul smelling defect in the skin over the right rump, extending from the stifle to the tarsal joints.  The margins of the wound are markedly jagged, but sharp, and there is scant evidence of contraction.  The wound bed is partially coated with tan to green purulent exudate and very early granulation tissue.  The wound extends down to, and exposes, the lateral aspect of the tarsal joints.  A semicircular pattern of linear excoriations (presumed shark bite wounds) are present along the left hip and thigh region.  The diameter of these semicircular areas range from 18 – 29 cm, with 2-2.5 cm distances between linear excoriations.  None of the linear excoriations penetrate the dermis.

An 8 cm diameter full thickness defect is present on the dorsal surface of the right rear flipper.  The wound margins are sharp and the wound bed is markedly erythematous.  The wound extends down to the underlying metatarsal joint.

There is a linear defect of the fifth left rear digit extending 13 cm long x 2 cm wide and exposing P1 and P2, with an open luxation of the P1-P2 joint.

Image:  Extensive purulent wound to rump with jagged skin margins. Semi-circular bite marks can be seen cranially over the back of the animal – suspect shark bites (Image by Leesa Pratt, ORRCA).

The teeth are heavily stained and worn.  The incisors are worn to the gum line.  The canine teeth are all in excellent condition, with intact apexes.  Several teeth are missing.  There are only 4 premolars on the lower right and 5 on the left.  Along the maxilla there are 4 premolars, a gap with missing or fractured teeth and then a caudal molar.

A large, hard granuloma is present along the mandibular symphysis, immediately behind the incisor teeth.  A fistula enters the granuloma just caudal to the middle incisors.  Laxity and crepitus is palpable along the mandibular symphysis.  Upon sectioning through the lesion, it is composed of a thin (2-3 mm) connective tissue capsule, which is lined by soft black material, surrounding a central core of necrotic bone.  The necrotic bone has a discoid shape, bearing stellate projections from the surface.  The lesion completely replaces the mandibular symphysis.

Image: Large, necrotic, granulomatous lesion replacing the mandibular symphysis making the jaw unstable (Image: Australian Registry of Wildlife Health)

Hydration: fair

Muscle mass: reduced/emaciated

Fat deposits: absent, <1 mm blubber depth and no visible mesenteric, peri-renal or pericardial fat deposits

Internal examination:  A single 1.1 cm diameter hole is present in the skin and underlying frontal bone immediately caudal to the orbit of the left eye (at the peak of the zygomatic arch).  The entire calvarium dorsally, laterally and ventrally is shattered and the brain has been pulverised and filled with large blood clots.

The cervical and axillary lymph nodes are markedly enlarged and firm.  The cervical lymph nodes measure 7.5 cm x 4.5 cm x 2.5 cm and the axillary lymph nodes measure 9 cm x 5 cm x 2 cm.  Other lymph nodes throughout the body (bronchial, mesenteric, colic, and inguinal) appear normal.

Several 4- 9 mm diameter cysts are present within the parenchyma of the thyroid glands.

The pulmonary parenchyma is mildly congested.  A small volume of red fluid is present in the trachea and bronchi.

The oesophagus and stomach contain one freshly ingested fish, numerous sets of fish skeletons, and numerous ascarid parasites.  Liquid ingesta is present along the small intestine, along with numerous

cestode parasites (20 mL volume of large, long, flat cestodes).  The large intestine contains a small volume of faeces.

Several 1 cm diameter tan structures are scattered throughout the ventral subcutaneous tissues.  Upon incising these lesions, a small, white plerocercoid emerges.

MICROBIOLOGY

Anaerobic culture:  Axillary Lymph Node – Moderate numbers of mixed anaerobes.

Aerobic culture:  Mandibular symphysis – Very heavy growth of Klebsiella pneumoniae, and heavy growth of Proteus mirabilis

Anaerobic culture:  Mandibular symphysis – Heavy growth of mixed anaerobes

MORPHOLOGIC MEASUREMENTS

Weight:                                                            145.2 kg

Sternal Blubber Depth:                                1 mm

Girth at Neck:                                                 0.94 m

Girth at Pectoral Flippers:                            1.3  m

Girth at Pelvis:                                                0.83 m

Pectoral Flipper Length:                               0.61 m

Pelvic Flipper Length:                                   0.44 m

Snout to Anus Length:                                  2.07 m

Snout to Base of Ear Length:                       0.265 m

Snout to Genital Aperture Length:              1.85 m

Snout to Origin of Flipper Length:               0.93 m

Standard Length:                                           2.4 m

DIAGNOSIS

Euthanasia – shot

Intermandibular osteomyelitis with sequestrum – focal, severe, chronic

Lymphadenomegaly – cervical and axillary

Emaciation – diffuse, severe, chronic

Thyroid cysts – multifocal, mild, chronic

Cataract – unilateral, moderate

Gastric nematodiasis – diffuse, moderate, chronic – presumed Anisakidae

Intestinal cestodiasis – segmental, moderate, chronic

Subcutaneous cestodiasis

COMMENT

The fur seal was euthansed by gun-shot due to its poor body condition and evidence of a debilitating injury.  The shape and nature of the wound along the right lateral leg was highly consistent with a shark bite.  Several semicircular patterns of excoriation along the left hip and flank were consistent with the impressions of the other jaw of the shark.  The wounds were consistent in age with the history provided, as they appeared to be of approximately two weeks duration or less.

The seal was found to be suffering from a chronic underlying infection in the bone at the point where the mandibles join.  This lesion is of many weeks duration and seems to coincide with the expected time-frame required for the animal to emaciate to such a degree and to develop such large, firm lymph nodes.  The cervical and axillary lymph nodes drain the area of the mandibular infection.  The mandibular infection did not appear to be associated with an abscess of a canine tooth root, but may have been associated with root abscess of an incisor, or from a penetrating wound.  Blood supply to a bony sequestrum is incredibly poor and therapeutic recovery rates are very poor.

The seal was severely emaciated, weighing only 145.2 kg, where a bull Australian Fur-seal would be expected to weigh 220 – 350kg.  Parasites described during post mortem examination are considered normal and incidental in an adult seal.  It seems that the shark was attempting to predate a thin and weak animal.

The process of euthanasia was conducted in an expert and commendable manner with complete and instant pulverisation of the calvarium and brain.  The cataract in the left eye may have been peracute, associated with euthanasia.

To summarise, while shark predation and euthanasia were the proximate causes of the animal’s death, chronic infection of the mandible was the most likely ultimate cause of death and treatment options for such an infection in a wild seal would not be considered possible/effective.