DISEASES
No significant infectious diseases, either at a population or individual level, have been reported in dugongs (Woods et al. 2008; Ladds 2009). Cetacean morbillivirus, Leptospira spp.
and Brucella spp. have not been detected in several serological surveys, suggesting exposure of the Australian dugong population has not occurred (D Blyde unpublished).Approximately 50 dugongs strand along the Qld coast annually (Meager 2016). Larger numbers strand in years of extreme weather events such as cyclones. In 2011, 238 dugongs stranded along the Qld coast, most likely the result of loss of seagrass beds caused by severe storms and flooding. Other reasons for stranding include disease, predation, vessel strikes, entanglements and non-fatal wounding or trauma following indigenous hunting; however, many are not investigated and the reason for stranding is undetermined. Necropsy findings in stranded dugongs include: severe subacute multifocal necrotising hepatitis and severe pulmonary oedema associated with Vibrio metaschikovii; cardiac fibrosis; haemopericardium, with resultant cardiac tamponade; tracheal trematodes; moderate to heavy gastric helminthiasis; bacterial pneumonia caused by mixed infection with Morganella morganii, Pasteurella multocida and Serratia marscesens; and fungal pneumonia (Meager 2016).
In a study by Owen et al. (2012), 55 necropsies were performed over a 14-yr period. The main causes of death were anthropogenic (e.g. boat strike, propeller wounds, fishing line or crab pot or net entanglement). Other causes of death included: parasitism; uterine abscess; clostridial septicaemia; toxoplasmosis; congestive heart failure from myocardial degeneration; and intestinal volvulus. Perinatal mortality was also common but generally a result of maternal abandonment or separation rather than infectious causes. A significant proportion of animals were found with non-specific signs of chronic debility without an apparent cause.
Starvation was unlikely in these cases, as they were often from areas where other animals were thriving.Of six dugongs necropsied in the Townsville area, one had bacterial septicaemia, verminous bronchopneumonia, cachexia and extensive skin and GI abscessation; another two died from acute peritonitis secondary to rupture of small intestinal abscesses (Marsh et al. 2000).
Nielsen et al. (2013) isolated bacteria from dugongs at necropsy. The main bacteria associated with pathological lesions were Aeromonas spp., Clostridium spp., Vibrio spp., Enterococcusfaecalis and Pseudomonas spp. All these organisms are normal GI inhabitants or water-borne and likely secondary opportunistic pathogens in these cases.
Woolford et al. (2015b) examined six dugongs hunted legally for human consumption. Pathological lesions were minimal and predominately caused by host reaction to trematodes within the GIT, liver and pancreas. Ascarid worm burdens were low. Hepatocellular lipofuscin and ferritin pigmentation were common and more pronounced in the livers of older animals in conjunction with periportal and bridging fibrosis. All these animals were drowned to kill them. Lesions attributable to drowning included: incomplete collapse of lungs; dorsal or diffuse pulmonary congestion; mild intra-alveolar haemorrhage and oedema; mild interstitial oedema; and rupture of peripheral alveolar septa with acute myofibre fragmentation and degeneration. No accumulation of foam or aspiration of water or particulate matter was present, suggesting that dugongs ‘dry drown’.
An adult female dugong was found dead in Moreton Bay, with a 360° mesenteric volvulus with infarction of the associated segment of small intestine and fibrinous peritonitis (Gillespie et al. 2011). The voluminous nature of the GIT of dugongs may predispose them to torsion.
6.1 Toxoplasmosis
Toxoplasmosis has been implicated in deaths of two free- ranging dugong (J Wilkie pers. comm.; Owen et al. 2012). Pathology included cachexia and multifocal hepatitis in both cases.
Other organs affected included lungs, brain, adrenal glands, lymph nodes and pancreas.6.2 Starvation
Starvation may occur when dugongs stray outside their normal range when water temperature and access to seagrass beds are inadequate. Seagrass beds may be damaged during or after floods, large tropical storms or cyclones or by reduced salinity and increased turbidity during prolonged periods of rain. This damage to seagrass beds results in starvation and may significantly affect dugong populations.
Signs of starvation include prominence of the dorsal vertebral processes and the ribs. At necropsy, fat is clear or gelatinous, most prominent ventrally, and fat around the heart may be affected in severe cases. The GIT may be empty or contains unusual material such as algae and sand. The gall bladder may be distended with bright yellow bile.
6.3 Cold stress
Cold stress syndrome (CSS) describes a range of clinical signs and acute and chronic disease processes that occur in Florida manatees exposed to water temperatures below 20°C for extended periods (Bossart et al. 2003). There is some evidence to suggest a similar syndrome occurs in dugongs exposed to similar climatic conditions (Owen et al. 2013). Of 14 carcasses submitted from Moreton Bay, 10 fulfilled the criteria for potential CSS. Histopathological findings included epidermal hyperplasia and secondary bacterial infection, serous atrophy of pericardial adipose tissue and multisystem abscessation. Water temperature data were correlated with the time of year that carcasses were submitted and this indicated that the majority of cases with evidence of CSS were seen in the colder months of the year. There have been numerous reports of dugongs south of Moreton Bay, their southernmost range on the
Fig. 47.5. Dugong (Dugong dugon) with rope entangled around the tail stock.
east coast of Australia.
Factors that drive these animals to stray outside their normal range are unknown. Some animals may head south in summer and return to their normal range in winter without ill effect. However, if they remain in southern waters during winter, water temperatures usually fall below 20°C, increasing the risk of CSS. In December 2015 a male dugong was rescued in Merimbula, NSW, well south of the normal range. The animal was thin and had extensive skin lesions over its dorsum and ventral and dorsal tail. Biopsies of the skin lesions revealed irregular hyperplasia with parakeratotic hyperkeratosis, multifocal spongiosis, erosion, ulceration and a neutrophilic exudate. Special stains confirmed a superficial overgrowth of bacteria and fungi. These histopathological changes are not aetiologically specific but are within the spectrum of changes reported in manatees with CSS. The animal was taken into care and recovered with supportive care, warmer water and antibiotics (Blyde and Mackie 2017).6.4 Marine toxins and contaminants
There have been no confirmed cases of marine toxins casing deaths of dugongs. However, given the susceptibility of manatees to marine toxins such as brevetoxin (Bossart 2001), this should be considered as a differential diagnosis in any dugong death where the cause is not identified, particularly if other species in the immediate area are also affected. Jarolimek et al. (2023) provide a comprehensive review of inorganic contaminants in Australian marine mammals, birds and turtles. High concentrations of iron and copper have been found in dugong livers and cadmium in kidneys.
6.5 Anthropogenic interactions
Boat strikes and entanglements in nets, fishing lines, ropes and crab pot lines are all common in dugongs (Woods et al. 2008; Owen et al. 2017) (Fig. 47.5). Indigenous hunting, both legal and illegal are significant causes of deaths in dugongs and may lead to local population declines, particularly in areas such as the Torres Strait (Heinsohn et al. 2004).
6.6 Non-anthropogenic interactions
Deaths in dugongs have also been attributed to stingray barbs and attacks from predators such as sharks and killer whales.
ACKNOWLEDGEMENTS
I thank Drs Janet Lanyon and Michael Walsh for their assistance in compiling this chapter, Dr Duan March for revision of the chapter and Mark Blyde for assistance with illustrations and figures.