David J. Blyde1
Australia and New Zealand have large coastlines with relatively small human populations which have allowed the marine environment in these countries to remain relatively pristine and free of contaminants.
In the last few decades, populations, particularly along the coast, have increased dramatically, bringing an increased risk of infectious diseases entering the marine environment. In the same time period industries such as mining and oil and gas have prospered leading to contamination of the marine environment. Global warming may also be playing a part in the emergence of infectious diseases in Australian marine mammals as it may alter the environment enough to make animals more susceptible to contracting these diseases.
Toxoplasmosis was first diagnosed in marine animals in Australia in 2000 when an Indo-Pacific humpback dolphin (Sousa chinensis) was found to have died from the disease.1 Since this time it has been diagnosed as the cause of death in other cetacean species and a dugong (Dugong dugon). Serological evidence of toxoplasmosis has been found in a range of cetacean species, including some pelagic species.
In addition, Toxoplasmosis gondii is recognised as a major mortality factor in Hector's dolphins (Cephalorhynchus hectori) in New Zealand.2
Recently, an outbreak of disease, possibly caused by Toxoplasma gondii, occurred in a marine park in Australia. Dolphins exhibited non-specific clinical signs including lethargy and decreased appetite. Serological evidence of exposure to toxoplasmosis was observed and response to treatment supported the diagnosis.
Despite ongoing investigations, the exact epidemiology of the disease has yet to be determined. Possible sources for the pathogen in marine animals include introduced infected bait fish and marine molluscs.
The author would like to thank Dr Michelle Dennis and Dr Brett Stone from Queensland Medical Laboratories.
1. Bowater RO, Norton J, Johnson S, Hill B, O'Donoghue P, Prior H. Toxoplasmosis in Indo-Pacific humpbacked dolphins (Sousa chinensis), from Queensland. Aust Vet J. 2003;81(10):627–632.
2. Roe WD, Howe L, Baker EJ, Burrows L, Hunter SA. An atypical genotype of Toxoplasma gondii as a cause of mortality in Hector's dolphins (Cephalorhynchus hectori). Vet Parasitol. 2013;192(1–3):67–74.