Toxoplasmosis is an infectious disease caused by parasite, Toxoplasma gondii. It is an important cause of disease in a wide range of species, including humans, domestic animals and wildlife species.
Cats are the only known source of the infectious stage (the oocyst) of the parasite, and can shed millions of these microscopic cysts in their faeces. Oocysts are extremely resistant in the environment, and infect a new host when they are accidentally swallowed in food or water that has been contaminated by cat faeces.
After they’ve been ingested, oocysts can spread throughout the body in the blood stream, causing clinical disease, or encyst and become latent in body tissues. As well as accidental ingestion, hosts can also be infected by eating encysted Toxoplasma organisms in tissues of prey or food items, or by transfer across the placenta to a fetus when a pregnant host becomes infected.
In humans, disease can be either mild and flu-like, or more severe and debilitating, especially in newborn children and in patients who are immunosuppressed. Some recent studies have also suggested a link between Toxoplasma infection and psychiatric disorders.
For many years toxoplasmosis was considered to be mainly a terrestrial disease, but more recently increasing numbers of water-borne infections have been recognised. Human infections have occurred overseas following contamination of water sources with faeces from wild large cats, and in several countries coastal marine mammals such as sea otters have been found to be infected.
Our research group is contracted by the Department of Conservation to investigate deaths in several marine mammal species, and between 2007 and 2016 we diagnosed fatal toxoplasmosis in eight endangered New Zealand Hector’s dolphins, including two critically endangered Maui dolphins. All of these infections were caused by a specific strain (genotype) of Toxoplasma, which appears to be identical to a genotype we have found in wild birds.
We believe that oocysts reach the New Zealand coast in freshwater runoff contaminated by cat faeces. The genotype we have found in dolphins and birds could be either extremely common in this country, or cause particularly severe infections in these species. To investigate this disease further we are conducting a series of studies looking at the frequency of infection and the genotypes present in domestic and feral cats in different parts of New Zealand, and possible transmission pathways for oocyst contamination of waterways. We are particularly interested in identifying risk factors for contamination of the marine environment, including high volumes of rainfall and proximity to urban settlements or large cat populations.
Toxoplasmosis isn’t the only infectious disease that we’ve found in Hector’s dolphins. One of the biggest impacts on the species is believed to be accidental capture in fishing nets, but our work at Massey shows that disease is an additional factor contributing to Hector’s dolphin deaths.
In one of our studies, 13/49 (27%) of the dolphins examined had died due to an infectious disease, either bacterial, fungal or parasitic. We’ve made similar findings in investigations of New Zealand sea lion deaths.
While it appears as though infectious disease is on the increase in our marine mammals, it is difficult to be absolutely sure about this, since our ability to diagnose these infections has also improved over the years.
Our findings show how important it is to investigate infectious disease in wildlife species, including going back to re-examine stored samples using newer techniques. As we continue to do this we will begin to build a better picture of the impacts we are having on our natural world, and the effects these changes will have on our own health and that of our environment.
Dr Wendi Roe is a veterinary pathologist and marine mammal researcher based at Wildbase, in the Veterinary School at Massey University, Palmerston North.
She began her veterinary career with 10 years in “normal” practice, working with cats, dogs, and farm species, before going back to university in 2000 to train in pathology and work with marine mammals. Current research carried out by Wendi and her group focusses on causes of disease and death in endangered New Zealand marine mammals.
Find out more about toxoplasmosis in a blog by Siouxsie Wiles.
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