Toxoplasmosis and Dolphins: One Health in Action

New Zealand is home to the world’s smallest dolphin species, the Hector’s Dolphin ( Cephalorhynchus hectori hectori) and its sub species, the Maui dolphin (C.h. maui), both of which measure 1.2 - 1.6 meters long and weigh up to 60Kg. The species are endemic to the New Zealand coastline and in the case of the Hectors dolphin, the population may exceed 15,000 individuals. They populate a number of South Island habitats, most notably Banks Peninsula. The situation with the Maui subspecies is more serious, with population estimates of significantly less than 100: this critically endangered subspecies is confined to the western coast of the North Island.

Recent research has indicated that a worrying cause of death for Maui dolphins is Toxoplasmosis. The research was reported at the One Health Aotearoa Conference in Wellington late in 2024 and provides a great example of One Health in action, the ever-present interaction between animals, humans and the environment.

Toxoplasmosis gondii is a common parasite which infects a wide range of species including humans. It is a common cause of abortion in sheep and vaccination is commonly undertaken in New Zealand but the vaccine is delicate and must be carefully handled to be fully effective [1]. The life cycle is a typical two stage process, with the sexual stage, which gives rise to the infective oocysts, only occurring in cats. Once excreted into the environment, the oocysts remain viable for many months. But initially, it seems incredible that oocysts excreted by cats on land can be such a significant risk to marine species, such as dolphins. The evidence, to the contrary however, is compelling.

In a series of postmortems on Hectors and Maui dolphins reported in 2013 [2], Toxoplasmosis gondii was the most important infectious cause of death, with 7/49 cases being infected. Histology of these cases showed the mammals to be systemically infected with the disease detected in numerous tissues including lungs and lymph nodes. Internationally other marine mammals, including sea otters and seals, have been shown to be infected with Toxoplasmosis; a case has also been reported in a New Zealand Sea lion.

How do the dolphins get infected?

New Zealand has a significant cat population with an estimated 1.2 million domestic cats and a large population of feral cats which could be as high as 2.4 million [3]; the latter are difficult to count but they present a serious risk to New Zealand native species, particularly birds. The seroprevalence of “Toxo” in cats has been reported as 61%, with no regional differences detected: the parasite is widespread [4]. Of course, New Zealand had no native land mammals until the arrival of the human species; it is thought that the first felines arrived with Captain Cook on the Endeavor, so the exposure of marine mammals to this parasite has been relatively recent.

Research has shown that Toxoplasmosis oocysts can survive for 2 years in salt water and they are highly infectious; a single oocyst can cause disease in rodents. Screening for the presence of oocysts can be compromised by dilution, but the use of natural filter feeders’ such as mussels, which concentrate the parasite, have improved detection rates and proved that oocysts are entering the marine environment from land. There is also some evidence that dolphins may become immunosuppressed, possibly during pregnancy, making them more susceptible to infection.

To complete the life cycle, feline faeces must be getting into freshwater sources. Control programs to date have focused on advising owners to keep cats indoors and avoid the flushing of cat litter down toilets. The spores are resistant to waste water standard treatment protocols. A national program, “Predator Free NZ” [5], has seen increased trapping of feral cats but a large population will take many years to control. And as noted previously, the disease is not confined to dolphins and continues to be a major ongoing cost to agriculture.

While the fate of the declining population of Maui dolphins hangs in the balance, the complex interaction of human behaviour (keeping pet cats), animal disease diagnosis and environmental contamination provide a fascinating One Health story, which once again demonstrates the role veterinarians can play in One Health issues.