Investigation of the risk of Toxoplasma gondii to the establishment of the ‘extinct in the wild’ eastern barred bandicoot (Perameles gunnii) on Phillip Island.

Abstract

The eastern barred bandicoot, Perameles gunnii, is currently considered extinct in the wild, and is extant only as intensively managed reintroduced populations. Phillip Island, in Westernport Bay, Victoria, has been proposed as a potential reintroduction site for this species. Feral cats, Felis catus, the definitive host for the coccidian parasite Toxoplasma gondii, are present on the island, and eastern barred bandicoots are known to be susceptible to toxoplasmosis. The aim of this study was to investigate the epidemiology of T. gondii on Phillip Island, and the potential risk to the establishment of eastern barred bandicoot populations.

The prevalence of T. gondii in the feral cat population on Phillip Island was determined using real time PCR (qPCR), and seroprevalence was determined using the modified agglutination test (MAT). A total of 161 feral cats was sampled between June 2016 and November 2017. Overall prevalence by qPCR was 79.5 % (95 % confidence interval [95% CI] 72.6 - 85.0), and overall seroprevalence was 91.8 % (95% CI 84.6 – 95.8). Additionally, the diagnostic sensitivity and specificity of both testing methods, and the true prevalence of T. gondii in feral cats on Phillip Island, were evaluated using Bayesian modelling. The sensitivity and specificity of the MAT in cats were estimated as 96.1 % (95 % credible interval [95% CrI] 91.5 – 98.8) and 82.0 % (95% CrI 65.1 – 93.3), respectively. The sensitivity and specificity of the qPCR method in cats were estimated as 90.1 % (95% CrI 83.5 – 95.6) and 96.2 % (95% CrI 82.2 – 99.8), respectively. The true prevalence of T. gondii infection in feral cats on Phillip Island was estimated as 90.2 % (95% CrI 83.1 – 95.2).

Environmental contamination with T. gondii oocysts was assessed. Soil was collected from two sites on the island, Summerland Peninsula and Cape Woolamai, in September and October 2017. A total of 412 soil samples, from 206 sites, was tested. Soil samples were processed to isolate any protozoan organisms present and qPCR was conducted to specifically detect T. gondii DNA. Toxoplasma gondii oocysts were not detected by qPCR in any of the soil samples collected. Additionally, European rabbits, Oryctolagus cuniculus, were used as an indicator species to predict environmental exposure in eastern barred bandicoots. A total of 134 feral rabbits was sampled from August 2016 to November 2017. Overall prevalence of T. gondii in feral rabbits, as determined by qPCR on tissue, was 10.5 % (95% CI 6.3 - 16.8). Toxoplasma gondii infection was not identified in any rabbits collected from the proposed release site for eastern barred bandicoots, the Summerland Peninsula.

Sixty-seven eastern barred bandicoots were released onto the Summerland Peninsula in October and November 2017 as part of an assisted colonisation trial. Sixty-one of these bandicoots were tested for prior exposure to T. gondii using the MAT. No animals were seropositive for T. gondii prior to release on to Phillip Island, and the population could be concluded to be free from disease, at a design prevalence of 6 %. Bandicoots were trapped for blood sample collection in November 2017 after approximately three weeks of habitation on the island (n = 23), and again in February 2018 after approximately four months (n = 21). No bandicoots demonstrated seroconversion to T. gondii on the MAT in either November or February. In November, this was sufficient to conclude that the population was free from infection with T. gondii, at a design prevalence of 6 %. However, in February, the sample size was too small to make this conclusion. Mortality data on the bandicoots was limited, due to difficulties associated with placing radio-transmitters on this species. However, one deceased animal was recovered. This animal had no evidence of T. gondii infection based on qPCR on tissues.

These findings suggest that while prevalence of T. gondii infection is very high in feral cats on Phillip Island, other factors, such as low cat density at the site, may translate to a low toxoplasmosis risk for eastern barred bandicoots released onto the Summerland Peninsula. Further studies are required to understand the factors contributing to the observed low level of environmental contamination with oocysts. Additionally, future studies should focus on increasing the sensitivity of mortality surveillance in the eastern barred bandicoot population on the Summerland Peninsula. This will enable the collection of more robust mortality data, further elucidating the importance of toxoplasmosis in this population.