Bonamia in Ostrea angasi: Diagnostic performance, field prevalence and intensity

Abstract

Bonamia spp. parasites threaten flat oyster (Ostrea spp.) farming worldwide. Understanding test performance is important for designing surveillance and interpreting diagnostic results. Following a pilot survey which found low Bonamia sp. intensity in farmed Ostrea angasi, we tested further oysters (n = 100–150) from each of three farms for Bonamia sp. using heart smear, histology and qPCR. We used a Bayesian Latent Class Model to assess diagnostic sensitivity (DSe) and specificity (DSp) of these tests individually or in combination, and to assess prevalence. Histology was the best individual test (DSe 0.76, DSp 0.93) compared to quantitative polymerase chain reaction (qPCR) (DSe 0.69, DSp 0.93) and heart smear (DSe 0.61, DSp 0.60). Histology combined with qPCR and defining a positive from either test as an infected case maximized test performance (DSe 0.91, DSp 0.88). Prevalence was higher at two farms in a high‐density oyster growing region than at a farm cultivating oysters at lower density. Parasite intensities were lower than in New Zealand and European studies, and this is probably contributed to differences in the performance of test when compared to other studies. Understanding diagnostic test performance in different populations can support the development of improved Bonamia surveillance programs.

Infection dynamics of Bonamia exitiosa on intertidal Ostrea angasi farms

“…Fourteen‐month‐old (7–10 mm) O. angasi that had not been used for any experiments were sourced from the South Australian Research and Development Institute (SARDI) Aquatic Sciences Centre (SAASC) Mollusc Hatchery (West Beach, Adelaide, South Australia). Testing by real‐time PCR (Corbeil et al, ) ( n = 150) and histology ( n = 150) did not detect Bonamia (mean Bayesian estimated prevalence, credible intervals: 0.017, 0.000–0.05; Buss et al, 2019b).…”

“…Heart smears were used to determine the prevalence and infection intensities of B. exitiosa in O. angasi . Within 24 hr of sampling, heart smears from sample oysters were made on slides, dried, fixed and stained as per Buss et al (2019b). Heart smears were viewed under a compound light microscope (Brightfield Olympus BX53), and B. exitiosa infection intensities were graded using a semi‐quantitative score following Buss et al (2019b).…”

“…Within 24 hr of sampling, heart smears from sample oysters were made on slides, dried, fixed and stained as per Buss et al (2019b). Heart smears were viewed under a compound light microscope (Brightfield Olympus BX53), and B. exitiosa infection intensities were graded using a semi‐quantitative score following Buss et al (2019b). Parasitology terminology used for B. exitiosa prevalence and infection intensity is consistent with Bush, Lafferty, Lotz, and Shostak ().…”

“…This model used uninformative beta (1, 1) priors for prevalence estimates. Priors for DSe (diagnostic sensitivity) and DSp (diagnostic specificity) of heart smear were informed by outputs from the analysis of Buss et al (2019b). These beta priors reflected 95% confidence that heart smear DSe and DSp fall within the credible interval with the mean specified in Table .…”

“…Large farmed oyster populations may have facilitated the proliferation of B . exitiosa in SA, leading to the high prevalence observed by Buss, Wiltshire, Prowse, Harris, and Deveney (2019b). Although the origin of SA B .…”