Comparison of diagnostic techniques for Bonamia exitiosus from flat oysters Ostrea chilensis in New Zealand

Two traditional diagnostic methods, namely ventricular heart imprints and histology, and two molecular-based methods, namely in situ hybridisation (ISH) and PCR amplification of a 304 bp region of 18S rDNA, were compared for detecting Bonamia exitiosus infections in a sample of 28 New Zealand flat oysters (Ostrea chilensis). The apparent prevalence of B. exitiosus determined by pooling the results of the two traditional techniques, was 60.7%, while the apparent prevalence obtained using the pooled results of the two molecular techniques was 96.4%. ISH was the most sensitive method examined, followed by PCR, heart imprints and histology. The results suggest that ISH is useful for screening smaller numbers of oysters for B. exitiosus in circumstances where high sensitivity is required and strict control over fixation and processing is possible. Heart imprints appear the most time and cost effective method for screening large numbers of oysters for B. exitiosus in situations where reduced sensitivity may be tolerated, but high specificity is required. Histology appears most useful in epidemiological studies where detection of physiological state, other disease agents or pathological lesions is required. The need for more extensive field studies, and optimisation of PCR for specific detection of B. exitiosus under New Zealand conditions is discussed. (C) 2003 Elsevier Science B.V. All rights reserved.