Tests undertaken by MPI’s Animal Health Laboratory show a combination of many factors is behind the recent poor condition of the scallop populations around Great Barrier and Mercury Islands and in Pelorus Sound.
Earlier this year callers to the Ministry for Primary Industries (MPI) 24-hour Exotic Pest and Disease Hotline (0800 80 99 66) reported deformed, watery and smaller than normal scallops in three locations.
MPI Animal Health Laboratory (AHL) tested scallops collected from all three areas to determine if the decline in the beds and/or deformation of the roe and muscle was caused by an exotic disease or organism. AHL staff applied general aquatic bacteriology testing, histopathology and molecular testing to the scallop specimens and concluded the mortality and poor condition of the scallop populations are likely the result of a combination of causes.
The specimens from Pelorus Sound showed a high load of an opportunistic bacterium common in New Zealand waters. Samples from all three sites were affected by a protozoan parasite that infects molluscs and is found in other shellfish species around the North Island and the top of the South Island. There was also inflammation and some degeneration of the digestive tract in some of the scallops. This digestive tract damage is found in samples from all three sites and is the most significant finding in the investigation.
The affected scallops do not pose a food safety risk. However, as is the case with any animal, shellfish that are obviously sick and dying should not be eaten.
MPI Manager Surveillance & Incursion Investigation, Brendan Gould says MPI will continue to keep a close eye on the wellbeing of these scallop fisheries and encourages fishers and members of the public to report any concerns or abnormalities they observe.
“MPI takes the health of the scallop fisheries very seriously. We also act on information received via the Exotic Pest and Disease Hotline so I’d urge people who gather shellfish to phone 0800 80 99 66 if they notice anything unusual.”
The laboratory results
AHL scientists analysed the scallops collected using molecular (polymerase chain reaction) testing. Molecular testing provides a rapid and highly specific diagnosis of diseases. One pathogen, Perkinsus olseni, was found at all three sites. Perkinsus olseni is a protozoan parasite that infects molluscs. It is established in New Zealand having been found in other shellfish species around the North Island and the top of the South Island. Overseas, high levels of infection by P. olseni have caused significant deaths in susceptible shellfish species. The presence of the parasite can also slow the growth rate of shellfish.
No mass mortalities from P. olseni infection have been observed in New Zealand shellfish. Further, scallops are not thought to be particularly susceptible to P. olseni and this is borne out by the low prevalence of the parasite found in the samples. Importantly, the presence of P. olseni has no implications for human health.
Examination also showed inflammation and some degeneration of the digestive tubules in some of the scallops. This damage is found in samples from all three sites and is the most significant finding in the investigation. Little scientific research has been done to determine how this damage is caused but there is a suggestion that a ‘virus-like’ particle may be a contributing factor.
Regardless of the causes, because the scallop can’t feed properly, damage to the digestive tract compromises wellbeing, reducing the ability to cope with other stresses. These could be environmental (for example, changes in salinity, temperature or levels of nutrients in the water) or biological (for example, opportunistic pathogens). Even natural events with high energy requirements, such as spawning, might be enough to cause serious effects if the scallops are unable to recover because they can’t feed properly.
All the Mercury Island scallops also had cysts present, and half had a marked immune response. The immune response is likely inflammation due to the presence of these metazoan cysts which indicate the presence of a parasite. The presence of a parasite is not thought to be a significant cause of disease in healthy animals, however, in a scallop that is already under stress, an infestation could further compromise health.
Bacteriological culture did not identify anything significant in the Great Barrier or Mercury Island specimens. However, samples from Pelorus Sound showed a high bacterial load of Vibrio splendidus, a common opportunistic bacterium in New Zealand waters. Stressed animals are more susceptible to bacterial attack and, therefore, the digestive tract damage may have contributed to the degree of infection.
In this investigation there was not one casual factor or disease and, therefore, the mortality and poor condition of the scallop populations are likely the result of a combination of many natural causes.
Let MPI know if you find something unusual
To report suspected exotic disease or pests in animals, plants, fish or bees, phone the MPI Exotic Pest and Disease Hotline 0800 809966
About the Animal Health Laboratory
The MPI Animal Health Laboratory is based within MPI’s Investigation and Diagnostic Centre at the National Centre for Biosecurity and Infectious Disease at Wallaceville, Upper Hutt. It is responsible for the identification of all suspected exotic, new and emerging diseases of production, companion and aquatic animals, wildlife and introduced fauna.
For more information:
- contact the MPI media phone on 029 894 0328
- email media@mpi.govt.nz
- to report suspected exotic disease or pests, phone the MPI Exotic Pest and Disease Hotline 0800 809966
