Australian-Linked Virus Suspected in Sardine Deaths
CAPE TOWN – Scientists investigating the mysterious deaths of sardines along South Africa’s coast are increasingly focusing on pilchard herpesvirus (PHV) after testing detected high levels of the virus in affected fish and identified a viral sequence with a 99.7% match to an Australian PHV gene.
The Department of Forestry, Fisheries and the Environment (DFFE) said no new mass-mortality incidents had been reported since 16 August, suggesting the acute phase of the event may have subsided. Investigations are continuing, however, as scientists attempt to establish what caused sardines to die on a scale that environmental conditions alone have so far struggled to explain.
Approximately 75 sardines have been included in the PHV testing programme. Initial work conducted by the South African Institute for Aquatic Biodiversity and Rhodes University detected viral genetic material in 15 fish examined, with 10 sardines collected from the mortality event showing consistently high viral levels. Five fish caught alive and appearing healthy also returned varying levels of the virus.
Around 60 additional samples have subsequently been processed for PHV screening. Preliminary results from approximately half of those samples have again indicated high viral levels, according to the department. Pathological examinations involving a further 40 samples are now being conducted to determine whether the fish display the characteristic gill inflammation and damage associated with active PHV disease.
Scientists have also investigated whether harmful algae, toxins or unusual environmental conditions could explain the deaths. Paralytic shellfish toxins were not detected, while domoic acid and yessotoxin were found only at low levels. Examination of stomach contents and material taken from the fishes’ gills similarly failed to identify harmful algae at concentrations considered sufficient to explain the mortality.
Unusually cold water and low oxygen levels were recorded along parts of the West Coast, but DFFE said comparable conditions had existed during a recent pelagic recruitment survey without dead or dying sardines being observed. Low oxygen also occurs periodically along the West Coast without previously producing sardine deaths on the scale currently under investigation.
“The apparent restriction of the mortality to sardines makes a general environmental cause less likely,” the department said, although it cautioned that environmental conditions could still have contributed additional stress.
The evidence consequently increasingly points towards PHV as the leading explanation, but authorities have stopped short of declaring the virus responsible until pathological examinations and broader surveillance are completed.
Despite the detection, DFFE stressed that PHV is a fish virus with no known human-health risk and said there was no evidence that it could infect or cause disease in people. The findings also do not mean commercially caught healthy sardines or canned sardine products are unsafe to eat.
A high-level task team involving DFFE, municipalities, management authorities, the fishing industry and other stakeholders has meanwhile agreed on a coordinated response covering expanded surveillance, testing, disposal of dead fish, public-health communication and an assessment of the possible impact on South Africa’s sardine resource.
Members of the public are being advised not to collect or consume dead or dying sardines, or feed them to pets, while scientists continue working towards a definitive explanation for the unusual deaths.
Anyone with additional information, photographs or observations relating to unusual sardine deaths along the South African coastline can contact TownPress at press@townpress.co.za.