The sardine mortality investigation is narrowing, with the Department of Forestry, Fisheries and the Environment (DFFE) saying evidence increasingly points to pilchard herpesvirus (PHV) as the leading explanation for the deaths along parts of South Africa’s coast.
No new mass-mortality incidents have been reported since Sunday, August 16, suggesting the acute phase may have eased.
Monitoring and testing continue as scientists work to establish whether PHV caused the deaths or if environmental conditions helped trigger or worsen the event.
PHV emerges as leading cause of sardine deaths
Pilchard herpesvirus (PHV) is a virus that infects sardines, also known as pilchards.
It belongs to the herpesvirus family of viruses, which are known for establishing infections that can persist in their hosts.
PHV has been linked to major sardine mortality events in Australia and New Zealand. However, the virus can also be found in apparently healthy fish, meaning its detection alone does not prove that it caused the current deaths.
A study on PHV and it’s associated disease by Murdoch University documents how the virus was identified during large-scale pilchard mortality events in the 1990s and detected in fish before, during and after a mortality event.
DFFE is now investigating whether PHV caused the recent sardine deaths in South Africa, or whether environmental conditions contributed to the mortality.
About 75 sardines have entered the PHV testing programme.
Initial testing by the South African Institute for Aquatic Biodiversity and Rhodes University detected PHV genetic material in all 15 fish examined.
Ten fish collected during the mortality event showed consistently high viral levels. Five apparently healthy fish caught alive also tested positive, although their viral levels varied.
The viral sequence showed a 99.7% match to an Australian PHV gene.
DFFE has processed about 60 additional samples. Preliminary results from roughly half indicate high PHV levels, while confirmatory testing continues.
Scientists are also conducting pathological examinations on another 40 samples. These tests will look for the characteristic gill inflammation and tissue damage associated with active PHV disease.
DFFE has stressed that the South African investigation has not yet established PHV as the definitive cause.
“Taken together, the evidence increasingly points to PHV as the leading explanation,” the department said.
It added that the virus’s causal role will only be confirmed once the pathological and wider surveillance results are available.
Harmful algae and low oxygen also tested
Scientists have also investigated whether harmful algae and low-oxygen conditions contributed to the sardine mortality.
Tests found no paralytic shellfish toxins. Domoic acid and yessotoxin occurred only at low levels, with the highest concentrations found in an apparently healthy comparison sample.
Examinations of stomach contents and material rinsed from the gills also found no harmful algae at densities likely to have caused the deaths.
Researchers recorded unusually cold water and low oxygen along parts of the West Coast. However, similar conditions occurred during an earlier pelagic recruitment survey, when scientists found no dead or dying sardines.
That makes a general environmental cause less likely. DFFE has not ruled out environmental conditions as contributing stressors.
Is PHV dangerous to people?
The department said PHV poses no known risk to human health. There is no evidence that the virus infects or causes disease in people.
Finding PHV in sardines also does not automatically mean that healthy commercially caught sardines or canned sardine products are unsafe to eat.
The public should nevertheless not collect, eat or feed dead or dying sardines to pets while the investigation continues.
Sardine investigation task team steps up response
DFFE has established a high-level task team involving the department, affected municipalities, management authorities, the fishing industry and other stakeholders.
The group has agreed on a coordinated response covering expanded surveillance and testing, guidance on disposing of dead fish, public-health communication and an assessment of the impact on the sardine resource.
No new mass-mortality events have been reported since August 16, but monitoring remains under way.
DFFE will provide further updates as confirmed laboratory and pathology results become available.
