WESTERN CAPE – A fish virus is increasingly emerging as the leading explanation for the mass sardine deaths reported along the West Coast, as investigations by the Department of Forestry, Fisheries and the Environment (DFFE) point to consistently high levels of pilchard herpesvirus (PHV) in affected fish.
No new mass-mortality incidents have been reported since Sunday 16 August, suggesting the acute phase of the event may have eased. Monitoring and testing are continuing.
DFFE spokesperson Zolile Nqayi said the evidence increasingly points to PHV as the leading explanation, but its causal role will only be confirmed once the pathological and wider surveillance results are available.
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More samples screened
“Approximately 75 sardines have so far been included in 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,” Nqayi said.
“Ten of those fish, collected during the mortality event, had consistently high viral levels. Viral levels varied among five fish that had been caught alive and appeared healthy when collected. The viral sequence was found to be a 99,7% match to an Australian PHV gene.”
DFFE has since processed about 60 additional samples for PHV screening. Preliminary results from roughly half of these indicate high PHV levels.
Confirmatory testing is continuing, while pathological examinations of another 40 samples are under way.
Researchers are looking for the characteristic gill inflammation and damage associated with active PHV disease.
Other causes investigated
Investigators have also been examining whether environmental conditions or harmful algae could have triggered the mortality. So far, there is no evidence that harmful algae were the primary cause.
Paralytic shellfish toxins were not detected, while domoic acid and yessotoxin were found only at low levels. The highest concentrations were recorded in an apparently healthy comparison sample.
Examinations of stomach contents and material rinsed from the sardines’ gills also found no harmful algae at densities considered likely to have caused the deaths.
Unusually cold water and low-oxygen conditions were recorded along parts of the West Coast. However, DFFE said these conditions were broadly similar to those recorded during a recent pelagic recruitment survey, before the mortality event, when no dead or dying sardines were observed.
Low oxygen also occurs periodically along the West Coast without previously causing sardine mortality on this scale.
The fact that the mortality appears to have been restricted to sardines makes a general environmental cause less likely, although DFFE said environmental conditions cannot yet be ruled out as contributing stressors.
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No known human health risk
DFFE stressed that PHV is a fish virus and poses no known risk to human health.
“There is no evidence that it can infect or cause disease in people,” Nqayi said.
The detection of PHV does not, on its own, mean that healthy sardines caught commercially or canned sardine products are unsafe to eat.
Nevertheless, members of the public are being urged not to collect, consume or feed dead or dying sardines to pets.
The plan includes expanded surveillance and testing, clearer guidance on the disposal of dead fish, public-health communication and an assessment of the possible impact on the sardine resource.
DFFE said it would issue further updates as confirmed results become available.
For now, the department says the investigation remains ongoing, but the accumulating laboratory evidence is putting PHV increasingly at the centre of the mystery behind the West Coast sardine deaths.
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