Beaches across New Zealand have been struck with a possible pathogen which contributed to the death of millions of pipi and tuatua this year.

In March, huge numbers of dead and dying shellfish washed up on beaches on the Horowhenua coast and in the Bay of Plenty.

Waihi Beach resident Jeannette McCallum was one of many concerned by the waves of dead shellfish which washed ashore.

The Ministry for Primary Industries tested live samples of the shellfish at both locations.

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Results released to Horowhenua Chronicle this week showed the shellfish were heavily affected by a Rickettsia-like organism (RLO), but MPI was unable to confirm where the organism originated or how to prevent further infection.

MPI found RLOs in cockles, tuatua, pipi, green-lipped mussels, toheroa and scallops across New Zealand, in Northland, Auckland, Bay of Plenty, Horowhenua, Marlborough and the West Coast.

Millions of pipi died on Waitarere and Hokio beaches. Photo/Ashleigh Collis
Millions of pipi died on Waitarere and Hokio beaches. Photo/Ashleigh Collis

RLOs are not new but could be considered emerging pathogens because they were relatively unknown until recent years, according to scientific study.

So far this year there have been five reported events of mass shellfish mortalities in New Zealand, four confirmed to be linked with RLOs.

One was at Ninety Mile Beach in January.

MPI Aquatic and Environmental Health Team Manager Dr Mike Taylor said given the importance of shellfish in New Zealand, RLOs had become a research priority.

Mr Taylor said MPI had also taken samples from healthy beaches for comparison but tests had not been conclusive.

A strategic sampling scheme would be triggered whenever shellfish mortalities were found, aiming to better understand how RLOs, environment and other potential factors contribute to shellfish mortality.

Mr Taylor said MPI was unsure whether pollution from the Manawatu River had any bearing on the shellfish deaths on the Horowhenua coast but environmental factors could stress shellfish.

"Stress can leave them predisposed to infection by opportunistic pathogens [such as RLOs]," he said.