Calicivirus keeping a lid on Otago rabbit numbers

The rabbit killing virus illegally imported by farmers nearly five years ago has successfully stemmed Otago's rabbit plague.

Monitoring of the performance of rabbit haemorrhagic disease (RHD) in Otago shows the lid is being kept on pest numbers.

Before 1997, council-owned company Regional Services spread between 3500 and 5500 tonnes of 1080 poison-laced carrots a year. Last year, it spread 100 tonnes and this year, after a successful rabbit breeding season, it has ordered 400 tonnes.

Numbers overall were lower than before RHD was released in the winter of 1997, but differ depending on the amount of secondary control work - shooting, fumigating, ripping burrows and poisoning - done by landowners.

The council's technical services manager, Darryl Lew, said the latest review of data collected since 1997 sent farmers the same messages as earlier reviews.

The best results were evident when the virus arrived naturally, rather than introduced through bait or infected carcasses, he said.

Secondary rabbit control was also vital to kill survivors which had been exposed to the virus but survived and could have developed immunity.

Farmers relying on the virus to keep their rabbit populations under control were seeing a resurgence. Some farmers said this summer had been the best breeding season in their lifetime.



Otago Regional Council chief executive Graeme Martin said that when farmers saw dead rabbits or more hawk activity indicating a virus wave, it was a sign for them to get out and mop up survivors.

Immunity to the virus varied between properties and how it had been introduced, but it was at acceptable levels, he said.

Immunity could be the result of rabbits being exposed to non-lethal doses of virus or the virus resurfacing when the young received antibodies from their mothers.

However, the immunity disappeared at weaning.

The author of the independent review of council data, Roger Lough, warned that the practice of biociding, or artificially introducing the virus into an area, should at best cease or at worst be done carefully and seldom.

Little was known about how the virus was spread but one theory was that it surfaced during long dry periods and was spread by flies attracted to moist parts of the rabbit, such as the eyes, nose or the anal area.

The flies picked it up either directly from infected rabbits or from infected residue in the dung.

This could have implications for stock management if a farmer wanted to encourage the virus.

The council intended doing more work on this theory and had spoken to AgResearch. However, the wet summer would prevent any work this season.

Another observation was that rabbits were more "flighty" since the arrival of RHD.

Ministry of Agriculture and Forestry enforcement unit manager Jockey Jensen said MAF had been unable to determine who had imported the rabbit killing disease or how they did it. The time limit for laying charges had lapsed, prompting the decision to end the investigation.

A 1997 application to MAF by several South Island local authorities and farmer groups to import RHD was declined, but in August of that year the discovery of thousands of dead rabbits on a Cromwell farm revealed the rabbit-killing virus had been illegally imported and released.

A story of secret deals and midnight meetings in the South Island was later revealed - with a group of Mackenzie Basin farmers admitting spreading the virus.

But just who imported it and how it got here remains a mystery.

Mr Jensen said he still hoped someone would reveal how they smuggled the virus through New Zealand's biosecurity borders so MAF could ensure that loophole was closed.

- NZPA

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