Its effectiveness during the past summer has puzzled the industry because the Otago Regional Council's two-yearly tests for the disease's presence in Otago show high levels of immunity to the virus.
Test results show the average immunity across the 10 sites was 68%, similar to 2009 levels, council regional services technical field adviser Don Robson says in a report to be presented to its regulatory committee tomorrow.
"Interpreting this year's serum results has been difficult, with most monitoring sites showing high immunity levels, yet the virus has been the most effective and widespread for many years."
The virus displayed its greatest activity in the Alexandra basin, Ida Valley, Upper Clutha and Strath Taieri.
"In fact, most of Otago appears to have received the benefits of good kills from the virus."
As well as good death rates, observations showed deaths due to the virus continued to occur longer than normal, which meant the antibodies were still present during testing - unlike in other years.
"This observation clearly indicates high immunity levels do not necessarily mean the virus is no longer effective.""Exceptional" conditions in Central Otago had seen grass growth throughout most of summer because of frequent rain, which meant more young rabbits survived, he said. It also meant more flies, which were thought to be an important vector of the virus.
"The unusually large number of young being born during mid-to-late summer this year, plus the long duration of the virus' presence, may mean that the high number of young rabbits now presented with long-term immunity."
This meant the increased effect of the virus this season might be offset by the breeding season which was "well beyond a normal year", he said.
"The need for secondary control remains."
He concluded that during an Otago season, the RHD virus was "highly unlikely" to cause significant population reduction.
The data supported the council's position that the virus provided the greatest benefits to properties with low to moderate rabbit levels - whether due to the land's inherent rabbit proneness or because of the amount of rabbit control carried out, Mr Robson said.
He recommended council continue with its biennial RHD serum testing, because the data would help future researchers better understand its impact, or lack of impact, in the field.
In separate inspections, the council found landowners breaching allowed rabbit-number limits in Arrowtown, Wanaka, Roxburgh and Cromwell, and had requested management plans from those owners, environmental services manager Martin King said.
There were eight aerial 1080 operations confirmed for this winter, mostly in Central Otago.
Good grass growth meant operations in the Strath Taieri and Dunback had to be put off.
