The rabbit-killing virus RHD has this year in Otago taken its biggest toll on the animal in at least five years.
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.
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