RHD - the solution to our rampant rabbit woes?
The virus RHD has been released into New Zealand for the second time. Pat Deavoll looks at how it might curb our rabbit problem.
Last week the rabbit haemorrhagic disease (RHD) was re-released into the countryside to curb an ever-burgeoning wild rabbit population.
The previous release in 1997 was the stuff of movies - a bunch of renegade farmers desperately taking the law into their own hands and using cloak-and-dagger measures to spread the virus throughout the high country. But although the release was successful to start with and millions of rabbits died, 20 years afterwards its effect is waning. Farmers are looking again for an answer to the pest that destroys their pasture and erodes their land and profitability.
It is understandable that last week’s release was welcomed with relief primarily by those in the rabbit-vulnerable areas of Otago, North Canterbury, the Mackenzie, the Canterbury high country, Marlborough and Hawke’s Bay. There were some ‘‘desperate’’ farmers out there, according to Mackenzie Country runholder Andrew Simpson, who represents Federated Farmers on the crosssector group coordinating the Rabbit Coordination Group (RCG).
‘‘The timing of this [RHD release] is critical in some areas. If another year goes by without the release of this virus the ecological damage to some properties would be a catastrophe. RHD was introduced into Australia last year and has reportedly been very effective, better than anticipated. Knockdown rates are averaging above 40 per cent at the moment and in some areas, they are achieving up to 80 per cent.’’
It’s been 20 years since RHD was first discovered on a Central Otago farm in August 1997. Known as New Zealand’s most significant intentional biosecurity breach, it was a huge slap in the face for New Zealand’s border officials. Not only had a highly infectious agent been smuggled into the country, but it also appeared the crime had been committed by a group of New Zealand farmers. Most pleaded ignorance but a few admitted to manufacturing and liberating the virus on their farms. They posed for the media alongside kitchen whizzes in which they’d mixed their viral cocktail. They felt their livelihood was threatened by the rabbit plague, and the government had let them down by failing to recognise their plight. The covert undertaking had been in effect for several weeks before the outbreak was officially recognised. In late August dead rabbits were found on a farm in Otago, and postmortem showed they had died of RHD. An emergency disease response was put in place by the then-Ministry of Agriculture and Fisheries (MAF), and the farm was put under quarantine.
Several months before the release, in June 1996, a group of farmers applied to import the RHD virus from Australia as a biocontrol agent. But after a protracted and often fractious period of public debate, MAF rejected the application, reasoning that too little was known about the virus’s performance in the New Zealand farmland. The rejection was received acrimoniously in farming circles. Anecdotal evidence suggests that the decision was anticipated and plans for the illegal release were already in place at this stage. MAF initiated an emergency disease response, but it was too late. By then, the virus was spreading like wildfire.
Rabbits were brought into New Zealand and released for both food and sport as early as the 1830s. By 1875 they had spread into Otago, Southland and Canterbury; by 1890 they had overrun the Mackenzie Country, causing a major drop in productivity on the early high country stations as they ate out the feed, leaving little for the sheep. Throughout the 1870s and 1880s, farmers walked off runs in Otago because of the impact of rabbits; come 1887, half a million hectares of land had been abandoned.
Canterbury escaped the worst of the first rabbit plague - but after WW1 stocking rates fell, in the same way as they had further south. Farmers tried a variety of ways to combat rabbits. At first, they relied on digging out burrows, hunting with dogs, shooting and trapping. Large gangs of men were employed to do this but were unable to cope with the vast numbers. However, there was a significant breakthrough after WW2 when experiments in dropping poisoned bait from planes proved successful. Remote and inaccessible areas, particularly in the South Island high country could now be included in large-scale programmes.
By 1950 more than 100 rabbit boards were administrating over 7.3m hectares of rabbit-prone country. Funding came from rates charged to landholders based on the area of their properties or the number of stock carried. The combination of aerial spraying and the use of carrots poisoned with 1080 allowed rabbit boards to reduce rabbit number in most areas. But in 1989 the administration of rabbit control was restructured entirely. The newly formed regional councils took over the role of the Agricultural Pest Destruction Council, and rabbit boards were disbanded. Farmers became responsible for meeting the costs of rabbit control undertaken on their account and imposed on them by the regional government. Coincidentally at the very time the funding declined and its administration changed, a prolonged drought in the South Island led to an explosion in rabbit numbers. It was now that RHD came clandestinely onto the scene and for the next few years, rabbit numbers declined dramatically. But the decline wasn’t to last.
Biosecurity team leader at Environment Canterbury (Ecan) Brent Glentworth says that since its release in 1997, RHD has become less effective. There are three lines of evidence from study sites in the Mackenzie Basin that support this claim, he says. ‘‘First, the proportion of rabbits of all ages with antibodies to RHD has increased each year since 1997. Second, the proportion of young rabbits that have antibodies to RHD has also increased. This is strong evidence that something has changed in the rabbit–virus interaction. The cause of this remains unknown but is reflected in the third line of evidence, that the abundance of rabbits (surveyed by spotlight counts) has increased since 1997. The rate of increase has, however, been much slower than that seen in the same populations as they recovered from conventional control before the arrival of RHD. Therefore, we conclude that RHD is still an effective biocontrol but its efficacy is waning.’’
ECan scientist Graham Sullivan says it will be a few weeks before it can confirm the success of the project. ‘‘While not the silver bullet for rabbit control, we anticipate that the new strain will greatly assist the control of wild rabbit populations by supplementing more traditional control methods. The impact of the RHD release will be monitored at a range of representative sites. The controlled release, using a highquality commercially prepared product, is being undertaken in Canterbury now because research suggests this is the optimal time to increase the effectiveness of the virus.’’
Next week is the annual Easter Bunny Hunt in Central Otago. In its twenty-fifth year, it has has a simple goal - to kill as many rabbits as possible in 24 hours. It’s not uncommon for the kill-count to number over 10,000. The proof will be in the pudding for RHD if next year’s count is a mere percentage of this.