Danger? New strain of rabbit disease released in New Zealand!

Imagine its August 1997. You’ve just been out to dinner with your family. You settle down for the night surrounded by everyone you love. You wake early in the morning for breakfast, everyone around you is still asleep. Or so they appear to be. You look closer at your mother laying nearby: she is not moving. You look around and you realise no one is moving. You race from your house to the neighbours to find your friend in a similar predicament. You run to another to find no one else is stirring. Everyone seemed fine yesterday but today they are all dead.

You are a European rabbit (Oryctolagus cuniculus). Your ancestors were brought here on a boat just like the humans who live in a big house across the paddocks from you. Last night you ate at that paddock. Free carrots! This morning you realise they infected your family with the deadly rabbit haemorrhagic disease virus (RHDV). You and your friend survived because you were naturally immune.

European rabbit surveys the land. Photo credit: JJ Harrison

Rabbit haemorrhagic disease virus was illegally released on central Otago farms, New Zealand in August 1997. According to Peter O’Hara, former Deputy Director General of the Ministry of Agriculture and Forestry, this action by the farmers was likened to bio-terrorism. RHDV was first reported in China in 1984, where many  rabbits were being farmed for meat and fur.

A strain of RHDV from the Czech Republic was introduced to an off-shore Australian island in 1991. This was to begin testing it as a bio-control of the plague-like numbers of wild rabbits that inhabited both Australia and New Zealand. The spread of these rabbits is known to be the fastest colonisation of any mammal worldwide. The test island in Australia had an outbreak and it was subsequently released onto the mainland, possibly by insects. By 1999, just two years after introduction, RHDV-v351 had been spread to most wild populations of rabbits in New Zealand.

Abundance declines of up to 88% following RHDV release were observed in Otago. Ben Reddiex and colleagues in 2002 also studied RHDV infected rabbit populations on farms and found that predation was important for keeping abundance at low levels. Once RHDV spread through an area, rabbit populations could once again be effected by predation. This could then prevent the rabbit numbers from soaring again. This finding is important because nestling mortality is attributed to predation rather than RHDV.

Since its release, the rabbit declines caused by RHDV have led to many flow-on environmental effects. Grant Norbury and colleagues summarised these effects in a 2002 scientific article.  They include increased pasture biomass, reduced the abundance of “rabbit-specialist” predators (stoats), a short-term increase in predation of native birds, an increase in other exotic herbivores, and increases in woody plant species such as Scotch broom (Cytisus scorparius), sweet brier (Rosa rubiginosa), and wilding pines (Pinus species).

Some areas, such as Otago, have rabbit numbers that are now similar to pre-RHDV levels, which has sparked the debate to introduce new methods. Rabbits with anti-bodies increase every year, showing that the efficiency of RHDV-v351 is declining, albeit much slower than those areas without exposure. In 2012 an application to the Ministry of Primary Industries (MPI) was made by Environment Canterbury (ECAN) to register a new strain of the virus, RHDV1a-K5 (or K5). K5 has been shown to overcome the immunity to the old virus strain in Australian, following the release there in 2017. It is not expected that K5 will kill as much of the population as v351 did because these are no longer naïve populations. However, it is hypothesised that it will kill 0-40% of wild rabbits, making it very likely that it will not establish here in New Zealand.

Under New Zealand Hazardous Substances and New Organisms act, the new RHDV did not need to be registered as a new infectious substance due to it being deemed a variant of the old strain. The application was met with 167 submissions, the majority against the release of K5. One application was signed by 4651 people who wished to halt the release of K5 until an effective vaccination was found.  The 116,000 companion rabbits in New Zealand are considered non-target animals and without an effective vaccination may die as a result. However, farmers hope that the virus will kill more than the predicted 40% due to the unmanageable populations they currently face. Permission was granted under the Bio-security Act for importation, clearance and release of K5 on the 21st of February 2018.

Manaaki Whenua-Landcare Research has an interactive map available that shows where RHDV is being released.

My pet rabbit, Dusty. CC-BY-NC 4.0 Kathryn Bugler

RHDV is seen to be more humane than other methods to cull wild rabbits. Rabbits contract “cold-like” symptoms, a rise in body temperature and lethargy. A majority of individuals show no symptoms and after a fever, death occurs within 12 hours. No other species, not even hares, have shown clinical signs of RHDV, but some predators such as foxes (not found in New Zealand), have shown anti-bodies. The lack of symptoms is what has most domestic rabbit owners worried, they are less likely to be able to seek veterinary help and often suddenly find dead rabbits, who seemed healthy the day before.

A few studies have considered the  effectiveness of the current vaccination developed by Cylap. Wishart and Cox suggest that the vaccination should work on both variants of the RHDV because they’re closely related variants of the same strain. Read and Kirkland show in their study that the Cylap vaccine provided enough protection from K5 as well as v351, because all vaccinated rabbits survived the exposure to high levels both orally and naturally (via insects, direct contact or predator’s faeces).

I’ve been talking to other people with domestic rabbits and the general feeling is that Read and Kirkland’s study was not considered well executed due to the length of study and the age of rabbits. The study notes its own limitations but still agrees the vaccination offers enough protection. The application for register of K5 deemed the current vaccination sufficient to protect non-target rabbits like it did for v351. There are other vaccinations available worldwide but are not registered in New Zealand and are very expensive. It is important to note that in Australia no deaths of vaccinated pet rabbits occurred from K5.

It is unclear how the release of K5 has affected wild rabbits but in the mean time it has caused an almost hysterical feeling among domestic rabbit owners. With many domestic rabbit Facebook groups being told the wrong, highly emotional information. Which is fair enough when our fur children are the ones who stand to die in this situation! Thankfully many of the companies involved with this have put out FAQs . MPI are also testing any domestic rabbits that are suspected to have died from the virus and were vaccinated. This is ongoing and to date no deaths of vaccinated rabbits have been caused by RHDV.

PS – In the last month a new RHDV virus has been found in Marlborough. It is a completely new strain (RHDV2) to New Zealand and posses a real threat to domestic rabbits as an effective vaccination is not available here. However, it is likely to be effective against wild rabbits as the kill rate for juveniles is much higher than the Czech Republic strain (RHDV1).

– Reddiex B., Hickling G.J., Norbury G.L. & Frampton. C. M. (2002) Effects of predation and rabbit haemorrhagic disease on population dynamics of rabbits (Oryctolagus cuniculus) in North Canterbury, New Zealand. Wildlife research, 29, 627-633.

The author Kathryn Bugler is a postgraduate student enrolled in the Master of  Science at Lincoln University. She wrote this article as part of his assessment for ECOL608 Research Methods in Ecology.

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