Nearly 4,000 blood samples from rabbits across Australia have helped explain the spread of RHDV2. The analysis reinforces the importance of adjusting rabbit control programs depending on the age of rabbits present, and explains one of the reasons why RHDV2 appears to be out-performing RHDV1.
Rabbit Haemorrhagic Disease Virus (RHDV), a calicivirus, is highly contagious to European rabbits. A strain of Rabbit Haemorrhagic Disease Virus (RHDV1) was formally released in Australia in 1996. It spread rapidly across mainland Australia, most likely assisted by flies, drastically reducing rabbit populations – although being less effective in cooler, higher rainfall areas. (Wishart & Cox, 2016). It is not lethal to young rabbits who may develop and retain immunity to it.
Benign (non-disease causing) strains of calicivirus arrived in Australia with the first European wild rabbits introduced to the country. They appear to be more prevalent in higher rainfall areas. Rabbit Calicivirus Australia 1 (RCV-A1) was isolated from wild rabbits in 2009 and found to reduce the likelihood of death from RHDV1, indicating a reason why RHDV1 had been less successful in such regions of Australia.
In 2015 a new form of RHDV – RHDV2 – was confirmed as present in Australia. It was first reported in Europe in 2010 and may have been in Australia from 2014. It is lethal to rabbits and hares and appears to outcompete strains of RHDV1. Its competitive advantages have been speculated to be a wider host range, the ability to clinically infect young rabbits, and an ability to partially overcome immunity to other variants of RHDV.
As early as 2017 there were reports of rabbits in Australia succumbing to RHDV2, even though they were immune to RHDV1 (Peacock et al, 2017). Now there is also evidence that the ability of RHDV2 to infect young rabbits enables it to spread several months earlier in the rabbit breeding season than can RHDV1 variants. It lowers the population rabbits earlier, reducing the number of rabbits that would otherwise have become susceptible to RHDV1 (Taggart et al, 2021).
A summary of the recent findings and their consequences has been provided by the Centre for Invasive Species Solutions (CISS). They suggest that localised rabbit control programs should consider the age and abundance of rabbits, and the time since benign strains, like RCV-A1, were prevalent. For example, the best results from releasing strains of RHDV1 will be when most rabbits are mature, rather than when there are high numbers of young rabbits.
References:
CISS (2021) ‘DOES THE RABBIT VIRUS ‘RHDV2’ HAVE AN ADVANTAGE OVER ‘RHDV1’ STRAINS?’
Peacock D, Kovaliski J, Sinclair R, Mutze G. (2017) ‘RHDV2 overcoming RHDV immunity in wild rabbits (Oryctolagus cuniculus) in Australia.’ Veterinary Record. Vol 180, Issue 11.
Taggart PL, Hall RN, Cox TE, Kovalski J, McLeod SR & Strive T. (2021) ‘Changes in virus transmission dynamics following the emergence of RHDV2 shed light on its competitive advantage over previously circulating variants.’ Transboundary and Emerging Diseases
Wishart J & Cox T. (2016). ‘Rollout of RHDV1 K5 in Australia: information guide. Second edition’. PestSmart Toolkit publication. The Centre for Invasive Species Solutions, Canberra, ACT