New challenges in rabbit protection: Highly virulent RHDV2 strains

Introduction 

Rabbit Haemorrhagic Disease Virus (RHDV) is a viral disease caused by a highly contagious calicivirus that affects European rabbits (Oryctolagus cuniculus). Since its discovery in 1984, RHD has caused significant losses in both domestic and wild rabbit populations. 

Over the years, the virus has evolved, even leading to a new serotype discovered in France in 2010: RHDV2. This virus is characterized by its high adaptability, quickly replacing the classic strain in most territories. 

Characteristics of Highly Virulent Strains

The classic RHDV strain has a mortality rate ranging between 80-90%. However, in the case of RHDV2, the mortality rate can vary greatly depending on the strain, with more recently isolated strains being much more virulent than those initially isolated.^1,2 

This variability can be seen in the results of Figure 1, which shows the mortality rate of rabbits that were not vaccinated but were infected with different strains at various ages. These results come from recently published studies .^3,4 

Regarding the RHDV2 strain isolated in 2013, mortality is higher in young rabbits (67%) compared to adult rabbits (53%). In the case of the classic strain (RHDV), the infection could not be carried out as with the other cases at 37 days of life, since in rabbits under 8 weeks old, the disease is subclinical. The mortality rate in unvaccinated rabbits aged 12 weeks and 1 year was very high, at 91% and 80%, respectively. 

When comparing these results with those of highly virulent RHDV2 strains, clear differences are observed: in none of the cases is the mortality rate below 90%. Even in the most recently isolated strain in 2022, a 100% mortality rate was observed. Furthermore, this high mortality is consistent across both young and adult rabbits.

These data underscore the importance of epidemiological surveillance and the revision of vaccination programs.

Figure 1. Mortality Rate in Unvaccinated Rabbits Based on the Strain Used and the Age of the Animals.

Challenges 

Controlling the rabbit haemorrhagic disease virus poses significant challenges in clinical settings and wild populations. Firstly, the virus's ability to evolve relatively easily into new serotypes, as in the case of RHDV2, and to increase its virulence over the years. 

Secondly, the virus has been able to expand its range of hosts, affecting not only the European rabbit (Oryctolagus cuniculus) but also some hare species, which were not affected by the classic RHDV strain. For all these reasons, it is essential to monitor the virus's evolution to anticipate possible mutations that could endanger rabbit health.

Prevention

For all these reasons, now more than ever, it is crucial to ensure that all rabbits follow appropriate vaccination plans, including vaccines capable of protecting them against these highly virulent strains .

Additionally, precautions should be taken not only for rabbits with outdoor access but also for indoor rabbits, as the virus is highly resistant in the environment. 

References

 1. Lavazza, A.; Capucci, L. Chapter 3.6.2.—Rabbit haemorrhagic disease. In Manual of Diagnostic Tests and Vaccines for Terrestrial Animals 2021; OIE: Paris, France, 2021; pp. 1389–1406. 

2.Le Minor, O., Boucher S., Joudou L., Mellet R., Sourice M., Le Moullec T., Nicolier A., Beilvert F., Sigognault-Flochlay A. 2019. Rabbit haemorrhagic disease: experimental study of a recent highly pathogenic GI. 2/RHDV2/b strain and evaluation of vaccine efficacy. World Rabbit Sci., 27.3, 143-156. 

3. Perozo E., Fontseca M., Acal L, Gascon-Torrens S., March R., Sitjà M. (2024). YURVAC RHD vaccine against RHDV and RHDV2 [Oral communication, O126]. ICARE Congress, Ghent, Belgium. 

4. Perozo E., Fontseca M., Nadal G., Montbrau C., Gascon-Torrens S., March R., Sitjà M. (2024). Efficacy evaluation of YURVAC RHD against a current highly virulent RHDV2 strain [Poster communication, P1-PH1-07]. WRC Congress, Tarragona, Spain.