Rabbit hemorrhagic disease
Rabbit hemorrhagic disease (RHD), also known as viral hemorrhagic disease (VHD), is a highly infectious and lethal form of viral hepatitis that affects European rabbits. Some viral strains also affect hares and cottontail rabbits. Mortality rates generally range from 70 to 100 percent.[4] The disease is caused by strains of rabbit hemorrhagic disease virus (RHDV), a lagovirus in the family Caliciviridae.
Epidemiology and transmission[edit]
Both viruses causing RHD are extremely contagious. Transmission occurs by direct contact with infected animals, carcasses, bodily fluids (urine, feces, respiratory secretions), and hair. Surviving rabbits may be contagious for up to 2 months.[6] Contaminated fomites such as clothing, food, cages, bedding, feeders, and water also spread the virus. Flies, fleas, and mosquitoes can carry the virus between rabbits.[10] Predators and scavengers can also spread the virus by shedding it in their feces.[10] Caliciviruses are highly resistant in the environment, and can survive freezing for prolonged periods. The virus can persist in infected meat for months, and for prolonged periods in decomposing carcasses. Importation of rabbit meat may be a major contributor in the spread of the virus to new geographic regions.[6]
RHD outbreaks tend to be seasonal in wild rabbit populations, where most adults have survived infection and are immune. As young kits grow up and stop nursing, they no longer receive the antibodies provided in their mother's milk and become susceptible to infection. Thus, RHD epizootics occur more often during the rabbits' breeding season.[10]
Generally, high host specificity exists among lagoviruses.[6] Classic RHDVa only affects European rabbits, a species native to Europe and from which the domestic rabbit is descended. The new variant RHDV2 affects European rabbits, as well, but also causes fatal RHD in various Lepus species, including Sardinian Cape hares (L. capensis mediterraneus), Italian hares (L. corsicanus), and mountain hares (L. timidus).[15] Reports of RHD in Sylvilagus species have been coming from the current outbreak in the United States.[16]
RHD caused by RHDV and RHDVa demonstrates high morbidity (up to 100%) and mortality (40-100%) in adult European rabbits. Young rabbits 6–8 weeks old are less likely to be infected, and kits younger than 4 weeks old do not become ill.[6] The more recently emerged RHDV2 causes death and disease in rabbits as young as 15 days old. Mortality rates from RHDV2 are more variable at 5-70%. Initially less virulent, the pathogenicity of RHDV2 has been increasing and is now similar to that found with RHDV and RHDVa. Deaths from RHDV2 have been confirmed in rabbits previously vaccinated against RHDVa.[6]
Pathophysiology[edit]
These viruses replicate in the liver and by mechanisms not fully elucidated, trigger the mass death of hepatocytes which can in turn lead to disseminated intravascular coagulation, hepatic encephalopathy, and nephrosis.[10] Bleeding may occur, as clotting factors and platelets are used up.
Diagnosis[edit]
A presumptive diagnosis of RHD can often be made based on clinical presentation, infection pattern within a population, and post mortem lesions. Definitive diagnosis requires detection of the virus. As most caliciviruses cannot be grown in cell culture, antibody and nucleic acid based methods of viral detection are often used.[6]
Complete blood counts from rabbits with RHD often show low levels of white blood cells and platelets, and chemistry panels show elevated liver enzymes. Evidence of liver failure may also be present, including increased bile acids and bilirubin, and decreased glucose and cholesterol. Prolonged prothrombin and activated partial thromboplastin times are typical. Urinalysis can show bilirubinuria, proteinuria, and high urinary GGT.[6]
The classic post mortem lesion seen in rabbits with RHD is extensive hepatic necrosis. Multifocal hemorrhages, splenomegaly, bronchopneumonia, pulmonary hemorrhage or edema, and myocardial necrosis may sometimes also be seen.[6]
RT-qPCR tests are a commonly used and accurate testing modality for RNA-based viruses. Other tests used include enzyme-linked immunosorbent assay, electron microscopy, immunostaining, Western blot, and in situ hybridization.[6] The tissue of choice for molecular testing is fresh or frozen liver, as it usually contains the largest numbers of virus, but if this is not available, spleen and serum can also be used. Identification of the strain of RHDV is needed so vaccination protocols can be adjusted accordingly.
Prevention and control[edit]
Vaccines[edit]
A number of vaccines available against RHD are sold in countries where the disease is endemic. All provide 12 months of protection against RHD viruses. Because RHD viruses cannot normally be grown in vitro,[17] how these vaccines are produced is affected. Inactivated RHD vaccines, including Eravac,[18] Felavac, and Cylap, are “liver-derived”, meaning that laboratory rabbits are intentionally infected with RHD and their livers and spleens harvested to make vaccines. Each rabbit used results in the production of thousands of vaccine doses. This has led to controversy among rabbit lovers, who question the ethics of some rabbits having to die to protect others[19] but is not an issue where rabbits are primarily farmed for meat. Another method of reproducing the virus is through recombinant technology, where antigenic portions of the RHD viruses are inserted into viruses that can be grown in culture. This is the method used to create Nobivac Myxo-RHD PLUS.[20]
Vaccines against only the classic RHDVa strain are: Cylap RCD Vaccine, made by Zoetis,[21] protects rabbits from two different strains of RHDVa (v351 and K5) that are used for wild rabbit control in Australia.[22] CUNIPRAVAC RHD,[23] manufactured by HIPRA, protects against the RHDVa strains found in Europe. Nobivac Myxo-RHD,[24] made by MSD Animal Health, is a live myxoma-vectored vaccine that offers one-year duration of immunity against both RHDVa and myxomatosis.
Vaccines against only the newer RHDV2 strain are: Eravac vaccine, manufactured by HIPRA,[25] protects rabbits against RHDV2 for a year.
Vaccines that protect against both RHDVa and RHDV2 strains include: Filavac VHD K C+V,[26] manufactured by Filavie, protects against both classical RHDVa and RHDV-2.[27] It is available in single dose and multidose vials. A soon-to-be-released vaccine from MSD Animal Health, Nobivac Myxo-RHD PLUS, is a live recombinant vector vaccine active against both RHDVa and RHDV2, as well as myxomatosis.[28]
Countries in which RHD is not considered endemic may place restrictions on importation of RHDV vaccines. Importation of these vaccines into the United States can only be done with the approval of the United States Department of Agriculture[29] and the appropriate state veterinarian.[30]
Disinfection[edit]
Caliciviruses are stable in the environment and difficult to inactivate. Products commonly used for household disinfection such as Clorox and Lysol disinfecting wipes do not work against these viruses. One effective option is to wipe down surfaces with a 10% bleach solution, allowing 10 minutes of contact time before rinsing. Other disinfectants shown to work include 10% sodium hydroxide, 2% One-Stroke Environ, Virkon S, Clorox Healthcare Bleach Germicidal Wipes, Trifectant, Rescue, and hydrogen peroxide cleaners. Surface debris must always be mechanically removed prior to disinfection. A list of disinfectants that are effective against calicivirus (in this case norovirus) can be found on the Environmental Protection Agency's website.[31] Studies have shown that many quaternary ammonium compound based disinfectants do not inactivate caliciviruses.[32]
Quarantine and other measures[edit]
Because of the highly infectious nature of the disease, strict quarantine is necessary when outbreaks occur. Depopulation, disinfection, vaccination, surveillance, and quarantine are the only way to properly and effectively eradicate the disease. Deceased rabbits must be removed immediately and discarded in a safe manner. Surviving rabbits should be quarantined or humanely euthanized. Test rabbits may be used to monitor the virus on vaccinated farms.[33]