Other names: RHD, Rabbit Calicivirus, Viral Hemorrhagic Disease (VHD) of Rabbits, European Brown Hare Syndrome.
Cause
Rabbit hemorrhagic disease virus (RHDV) is a highly contagious and deadly calicivirus of the genus Lagovirus that affects some species of animals in the taxonomic group (order) of lagomorphs, including rabbits, hares, and pikas1. The first serotype identified, RHDV1, was discovered in 1984 in China and has since spread around the world causing hundreds of millions of domestic rabbit mortalities. All caliciviruses are capable of frequent mutation, so multiple strains of RHDV exist, with most pathogenic strains belonging to the RHDV1 serotype. A new pathogenic serotype, RHDV2, emerged in Europe in 2010 and was confirmed in wild lagomorphs for the first time in the United States in 2020.1,2
Significance
Mortality from RHDV can reach 70% or higher, so it can cause dramatic declines in wild rabbit populations, especially when first introduced2. In Spain’s Iberian Peninsula, steep rabbit population declines negatively impacted ecosystems by dramatically reducing a source of food for predators, including the endangered Iberian lynx. RHDV could have particularly devastating effects on small populations. Because of its high mortality rate, RHDV1 has been used intentionally in Australia in an attempt to control populations of introduced, non-native wild rabbits.
In early 2020, RDHV2 was confirmed in the United States in both domestic rabbits and in wild cottontails and jackrabbits. It has quickly become established in wild rabbit populations throughout the western half of the U.S. but has not yet been found in wild rabbits East of the Mississippi River. There is concern that the virus could significantly impact the New England Cottontail, a species still recovering from population declines1, as well as the Appalachian Cottontail and Snowshoe Hare, species of conservation concern in Pennsylvania.4
Species Affected
Only members of the rabbit family (Lagomorphidae) are susceptible including all domestic, feral, and wild rabbits and hares. RHDV is not a zoonotic disease affecting humans.1
Distribution
RHDV1 has been confirmed in 40 countries across Europe, the Mediterranean, Africa, Asia, North America, Australia, and New Zealand. Because the domestic European rabbit is the main reservoir host and does not occur in the wild in the US, RHDV1 is not thought to be established (enzootic) here. However, sporadic RHDV1 outbreaks have appeared intermittently among domesticated rabbits in the US. Cases were confirmed in Iowa in 2000; Utah, Illinois, and New York in 2001; Indiana in 2005; Maryland in 2008; Minnesota in 2010; and Pennsylvania in 2018.
The RHDV2 strain emerged in France in 2010 and spread rapidly across Europe and the Mediterranean. It was detected in Australia in 2015 and in feral rabbits in British Columbia in 2018. A few months later it was confirmed in a pet rabbit in Ohio as well as pet and feral rabbits in Washington state. In early 2020, RHDV2 was identified as the cause of death in both domestic and wild rabbits in Texas, New Mexico, Arizona, Nevada, Colorado, and California. In 2021, Oregon, Montana, Idaho, Utah, and Wyoming reported both domestic and wild cases and Florida and Georgia reported single cases of RHDV2 in domestic rabbits. In 2022, RHDV2 has been detected in domestic rabbits in Washington, Kansas, Oregon, Colorado, Minnesota, Wisconsin, and Pennsylvania. The distribution of RHDV can be seen on the United States Department of Agriculture’s Animal and Plant Health Inspection Service interactive map or at https://rhdv2.org/where-is-rhdv2/
Transmission
Rabbits infected with RHDV can spread the virus for at least a month in their urine, feces, blood, and other secretions, and the shedding period may be substantially longer.3 The virus can then infect other rabbits through oral, nasal, or conjunctival routes. The incubation period for RHDV ranges from 16 to 48 hours, and mortalities usually occur 12 to 36 hours after onset of fever.1
Spread of RHDV can occur through direct contact; exposure to infected carcasses or animal products, rabbit fur, food, water or clothing; and mechanical vectors such as insects, scavengers, and predators.
RHDV2 is very resistant to environmental degradation and can survive long periods outside the host. It has survived at room temperature for as long as 105 days, it can survive for one hour at freezing temperatures as well as heat of 122°F, and it has been detected in decaying tissue after 90 days outdoors.5
Clinical Signs
The clinical signs of disease do not differ between the various viral strains of rabbit hemorrhagic disease and most rabbits die within 12 to 36 hours of infection. There are four main clinical forms of RHDV infection. In the rapid, peracute form of the disease, clinical signs are usually absent, and animals collapse and die suddenly, sometimes with vocalizations.
The less rapid, acute form may present with a wide range of signs including bleeding from the mouth, nose or rectum, fever, difficulty breathing, neurological signs such as loss of coordination and seizures, depression, lethargy, and loss of appetite. Hemorrhages in the eye, blood in the feces, and yellow mucus membranes may also be observed secondary to anemia and coagulation abnormalities caused by the virus.
The subacute form is characterized by the same clinical signs, but they are generally less severe.
Lastly, the chronic or subclinical form of the infection may occur in a small percentage of animals. The affected animals are usually lethargic, jaundiced, and in poor body condition and may or may not die. Survivors of either this or the subacute form are protected with apparent life-long immunity.
Diagnosis
RHDV is diagnosed through necropsy and molecular tests. On necropsy, the most common lesions are a pale liver and an enlarged spleen. Hemorrhages in other organs like the trachea, lungs, heart, liver, spleen, and kidney can also be observed.
Treatment
There is no treatment available for rabbit hemorrhagic disease apart from supportive care. In the wild, rabbits are often found dead.
Management
RHDV is classified as a foreign animal disease. When it is detected in the US, it must be immediately reported to state agricultural authorities who act on behalf of and coordinate with the US Department of Agriculture (USDA). The Wildlife Futures Program (WFP), the USGS National Wildlife Health Center (NWHC), and others are alerting partners to be vigilant for morbidity or mortality events involving wild rabbits. In Pennsylvania, the WFP will facilitate submission of rabbits and hares suspected of this disease to USDA. Outbreaks of RHDV in domestic rabbits have been managed through quarantine, depopulation, and disease tracing, along with cleaning and disinfection. However, eradication of RHDV2 is not likely to be possible once the virus is established in wild hare or rabbit populations due to the ease of transmission and long persistence of the virus in the environment. Therefore, vigilance and strict biosecurity measures are key to preventing RHDV2 from becoming established in wild populations. Best Management Practices for groups such as zoos and rehabilitators who might encounter suspects are being developed. Though a vaccine for RHDV2 exists, it has limited availability in the US. Medgene Labs’ experimental inactivated (killed) recombinant subunit Rabbit Hemorrhagic Disease Virus vaccine is the only one that has been approved for sale in Pennsylvania and can only be sold to licensed veterinarians. This experimental vaccine must be administered according to the label instructions to domestic animals of the Leporidae family only — wild rabbits and hares may not be vaccinated with this product.
Wildlife professionals, hunters, or individuals who handle rabbits in the field should wear personal protective equipment (PPE) and decontaminate all field gear, clothes, and vehicles to prevent the spread of RHDV2 within populations of wild rabbits or to domestic rabbits. Field necropsies of RHDV suspects are strongly discouraged and instead, it is recommended that the whole carcass be collected, double bagged, and refrigerated before notifying state agricultural authorities.
The following additional guidance is provided for rabbit hunters:
- Do not harvest rabbits that appear sick.
- Wear rubber or disposable nitrile gloves while handling and cleaning game.
- When cleaning game, bag and dispose of remains in an approved fashion, including burying them deeply enough to prevent scavenging, or placing them in the commercial waste stream.
- When done handling game, wash hands thoroughly with soap and water, then clean and disinfect knives, equipment, and surfaces that were in contact with game. A viricidal solution can be made form 1 part bleach to 9 parts water. Clothing should be immediately washed and thoroughly dried.
- All game should be thoroughly cooked to an internal temperature of at least 165°F.
Literature Cited
1. Abrantes, J., van der Loo, W., Le Pendu, J. et al. 2012. Rabbit haemorrhagic disease (RHD) and rabbit haemorrhagic disease virus (RHDV): a review. Vet Res 43 (12). https://doi.org/10.1186/1297-9716-43-12
2. World Organization for Animal Health. 2019. Rabbit Haemorrhagic Disease. OIE.
3. Spickler, Anna Rovid. 2020. Rabbit Hemorrhagic Disease. USDA. http://www.cfsph.iastate.edu/DiseaseInfo/factsheets.php .
4. Pennsylvania Rabbit Hemorrhagic Disease Response Plan, approved April 2022.
5. Iowa State University | Rabbit Hemorrhagic Disease and Other Lago viruses
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