Rabbit haemorrhagic disease

Rabbit haemorrhagic disease (RHD), also known as rabbit calicivirus disease (RCD) or viral haemorrhagic disease (VHD), is a highly infectious and often fatal disease that affects rabbits of the species "Oryctolagus cuniculus". The infectious agent responsible for the disease is rabbit haemorrhagic disease virus (RHDV), or rabbit calicivirus (RCV), genus "Lagovirus" of the family Caliciviridae.

History

RHD first appeared in 1984 in the People's Republic of China. Scientists can not pinpoint its exact origins; however, it is believed the disease emerged from a virulent calicivirus spreading asymptomatically in European rabbit populations, particularly in the German Democratic Republic. The Chinese outbreak was spread by the angora rabbit, which had originated in Europe.Fact|date=July 2008

In 1992, the United Kingdom reported its first case of RHD in domestic show rabbits.Fact|date=July 2008 By the late 1990s, RHD stretched to forty countries and had become a serious endemic in wild rabbit populations in Europe, Australia, New Zealand and Cuba. In Europe there was a rapid increase in research into RHD, due to the commercial value of rabbit meat and fur production.

The first reported case in the United States was in Iowa on March 9, 2000.Fact|date=July 2008 The affected population included Palominos and California Whites. By April 6th, 25 of the 27 affected rabbits had died as a result of RHD. In order to contain the disease, the remaining two rabbits were euthanized. No new introductions of rabbits were placed on the farm for two years after the discovery of RHD and August 1999 was the last time rabbits left and/or returned to the farm. The United States experienced other outbreaks of RHD in 2001 (Utah, Illinois, New York) and 2005 (Indiana).

World geographic distribution

Asia

Within a few months of RHD being reported in China in 1984, the disease was widely seen in many commercial rabbitries and had reached the Republic of Korea. RHD has also been reported in India and the Middle East.

Americas (North and South)

Since 1993, RHD has been endemic in Cuba; it is also believed to be thriving in Bolivia. From 1988 to 1992 Mexico dealt with an RHD endemic in domestic rabbits.Fact|date=July 2008

After outbreaks of RHD in 2000, 2001, and 2005, the United States has eradicated RHD from its rabbit populations.Fact|date=July 2008

Australia/New Zealand

In 1991 the virus that caused RHD was imported to Australia under strict quarantine conditions to research the safety and usefulness of the virus if it was used as a biological control agent against Australia and New Zealand's rabbit pest problem. However, in 1995 a laboratory accident in Southern Australia lead to the escape of the RHD virus and consequently killed 10 million rabbits within 8 weeks of its exposure.Fact|date=July 2008 The disease is currently endemic throughout Australia and New Zealand.

Europe

RHD is endemic throughout most of Europe. Italy's first case of RHD was recorded in 1986 and Spain's in 1988.Fact|date=July 2008 France and Scandinavia followed in 1990. Within a few years of RHD's first appearance in Europe it had caused the largest mortality in domestic and wild rabbits in Germany, Austria, Spain and Italy. Spain was the worst affected by RHD.

When the United Kingdom's first case of RHD in 1992 was discovered, the disease was transmitted into the wild by domesticated pet rabbits.Fact|date=July 2008 Sources vary in the number of confirmed cases of RHD; there were 9 known outbreaks in 1994, 32 cases but some sources believe there were as many as 512 cases of RHD in 1995, and around 30 RHD cases in 1996 throughout Scotland, England and Wales.Fact|date=July 2008

Transmission

Transmission of RHD occurs by direct contact with an infected animal and fomites. Rabbits acquire RHD through oral, nasal or conjunctival pathways. Urine, faeces and respiratory secretions may also shed the virus. Carriers of the virus may remain infectious for up to a month depending on climate conditions; however, the virus has been known to persist for as little as 2 days and as long as 215 days. An infected carcass or hairs from an infected animal may also transmit RHD. Fomites such as clothing, contaminated food, cages, bedding, feeders and water will also harbour the virus. Even though the virus cannot reproduce in other mammals, it is believedweal that predators and scavengers such as foxes, ferrets and some birds can excrete the virus through their faeces after ingesting an infected rabbit carcass. Flies, rabbit fleas, and mosquitoes can also spread the virus between rabbits.

Climate appears to play a crucial role in the transmission of RHD.Fact|date=July 2008 In normal conditions, most outbreaks of RHD occur in winter or spring. High temperatures in late spring and summer will considerably reduce the spreadof the virus. RHD will also be more prevalent in dry and semi-dry areas than in areas that are relatively cool and humid.

Symptoms

RHD primarily infects only adult rabbits. In fact, research has shownFact|date=July 2008 that rabbits younger than 8 weeks of age are resistant to the virus. The incubation period for the RHD virus is between 1 to 3 days, with death following 1 to 2 days after the infection. There is a wide range of RHD symptoms. Most rabbits will show no signs of external symptoms of RHD.

Symptomatic cases of RHD will display fever, squeals, and often coma leading to death within 12 to 36 hours. In less severe cases, rabbits may display uneasiness, excitement, anorexia, swollen eyelids, paralysis, ocular haemorrhages, and paddling. Convulsions may be seen as well. A fatal bloody discharge from the nose as been exhibited along with blood-stained cage floors, though these symptoms may have occurred after death. Rabbits who have recovered from the less severe symptoms usually develop severe jaundice with weight loss and lethargy. Diarrhoea, constipation and abdominal cramping are then exhibited right before death a few weeks later.

Research conducted at the CSIRO Australian Animal Health Laboratory showed that rabbits did not show any outward signs of pain or suffering as a result of having RHD. In fact, most infected rabbits became quieter and died quietly with no pain or suffering.

RHD causes rapid development of blood clot formation in major organs such as the heart, lungs and kidneys. The clots block blood vessels causing heart and respiratory failure. An infected rabbit that has died from RHD will often have its legs straight out and head over its neck as if trying to catch its breath.Fact|date=July 2008

Diagnosis

RHD may be indicated when several animals in the herd die after experiencing a fever and lethargy. Differential diagnosis includes pasteurellosis, myxomatosis, poisoning, heat exhaustion, and "E. coli" or "Clostridium perfringens" type E enterotoxemia.

Rabbits that die from RHD are usually in good outward state. However, the most frequent post-mortem lesions are hepatic necrosis and splenomegaly. The liver of RHD rabbits may have a fine reticular pattern of necrosis outline each lobule and maybe yellow, gray or pale in colour. The liver is also usually friable and swollen. The spleen will be black in colour and also swollen with rounded edges, while the kidneys are dark brown in colour. Haemorrhages will also be seen many other organs and tissues. The trachea may present a foamy, bloody mucous. Enteritis of the small intestine and swollen meninges may also occur.

Laboratory tests such as reverse transcription polymerase chain reaction (RT-PCR), Western blotting, negative-staining immunoelectron microscopy and ELISAs may be performed on samples from the liver, blood, spleen or other organs.

Morbidity, mortality, and immunity

RHD is extremely hard to locate in the wild since about 75% of rabbits with RHD will die in their burrows underground. Due to this difficulty, the morbidity and mortality estimates for RHD have a broad range. The morbidity rate ranges from 30% to 100% and the mortality rate from 40% to 100%; however, the typical mortality rate is usually around 90%.Fact|date=July 2008

In the wild, outbreaks in rabbits vary depending on the season, breeding cycles and geographical location. Some areas will see a high morbidity and mortality among its rabbit populations followed by calmer outbreaks.

Maternal antibodies such as immunoglobulin G (IgG), which are readily transmitted to the young across the placenta, may explain why very young rabbits are resistant to RHD.Fact|date=July 2008 Some scientists also believe that the immature immune system of a young rabbit cannot produce the number of chemicals needs to initiate clotting in order to kill.Fact|date=July 2008 Rabbits may develop immunity against other strains of the RHD virus, while others may endure persistent infections. The immunity does not survive through the next generation, leaving open the possibility of further outbreaks in the population.

A population of rabbits on the San Juan Islands, Washington, USA appears to be immune to RHD. The cottontail rabbit and jackrabbit are not affected by the disease; nor are humans or other mammals.

Control

Countries that are uninfected by RHD may place restrictions on importation from endemic countries. According to the Merck/Merial Manual For Pet Health, Home Edition, 2007, RHD is a reportable disease in the United States. If a diagnosis is made by a veterinarian, a notification of the "appropriate government authorities" must be made.Fact|date=July 2008

Because of the highly infectious nature of the disease, it is recommendedweal that animals be quarantined when outbreaks occur. Depopulation, disinfection, surveillance and quarantines are the only way to properly and effectively eradicate the disease. Good disinfectants include 10% sodium hydroxide, 1-2% formalin, 2% One-Stroke Environ®, and 10% household bleach. The RHD virus is resistant to ethers or chloroform. 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.

An effective, safe "killed" vaccine has been developed in various countries, particularly Spain, and is believed to give protection for 5 to 15 monthsFact|date=July 2008; however, vaccine manufacturers recommend a booster be administered annually. The vaccine is also recommended in areas where the disease is widespread and eradication efforts have been made.

RHD as a biological control agent

The European rabbit is the second most serious pest in New Zealand. Rabbits compete with livestock for grazing pasture, kill tress, shrubs, and have contributed to the extinction of some native plants. Consequently, rabbits contribute to soil erosion by eliminating the protective vegetation and disturb the soil by burrowing.

The estimated combined cost of control and production losses in New Zealand as a result of rabbits is about $22 million annually.Fact|date=July 2008 This figure is only a small portion of the damage caused by rabbits.

Control measures used against rabbits in New Zealand include poisoning, shooting, ripping, blasting, releasing predators, the myxoma virus (a biological control agent) and fencing. Additional control methods, such the introduction of RHD, are needed to control rabbits so that the labour-intensive conventional control methods are more successful.

After the safety of RHD was confirmed by laboratory research, RHD was approved for release as a biological control agent in New Zealand. RHD is safe because it infects rabbits, but not other animals or humans. It is also safe to eat the meat of infected RHD rabbits.Fact|date=July 2008 Virus mutation is not a concern&em;many years of research show no evidence that the virus has changed to affect any other species other than European rabbit.

However, it is possible that widespread rabbit deaths might cause predators to prey upon other food sources, such as endangered or rare native species. With proper vaccination plans, the safety of domesticated rabbits should not be a concern.

ee also

*Rabbits in Australia

References

* Cooke, B.D. Rabbit Haemorrhagic Disease: Field epidemiology and the management of wild rabbit populations. "Common Wealth Scientific and Industrial Research Organisation (CSIRO) Sustainable Ecosystems". http://www.oie.int/eng/publicat/rt/2102/COOKE.pdf. Retrieved on 2008-02-06.

* RCD – Frequently Asked Questions. "Rabbit Biocontrol Advisory Group". http://www.maf.govt.ns/mafret/publications/rabbit-biocontrol-advisory-group/rbag0002.htm. Retrieved on 2008-02-06.

* Rabbit Calicivirus Disease. "Center for Emerging Issues". http://www.aphis.gov/us/ceah/cei/taf/iw_2000_files/domestic/rabbitcal.htm. Retrieved on 2008-02-06.

* Rabbit Calicivirus Disease. "The Merck/Merial Manual For Pet Health 2007 Home Edition". Page 995-996.

* Rabbit Calicivirus Disease. "The Merck Veterinary Manual, Eighth Edition" (1998). Page 1398-1399.

* RHD – A New Rabbit Disease. "The British Association For Shooting and Conservation Ltd." http://www.basc.org.uk/media/rabbits.pdf. Retrieved on 2008-02-06.

* Rabbit Hemorrhagic. "Center for Food Security & Public Health Iowa State University College of Veterinary Medicine". http://www.cfsph.iastate.edu/Factsheets/pdfs/rabbit_hemorrhagic_disease.pdf. Retrieved on 2008-02-06. ]

* Rabbit Viral Haemorrhagic Disease (RVHD). "Scottish Natural Heritage". http://www.snh.org/uk/publications/on-line/advisorynotes/31/31.htm. Retrieved on 2008-02-06.