Rabbit haemorrhagic disease

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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. There is an exception to the genus Oryctolagus: a population of rabbits that live on San Juan Islands, Washington cannot fall ill to RHD. The cottontail rabbit and jackrabbit are not affected by the disease, as well as humans and other mammals. The rabbit haemorrhagic disease virus (RHDV), or rabbit calicivirus (RCV), causes rabbit haemorrhagic disease and is from the genus Lagovirus and family Caliciviridae. According to the Merck/Merial Manual For Pet Health, Home Edition, 2007, RHD is a reportable disease and that if a diagnosis is made by a veterinarian, a notification of the "appropriate government authorities" must be made.

As a result of RHD, some wild rabbit populations have severely declined. The causative virus is extremely hardy due to a protective organic covering on the virus. The virus may still be found in harvested meat and in decomposing carcasses.

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[edit] History

RHD first appeared in 1984 in the People's Republic of China; however, scientists can not pin point the exact origins of RHD. But it is believed the disease emerged from a virulent caliciviruses making its way around asymptomatically in European rabbit populations, particularly from the German Democratic Republic. The Chinese outbreak was spread by the angora rabbit breed that had originally come from Europe.

In 1992, the United Kingdom reported its first case of RHD in domestic show rabbits. By the late 1990s, RHD had 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 response to research RHD due to the commercial value of rabbit meat and fur production.

The United States saw its first confirmed case on April 7, 2000 when a group of 27 backyard rabbits in West Central Iowa became ill with RHD. The identity of the source of infection could not be determined. The outbreak was contained to the 27 rabbits and the virus was eliminated. Today, the United States has eradicated RHD from its rabbit populations.

[edit] World geographic distribution

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. Other endemic locations of RHD include Australia, New Zealand, Cuba, parts of Asia and Africa, and most of Europe. Other locations where RHD have been reported in domesticated rabbits are the Russian Federation, India, the Middle East and North and South America.

Italy's first case of RHD was recorded in 1986 and Spain's in 1988. France and Scandinavia would soon follow 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.

It is speculated that when the United Kingdom's first case of RHD in 1992 was discovered, the disease was then transmitted into the wild by domesticated pet rabbits. 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.

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.

Since 1993, RHD has been endemic in Cuba and it is also believed that RHD is thriving in Bolivia. From 1988 to 1992 Mexico dealt with an RHD endemic in domestic rabbits. The first reported case in the United States was in Iowa on March 9, 2000. The rabbits consisted of the breeds Palominos and California Whites. By April 6th, 25 of the 27 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).

[edit] Transmission

The transmission of the virus that causes 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 believed 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.

Through much intense research, it is believed that climate plays a crucial role in the transmission of RHD. In normal conditions, RHD will incur most of its outbreaks in the winter and/or spring. Due to high temperatures in late spring and summer the devastation of the virus is slowed down considerably. RHD will also have higher outbreak rates in dry and semi-dry areas compared to areas that are cooler and humid.

[edit] Symptoms

RHD has been known to be a disease that primarily infects only adult rabbits. In fact, research has shown 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 followed by death of the individual 1 to 2 days after the infection. There is a wide range of RHD symptoms but most rabbits will show no signs of external symptoms of RHD and more often than not a seemingly healthy rabbit will just suddenly die for no reason. Those rabbits that have been studied and suddenly died as a result of RHD with no apparent symptoms contained fresh grass in their mouths, indicating the rapid onset of death as a result of RHD.

Usually in the severe, symptomatic cases, rabbits with RHD will have a fever and die within 12 to 36 hours later. Some of the symptoms may include terminal squeals followed by the sudden collapse of the animal such as a coma and then eventually death. In less severe cases, symptoms are anorexia, swollen eyelids, inability to properly move or uneasiness, excitement, ocular haemorrhages and paddling. Convulsions or mania-type behaviour may be seen such as thrashing about in the cage. A fatal bloody discharge from the nose as been exhibited along with blood-stained cage floors, though sources vary it is believed that these symptoms occur after the rabbit has died. 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.

Ultimately, RHD causes rapid development of blood clot formation in major organs such as the heart, lungs and kidneys. As a result, the clots block blood vessels causing heart and respiratory failure. Infected rabbits that have died from RHD will have their legs straight out and their head pulls back over their necks as if trying to catch their breath. RHD is extremely hard to locate in the wild since about 75% of rabbits with RHD will die in their burrows underground.

[edit] Diagnosis

A good sign of RHD is when several animals in the herd die after experiencing a fever and lethargy. It is crucial to prevent the misdiagnosis of RHD with conditions such as pasteurellosis, myxomatosis, poisoning, heat exhaustion, 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.

Usually 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.

It is always important to remember to contact the proper authorities if any suspected foreign animal disease, such as RHD is discovered. Collected samples must be transported under secure settings and to the proper laboratories in order to contain the disease.

[edit] Morbidity and mortality

Due to the difficulty of collecting data on RHD in the wild the morbidity and mortality numbers have a broad range. The morbidity rate ranges from 30% to 100% and the mortality rate from 40% to 100%; however, the average mortality rate is usually around 90%.

Maternal antibodies such as immunoglobulin G (IgG), which are readily transmitted to the young across the placenta, are the only explanation as to why very young rabbits are resistant to RHD. Rabbits may even develop immunity against other strains of the RHD virus, while others may endure persistent infections. The immunity does not survive through the next generation, thus another outbreak could occur within the population. 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.

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.

[edit] Control

Countries that are uninfected by RHD may place restrictions on importation from endemic countries. It is recommended that quarantines be place when outbreaks occur because of the contagious form of the RHD virus. 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 against 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 months; however it is recommended by vaccine manufacturers to booster the vaccine annually. It is also recommended to use the vaccine in areas were the disease is widespread and eradication efforts have been made.

[edit] 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 small cost scale of control and production losses in New Zealand as a result of rabbits is about $22 million annually. This figure is only a small portion of the damage caused by rabbits.

Some 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 only infects rabbits and not other animals and humans. It is also safe to the eat meat of infected RHD rabbits. There is also no concern of virus mutation because mutations only affect the virus itself and its impact on the host. Many years of research has shown no evidence that the virus has changed to affect any other species other than European rabbit.

The only real concerns would be the result of widespread deaths of rabbits would cause predators to prey upon other food sources such as endangered or rare native species. The other concern would be for the safety of domesticated rabbits, but with proper vaccination plans there should be no concern.

[edit] See also

[edit] 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.
  • 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.
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