VN October 2023

Vetnuus | October 2023 34 Technical I Article In fact, the virulent RHDV/RHDVa may cause death of most or all the rabbits (80–90% lethality) whereas a variable rate of mortality is observed in rabbits infected by RHDV2 (from 5 to 70%). This disease, independently from the causative strain, has also caused dramatic declines in some wild rabbit populations, particularly when it is first introduced. RHD spreads very readily. Hosts • RHD affects wild and domesticated members of the species Oryctolagus cuniculus , the European rabbit. • Other rabbit species including cottontails ( Sylvilagus floridanus ), black-tailed jackrabbits ( Lepus californicus ) and volcano rabbits ( Romerolagus diazzi ) are not susceptible. • Among hare species, the European brown hare ( Lepus europaeus ) and other hare species ( L. timidus, L. corsicanus, L. capensis ) are not affected by RHDV/RHDVa classical strains. However, the recently emerged RHDV2 was shown to be able to infect and cause a RHDV-like disease in some hare species, namely the Sardinian cape hare ( L. capensis var mediterraneus ), the Italian hare ( L. corsicanus ), the European brown hare ( L. europaeus ) and the mountain hare ( L. timidus ). • Among hare species, Lepus europaeus, L. timidus and L. corsicanus, but apparently not L. granatensis , L. castroviejoi and L. capensis, are affected by a disease caused by a different lagovirus (European brown hare syndrome – EBHS). • EBHSV was also shown to occasionally infect and cause an EBHS-like disease in cottontails ( Sylvilagus floridanus ). • Although rabbits of all ages can be infected, the infection is subclinical in animals younger than 6–8 weeks old if the causative agents are the classical RHDV/RHDVa. Conversely, RHDV2 causes disease and mortality even in young animals from 15–20 days old onwards. • RHDV/RHDVa replication has not been reported in other mammals, including rabbit predators, although seroconversion can occur. Inoculation of tissue suspensions from RHDV- infected rabbits into 28 different vertebrate species other than rabbits failed to produce disease, and no replication of the virus was detected by reverse-transcription polymerase chain reaction (RT-PCR). Transmission • Direct contact with infected animals through the oral, nasal or conjunctival routes. • Exposure to an infected carcass or hair froman infected animal. • By means of fomites, including contaminated food, bedding and water. • Experimental transmission by oral, nasal, subcutaneous, intramuscular, or intravenous routes. • Importation of infected rabbit meat. This could be one of the main means of transmission of RHD to a new area. Meat contains high levels of virus-infected blood, which survives freezing well. • Mechanical transmission. Flies and other insects are very efficient mechanical vectors; only a few virions are needed to infect a rabbit by the conjunctival route. Wild animals can transmit the virus mechanically. Although virus replication does not seem to occur in predators or scavengers, these animals (dogs, foxes, etc.) can excrete RHDV in faeces after eating infected rabbits. • How long rabbits that have recovered from RHD may remain infectious remains unknown. A low level of serum antibodies is sufficient to protect rabbits from the disease, but infection at the intestinal level could occur with shedding of the virus in the faeces. High sensitivity PCR demonstrated a long-term persistence (up to 2 months) of the viral RNA in recovered or in vaccinated and then infected rabbits. Whether this is due to real and active persistent or latent RHDV infections is still to be demonstrated. Sources of virus • The liver has the highest virus titre, followed by the spleen and serum. • Most or all excretions, including urine, faeces and respiratory secretions, are thought to contain virus. • Rabbit meat contains virus by virtue of its high blood supply. Occurrence RHDwas first reported in 1984 in China (People’s Rep. of) and 2 years later in Europe. To date, RHD has been reported in over 40 countries in Africa, the Americas, Asia, Europe and Oceania and is endemic in most parts of the world. The RHDVa subtype has been identified in Europe since 1996–97 and is also reported in other continents (Asia, Australia and the Americas). RHDV2 was firstly detected on 2010 in France in wild and farmed rabbits and then it rapidly spread to Europe and Mediterranean basin causing significant losses in farmed and wild rabbits in many countries (to date [mid-2015] France, Germany, Italy, Malta, Norway, Portugal, Spain, Sweden, Tunisia and the United Kingdom). For more recent, detailed information on the occurrence of this disease worldwide, see the OIEWorld Animal Health Information Database (WAHID) Interface [http://www.oie.int/ wahis/public.php?page=home] or refer to the latest issues of theWorld Animal Health and the OIE Bulletin . DIAGNOSIS • A presumptive diagnosis can be made in an unvaccinated rabbitry when there are multiple cases of sudden death following a short periodof lethargy and fever, and characteristic hepatic necrosis and haemorrhages are visible at necropsy • A field diagnosis is more difficult when there are few rabbits on the premises, rabbits are relatively isolated, as in research colonies, or the rabbitries are partially vaccinated or vaccination is only partially protective, such as in the case of infection with RHDV2 of premises vaccinated against RHDV/ RHDVa. The clinical manifestations have been described mainly in the acute infection (nervous and respiratory signs, apathy and anorexia). Clear and specific lesions, both gross and microscopic, are present. There is primary liver necrosis and a massive disseminated intravascular coagulopathy in all organs and tissues. The most severe lesions are in the liver, trachea and lungs. Petechiae are evident in almost all organs and are accompanied by poor blood coagulation. The incubation period of classical RHDV/RHDVa infection is 1–3 days, and death usually occurs 12–36 hours after the onset of fever. The disease is characterised by a high morbidity (almost 100%) and a mortality rate that usually ranges between 70 and 90%.