SWINE VESICULAR DISEASE VIRUS
FRANCISCO RUIZ-FONS
Instituto de Investigation en Recursos Cinegeticos IREC (CSIC-UCLM-JCCM), Ciudad Real, Spain
Swine vesicular disease virus (SVDV) belongs to the genus Enterovirus of the family Picornaviridae.
SVDV is a singlestranded RNA virus ranging from 20 to 32 nm in size. Currently four antigenic variants are known, all of them placed in a single serotype. SVD is clinically indistinguishable from other porcine vesicular diseases caused by Foo t- and-mouth disease virus, Vesicular stomatitis virus and Swine vesicular exanthema virus.Swine vesicular disease (SVD) was first diagnosed in Italy in 1966 in domestic pigs and was later reported from several European countries. Currently in Europe SVD is enzootic only in some areas of Italy(15). European wild boar populations from Lithuania, Slovenia, Sweden, the Czech Republic and the Netherlands have been monitored for anti-SVDV antibodies and only a single reactor was found in Sweden in 2008(16). Unlike Foot-and-mouth disease virus, SVDV does not affect ruminants, although sheep can become seropositive after exposure to the virus. Until now, the only species found to become infected, display clinical signs and excrete SVDV is the domestic pig. The wild boar (Sus scrofa), being the same species as the domestic pig, may be susceptible to SVDV infection.
SVDV in the environment is highly resistant and is able to survive up to 2 years in meat products. It resists destruction by common disinfectants. The domestic pig is assumed to be the natural reservoir of SVDV Wild boar are potential reservoirs; however, no evidence of SVDV infecting wild boar has been found in Europe so far, except for the single reactor found in Sweden.
Transmission takes place by direct contact, through fomites and by ingestion of infected meat products. The gastrointestinal tract is the main site of entry.
Infection by the oral route may require up to 300,000 infectious units, whereas infection through skin abrasions or wounds requires a much lower dose, of 100 infectious units(17). The virus first replicates in the gastrointestinal tract. Viraemia peaks at 2—4 days after infection and the virus persists for up to 7 days in blood and various tissues. Experimentally the first replication site of SVDV in the epithelium is the stratum spinosum. SVDV is excreted by oronasal secretions from 48 hours before the onset of clinical signs to 2 weeks after it. Faecal excretion may last for 4 months. Vesicle formation is the only gross pathologic finding. Microscopic lesions in the epithelium consist of hydropic degeneration and intracellular oedema followed by necrosis.Clinical manifestations of SVD appear 1—5 days after infection, and affected animals usually recover 7 to 14 days after this. Clinical signs may vary, depending on the individual, the viral strain and the infectious dose. Animals develop a high fever, followed by lameness and formation of vesicles in the mouth, snout, feet and teats. Vesicles may affect the whole coronary band, resulting in loss of the hoof. A subclinical status has occurred frequently in Italian domestic pigs(15).
A presumptive diagnosis is based on clinical signs; however, some cases show no clinical manifestations. The diagnosis of SVDV is confirmed by virus isolation in cell cultures, or virus detection by the antigen-capture ELISA followed by RT-PCR to avoid inhibition of the polymerase activity. ELISA serology is useful as well, but some infected animals may be seronegative. The performance of serological tests has not been evaluated in wild boar.
SVD is a notifiable disease, because it is clinically and pathologically indistinguishable from foot-and-mouth disease in pigs. Where SVDV is diagnosed in animals, eradication by slaughter is performed, with consequent pig-trade restrictions.
The low virulence of SVDV, together with its proven absence in many European wild boar populations, suggests that there is no effect on the population dynamics of wild boar.
Encephalomyocarditis
FRANK KOENEN
CODA-CERVA, OD: Interaction and Surveillance, Belgium
aetiology
Encephalomyocarditis virus (EMCV) is an RNA virus and belongs to the genus Cardiovirus of the family Picornaviri- dae. The encephalomyocarditis (EMC) virions contain a single strand of RNA of 2.6 ? 106 daltons, and are enclosed in a protein capsid shell. In contrast to the high antigenic stability, the D region of EMCV (coding for capsid protein VP1) displays considerable genetic variability. A single mutation in a nucleotide sequence can be involved in attenuation or responsible for the diabetogenic nature of a particular EMCV strain(18). Limited variability between pig and rodent EMCV isolates has been noticed(19).
epidemiology
Little is known about the distribution of EMCV in European wildlife. A research project funded by the EU detected evidence of exposure in rats (Rattus rattus, Rattus norvegi- cus), voles (Apodemus sylvaticus, Microtus arvalis), mice (Mus musculus) and wild boar (Sus scrofa). The seroprevalence in wild boar varied between 0.6 and 10.8%, and a study in Belgium found a prevalence of viral infection of 3.3%(20). Even though field samples from red foxes ( Vulpes vulpes) were negative, animal experiments proved foxes to be susceptible to EMCV infection(21).
Rodents are considered to be the natural hosts of EMCV. Rattus rattus, Rattus norvegicus, Apodemus sylvaticus, Sciurus vulgaris leucorourus, Microtus arvalis, Myoxus glis and Mus musculus were found to be susceptible to EMCV. In these rodents, the virus usually persists without causing disease. Conversely, African multimammate rats (Mastomys spp.) develop clinical disease and die. Infected rodents show high levels of the virus in their tissues, and excrete the virus in faeces and urine. Rodents are thought to play a role as reservoir, introducing and spreading the virus via their faeces or maintaining the infection as infected carcasses.
As rodents are the natural hosts, the presence of EMCV or EMC antibodies often indicates virus circulation among a wide range of other species. EMC viruses have been isolated from over 30 host species, including mammals, birds and insects. In mammals, the host range includes monkeys, chimpanzees, elephants, lions, squirrels, mongooses, raccoons and swine (domestic and wild boar). An episode of lion deaths at a zoo was found to be due to feeding carcasses of African elephants that died of EMCV infection1-22). From all outbreaks in wildlife those in elephants are most documented, including an outbreak in Taronga Zoo, Australia1-23) and in wild elephants in the Kruger National Park(24).EMCV has been isolated from a variety of arthropods, including mosquitoes, ticks, houseflies and fleas. Experimental infection of arthropods and arthropod cells with EMCV failed to show any virus replication; however, in some cases virus could be detected for up to 3 months (R.S. O’Hara, L. Bell-Sakyi and N.J. Knowles, unpublished data).
In certain European countries a seasonal pattern of the outbreaks in pigs, with peaks in autumn, was noticed(20), but similar data are not available for wildlife.
Experimentally, EMCV may be transmitted by the oral, intranasal, aerosol, intratracheal and parenteral routes. Infection appears to be influenced by virus strain, viral dose, passage history and susceptibility of the individual animal(19). In elephants, rodents have been demonstrated to be the source of infection. Feed, including grass, and water contaminated with EMCV by rodent excreta, or infected rodent carcasses and tissues from infected animals, are considered important sources of natural infection.
PATHOGENESIS, PATHOLOGY AND IMMUNITY
Natural infection is most likely to occur by the oral route. After experimental oral infections in young pigs, virus was isolated at 6 hours post-inoculation (hpi) from the intestinal tract and lymph nodes and was observed in macrophages in the tonsils.
The tonsils were considered to be the portal of entry of the virus and migration of infected macrophages the route or viral dissemination to all organs. At 12hpi EMCV was observed by immunolabelling in scattered degenerated myocardiocytes; myocarditis developed subsequently and progressed in severity. The virus replicated intensively in the cytoplasm of myocardiocytes, and this was followed by viraemia. In this second phase of viral replication, some animals died with typical post mortem lesions, including discoloration of the heart. EMCV was observed by immunohistochemistry in the tonsils and in the heart. Three days post-inoculation (dpi), virus was also isolated from liver, kidneys, spleen and lungs. Animals that survived the infection produced EMCV antibodies. After antibody formation, the virus detection decreased(25).The pathogenesis of transplacental infection with EMCV in pregnant suidae is not well understood. In rats experimentally infected with a myocardial EMCV strain, no clinical or macroscopic lesions were observed in any organs. Virus was isolated most frequently from Peyer’s patches and thymus, indicating that these tissues represent a site of persistence after oral infection(26). In mice, certain strains cause predominantly fatal encephalitis, widespread myocardial damage or even specific destruction of pancreatic beta cells, but there are no studies of pathogenesis in wild rodents.
Not all suidae dying from the acute phase of cardiac failure show gross lesions — the only lesion may be epicardial haemorrhage. Hydropericardium, hydrothorax and pulmonary oedema develop as a result of cardiac dysfunction and are frequently observed at necropsy. The heart is usually enlarged, soft and pale. The most striking lesions are found in the myocardium and consist of multiple greyish-white foci of variable size. Infected fetuses usually appear normal but can be haemorrhagic and oedematous. With some strains, fetuses can become mummified in various states of development, depending on the time of infection.
Macroscopic myocardial lesions are exceptionally rare(19). Some EMCV strains can induce encephalitis, myositis or pancreatitis. However, a species predilection for the development of lesions in particular organs has been reported(27).Histopathologically, the most significant findings are seen in the heart, the target organ of EMCV. The positive immunohistochemical reaction is mainly localized in the cytoplasm of myocardial cells as a fine and granular signal. Sometimes, the antigen is detected in the Purkinje fibres and in the endothelial cells next to those. Mineralization of myocardium is common but not always present. In the tonsils, the antigen is located in necrotic debris filling the crypts and in the cytoplasm of monocyte-macrophage lineage cells. This last finding also occurs in lymph nodes(25). In the brain, congestion, meningitis, perivascular infiltration by mononuclear cells and neural degeneration may be observed.
CLINICAL SIGNS AND TREATMENT
I nfection with ECMV is generally subclinical in a wide range of species. Sometimes animals are found dead without prior signs of illness. In young pigs, in fast-growing fattener pigs and in elephants the most striking clinical signs are acute congestive heart failure with massive pulmonary oedema. In other age groups dyspnoea can be noticed. In breeding females clinical signs may vary from none to various forms of reproductive failure, including abortion and increased numbers of mummified and stillborn fetuses(19).
There is no treatment but, in the acute phase, mortality may be minimized by avoiding stress or excitement.
DIAGNOSIS
A conclusive diagnosis of EMCV is demonstrated by virus isolation. BHK-21 cell culture is the most sensitive method for virus isolation. Infected cell monolayers show a rapid and complete cythopathic effect. Serial passage of EMCV in cell culture can alter in vitro growth characteristics, reduce virulence and affect haemagglutinating activity1-28). The development of nucleic acid probes or RT-PCR for the detection of EMCV has been reported1-19). Of all serological tests virus neutralization (VN) and ELISA are the most frequently used methods and have been shown to be specific1-28). For VN, antibody titres of >1:16 appear to be significant. VN- antibody reaction starts as soon as 7 dpi and may persist for an extended period of 6 months or one year(19).
MANAGEMENT, CONTROL AND REGULATIONS
An inactivated EMCV vaccine for pigs is commercially available in the USA. The vaccine appears to be effective, as high humoral immunity is detected in vaccinated pigs. Vaccinates were protected from clinical disease when challenged with virulent EMCV that killed 60% of unvaccinated controls, but its efficacy in wildlife has not been documented. A safe and effective aziradine-inactivated vaccine containing high levels of viral antigen and an oil adjuvant was developed following the EMC epidemic in free-ranging elephants in South Africa)24).
PUBLIC HEALTH CONCERN
Currently, the impact of EMCV on public health is believed to be minimal. Despite the frequency of infection in swine, no association between infection and transmission of disease to humans has been recorded)28), even in persons at the greatest risk (veterinarians, animal caretakers, laboratory staff). A serological survey in Austria demonstrated a seroprevalence in hunters of 15% and of 8% in the urban control group. In the light of the ubiquitous presence of EMCV around the world in several animal species, including primates after a rodent plague)29), secondary infections in immunocompromised persons may possibly be anticipated. Recently, EMCV has been isolated from cases of febrile illness in humans in Peru)30); interestingly, these viruses were most closely related to EMC viruses isolated from European pigs.
Significance and implications for
ANIMAL HEALTH
Several reports show the role of wild rodents in the transmission of EMCV to pigs, but no data are available to prove the spread from EMCV from wild boar to domestic pigs)21). Transmission from wild animals to other domesticated species has not been reported.