Kerala, India
Arboviruses - Viral Disease in Emu
Eastern equine encephalomyelitis (EEE), western equine encephalomyelitis (WEE) and St Louis encephalitis viruses have been isolated from ratites in the USA. Initial reports indicated that emus were the only birds affected on multi-species farms. A recent report indicates that an apparent outbreak of EEE occurred in a flock of rheas in southern Florida. The concentration of non-native avian species adjacent to wooded land, swamp or irrigated areas provides an environment suitable for mosquitoes and other arthropod vectors. Mosquitoes transmit infection from passeriform reservoirs to emus and rheas which are accidental hosts.
The clinical manifestation of EEE in emus is a peracute haemorrhagic gastroenteritis. Cases of WEE and St Louis encephalitis develop the typical neurological signs of progressive depression and paralysis. The encephalitis viruses are zoonotic, and appropriate precautions should be taken when performing necropsy procedures on viraemic subjects. Ratites are not involved as amplifiers of equine encephalitis or St Louis encephalitis viruses within endemic areas. Vaccinated birds develop a protective antibody titre of unknown duration. It is currently recommended that a bivalent EEE/WEE inactivated tissue-culture origin vaccine be administered twice-yearly to emus and rheas raised in an area endemic for these arboviruses. Young birds should receive the first dose at six weeks of age, with a booster four weeks later. Unvaccinated birds should be vaccinated twice, at four-week intervals, before being subjected to the twice-yearly programme. A full dose of the equine vaccine should be administered to birds of all ages. This is not an approved vaccine for ratites in the USA, and administration is subject to the ethical and legal restraints of using the vaccine in non-approved species. Owners must receive relevant information before informed consent is granted for vaccination. Antibody response to vaccination can be monitored using the complement fixation procedure.
Avian Influenza in Emu
Type A orthomyxoviruses are classified according to haemagglutinating and neuraminidase activity of glycoprotein in the viral envelope. A range of avian influenza strains have been isolated from ostriches in South Africa and rheas in the USA exhibiting various clinical syndromes ranging from high mortality to inapparent infection.
The ability of avian influenza virus to mutate and spread among avian species makes this a potentially dangerous infection for commercial poultry. Transmission can occur from wild birds coming into contact with ratites, from newly-introduced, non-quarantined stock or indirectly via the clothing of visitors and transport trailers. The risk of exposure can be reduced by operating closed facilities with strict biosecurity measures, including control over entry of personnel.
Infection with influenza strains of low pathogenicity may be treated with supportive therapy. Virulent strains result in high mortality in young ostriches, which die despite administration of fluids and antibiotics. Prevention requires application of biosecurity measures and quarantine of newly-introduced birds, which should be monitored for the presence of antibodies using the agar gel precipitation procedure prior to purchase or delivery. It is possible that inactivated vaccines against specific strains of influenza may be permitted by the United States Department of Agriculture, Animal and Plant Health Inspection Service, which is the Federal agency responsible for control of exotic and catastrophic diseases in the USA.
Newcastle Disease Virus in Emu
Newcastle disease (ND) is caused by paramyxovirus type 1, which affects galliforms, passeriforms and psittacines in addition to other families, including ratites. Velogenic viscerotropic ND (VVND) has been isolated from ostriches showing high mortality in Israel and from rheas in Brazil. The disease manifests clinically as nervous signs, and 80% mortality occurs experimentally with infected ostrich chicks. Countries which are free of VVND, such as the USA, maintain restrictions on the importation of domestic, companion and exotic bird species, together with quarantine at the point of entry. The disease is spread by direct and indirect contact with infected carriers and fomites. Air-borne dissemination of virus may occur for up to 4 km.
Ratites should be vaccinated only in areas where VVND is endemic. Vaccinated ratites produce antibodies which will be detected if a test and depletion control programme is implemented in a non-endemic area. This may result in compulsory slaughter or rigid and extended quarantine.
Adenovirus in Emu
Adenoviruses have been implicated as the cause of wasting disease in young ostriches in the USA. The clinical signs associated with these outbreaks included non-specific wasting, anorexia and depression. Specific adenoviruses are pathogenic in poultry and are responsible for haemorrhagic enteritis in turkeys, and inclusion body hepatitis/hydropericardium syndrome and egg drop syndrome in chickens.
Adenoviruses are transmitted vertically by the transovarian route. Adenoviral infection may remain dormant in chickens until onset of production or exposure to an environmental stress. In the cases observed in the USA, mortality in ostriches occurred at approximately two months of age. Affected birds showed anorexia, emaciation and diarrhoea. Isolation and identification of an adenovirus from moribund or dead birds is required to confirm the diagnosis. It is noted that adenoviruses are ubiquitous, and considerable research is still required to define the role of the agent in the ‘fading chick’ syndrome. There is no diagnostic procedure to determine the carrier status of an asymptomatic breeder ostrich. Poultry producers in countries other than the USA administer homologous, inactivated oil-emulsion vaccine to control specific infections including egg drop syndrome, which is exogenous to the USA. At this time, it is considered advisable to maintain a closed breeding flock or purchase immature ostriches from a facility known to be free of the condition. Custom hatching and brooding may disseminate the infection, which can be spread by lateral exposure to ‘shedders’. The total impact of adenovirus infections in the ratite industry is not known at this time, but biosecurity measures are required to prevent adenoviral infection restraining the expansion of production, especially in the large units required for commercial multiplication of slaughter stock.
Avibirnavirus
Infectious bursal disease virus (IBDV) is responsible for immune suppression in young chickens as a result of the destruction of immature lymphocytes in the bursa of Fabricius. Avibirnavirus identical to IBDV has been isolated from the bursa of immature ostriches in flocks in California and Florida where high flock mortality was observed. Affected birds do not show specific clinical signs, but depression and anorexia may precede death. Isolation and identification of the virus is required for confirmation. Supportive treatment and antibiotic therapy is recommended due to immune suppression. Administration of infectious bursal disease vaccines licensed for chickens is not recommended. The response of a ratite to live attenuated chicken vaccine is unknown, and the vaccine virus could be deleterious to an unnatural recipient. It must be remembered that IBDV can persist for up to ninety days in biological material, necessitating thorough disinfection of brooding facilities. Diligent biosecurity is necessary to prevent exposure to the highly infectious agent.
Avipoxvirus
Pox has been diagnosed in ostriches in the USA, Israel and South Africa. The virus can be transmitted by mosquitoes or by direct contact with a pox lesion.
Immature ostrich chicks aged between two weeks and one year are most susceptible. The cutaneous form of pox is characterized by persistent proliferative lesions of 0.5-2.0 cm in size, on the eyelids, beak, wing and toes. The diphtheritic form produces tracheitis, stomatitis and severe dyspnoea in affected birds. Pox can be confirmed by histopathological examination of affected tissue, which shows the presence of intracytoplasmic Bollinger bodies. Affected birds should be separated and given supportive therapy, including systemic and topical antibiotics, to prevent secondary bacterial infections. All unaffected birds should be vaccinated with a commercial fowlpox vaccine via the intradermal route, in accordance with the recommendation of the manufacturer for reconstitution and administration. Control of mosquitoes may be possible but rapid diagnosis and vaccination is the most appropriate preventive measure.
Viral Enteritis
A wide range of bacteria and viruses have been isolated from the gastrointestinal tract of one- to three-week-old ostriches from flocks with high levels of morbidity and mortality. Isolates include coronaviruses and adenoviruses which may not be the primary cause of death. Intra-flock transmission usually occurs rapidly through faecal/oral contamination, and may be exacerbated by high stocking density or defects in biosecurity and hygiene. Clinical signs associated with viral enteritis are non-specific but generally include anorexia and diarrhoea. Due to the presence of opportunistic bacteria, it may be difficult to identify a viral agent during pathological examination. Virus identification by histopathology or electron microscopy and examination of faecal material may help in identifying the viral organism responsible for the clinical illness. Supportive therapy, intravenous fluid administration, tube feeding and antibiotics may help reduce flock deaths. The best way to prevent outbreaks of viral enteritis is through proper management hygiene before the introduction of new birds into a production unit, and through biosecurity in established flocks.
Borna Disease Virus
Borna disease virus (BDV) has been identified in Israel in ostrich chicks (two to eight weeks old) raised under intensive conditions. Clinical signs are initiated by paresis and general malaise, which contribute to anorexia and depression. The affected birds usually die within four to eight days, due to dehydration. Intensive farming methods seem to be a contributing factor in BDV outbreaks, with insect vectors believed to be responsible for transmission of the virus.
Histopathological lesions in affected birds show neuronal degeneration and lymphocytic perivascular cuffing. An enzyme-linked immunosorbent assay has been developed in Israel to demonstrate the presence of BDV in brain tissue of infected birds. Prevention of Borna disease outbreaks in intensive ostrich-raising facilities will require stress-reduction management programmes. For a Borna disease management programme, an inactivated viral vaccine must be developed, to immunize parent stock and thus provide maternal immunity to the offspring. At present, BDV is restricted to intensive ostrich-farming facilities in Israel, but the current popularity of ratite importation increases the likelihood of virus spread. Good husbandry, management and biosecurity will reduce the stress on birds and consequently, reduce the risk presented by this viral organism to susceptible birds in Israel and other parts of the world which are affected.
Other Viruses
For some of the viral mortality associated with disease outbreaks in ratite facilities, no aetiological agent has been identified. The capacity of ratites to harbour and grow multiple pathogens within the gastrointestinal and respiratory tracts makes the isolation of unknown viral agents difficult. Rotaviruses, reoviruses, parvoviruses and picornaviruses are all potential pathogens which may contribute to ratite morbidity and mortality, but have not yet been identified in these species. With increased observation and diligence by clinical veterinarians and pathologists, other ratite viral pathogens will be discovered in the future, thereby helping to ensure healthy production within the industry.