Newcastle disease is caused by viruses in the serotype avian paramyxovirus type 1 (APMV-1), also called Newcastle disease viruses (NDV). The viruses are members of the genus Avulavirus in the family Paramyxoviridae. APMV-1 strains are classified into three pathotypes based on their virulence in chickens. Lentogenic strains are the least virulent, mesogenic strains are moderately virulent and velogenic strains are the most virulent.
Velogenic NDV is endemic in Asia, the Middle East, Africa, Central and South America, and in wild birds of North America. Lentogenic isolates are found in poultry throughout the world.
Outbreaks of velogenic ND have a tremendous impact on backyard chickens in developing countries where these birds are a significant source of protein. Morbidity and mortality rates can be up to 90%. The total yearly cost of the disease is estimated to be US $2 billion.
NDV can be transmitted by inhalation or ingestion (faecal/oral route). Birds shed virus in faeces and respiratory secretions. APMV-1 is readily transmitted on fomites. Virus survival is prolonged on eggshells and in faeces.
The incubation period in poultry varies from two to 15 days. Lentogenic strains usually cause subclinical infections or mild respiratory disease. Mesogenic strains can cause acute respiratory disease and neurologic signs in some chickens but the mortality rate is low. Velogenic strains cause clinical signs, which vary from sudden death to lethargy, conjunctival reddening and swelling of the head, diarrhoea and respiratory signs. They also cause neurologic signs such as tremors, paralysis and torticollis (twisted neck).
Outbreaks in large flocks are eradicated with quarantine, depopulation of all infected and exposed birds, and thorough cleaning and disinfection of the aviaries. Vaccination against ND is available for the free-range chickens in endemic areas.
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