New Castle Disease

Newcastle disease is an acute viral disease of domestic poultry and many other bird species. It is a worldwide problem that known as a respiratory disease, but depression, nervous manifestations, or diarrhea may be the predominant clinical form. Mortality is variable. If occurrence of a virulent form of the disease is reported, that may result in trade restrictions.

Etiology and Pathogenesis:

Newcastle disease is caused by an RNA virus, Newcastle disease virus (NDV), synonymous with avian paramyxovirus-1 which is in the genusAvulavirus, family Paramyxoviridae. Isolates are classified into 1 of 3 virulence groups by chicken embryo and chicken inoculation as virulent (velogenic), moderately virulent (mesogenic), or of low virulence (lentogenic). Lentogenic strains are used widely as live vaccines in healthy chickens. Clinical signs vary from high morbidity and mortality to asymptomatic infections. Mostly Chickens and rarely waterfowl are susceptible.

Epidemiology and Transmission:

Virulent NDV strains are endemic in poultry in most of Asia, Africa, and some countries of North, Central, and South America. Other countries, including the USA and Canada, are free of these strains and maintain this status by import restrictions and eradication by destroying diseased poultry. Cormorants, pigeons, and imported psittacine species have also been sources of virulent NDV infections of poultry. Low virulence NDV is prevalent in poultry and wild birds, especially waterfowl which leads to low production.

Infected birds shed virus in exhaled air, respiratory discharges, feces, during incubation, during disease, for a varying but limited period during convalescence, in eggs laid during clinical disease and in whole carcass during acute virulent infections. Chickens get infected by aerosols and by ingesting contaminated water or food. Infected chickens are the primary source of virus, but other domestic and wild birds may be sources of NDV.Horizontal Transmission of virus happens through  infected feces, by the workers and contaminated equipment.

Clinical Findings:

Onset of disease is fast, signs appear throughout the flock within 2-12 days (average 5) after aerosol exposure. Spread is slower in case of fecal-oral route, particularly for caged birds. Young birds are the most susceptible. The clinical signs depend on whether the infecting virus has a predilection for respiratory, digestive or nervous systems. In low virulence infections respiratory signs of gasping, coughing, sneezing, and rales predominate. Nervous signs of tremors, paralyzed wings and legs, twisted necks, circling, colonic spasms, and complete paralysis may accompany, but usually follow the respiratory signs in neurotropic velogenic disease. Nervous signs with diarrhea are typical in pigeons, and nervous signs are frequently seen in cormorants and exotic bird species.In most virulent form of the disease, respiratory signs with depression,watery-greenish diarrhea, and swelling of the tissues of the head and neck. In viscerotropic velogenic Newcastle disease (VVND, also called exotic Newcastle disease), although nervous signs may also be seen. Varying degrees of depression and inappetence, a partial or complete loss of egg production. abnormality in eggs color, shape, surface,with watery albumen. Mortality is variable but can be as high as 100%.

Lesions:

Remarkable gross lesions are usually observed only with VVND. Petechiae may be seen on the serous membranes; hemorrhages of the proventricular mucosa and intestinal serosa are accompanied by multifocal, necrotic hemorrhagic areas on the mucosal surface of the intestine, especially at lymphoid foci such as cecal tonsils. Splenic necrosis and hemorrhage and edema around the thymus may also be observed. In contrast, the lesions in birds infected with lower virulence NDV strains may be limited to congestion and mucoid exudates seen in the respiratory tract with opacity and thickening of the air sacs, lesions increase with secondary bacterial infections.

Diagnosis:

Diagnosis may be confirmed by isolation of a hemagglutinating virus identified by inhibition with Newcastle disease antiserum. A rise in hemagglutination-inhibition antibodies in paired serum samples also confirms the disease. Diffential diagnosis in acute form with highly pathogenic avian influenza is required. Virulence of an isolate is established by the rapidity of killing day-old chicks inoculated by the intra-cerebral route, the intra-cerebral pathogenicity index, or by the presence of a specified amino acid motif at the cleavage site of the fusion protein (F) precursor (FO). Reference laboratories use monoclonal antibodies to detect antigenic differences and nucleotide sequence analysis to detect genetic differences for comparison of isolates from different outbreaks and to identify the source of those infections.

Prevention:

Live lentogenic vaccines, chiefly B1 and LaSota strains, are widely used in poultry by mass application in drinking water or by spray. Alternatively, individual administration is via the nares or conjunctival sac. Healthy chicks are vaccinated as early as day 1-4 of life. However, delaying vaccination until the second or third week avoids maternal antibody interference with an active immune response.Mycoplasma, some other bacteria, and other viruses affecting the respiratory tract, sometimes aggravate the vaccine reaction after spray administration.

Oil-adjuvanted inactivated vaccines are also used following live vaccine in breeders and layers or alone, when use of live virus may be contraindicated. Where virulent NDV is endemic, a combination of live virus and inactivated vaccine can be used. Alternatively, if law permits, a live mesogenic strain vaccine may be used in older birds. The frequency of revaccination largely depends on the risk of exposure and virulence of the field virus.

Zoonotic Risk:

Newcastle disease viruses, whether virulent field viruses or live vaccine, can cause transitory conjunctivitis in humans. But this happens to people working in laboratory and vaccination production, exposed to large quantities of virus. Conjunctivitis from NDV infection used to occur in workers of poultry processing plants, when poultry vaccination was not in practice. The disease is not reported in people who rear poultry or consume poultry products.
Reference :- Merck Veterinary Manual
 
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