Newcastle disease (ND) is a highly contagious viral disease that affects both domestic and wild birds worldwide. ND is caused by the Newcastle disease virus (NDV), also referred to as avian paramyxovirus (APMV). NDV belongs to the genus Avulavirus
within the family Paramyxoviridae
. There are several different strains of the NDV.
NDV strains were originally classified into three virulence groups by chicken embryo inoculation as virulent (velogenic), moderately virulent (mesogenic), or as low virulence (lentogenic). For regulatory purposes, this classification system was modified so that velogenic and mesogenic viruses are now classified as virulent NDV, the cause of ND, and lentogenic viruses are the low virulence NDV that is used in the manufacturing of the vaccines. The most severe strain of the virus is called viscerotropic velogenic Newcastle disease (VVND), which is often referred to as 'Exotic Newcastle Disease'.
Symptoms of Newcastle Disease in Chickens
Clinical signs vary depending on the system affected. Some virus strains attack the nervous system, others the respiratory, or digestive system. The most characteristic signs of ND in chickens are manifested as a result of central nervous system dysfunction, which consist of:
- Incoordinated (ataxia) walk and movements, where affected birds may be seen stumbling frequently, and appear to lack proper balance.
- Abnormal head and neck position, resulting in head tilt or wry neck appearance.
- Unilateral or bilateral partial or complete paralysis of their legs and wings.
However, the above-described clinical signs are not always seen in chickens with ND. Many affected birds may only show signs of general weakness and prostration. Diarrhea with hemorrhage is actually a classical clinical sign of the highly pathogenic (visceral, velogenic) ND in domestic chickens.
The severity of ND varies widely and is dependent on factors such as: the strain of the virus, the age of the chicken (young chicks are more susceptible), concurrent infection with other organisms, stress and immune status.
How Newcastle Disease is Transmitted to Chickens
NDV is spread by infected birds shedding the virus in their feces, body fluids (such as respiratory secretions from the nares, mouth, and eyes), and eggs. When birds are infected with NDV that may shed large amounts of the virus. Usually all members of the flock become infected within 2 to 6 days. Infected wild waterfowl (ducks and geese) can maintain low virulence strains of the NDV to themselves and to other species, including chickens.
How Long the Virus survives in the Environment
NDV can survive for long periods of time outside of a living host, and can remain infective for several weeks in the environment, especially at low temperatures or when protected by associated organic material (such as bedding litter, carcasses, water, eggs, and feathers). However, the virus is destroyed rapidly by exposure to ultraviolet rays in sunlight.
NDV Incubation period
The period between infection and the appearance of the first clinical signs of disease is generally between two and six days, but can be up to 15 to 21 days. It spreads rapidly through a flock. Incidents have occurred in severe virus strains, where the majority of the flock dies within 72 hours of infection, without showing any prior signs of disease.
How Newcastle disease is Diagnosed
ND cannot be diagnosed based on clinical signs or post mortem
lesions alone. In order to obtain a definite diagnosis of whether a chicken is infected with the NDV, one of the following three events needs to be confirmed via special laboratory tests :
- Identification and pathotyping of the virus.
- Demonstration of viral genetic material within the lesions.
- A significant rise in antibody titre between acute and convalescent sera coinciding with a disease outbreak.