Newcastle disease is endemic in Nigeria. The disease has been the most important disease of chickens in the country. Newcastle disease continues to be a serious economic threat to the poultry industry causing increased morbidity and mortality and loss of eggs for both breeding and human consumption. The disease is also known to cause some eggshell defects. The impact of Newcastle disease is most notable in domestic poultry due to the high susceptibility of poultry and the severe consequences of virulent strains on the poultry industries. The disease can result in significant economic impact from high morbidity and mortality accompanied by the inability to export product from flocks not known to be infected. There are numerous examples throughout the world in recent years creating sizable trade barriers. Additionally, when commercial poultry are infected with NDV strains of low virulence there may also be a negative economic impact, because of a reduction in productivity of the infected birds.
Causes of Newcastle disease
Newcastle disease is caused by Newcastle disease Virus (NDV) known as avian paramyxovirus-1 (APMV-1). It is one of the most important infectious diseases of poultry. It is distributed worldwide and has the potential to cause large economic losses in the poultry industry. Newcastle disease is a reoccurring concern to poultry industries internationally and outbreaks caused by virulent strains of Newcastle disease virus (NDV) are referred to as Exotic Newcastle disease (END) in the United State.

Clinical signs of Newcastle disease
Newcastle disease can cause clinical signs varying from subclinical infections to highly virulent (up to 100% mortality), depending on the susceptibility of the host and the pathotype of the NDV.
The Newcastle disease virus is categorized into velogenic mesogenic, lentogenic and asymptomatic enteric strains on the basis of their pathogenesis and virulence.
The velogenic strains cause acute fatal infection of chickens of all age groups with clinical findings of nervous signs or extensive hemorrhagic lesions in the gastrointestinal tract. The mesogenic strains are of intermediate virulence and cause moderate respiratory signs with occasional nervous signs and can seriously reduce egg production while the lentogenic strains cause mild inapparent infections.
Velogenic strains often produce a rapidly developing weakness and birds may die without showing any other clinical signs. Birds that survive for a long time may experience gasping breath due to the involvement of the respiratory organs, or muscular tremors and spasm, torticollis and paralysis. Green diarrhea is sometimes present. Mortality is very high with strains of high pathogenicity. Respiratory signs dominate the early clinical phase. There is severe respiratory distress with gasping and sometimes coughing. Egg production falls dramatically in surviving birds and signs of nervous involvement may develop. These include paralysis and torticollis. Mortality is variable. Lentogenic strains tend to infect adult birds without causing severe disease but produce mortalities in young chickens when other disease agents are present. The respiratory organs are usually affected. Immune levels of the flock modify clinical signs. Chicks of immune hens are protected for some time by antibodies that are transmitted through the yolk. Birds whose immunity is sufficient to suppress clinical signs completely may still shed the virus in their faeces.
Prevention and control of Newcastle disease
The most important measures for prevention of introduction of NDV and its spread in a farm during outbreaks are the control of those conditions under which the birds are reared and the maintenance of high degree of biosecurity practices at the farm. Vaccination is never an alternative to good management practice, biosecurity and good hygiene in rearing domestic poultry but Vaccination should be employed in conjunction with them. Vaccination is usually protective, although it is easier to protect against fatal disease than against loss of production.

http://www.agricincome.com/newcastle-disease-poultry/
cc;lalasticlala