Etiology
Antigenically related strains of Avian paramyxovirus, type 1.

Occurrence and Economic Significance
Three categories of viral pathogenicity result in different clinical forms of the disease.

• Velogenic-viscerotropic virus (vvNCD) infection results in acute onset, highly lethal disease.
• Mesogenic virus causes acute, moderately lethal disease with nervous and respiratory signs.
• Lentogenic virus is responsible for mild respiratory infection.

The lentogenic form is responsible for erosive losses in broilers including lowered gain and feed conversion efficiency and elevated mortality and condemnation. The severity and financial impact depends on climatic and management stress and intercurrent exposure to pathogenic E. coli and other viral respiratory disease and immunosuppressive agents.

The cost and consequences (respiratory stress) of vaccination are significant, especially during winter and following immunosuppression.

Transmission
NCD virus is highly contagious. Infection occurs either by the inhalation of virus in aerosol form or ingestion of contaminated feed or litter.

• Wind dispersal may occur over distances of 5 km.
• Direct and indirect contact with contaminated material (fomites) is associated with deficiencies in biosecurity.
• Companion birds, backyard flocks and gamefowl serve as reservoirs.

Mesogenic
Variable to high morbidity is evident in the flock with moderate mortality characterized by nervous and respiratory signs. An acute drop in egg production occurs in mature flocks.

Lentogenic
Acute onset with moderate to high morbidity. Mild to inapparent respiratory signs are noted but negligible mortality occurs in uncomplicated cases. Lentogenic NCD may be responsible for asymptomatic drops in egg production in incompletely immunized commercial layer or breeder flocks.

Pathology
Velogenic
Prominent hemorrhages occur throughout the digestive tract especially in the mucosa of the proventriculus and gut-associated lymphoid tissue. Severe tracheitis and pulmonary congestion are evident in acute cases. These changes are not specific to vvNCD and may be observed with highly pathogenic strains of avian influenza.

Lentogenic
Mild conjunctivitis and tracheitis are observed. Recovered flocks may show airsacculitis due to secondary infection with E. coli.

Diagnosis
Isolation, identification and characterization of the virus by a suitably equipped laboratory is the usual confirmatory procedure.

Retrospective serology (ELISA, hemagglutination inhibition and serum-virus neutralization) demonstrates the presence of antibodies which indicates exposure to NCD virus and the titer (level) can differentiate between field infection and previous vaccination.

Prevention
Vaccination. Conventional programs:

• Lentogenic infection of broilers can be prevented by day old administration of aerosol or eye drop vaccine using Hitchner B1 with subsequent boosters in drinking water or by the aerosol route.
• Lentogenic infection of breeders can be prevented by 10 day administration of Hitchner B1, by aerosol or eye drop route.
• Subsequent vaccinations include 24 day, and 8 week Hitchner B1 or LaSota in water, followed by multivalent oil inactivated emulsion at 18-20 weeks. An optional 45 week multivalent oil inactivated emulsion may be administered depending on antibody titer of the flock, risk of exposure, and other factors relating to the operation.

In countries with endemic vvNCD, more rigorous programs are implemented, incorporating day-old subcutaneous emulsion vaccine together with attenuated live vaccine by the eye-drop route. Hitchner or LaSota vaccine is administered to broilers by the aerosol route at 10 day intervals thereafter. Breeders may be immunized with mesogenic-strain vaccines in some countries. This expedient is only justified if birds have previously received one or more live attenuated lentogenic vaccines.