As you know there are respiratory; nervous, intestinal symptoms for both clinical and subclinical infections!
Newcastle Disease can be classified into five different categories:
■ Viscerotropic velogenic, a highly pathogenic form in which hemorrhagic intestinal lesions are frequently seen.
■ Neurotropic velogenic, a form that presents with high mortality, usually following respiratory and nervous signs.
■ Mesogenic, a form that presents with respiratory signs, occasional nervous signs, but low mortality.
■ Lentogenic or respiratory, a form that presents with mild or subclinical respiratory infection.
■ Asymptomatic: a form that usually consists of a subclinical enteric infection.
With velogenic viruses, the disease may appear suddenly, with high mortality occurring in the absence of any other clinical signs. In other cases, clinical signs often begin with listlessness, increased respiration and weakness, diarrhea, ending with prostration and death.
In cases involving the neurotropic velogenic isolates, neurological signs such as torticollis or colonic movements of the head or the legs, are commonly observed a few days after infection has started. A dramatic drop in egg production can be seen in layers and breeders. Morbidity rate may reach 100%.
The clinical signs induced by isolates classified as mesogenic are usually limited to respiratory signs and drop in egg production in laying hens. Nervous signs may occur but are not common. Mortality rate is generally low.
Finally, lentogenic strains do not usually cause disease in adults, but in young, fully susceptible birds, respiratory problems can be observed. Such reactions can be complicated by infections with other pathogens.
Apathogenic strains induce no clinical sign (asymptomatic form). Likewise, for clinical signs, the extent and location of the gross lesions depend on the virus strain, the host conditions and all those aforementioned factors affecting the severity of the disease. Additionally, there are no pathognomonic lesions associated to ND.
In the respiratory tract, mucosal hemorrhages, marked congestion of the trachea and airsacculitis may be observed. Hemorrhagic lesions in the digestive tract particularly in the mucosa of the proventriculus, caeca, small intestine and lymphoid tissues such as caecal tonsils and Peyer’s patches are commonly seen.
In laying hens, egg yolk in the abdominal cavity and flaccid and degenerative ovarian follicles are often found. Finally, even with chickens showing nervous signs, gross lesions are not observed in the central nervous system.
ND in its velogenic form is a notifiable disease hence proper diagnosis has to be reached. As clinical signs or macroscopic lesions are not pathognomonic, laboratory tools are needed to confirm infection with NDV. And since infections with lentogenic and/or vaccine strains are not reportable, determination of the virulence of the virus is necessary.
Indeed, several laboratory techniques are available. Although serology should not be used as a definitive diagnostic tool in countries where vaccination against ND is routinely used, it can give very useful information if performed in paired samples taken two to three weeks apart.
Isolation and identification of the virus remain the golden tools to reach the diagnosis. Suspected material collected from tracheal, oropharyngeal or cloacal swabs or from organ samples are processed and inoculated into 9 to 11 day-old SPF embryonated eggs. After seven days of incubation, hemagglutinating (HA) activity in the allantoic fluid is tested. The HA positive samples have to be tested for specific inhibition with antiserum to NDV.
Direct detection of viral RNA from swab or tissue samples using molecular techniques such as RT-PCR and sequencing are widely used nowadays. These techniques are extremely useful to identify NDV, determine its virulence and genotype, but are still costly and require expertise. Reference laboratories can be used to reach proper diagnosis of this disease.
NDV may be employed as an antigen in a wide range of serological tests, enabling neutralization or enzyme-linked immunosorbent assays (ELISA) and HI to be used for assessing antibody levels in birds. At present, the HI test is most widely used for detecting antibodies to APMV-1 in birds while the use of commercial ELISA kits to assess post-vaccination antibody levels is common.
In general, virus neutralization or HI titers and ELISA-derived titers correlate at the flock level rather than at the level of individual birds. Serological assays are also used in diagnostic laboratories to assess antibody response following vaccination, but have limited value in surveillance and diagnosis of ND because of the almost universal use of vaccines in domestic poultry.
There are a variety of commercial ELISA kits available and these are based on several different strategies for the detection of NDV antibodies, including indirect, sandwich and blocking or competitive ELISAs using MAbs. At least one kit uses a subunit antigen. Usually such tests have been evaluated and validated by the manufacturer, and it is therefore important that the instructions specified for their use be followed carefully.
The HI test and ELISA may measure antibodies to different antigens; depending on the system used ELISAs may detect antibodies to more than one antigen while the HI test is probably restricted to those directed against the HN protein. However, comparative studies have demonstrated that the ELISAs are reproducible and have high sensitivity and specificity; they have been found to correlate well with the HI test (Brown et al., 1990)