While some species may require a higher infectious dose of NDV than other species to become infected, as with turkeys
compared to chickens, infection with NDV has been documented in at least 241 species . Likely all avian species are susceptible to NDV infection; however, infectious dose and clinical signs depend not only on the species of the host, but also vary with individual NDV isolates.
Some books demonstrated that over 250 species of bird have been reported to be susceptible to natural or experimental infections of NDV and it seems probable that many more if not all species are susceptible to infection. NDV strains have been shown to infect all the major and minor species of domestic poultry, although some species (e.g. ducks) tend to show few signs of disease even when infected with the strains of NDV most virulent for chickens.
Horizontal transmission of NDV has been documented many times.
Infected birds shed NDV in oropharyngeal secretions and fecal matter . Susceptible birds may become infected by inhaling contaminated dust or aerosolized virus or by ingesting such material. Infection by inhaling aerosolized virus is illustrated by the success of applying live NDV vaccines using nebulizers.
The ingestion of contaminated feces or contaminated carcasses can cause infection in chickens and raptors.
Applying vaccine to chickens via the drinking water is another demonstration of oral infection. While immunized chickens may shed vNDV for 6–9 days after challenge or until they succumb), parrots, cormorants, and pigeons may have prolonged shedding of vNDV,usually in feces, without clinical signs.
Cases of vertical transmission of NDV from parent to offspring are difficult to prove definitively due to the possibility
of hatchlings being infected by contaminated feces through eggshell cracks or by exposure to a contaminated environment.
At least 2 reports of virulent NDV isolated from embryonating chicken eggs, day-old hatchlings, and
dead-in-shell birds have been documented.
Some NDV may have a tropism for the oviduct, which can be observed indirectly through microscopic lesions and directly using immunohistochemistry. Reports of embryos infected with virulent NDV from naturally infected layer hens have been reported, but as expected, the embryos usually die before hatching.
Vaccine virus has been found in reproductive organs after vaccination. A small number of embryonating chicken eggs experimentally inoculated with 1 mean egg lethal dose of virulent NDV survived and shed virus in feces (60).
Until eggs from naturally infected birds are cleaned and hatched under experimental conditions in a
negative pressure isolator and shown to be shedding virulent NDV identical to the outbreak virus, doubt will exist as to
the importance of vertical transmission. While this topic is especially critical for endemic countries, it is also important
for countries free of ND
Any bird can be infected with NDV of low virulence, mount an immune response, be subsequently infected with vNDV,
and without demonstrating clinical signs shed the vNDV to susceptible birds, acting as a carrier of the ND.
However, as mentioned above, apparently healthy adult cormorants pigeons, and parrots ) are often found to be infected
and possibly chronically shedding NDV. That being said molecular epidemiology studies examining outbreak viral
sequences in poultry do not implicate the viruses from these species.
That is, strains isolated from pigeons are usually pigeon variants of PPMV-1, and viruses isolated from cormorants are different from chicken strains Wild birds probably are not important in the secondary spread of vNDV.
While some NDV strains have a tropism for a specific species, this tropism can change if the virus is passed multiple times in a new avian species which may occur if biosecurity measures are not fully implemented.
Evidence that humans, mammals, or insects are biological vectors of spread of ND is lacking, although humans can be infected and develop conjunctivitis. It is possible that with the use of NDV strains—for instance, some virulent strains such
as those used as treatments for human cancer therapy—this potential risk could increase as virus is shed in saliva and urine.
Two different fly speciesfrom ND outbreak locations were confirmed to be carrying vNDV, albeit in amounts too small for the flies to be considered competent vectors.
spread by humans is more likely mediated by transport of contaminated fomites.
NDV contaminated equipment, clothing, shoes, feed, water,vaccines, and poultry products, and NDV-infected birds (wild
or domestic) moved or placed into contact with susceptible birds where virus can be inhaled or ingested can lead to the
spread of ND.
Migratory birds and illegally imported birds have been implicated in the introduction of the disease in certain regions .