Newcastle Disease in Poultry
Newcastle
disease is an infection of domestic poultry and other bird species with
virulent Newcastle disease virus (NDV). It is a worldwide problem that
presents primarily as an acute respiratory disease, but depression,
nervous manifestations, or diarrhea may be the predominant clinical
form. Severity depends on the virulence of the infecting virus and host
susceptibility. Occurrence of the disease is reportable and may result
in trade restrictions.
Etiology and Pathogenesis
NDV, synonymous with avian paramyxovirus serotype 1
(PMV-1), is an RNA virus and the most important of the 11 known PMV
serotypes as a pathogen for poultry. The original classification of NDV
isolates into one of three virulence groups by chicken embryo and
chicken inoculation as virulent (velogenic), moderately virulent
(mesogenic), or of low virulence (lentogenic) and has been abbreviated
for regulatory purposes. Velogens and mesogens are now classified as
virulent NDV (vNDV), the cause of Newcastle disease and reportable
infection, whereas infections with lentogens, the low virulence NDV
(loNDV) widely used as live vaccines, are not reportable. Clinical
manifestations vary from high morbidity and mortality to asymptomatic
infections. Severity of infection depends on virus virulence and age,
immune status, and susceptibility of the host species. Chickens are the
most and waterfowl the least susceptible of domestic poultry; however,
some differences may be seen if the NDV strain is adapted to a
particular species.
Epidemiology and Transmission
Virulent NDV strains are endemic in poultry in most of
Asia, Africa, and some countries of North and South America. Other
countries, including the USA and Canada, are free of those strains in
poultry and maintain that status with import restrictions and
eradication by destroying infected poultry. Cormorants, pigeons, and
imported psittacine species are more commonly infected with vNDV and
have also been sources of vNDV infections of poultry. NDV strains of low
virulence are prevalent in poultry and wild birds, especially
waterfowl. Infection of domestic poultry with loNDV contributes to lower
productivity.
Infected birds shed virus in exhaled air, respiratory
discharges, and feces. Virus is shed during incubation, during the
clinical stage, and for a varying but limited period during
convalescence. Virus may also be present in eggs laid during clinical
disease and in all parts of the carcass during acute vNDV infections.
Chickens are readily infected by aerosols and by ingesting contaminated
water or food. Infected chickens and other domestic and wild birds may
be sources of NDV. Movement of infected birds and transfer of virus,
especially in infective feces, by the movement of people and
contaminated equipment or litter are the main methods of virus spread
between poultry flocks.
Clinical Findings
Onset is rapid, and signs appear throughout the flock
within 2–12 days (average 5) after aerosol exposure. Spread is slower if
the fecal-oral route is the primary means of transmission, particularly
for caged birds. Young birds are the most susceptible. Observed signs
depend on whether the infecting virus has a predilection for
respiratory, digestive, or nervous systems. Respiratory signs of
gasping, coughing, sneezing, and rales predominate in infections with
loNDV. Nervous signs of tremors, paralyzed wings and legs, twisted
necks, circling, clonic spasms, and complete paralysis may accompany,
but usually follow, the respiratory signs in neurotropic velogenic
disease. Nervous signs with diarrhea are typical in pigeons, and nervous
signs are frequently seen in cormorants and exotic bird species.
Respiratory signs with depression, watery greenish diarrhea, and
swelling of the tissues of the head and neck are typical of the most
virulent form of the disease, viscerotropic velogenic Newcastle disease,
although nervous signs are often seen, especially in vaccinated
poultry. Varying degrees of depression and inappetence are seen. Partial
or complete cessation of egg production may occur. Eggs may be abnormal
in color, shape, or surface and have watery albumen. Mortality is
variable but can be as high as 100% with vNDV infections.
Well-vaccinated birds may not show any signs of being infected except
for a decrease in egg production, but these birds will shed virus in
saliva and feces. Poorly vaccinated birds may develop torticollis,
ataxia, or body and head tremors 10–14 days after infection and may
recover with supportive care.
Lesions
Remarkable gross lesions are usually seen only with
viscerotropic velogenic Newcastle disease. Petechiae may be seen on the
serous membranes; hemorrhages of the proventricular mucosa and
intestinal serosa are accompanied by multifocal, necrotic hemorrhagic
areas on the mucosal surface of the intestine, especially at lymphoid
foci such as cecal tonsils. Splenic necrosis and hemorrhage and edema
around the thymus may also be seen. In contrast, lesions in birds
infected with loNDV strains may be limited to congestion and mucoid
exudates seen in the respiratory tract with opacity and thickening of
the air sacs. Secondary bacterial infections increase the severity of
respiratory lesions.
Diagnosis
NDV can be isolated from oropharyngeal or cloacal
swabs or tissues from infected birds by inoculation of the allantoic
cavity of 9- to 11-day-old SPF embryonated chicken eggs. Infection is
confirmed by recovery of a hemagglutinating virus that is inhibited with
NDV antiserum or by detection of NDV RNA by reverse transcriptase PCR. A
rise in NDV antibody titer by hemagglutination-inhibition or ELISA of
paired serum samples indicates NDV infection. To confirm diagnosis,
identification of an isolate such as vNDV is established by the rapidity
of killing day-old SPF chicks inoculated by the intracerebral route,
the intracerebral pathogenicity index, or by the presence of a specified
amino acid motif at the cleavage site of the fusion protein (F)
precursor (FO). Reference laboratories use nucleotide sequence analysis
to detect genetic differences for comparison of isolates from different
outbreaks and to identify the source of those infections. The acute form
of ND should be differentiated from other diseases known to cause high
mortality, such as highly pathogenic avian influenza (see Avian Influenza).
Prevention
Vaccines are available for chickens, turkeys, and
pigeons and are used to induce an antibody response, so vaccinated bids
must be exposed to a larger dose of vNDV to be infected. Unfortunately,
ND vaccines do not provide sterile immunity, and in many areas of the
world vaccines are used to prevent losses from sickness and death. Live
lentogenic vaccines, chiefly B1 and LaSota strains, are widely used and
typically administered to poultry by mass application in drinking water
or by spray. Mucosal immunity induced in birds vaccinated by live
vaccines applied by these routes decreases the amount of vNDV the
vaccinated birds will shed if infected with vNDV, compared with the
immune response induced by an inactivated vaccine. Mass vaccination
methods are less labor intensive but if not applied properly may lead to
<85% of the flock being immunized, which is needed for herd
immunity. Alternatively, individual administration of live vaccines is
via the nares or conjunctival sac. Healthy chicks are vaccinated as
early as day 1–4 of life. However, delaying vaccination until the second
or third week avoids maternal antibody interference with an active
immune response. Mycoplasma, some other bacteria, and other
viruses affecting the respiratory tract, if present, may act
synergistically with some vaccines to aggravate the vaccine reaction
after spray administration.
Oil-adjuvanted inactivated vaccines are also used
after live vaccine in breeders and layers and may be used alone in
situations where use of live virus may be contraindicated (eg, in
pigeons). In countries where vNDV is endemic, a combination of live
virus and inactivated vaccine can be used; or alternatively, if
permitted by law, a live mesogenic strain vaccine may be used in older
birds. The frequency of revaccination to protect chickens throughout
life largely depends on the risk of exposure and virulence of the field
virus challenge. Administering inactivated vaccines is more labor
intensive, because each bird has to be handled individually. Accidental
inoculation of human tissues with oil-based vaccines requires prompt
medical treatment.
Fowlpox or turkey herpesvirus–vectored NDV vaccines
are commercially available for chickens and have the advantage of being
able to be administered in ovo at the hatchery. These vaccines must be
reconstituted as directed by the manufacturer and, because they take 3–4
wk to produce a protective level of immunity, biosecurity is even more
important. A commercial kit to detect levels of antibodies induced by
these vaccines is not yet available.
Zoonotic Risk
All NDV strains can produce a transitory
conjunctivitis in people, but the condition has been limited primarily
to laboratory workers and vaccination teams exposed to large quantities
of virus. Before poultry vaccination was widely practiced,
conjunctivitis from NDV infection occurred in crews eviscerating poultry
in processing plants. The disease has not been reported in people who
rear poultry or consume poultry products.
Last full review/revision January 2014 by Patti J. Miller, DVM, PhD
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