Newcastle disease (ND) is an acute viral infection of great economic significance to the poultry industry worldwide. It is caused by the Newcastle disease virus, also called avian paramyxovirus-1 (APMV-1), that is an RNA, negative sense, single stranded, non-segmented, enveloped virus belonging to the family Paramyxoviridae and the genus Avulavirus.
Although the serotype is only one, there are different strains of Newcastle disease virus. Variations in Newcastle virus strains are associated with a wide range of clinical signs. Variations are the result of the differences in 2 surface glycoproteins, hemagglutinin-neurominidase (HN) and fusion (F), which play an important role in the binding and fusion of the virus to host cells, initiating infection. The key to viral immunity involves a response to these glycoproteins.
ND Strains are grouped into five pathotypes or forms, based on the clinical signs seen in infected chickens. These have been defined as:
- Viscerotropic velogenic: a highly pathogenic form in which hemorrhagic intestinal lesions are frequently seen.
- Neurotropic velogenic shows high mortality, usually following respiratory and nervous signs.
- Mesogenic presents respiratory signs, occasional nervous signs, but low mortality.
- Lentogenic or respiratory: a form that presents mild or subclinical respiratory infection.
- Asymptomatic enteric: usually consisting of a subclinical enteric infections.
ND virus strains are classified according to the World Organization for Animal Health (OIE) method. This classification is used to determine whether an outbreak of ND should be reported to the OIE and other local veterinary authorities to facilitate control measures.
The most severe ND strain is the viscerotropic velogenic Newcastle disease (VVND). It is often referred to as “Exotic Newcastle Disease” and infection with this form usually causes high mortality. The milder form of the disease is called “mesogenic” Newcastle disease.
Newcastle disease is highly contagious. All birds in a flock usually become infected within three to four days. The virus can be transmitted by contaminated equipment, shoes, clothing and free-flying birds. During the active respiratory stage, it can be transmitted through the air. The virus is not thought to travel any great distance. Recovered birds are not considered carriers and the virus usually does not live longer than thirty days on the premises.
Signs of Newcastle disease are not greatly different from those of other respiratory diseases. The signs most frequently observed are nasal discharge, excessive mucous in the trachea, cloudy air sacs, casts or plugs in the air passages of the lungs and cloudiness in the cornea of the eye.
The Newcastle disease in young chickens begins with difficult breathing, gasping and sneezing. This phase continues for ten to fourteen days and may be followed by nervous symptoms. These nervous disorders, when occurring, may consist of paralysis of one or both wings and legs or a twisting of the head and neck. The head often is drawn over the back or down between the legs. Mortality may vary from none to total loss of the flock.
In adult chickens, respiratory symptoms are more evident. Egg production usually drops rapidly in layer farms. During the outbreak, small, soft-shelled, off-coloured and irregular-shaped eggs are produced. Mortality in adult birds is usually low.
In turkeys, the symptoms are usually mild and may be unnoticed unless nervous disorders develop. During an outbreak, turkeys will produce eggs with a chalky white shell. Reduced production in breeder flocks is the main economic loss from this disease in turkeys.
Unfortunately, ND disease cannot be differentiated from infectious bronchitis and some of the other respiratory infections, except by laboratory methods. So it is very difficult to recognize it at first sight.
Vaccination is practiced widely and is the recommended method for prevention. Several types of vaccines are available but the most successful and widely used are the mild live virus vaccine known as the B1 and La Sota types. The vaccines may be used by drops into the nostril or eye, addition to the drinking water or applied in spray form. Broiler chickens are usually vaccinated when seven to ten days of age.
Chickens kept for egg production are usually vaccinated at least three times. The vaccine is given when birds are approximately seven days, again at about four weeks and a third time at about four months of age. Revaccination is commonly practiced during the laying period.
Vaccination is not widely used in turkeys. It is used to protect egg producing breeder flocks. One dose of the mild type vaccine is given after selecting breeder birds.
There is no treatment for Newcastle disease. Good “biosecurity” practices will help reduce the possibility of exposure to Newcastle disease virus.