Newcastle disease is a viral infection of bird species, primarily seen as an acute respiratory disease, caused by virulent strains of avian paramyxovirus serotype 1 (APMV-1). It is a highly contagious disease of great relevance in the poultry industry.

This review will be focused on domestic poultry, especially chicken.

Etiology

APMV-1 can also be named Newcastle disease virus (NDV), and it is a member of the Paramyxoviridae family in the genus Avulavirus. APMV-1 is categorized into three pathotypes depending on its virulence: velogenic (the most virulent), mesogenic (moderately virulent) and lentogenic (the lowest virulent). Most strains cluster to the two extremes of virulence.

Velogenic and mesogenic strains are included as the cause of Newcastle disease, a reportable disease, while lentogenic is not reportable. Velogenic strains are also divided into two groups depending on their tropism: viscerotropic or neurotropic and respiratory. Groupings by tropism are not clear-cut and they can overlap.

Velogenic strain is endemic in Asia, Middle East, Africa and Latin America. Lentogenic strains are present worldwide. Other areas, like Europe, the USA or Canada, are free of virulent strains in domestic poultry. Outbreaks of ND must be reported to authorities and result in sacrifice of infected birds and trade restrictions, causing significant economic losses during outbreaks.

Newcastle disease transmission

Birds get infected by inhalation or ingestion of the virus. Infected domestic or wild birds shed the virus in exhaled air, respiratory discharges and faeces. Virus is shed during incubation, during the clinical stage and, after recovery, for a limited period of time.

Fomites are also a source of infection, like contaminated feed or water, tools and machinery and farm staff. Virus may also be present in soiled eggshells and carcass. Persistence of APMV-1 is highly variable depending on environmental factors, such as temperature and humidity, but it is especially able to survive for long periods in faeces, compared to inorganic surfaces.

Wild birds and waterfowl may act as reservoir hosts of lentogenic serotypes. These viruses may mutate and become virulent once stablished in domestic poultry. Some psittacine birds may shed the virus intermittently for months and over a year.

APMV-1 can be transmitted to humans. It causes transitory conjunctivitis in people exposed to high quantities of virus, like laboratory workers or vaccination teams. Disease has not been reported to affect farmers nor consumers.

Predisposing factors

Manifestation of the disease depends on:

• Susceptibility of host species, age and immune status and vaccinal immunity.
• Pathotype of the virus and its tropism for respiratory, digestive or nervous systems.
• Co-infection with other microorganisms.
• Environmental conditions (stress).

Chickens are the most susceptible and become severely ill if infected with velogenic strains, while turkeys do not tend to develop severe signs. Waterfowls are the least susceptible domestic poultry, infection in ducks and geese may be subclinical, like in wild birds.

Clinical signs in Newcastle disease

Incubation period ranges from 2 to 15 days, with 5- 6 days on average in chickens with velogenic strains. Clinical signs vary from high morbidity and mortality to asymptomatic infections, depending on factors mentioned in the previous chapter.

Lentogenic pathotype is usually linked to the subclinical disease, which may include mild respiratory signs and minimal mortality. Respiratory signs include gasping, coughing, sneezing and rales. Mesogenic strains may cause an acute respiratory disease with nervous symptoms,decrease in egg production and low mortality (<10%). Lentogenic and mesogenic pathotypes may show severe signs if co-infections are present.

Velogenic pathotype causes severe and sometimes fatal disease in chickens. Clinical picture includes respiratory signs that may be followed by nervous symptoms. Birds appear lethargic with ruffled feathers. Dyspnea and cyanosis may appear, and birds may also develop greenish or watery diarrhea. Nervous signs range from tremors, spasms, paralyzed wings and legs, torticollis and swelling of the neck, circling to complete paralysis. Sharp drop or complete cessation of egg production can occur. Eggs may be abnormal in colour, shape or surface (rough or thin eggshell) with watery albumen.

Velogenic pathotype may result in sudden death. Morbidity and mortality can be as high as 100% in unvaccinated chickens. Surviving chickens may have permanent neurological damage and persistent decreased egg production. Vaccinated birds may not show any signs except for the decrease in egg production.

Post-mortem lesions

Gross lesions are usually seen in birds affected by velogenic strains of the virus. They have been characterized especially in chickens with viscerotropic strain, but they are not pathognomonic of the disease. Sometimes, birds with neurological signs or that died suddenly show few or no gross lesions. Lesions may include:

• Swelling of the head, periorbital area and/or neck.
• Petechiae or haemorrhages in the serous membranes of the digestive system and the mucosa of the proventriculus and intestine.
• Haemorrhages in the lymphoid tissue of the respiratory tract and digestive system, especially in the cecal tonsils and Peyer’s patches.
• Enlarged spleen, friable and spotted with necrotic/haemorrhagic areas.
• Oedema around the thymus and bursa of fabricious of young birds.
• Oedema, haemorrhages or degeneration of ovaries.

Lesions caused by lentogenic pathotypes are usually limited to congestion and mucous exudates in the respiratory tract, with opacity and thickening of the air sacs.

Diagnosis

Clinical signs are not a reliable basis for diagnosis of ND, and gross lesions may orientate the differential diagnosis.

ND can be diagnosed by isolation of APMV-1 from live or recently dead birds.

• • Tracheal and cloacal swabs are usually taken from live birds or fresh faeces.
  • Collected tissues of spleen, lung, intestines (especially cecal tonsils) or intestinal content, liver, kidneys, heart and brain in dead birds or oronasal swabs from the carcass.

• Laboratory tests are run from SPF embryonated chicken eggs, cell cultures or directly from clinical specimens.
  • Hemagglutination inhibition (HI) assay for APMV-1 on embryonated chicken eggs is an effective tool for diagnosis. Other serological tests, such as virus neutralization and ELISA might be performed, but vaccination or previous exposure may interfere with results.
  • RT-PCR of cell culture. This test does not detect all strains, but it works for some strains which may grow in cell cultures but not in embryonated eggs. Therefore, both culture systems should be used in suspected cases.
  • RT-PCR of clinical specimens.
  • Molecular tests, like gene sequencing may also be employed, and they will be useful for the identification process.

The pathogenicity of APMV-1 may be also quantified with different assays:

• Gene sequencing. Most velogenic strains have a specific sequence at the viral F2 protein and at residue of the F1 protein. The presence of this sequence defines the isolate as a virulent strain, a useful tool for international trade purposes. If this sequence is not present, the pathogenicity of the virus must be evaluated in live birds.
• Intracerebral pathogenicity index (ICPI). This is the current international standard test for chickens, which evaluates disease and death in 1-day old chicks. It is valued from 0 to 2, where lentogenic viruses approach to 0 and virulent strains are close to 2.

Differential diagnosis

Differential diagnosis includes other illnesses showing respiratory signs together with high mortality, fowl cholera, HPAI, Fowl pox, laryngotracheitis, mycoplasmosis, infectious bronchitis. Laboratory test should be run to discard other etiology.

Prevention against Newcastle disease

Disease reporting should follow national guidelines, including control of movement and quarantines, destruction of exposed and infected birds, cleaning and disinfection and compliance of safety period before restocking. Prevention is based on good biosecurity measures and vaccination plans.

Biosecurity

Biosecurity plans are based on proper farm isolation and management practices:

• Isolation of farm, feed and water supplies of wild birds.
• Pest control of the facilities to avoid presence of rodents and insects.
• Control of human and vehicular traffic: minimizing travel on and off the facility and strict disinfections of all equipment entering the farm.
• Workers policy of shower-in and dedicated clothing. They should avoid contact with pet or wild birds outside the farm.
• Perform “all in-all out” system, to avoid contact of groups of different ages.
• Effective disinfectants include ether, formalin, glutaraldehyde, phenolics and oxidising agents, chlorhexidine and sodium hypochlorite. The virus can also be inactivated by heat treatments.

Vaccination against Newcastle disease

Vaccination is widely used worldwide to protect chicken, especially in countries where velogenic strains are endemic. Some countries free of ND allow vaccination to prevent infection from lentogenic strains.

Vaccination may work to decrease shedding and transmission and reduce losses and severity of the presentation. Sentinel chickens can be used to monitor vaccinated flocks.

It is important to consider the type of vaccine to be used, the field strain of the virus, the immune and sanitary status of birds and the level of maternal antibodies. Vaccination plan can include conventional live vaccines, inactivated vaccines or recombinant vaccines.

Live lentogenic vaccines are commonly used by mass application through the drinking water or by sprayed aerosol. The most used are B1 and LaSota strains. These application methods are less labour intensive but, if they are not administered properly, may lead to poorly vaccinated flock (<85%) that does not reach flock immunity. Administration of live vaccines can also be done individually through intranasal or conjunctival instillation. Chicks can be vaccinated at 1-4 days of life and revaccinated at 2-4 weeks later or initial vaccination can be delayed until the second or third week of life avoids interference with maternal antibodies.

Inactivated vaccines, which may be of virulent or avirulent strains, are incorporated into an emulsion and are applied intramuscularly or subcutaneously. Therefore, it is ensured that each bird receives a standard dosage, but a larger amount of virus is required for immunisation than in live vaccines. In addition, they are more expensive and labour intensive.

The frequency of revaccination depends on the risk of exposure and virulence of the field virus. Different strategies are performed to ensure maintenance of flock immunity. Layers and breeders under low challenge (NDV free countries) may employ initially inactivated vaccines or live vaccines and the following vaccinations will include slightly more pathogenic live vaccines. On the other hand, flocks under high challenge may use inactivated vaccines after several live vaccinations.

Vaccination immunisation should be evaluated by serological testing of the flock. Commercial ELISA kits are available for such purposes.

Recombinant vaccines, like fowl pox or herpesvirus-vectored NDV vaccines, where viruses are expressing the HN and/or F genes are also commercially available in chickens. These vaccines can be administered in ovo at the hatchery, but they there is still no commercial kit available to evaluate the induced immunity.

Natural Solutions to combat Newcastle disease

Commercial lines available nowadays in the chicken industry have been selected for their productive parameters, which had a negative correlation with their resistance to infections. In addition, there are situations where the immune system is not working effectively, like in young chicks that do not have a developed immune system yet; and birds under stressful conditions because of the intensive production and management practices.

In these situations, the immune system loses part of the ability to protect the organism. This fact may result in lower vaccine efficacy and higher risk of infection when challenge is present. Use of immunostimulants is indicated for these cases, in order to boost the immune system and allow the development of a proper immune response in vaccination.

Natural immunostimulants based on pronutrients, active molecules of plant origin, are able to physiologically promote the innate and adaptative immune systems activity to ensure an adequate antibody production and persistence and allow to reach flock immunity after vaccination.

Conclusions

Newcastle disease is one of the most important viral infections occurring in the chicken industry. It is a reportable disease that causes great economic losses when there is an outbreak because of the sacrifice of birds and trade restrictions it implies.

Strong preventive measures should be applied to avoid the outbreak or the effect of subclinical presentations, including proper sanitary prophylaxis and vaccination.

Commercial lines used nowadays in the chicken industry are genetically weaker against external threats, added to the stress conditions of the high productive demand that may affect the activity of the immune system.

The use of natural immunostimulants based on pronutrients will physiologically enhance the innate and adaptative immune response, ensuring proper antibody production and its persistence after vaccination.

Bibliography

• Newcastle disease, Technical Disease Card.
• Merck Veterinary Manual. Newcastle disease in poultry.
• Iowa State University – The Center for Food Security and Public Health. Newcastle disease, Technical Fact Sheet.