Newcastle disease (ND) is one of the most devastating viral infections in poultry, causing severe economic losses worldwide. The disease was first identified in 1926 in Newcastle-upon-Tyne, England, after an outbreak in imported birds. Since then, it has spread globally, with outbreaks reported in Asia, Africa, Europe, and the America.
The virus is highly contagious, capable of wiping out entire flocks within days if left unchecked. It impacts developing nations with small-scale poultry farming most severely. Major epidemics result in substantial economic losses. Due to its rapid spread and high mortality rate, Newcastle disease is classified as a notifiable disease by the World Organisation for Animal Health (OIE), meaning outbreaks must be reported to authorities.
Etiology (Causative Agent)
Newcastle disease is caused by the Newcastle disease virus (NDV), a single-stranded RNA virus belonging to the Avulavirus genus in the Paramyxoviridae family. Scientists classify it as avian orthoavulavirus 1 (AOAV-1). Many refer to it as avian paramyxovirus type 1 (APMV-1).
Taxonomic Classification
NDV fits into a structured hierarchy. This classification reflects its genetic and structural traits. The virus genome spans about 15 kilobases. It encodes six main proteins: nucleocapsid, phosphoprotein, matrix, fusion, hemagglutinin-neuraminidase, and large polymerase. The F (fusion) protein of the virus determines its virulence—highly pathogenic strains have a specific amino acid sequence that allows systemic infection.
Genomic and Phylogenetic Classification
Experts divide NDV strains into two classes based on genome length and phylogeny. Class I viruses have genomes of 15,198 nucleotides. They form a single genotype. These strains mainly infect wild birds. They show low virulence in poultry. Class II viruses have genomes of 15,186 or 15,192 nucleotides. They include both virulent and avirulent strains. This class divides into at least 21 genotypes (I to XXI). Genotypes reflect evolutionary branches. Class II strains dominate in poultry outbreaks. Recent updates unify this system. They use phylogenetic topology, genetic distances, and branch support. For example, genotype VII prevails in many regions. It splits into subgenotypes like VII.1 and VII.2. These cause severe disease in Asia and Africa. Genotype II includes common vaccine strains like LaSota.
Virus Strains and Pathogenicity
NDV strains are classified into three main pathotypes based on their virulence in chickens:
Velogenic (Highly Virulent): Velogenic strains induce severe disease and divide into two subtypes.
Viscerotropic Velogenic (VVND): Causes severe internal bleeding and high mortality (up to 100%).
Neurotropic Velogenic (NVND): Attacks the nervous system, leading to paralysis and tremors.
Mesogenic (Moderately Virulent)
Causes respiratory and nervous signs but lower mortality (10-50%).
Lentogenic (Mild)
Mostly causes mild respiratory symptoms with very low death rates.
Virulence stems from the fusion protein cleavage site. Multiple basic amino acids there mark virulent strains. Tests like mean death time in embryos assess pathotypes.
Host Range and Susceptibility
Over 250 avian species can host NDV.
Primary Hosts
Chickens: Most susceptible, especially young birds.
Turkeys, Quail, and Ducks: Can be infected but often show milder symptoms.
Secondary Hosts and Carriers
Wild Birds (e.g., pigeons, sparrows, crows): Can carry and spread the virus without showing symptoms.
Humans: Rare cases of mild conjunctivitis or flu-like symptoms in poultry workers.
Incubation Period
2 to 15 days, depending on:
Virus strain (velogenic strains act fastest).
Bird age (young chicks develop symptoms quicker).
Immune status (vaccinated birds may resist infection longer).
In severe outbreaks, death can occur within 2–3 days after the first signs appear.
Morbidity and Mortality Rates
Morbidity (Infection Rate):
Nearly 100% in unvaccinated flocks.
Even vaccinated birds may show mild symptoms if exposed to a highly virulent strain.
Mortality (Death Rate):
Velogenic strains: 90–100% in chickens.
Mesogenic strains: 10–50%.
Lentogenic strains: Less than 5%.
Pathogenesis (Step-by-Step Disease Development)
Entry & Initial Infection
Virus enters through eyes, nose, mouth, or wounds.
Replicates in respiratory and intestinal epithelial cells.
Primary Replication & Spread
Virus moves to tonsils, trachea, and lungs, causing inflammation.
Enters the bloodstream (viremia), spreading to organs.
Systemic Invasion
Lungs: Hemorrhages, fluid buildup (pulmonary edema).
Brain: Encephalitis (leads to paralysis, twisted neck).
Intestines: Severe diarrhea, bleeding in gut lining.
Spleen & Liver: Swelling and tissue death.
Final Stage (Severe Cases)
Multi-organ failure → rapid death.
Clinical Signs (Symptoms)
Acute Form (Velogenic Strains)
Respiratory: Gasping, coughing, nasal discharge.
Nervous: Muscle tremors, paralysis, twisted neck (torticollis), circling.
Digestive: Greenish, watery diarrhea, loss of appetite.
Reproductive: Sudden drop in egg production, soft-shelled eggs.
Sudden death (often before symptoms appear).
Mild Form (Lentogenic Strains)
Mild sneezing, coughing.
Slightly reduced egg production.
Necropsy (Post-Mortem) Findings
Hemorrhages in trachea, intestines, and proventriculus.
Swollen spleen (splenomegaly) and congested lungs.
Edema (fluid buildup) in the head and neck.
Lesions in the brain (encephalitis).
Necrotic patches in the liver and kidneys.
Prevention and Control
1. Biosecurity Measures
Isolate new birds for at least 2 weeks before introducing them to the flock.
Disinfect footwear, equipment, and vehicles entering farms.
Control wild birds and rodents (potential carriers).
2. Vaccination (Most Effective Prevention)
Types of Vaccines
Live Attenuated Vaccines (B1, La Sota, VG/GA):
Administered via drinking water, spray, or eye drops.
Fast immunity (within 7 days) but may cause mild respiratory signs.
Inactivated (Killed) Vaccines:
Injectable, longer-lasting immunity (used in breeders/layers).
Often combined with live vaccines for stronger protection.
Vaccination Schedule
Day 1–7: Live vaccine (B1 strain) in chicks.
Week 4–6: Booster with La Sota strain.
Every 6–12 months: Revaccination for long-term protection.
3. Outbreak Management
Immediate quarantine of infected farms.
Culling and safe disposal of infected birds.
Deep cleaning with virucidal disinfectants (e.g., formaldehyde, iodine).
Conclusion
Newcastle disease remains a major threat to poultry industries worldwide. Vaccination, strict biosecurity, and rapid response to outbreaks are critical to preventing massive losses. Farmers must stay vigilant, as wild birds and illegal poultry trade can reintroduce the virus.