Updated: Jul 7
Prachi Thatte PhD student, National Centre for Biological Sciences, Bangalore, India
‘Sunday ho ya Monday… Roz khao ande!!’ says the tagline of National Egg Co-ordination Committee. Loosely translated it means ‘doesn’t matter what day of the week it is, have an egg every day!’ It was also made into a television commercial which became hugely popular in the 1990s in India. It is difficult to say how much that advertisement contributed to the demand, but egg production has been increasing steadily over the last 4 decades at around 6-8% every year. Not just eggs, the demand for chicken meat has also been increasing. This has transformed the once unorganized backyard poultry farming into a commercial production system.
One of the major challenges facing the poultry industry is controlling disease outbreaks. The birds are routinely pumped with antibiotics and vaccines. However, vaccines do not always ensure that the diseases are kept at bay. One such disease that can affect the birds despite regular vaccination against it is the Newcastle disease. It is named after the populous city in North Eastern England where it was first identified. It is a fatal disease caused by a virus that goes by the same name, the Newcastle disease virus. This disease is reported from across the globe and is caused by 18 known variants, numbered from I to XVIII, of the Newcastle disease virus (NDV). Some variants are highly virulent, causing severe, often fatal infection and some cause mild symptoms.
Five Newcastle disease outbreaks were reported from Nagpur region of central India between 2006 and 2012. Researchers from Nagpur Veterinary College and Indian Institute of Technology, Guwahati carried out a study to investigate the outbreaks, examine the variant of Newcastle disease virus that was responsible for them and virulence (ability to cause severe infection) of the virus.
The researchers collected tissue samples from brain, lungs, liver and other organs of chickens that had died in each of the outbreaks. Additionally, they took blood from both ailing and dead chickens.
In order to examine the virulence, the researchers injected the virus obtained from the samples collected into the brain of healthy one day old chicks. These chicks were observed for 8 days and they were given a score based on the severity of disease symptoms. Higher the score, higher is the virulence. The average time to death of the chicks was also used as an indicator of virulence. If the virus is highly virulent, it causes severe infection leading to death within a few days. Researchers found that the virus variant responsible for the disease outbreaks in Nagpur region was highly virulent.
Furthermore, the researchers also identified the variant, among I to XVIII, of the virus that was responsible for the outbreaks. For this the researchers extracted the genetic material of the virus obtained from the samples collected. They compared the genetic data generated from the Nagpur samples with the known variants of NDV reported from around the world. Researchers identified the Nagpur variant as XIIIb.
Why is knowing the variant important? Similarity or ‘match’ between the virus variant and the vaccine is one of the main factors that determines the effectiveness of a vaccine to prevent disease.
Vaccines against viral diseases are often made of less virulent live viruses or chemical look-alikes of the viruses. Live viruses used can be variants that do not cause severe infection in the animal but can induce defense response against the virus. Once such defense response is induced, the animal is prepared for future attacks by that virus and other closely resembling variants.
Virus variants are formed due to changes in the genetic make-up of the virus. Changes that make a variant very different from the vaccine can render the vaccine ineffective, necessitating a vaccine update.
Newcastle disease vaccination in Mayurbhanj district, Odisha state, India
A less virulent variant of Newcastle disease virus, called ‘La Sota’, is used for vaccination of chicken in India. La Sota is variant II of the Newcastle disease virus. This is genetically different from XIIIb that was found to be responsible for the outbreaks in Nagpur. This genetic mismatch can be one of the reasons for inefficacy of the vaccination against Newcastle disease, researchers suggest. Variant XIIb of the Newcastle disease virus found in this study along with XIIIa has also been reported from other parts of India and Pakistan. The researchers caution that the widespread presence of variant XIII (a and b) in the Indian sub-continent can potentially cause a ‘panzootic’, an outbreak of the disease that can spread across a wide region. Newcastle disease was first reported from India in 1928. Despite easy availability of vaccine against it, the disease remains a major concern in India, causing huge losses to the poultry industry. Researchers suggest that a variant XIII matched vaccine may provide better protection to birds in central India. Improper vaccination schedule, uneven vaccine dose and lack of facilities to maintain the vaccine at cold temperature in rural areas are other challenges that need to be addressed, researchers note. More research on regional circulating NDV variants in India and testing effectiveness of La Sota vaccine against them would be important in order to formulate an effective control strategy for the disease.
Original Paper: Morla, S., Shah, M., Kaore, M., Kurkure, N. V., Kumar, S. (2016). Molecular characterization of genotype XIIIb Newcastle disease virus from central India during 2006–2012: Evidence of its panzootic potential. Microbial Pathogenesis 99, 83-86