Dr Sameera Akhtar – Being endemic in Pakistan, Newcastle disease (ND), also known as Ranikhet in the subcontinent, causes significant economic losses to the poultry farmers each year.
Though vaccines and their schedules are available from various poultry vaccine manufactures worldwide but still the disease is a major threat to the poultry industry.
Once infected, chicken and other avian species like pigeons, parakeets as well as backyard chicken, crows, kite, sparrows, doves etc act as a continuous source of infection for healthy chickens.
Newcastle disease virus is an enveloped ribonucleic acid (RNA) containing virus and has a wide host range and can infect many species of birds including chickens, turkeys, ducks, pigeons, guinea fowl, Japanese quail, pheasant, peacock, chuwkour, parrot and many wild and captive caged birds of all ages.
The LaSota strain of ND vaccine is the most common and popular vaccine for decades to protect against NDV outbreaks, as it canbe administered through drinking water, has no labour cost and no stress on chicken flocks.
Live virus vaccines and killed oil-based vaccines are used in many countries including Pakistan to prevent the disease in chickens.
Despite the extensive use of vaccines, outbreaks are continuously occurring in different areas of the country, resulting in huge economic losses.
Diagnosis of the disease is based on history, clinical signs and lesions and requires an endorsement of laboratory confirmation.
In recent years it has been observed that the LaSota strain is not providing protection especially to broiler birds against fatal field strain of NDV.
Factors that can be incriminated for this situation may be genetic variation in the circulating field virus and other factors that may cause immunosuppression in these birds.
Reasons for immune suppression includes encounter of birds either with bacteria, virus or any other toxic material besides poor management, including negligence by the poultry farmers.
Outbreaks of ND keep surfacing from time to time in Pakistan.
Previous studies on ND viruses isolated from various parts of Pakistan in commercial, backyard and wild poultry birds indicated the prevalence of multiple genotypes.
Generally, most research investigations performed on NDV have focused on immunological aspects of the viruses whereas genomic aspects have received little attention; may be due to the lack of services provided for the genomic studies.
As with enhancement in the field of molecular biology during the last decade, the genetic variation in the field circulating virus was determined not only in the commercial broilerchickens but other species as the incidence of the emerging ND signs and symptoms as well as mortality due to NDV.
Besides the above-mentioned predisposing factors, genetic variation in the causative agent (NDV) has been studied in the present study undertaken at the Department of Microbiology, involving internationally accredited Quality Operations (QOL) and the University Diagnostic Labs (UDL) of the University of Veterinary and Animal Sciences (UVAS), Lahore.
This study indicates that there has been genetic variation, which might lead to its expression in the form of changed outer surface proteins of virussuggesting that we need to consider alternatives to LaSotastrain vaccine for better protection against emerging NDV strains.
Interestingly, when compared with LaSotavaccine, the results of present study have shown differences in the antigenic epitopes of F and HN proteins (involved in facilitating virus for entry into cell and for providing protection or immunity against virus) forcurrently circulating ND viruses isolated from peacocks and pigeons.
In this study, ND viruses were isolated from two different outbreaks in pigeon and peacock farms.
The birds showed characteristic signs of disease, with high morbidity and mortality.
Fusion and haemagglutinin genes of the isolated viruses were sequenced and phylogenetically analysed with previously published NDV sequences to establish genotypic relationships and epitope mapping.
The evaluation proposed that pigeon isolate belonged to genotype VI and subgenotype ‘b’ whereas peacock isolate was found to be associated withNDV genotype VII and subgenotype ‘e’.
LaSota vaccine was tested against the pigeon and peacock isolates for its efficacy in providing protection in chickens.
Forty days-old broilers chicks were raised and tested against each isolate.
The birds were divided in two groups: one group received NDV vaccination while the other was keptunvaccinated.
The birds in vaccinated group were primed with live LaSotavaccine on day 7 followed by a booster on day 12 with oil-based killed ND vaccine containing genotype VII strain.
Another booster with live LaSota was given on day 25 and blood samples were collected to check their immune status.
The birds were divided into two groups and were challenged with the field isolates from pigeon and peacock on day 27.
Signs of ND were observed for 10 days post-challenge and birds showing ND specific signs were sacrificed for virus presence in various tissues.
Necropsy was performed on dead and sacrificed birds and typical NDlesions were noted in gizzard andintestines of infected birds. The mortality was more in chickens challenged with peacock isolates (65%) than with pigeon isolates (55%). The mortality was 100% in non-vaccinatedchallenge.
Furthermore, the clinical symptoms of ND were more severe in birds challenged with peacock isolates.
The results of the challenge study indicated that currently used vaccines did not provide complete protection against currently circulating virulent viruses of different genotypes.
The study further indicated that the viruses were present in almost all tissues of the infected birds and there was shedding of the virus in the feces and in pharyngeal/tracheal secretions.
Source: The Nation