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Rural Industries Research & Development Corporation
Detection of vvIBDV strains
and Australian variants in poultryby J. Ignjatovic, S. Sapats and G. Gould
November 2001
RIRDC Publication No. 01/147 RIRDC Project No. CSA-2J
Objectives of the study
Develop diagnostic tests to detect exposure of poultry to very virulent infectious bursal disease virus (IBDV).
Determine the nucleotide sequence of very virulent IBDV strains from neighbouring countries in order to confirm that all very virulent strains can be differentiated by the methods developed.
Determine the prevalence of variant IBDV strains on commercial poultry sites in Australia.
Further characterise the antigenic and protective capacity of Australian variant IBDV strains.
Background
VvIBDV strains emerged in Europe in 1987 and since then have been detected in many countries including the Middle East, Asia, Africa, and more recently in South America.
Australia, New Zealand and the USA are the only countries that remain free from this devastating disease that causes high mortalities and is difficult to control. The potential for transmission of vvIBDV from other countries into Australia is high and, as in other countries, would result in crippling losses. The threat is exacerbated by the highly stable and infectious nature of the virus and more freedom in international trade. The probability that vvIBDV strains can emerge in Australia by mutation of endemic strains should also be considered. For these reasons it would be prudent to ensure that adequate techniques and strategies exist to detect such exposure in the fastest possible way. The current strategy for IBDV strain characterisation is based on nucleotide sequencing, followed by pathogenicity testing and protection studies. The intention in this study was to develop a competitive ELISA in which sera from an already immune flock could be tested in a blocking ELISA to detect exposure to an exotic vvIBDV strains. This was to be achieved by developing a reagent (chicken recombinant antibody) specific for vvIBDV which would then be used in a competitive ELISA.
Initially when this project was conceived only one vvIBDV strain from Asia (OKYM strain from Japan) was characterised although it has been reported that vvIBDV are prevalent in almost all countries throughout Asia. As the presence of vvIBDV strains in neighbouring countries represents the most likely source of incursion of vvIBDV strains into Australia, we aimed to obtain IBDV strains from neighbouring countries and determine their nucleotide sequence. Such sequences would then be compared to sequences of other vvIBDV, as well Australian IBDV strains. This would strengthen our confidence of being able to discriminate an incursion of any vvIBDV, regardless of its origin. We, along with others, have shown previously that nucleotide sequencing of a region termed the hyper variable region within the VP2 gene of IBDV is the only available method that can differentiate vvIBDV strains. This method is of particular importance for Australia as it enables our strains to be clearly differentiated from vvIBDV strains.
In Australia clinical IBD has been uncommon, but in recent times clinically more severe IBD has been observed in some broiler flocks. Although only two strains, 002-73 and V877, have been available since the disease was first described in 1974, prevailing field strains were considered to be of low pathogenicity. During 1995-1997 RIRDC funded project on IBDV, we showed that in addition to classical strains, variant strains are also present in Australia. Variant strains were isolated from 4 different farms in the state of Victoria, whereas two classical strains were isolated from NSW. Antigenic variation was identified in these strains by using both monoclonal antibodies (in ELISA), and polyclonal chick sera (in virus neutralisation test). Nucleotide sequencing confirmed that strains from Victoria and NSW differ and that they belong to two separate genetic groups, both distant from all overseas strains. In this study we proposed to characterise an additional number of field IBDV isolates that originated from all major poultry producing area in order to determine the distribution of classical and variant types of IBDV across Australia. Additionally, such information would be used to confirm that all Australian strains can be differentiated form overseas IBDV strains at the genetic level, regardless of the site of collection.
During RIRDC 1995-1997 project on IBDV it was also shown that progeny chicks from hens vaccinated with classical vaccines were not protected against challenge with variant strains. Broiler chicks with maternal antibody titres of below 2,000 were susceptible to challenge with variants. In the USA, where antigenic variants have also been detected, bivalent vaccines that include classical and variant strains have been used for vaccination of breeder hens. Also, it has been suggested that vaccines based on the USA variant strains are superior to those based on classical strains. We proposed to compare the protective capacity of inactivated vaccines based on local variant and classical strains, and assess if there is an advantage of variant vaccines in protecting chicks against virulent challenge. Further, although it has been shown by us that the USA and Australian variants differ both antigenically and genetically, it was of interest to assess if inactivated vaccines based on the USA variants are able to provide protection against a challenge with the Australian variants.
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