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Rural Industries
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|| Introduction || Project listing|| Project Summaries ||
The Australian chicken meat industry produces approximately 460,000 tonnes of chicken meat annually with a retail value in excess of $2 billion. Production has increased dramatically over the past thirty years, to the point where chicken meat is now Australian consumers' second most popular meat.
Most production is consumed domestically. Opportunities for increased exports are believed to lie mainly in the further processed products area.
The major challenge likely to be faced by the industry in the next few years relates to the need for it to restructure its operations and improve its international competitiveness in the face of possible future imports of cooked chicken meat products and to its need to address concerns being raised in the community with respect to its environmental performance.
Research Highlights for 1996-97:
AVIAN COLIBACILLOSIS IN AUSTRALIA: UNDERSTANDING AND CONTROL
Objectives
· To establish methods for distinguishing strains of E.coli
(which can cause colibacillosis) in chickens from those which cannot.
· To find the genes responsible for virulence in E.coli.
· To produce strains of E.coli which had the virulence genes deleted,
and hence could no longer cause disease.
· To examine whether the virulence associated genes could be used
to rapidly distinguish virulent and avirulent E.coli.
· To assess the use of the E.coli strains lacking the virulence
genes as vaccines to prevent colibacillosis.
Background
Colibacillosis is a complex disease of major economic importance to the Australian poultry industry, conservatively costing $6 million per year. By focussing on genes carried on virulence plasmids of pathogenic avian E.coli this project aimed to provide the industry with tools to more effectively control this disease; namely improved diagnostic procedures to accurately distinguish disease causing strains, and plasmid cured strains which might be used as attenuated vaccines.
Research
The role of plasmid encoded genes as virulence determinants was assessed by comparing the virulence of plasmid cured variants (produced in the course and an earlier CMRDC project) with the wild-type strain and with variants into which plasmids had been selectively reintroduced.
The potential of the virulence associated plasmids as DNA probes was also assessed. E.coli isolates collected and categorised (using techniques such as antibiogram analysis and multilocus enzyme electrophoresis) during a longitudinal epidemiological study and from outbreaks of colibacillosis in the predecessor project, were used for this purpose. The suitability of plasmid cured variants as potential vaccine strains was also assessed.
Outcomes
A reliable aerosol challenge method for reproduction of avian colibacillosis was developed. It was shown that different field isolates of E.coli ranged from highly virulent to avirulent and that changes in the virulence of E.coli isolated from a single flock of broiler breeders was linked to the administration of antimicrobial agents to the birds.
Virulence genes were found on a single plasmid, which could be removed from virulent strains of E.coli, such that they no longer caused disease.
It was found that the plasmid could not be used as a probe for detecting virulent E.coli because it was too large to be sufficiently specific.
The use of killed vaccines made from virulent E.coli and avirulent E.coli (with the virulence genes removed) was assessed. It was found that administration of either of these vaccines to hens could help prevent colibacillosis in chicks.
Implications
This project has established that only specific strains of E.coli, carrying particular genes, can cause colibacillosis, and that the disease can be controlled by vaccination. Further work is necessary to develop a commercially viable vaccine, and to develop a sensitive and specific probe to detect virulent strains of E.coli, but it is clear from this project that these are achievable aims for future work.
RIRDC Project No: UM-20CM
RESEARCHERS: Dr Kevin Whithear
ORGANISATION: Department of Veterinary Science Veterinary Clinical Centre
University of Melbourne WERRIBEE VIC 3030
PHONE: 03 9742 8265 FAX: 03 9741 0401
PUBLICATIONS: Ginns, C. A., Browning, G. F., Benham, M. L., Anderson, G. A., and Whithear, K. G. (1996) - Antimicrobial resistance and epidemiology of Escherichia coli in broiler breeder chickens. Avian Pathology, 25:593-607 Ginns, C. A., Browning, G. F., Benham, M. L. and Whithear, K. G. - Development and application of an aerosol challenge method for reproduction of avian colibacillosis. Submitted. Ginns, C. A., Browning, G. F., Benham, M. L., Anderson, G. A., and Whithear, K. G. - Plasmid mediated virulence in avian colibacillosis. In preparation.
EMBRYONIC MANIPULATION TO IMPROVE GROWTH RATE OF BROILERS
Objectives
To test the following two hypotheses:
· Genetic female embryos can be manipulated during their development
so that they will grow testes and hatch as males.
· That these transformed females will grow more efficiently that
normal females and potentially as well as normal males.
Background
There are significant economic advantages to the broiler industry in having a greater proportion of male chickens than female chickens in each hatch. For instance, male broiler chickens grow more rapidly, are more efficient at converting food to muscle tissue and are less fat than female broiler chickens. As a consequence, males can be slaughtered up to five days earlier than females. The financial benefit of this earlier marketing can be up to 5 cents/day which, for an average broiler farm of 60,000 birds, translates to $7,500 extra profit per batch if all the birds were male.
The successful manipulation of genetically female embryos will result in their transformation into males which should exhibit superior growth characteristics compared to untransformed females. Furthermore, since all eggs would be treated similarly it is possible that genetic male embryos will also exhibit improved growth characteristics. The availability of multi-egg injectors should make it feasible for the industry to readily adopt any transformation procedures which are developed.
Research
In this project two approaches were used to manipulate sexual development in genetic female embryos. The first employed substances, called aromatase inhibitors, which prevent the formation of oestradiol by the developing embryo. In the second approach, testicular tissue from 14-day-old developing embryos was transplanted into 3 to 5-day-old embryos. This technique simply acts as a model in which, theoretically, all the factors which control testicular differentiation and development are presented to the developing genetic female embryo.
Obviously this procedure would not be a practical approach on a commercial scale. However, if successful it could lead to the isolation and identification of the "transformation factors" in testes. The use of the factors may be a viable option on a commercial scale.
Outcomes
It was found that transformation of genetic females to males could be achieved using aromatase inhibitors. Aromatase inhibitors are substances which prevent the formation of the female hormone oestradiol. However, at the concentrations used in these experiments the aromatase inhibitors appeared to have toxic effect leading to high embryonic mortality. This apparent toxicity was reduced when a cross-bred layer strain was used.
While aromatase inhibitors appeared to result in transformation of the gonads of genetic females examined at hatch, we were unable to detect "abnormal" looking or transformed gonads in older birds, suggesting that the effects of the aromatase inhibitors may not have been permanent or complete.
When testicular tissue was implanted into developing embryos the success in transforming females into apparent males was approximately 30%. On gross post mortem examination these transformed genetic females had two apparently normal looking testicles and plasma testosterone concentrations equivalent to genetic males, suggesting that the testes were functioning as normal male testes. While the growth rate of these transformed females was about 8% higher and the abdominal fat pad was smaller than untreated females, these differences were not statistically significant.
Implications
Clearly more work is required to modify the procedures to achieve a high rate of transformation. However, with the advent of multi-egg injectors it is now possible to manipulate developing embryos on a large scale. There are likely to be significant economic advantages to the broiler industry in having a greater proportion of male chickens than female chickens in each hatch.
RIRDC Project No: DAV-35CM
RESEARCHER: Dr David Hennessy
ORGANISATION: Victorian Institute of Animal Science 475 Mickleham Road
ATTWOOD VIC 3049
PHONE: 03 9217 4200 FAX: 03 9217 4299
AN IMPROVED ASSAY FOR CHICKEN ANAEMIA VIRUS
Objectives
· To improve in vitro expression of the gene which codes for
the major antigen of CAV.
· To formulate an ELISA to measure antibody to CAV using recombinant
protein.
· To validate the assay, thereby completing the development of a
reliable test for the detection of antibodies to CAV.
Background
Chicken anaemia virus (CAV) causes an immunosuppressive disease of poultry and, as a consequence, exacerbates infection by other microorganisms. It has been estimated that CAV infection results in lower body weights and higher mortality.
Infection appears to be controlled in the chicken by the presence of serum antibody. It is therefore possible to achieve improvements in performance by ensuring that breeder flocks have an adequate level of immunity as a result of vaccination or natural exposure. Monitoring antibody levels in breeder flocks is important to ensure the virus is not passed onto the flock's progeny.
It has proven very difficult to develop a reliable test for the detection of antibody to CAV. The development of such a test (an antibody ELISA test) has been hampered by difficulties encountered in growing CAV in vitro in a manner suitable for ELISA antigen preparation. Current assays use native virus grown in tissue culture. The hypothesis behind this work was that the use of alternative antigens would improve the reliability of the assay. The strategy adopted in this project therefore was to attempt to produce a more reliable source of antigen by expression of viral genes which had been cloned into E.coli or baculovirus.
Research
The genes coding for the major viral proteins of CAV were cloned and expressed in various expression systems. The recombinant proteins so produced were then used as the antigens in ELISAs of various formats and tested against a panel of known positive and negative sera for CAV.
Initial analysis of some of the expressed proteins demonstrated strong reactions with CAV positive sera. The antigens were tested in an ELISA after optimisation of the conditions. However in all systems there were substantial problems in terms of non-specific (or background) reactions with chicken serum. An intense effort was initiated to reduce or remove these high backgrounds but with only limited success.
Outcomes and Implications
The initial hypothesis was that the problem with the specificity of the CAV ELISA could be solved by the use of alternative specific antigens. However, it now appears that the major problem is in the nature of the sera. It is clear that this background problem requires further investigation. Unfortunately, until this problem is resolved the industry will have to rely on tests for CAV infection which are not entirely satisfactory in terms of their specificity and cost.
RIRDC Project No: CSU-14CM
RESEARCHER: Dr Stephen Prowse ORGANISATION: CSIRO Animal Health Private Bag 24 GEELONG VIC 3228 PHONE: 03 5227 5273 FAX: 03 5227 5555 EMAIL: stephen.prowse@aahl.dah.csiro.au
PUBLICATIONS Coombes, AL & Crawford, GR. (1996). Chicken anaemia virus: a short review. World's Poultry Science Journal 52, 267-277. Coombes, AL & Crawford, GR (1996). Chickens don't weigh up. Microbiology Australia 17(3), 11-12. Crawford, GR. (1996). Development of an Australian ELISA for chicken anaemia virus. Australian Veterinary Poultry Association (AVPA) Scientific Meeting, Gold Coast, April 1996.
Last updated: 22 October1997
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