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Progress in the Diagnosis of Infectious Diseases

 

 

Mrs Ruth E Lysons,
Head of Laboratory Testing,
Veterinary Laboratories Agency,
New Haw, Weybridge, Surrey, KT15 3NB,
United Kingdom

Introduction

There is a huge variety of veterinary diagnostic tests available, and technological advances mean that new tests are emerging all the time. The selection of a laboratory test should be dependent upon the purpose for which it is being used. There is increasingly a requirement for tests to be rapid and of low cost. At the same time, the test must be adequately validated and quality assured to ensure consistent results.

The main reasons for use of diagnostic tests in farm animals are for individual animal diagnosis, herd investigations, disease control or disease surveillance. In Great Britain the requirement for farm animals is increasingly for large-scale surveillance programmes. These are usually designed with a mathematical basis to answer specific epidemiological questions. In Britain there is currently a particular emphasis on surveillance of livestock for prevalence of specified food borne zoonoses such as salmonella, E. coli O157. Large-scale testing requires cheap and robust tests.

Test costs can be influenced considerably by careful selection of sample type, sample handling methodology, and by the available test technology.

Quality assurance of testing is essential and should comprise suitable within-test quality control and external quality assurance. Independent accreditation e.g. ISO9000 or UKAS/ISO 17025 adds an additional level of quality assurance.

PrP Genotyping

The TaqMan PCR technique can be used to demonstrate single base differences in a gene. In sheep, polymorphisms exists in the amino acids encoded at codon 136, 154 and 171 of the PrP gene. The significance of different polymorphisms varies with different breeds of sheep, but certain genotypes are always associated with scrapie resistance.

Cryptosporidium

Cryptosporidium parvum is believed to be associated with human illness. A new multi allele specific PCR has been developed which can differentiate the two main genotypes of C. parvum. Although still in a developmental phase, this is an example of a PCR on several genes which can be carried out in a single tube.

Brucella ELISA

Although Britain, in common with many European countries, is free from brucella, our national brucella monitoring programme means that a conventional brucella ELISA is our highest throughput test. This is an instance where false positive results can be tolerated in the primary screening test, as the vast majority of samples are negative. Any samples proving positive on ELISA can be re-tested by confirmatory assays.

Rabies

The Pet Travel Scheme (PETS) was launched in Britain this year. Britain is rabies-free and has previously required incoming dogs and cats to spend six months in quarantine. The PETS enables eligible animals to enter Great Britain without quarantine provided they are identified by microchip, vaccinated against rabies, blood tested for evidence of sero-conversion following vaccination, and that they carry appropriate veterinary certification. The official test used under this scheme is the rabies fluorescent antibody virus neutralisation assay. Increasing the throughput of this test presents logistical difficulties due to the requirement for high level laboratory containment facilities.

Tuberculosis

Unlike many other European countries, bovine tuberculosis remains problematical in Great Britain due to a persistent reservoir of infection in wildlife. A blood test is needed to replace the comparative intradermal tuberculin test currently used. A conventional antibody-based assay is not appropriate because M. bovis generates a cellular (T-cell) response. Detection of cytokines expressed by antigen-stimulated white blood cells is the basis of a test which is about to be used in a field trial involving British cattle.

Conclusion

Many advances have been made in diagnostic testing in recent years. There remains a requirement to select tests carefully, according to their intended purpose, and to develop procedures and protocols which keep test costs to a minimum and permit fast turnaround of results. Some specific examples have been described.

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