How many veterinary surgeons in cattle practice would you expect to want to spend a day at Bristol listening to details about testing, interpretation and strategies for the control of infectious disease?
The BCVA office planned for 40. Registrations built and a second room was booked with over 80 attending on the day. It would appear that cattle vets are taking every opportunity to fully understand about diagnosis and how to make the best use of the tools available.
Keith Cutler opened the day with the news that Jim Willshire from Sutton Bonington, primed to give a review of the technical aspects of testing for each disease, had arrived with his wife the evening before. Unfortunately an ambulance had to be called during the night to take Mrs Willshire to hospital where she had an emergency operation.
Later in the day delegates were informed that the operation had been successful and everyone wishes her a full and speedy recovery. At short notice, colleagues Bob Monies (Langford), Eamon Watson (Winchester) and Alan Murphy (Penrith) made reference to the prepared overheads, spoke unhesitantly and carried all before them.
The selection and interpretation of available tests requires considerable foreknowledge. The situations were highlighted where there are clinical signs but a negative or inconclusive test result, and no clinical signs but a positive test result.
Initial questions raised include: are the tests to detect the presence of disease, or to confirm a diagnosis or to establish a diagnosis and is there concern about a current infection, previous exposure or vaccination history?
Moving on to selection of samples and selection of specific tests, the message is “phone the lab”.
Which test, serology, virology, PCR, culture, and which sample, blood (clotted or heparin), milk or joint fluid? What is the test telling you? Tests are not perfect but information about the sensitivity, specificity and predictive value are important if the clinician is to believe the result.
Having mastered the technical issues to this point there is the question of sample size and whether the intention is to identify every positive animal or detect disease from a selected population or determine freedom from disease.
Fortunately, the lab has a computer program whereby the known factors can be entered and the correct sample size established to provide a level of confidence in the outcome.
Assessing status
David Graham, of the Agrifood and Biosciences Institute, Belfast, shared his observations on assessing herd health status, disease control and eradication strategies. The Cattle Health Certification Standards (CheCS) promote “assess, control and eradicate”. Who has ownership of endemic disease? The Government is focusing on statutory requirements and exotic disease, industry clearly has a role and there is a need for partnership between the herd owner, the veterinary practitioner and the testing laboratory.
There is a substantial list of the reasons why there should be assessment, control and eradication of infectious disease. An increased sale price through disease reduction heads the financial list with cost benefit information. Details are available at www.reading.ac.uk/fhpmodels/models.htm. It is worth the farmer and the vet looking together at the information to have a clear idea of the best way forward.
The notion that prevention is better than cure appears very important and that having a recent history of low disease diagnosis may not be a sound reason for inaction.
Pedigree sales are demanding health declarations and these can include the health of the herd as well as the individual animal. Specific legislative requirements control the use of vaccines and vaccine markers.
The CHeCS scheme is applied to BVD, IBR, Lepto and Johne’s and develops through four stages: monitoring, an accreditation programme, screening and eradication, and for BVDV only a vaccination and monitoring disease-free programme. Full information about the scheme, including the biosecurity requirements, is available at www.
checs.co.uk.
From April, the BVD Erns ELISA will be available from the VLA to be used for animals over three weeks of age. Antigen (Ag) ELISA plus leucocyte extraction is available for three to six months of age and Ag ELISA from six months onwards.
“These tests are geared towards the detection of persistently infected animals but may occasionally identify acute infections, therefore all positives should be retested using the ELISA in three to four weeks time to eliminate acute infections,” Mr Graham said.
A BVD RT PCR (Reverse Transcriptase Polymerase Chain Reaction) can detect virus shed into a bulk tank, can be used to test pooled blood samples and can be used in animals over six weeks of age, but the test is extremely sensitive to contamination and samples tested positive will still be positive two months later regardless of whether the positive was a result of acute or persistent infection.
BVD antibodies are long lived so individuals are considered positive or negative and seroconversion is life-long. A PI cannot be Ab +ve and pregnant Ab –ve cows that are Ag –ve cannot be carrying a PI calf. Bulk milk BVD Ab ELISAs provide information regarding the general exposure of the herd to BVDV and are particularly useful when viewed serially. Individual cow milk Ab ELISAs are now accepted by CheCS as a testing method but samples must be taken directly from the cow’s teat.
A simple disease…
With all the technical build-up, it was a delight to hear Frank Stephen, of DS McGregor & Partners at Thurso, refer to BVD as a “simple disease”. Since 2000 the practice has operated a control and eradication strategy for commercial beef clients that is easy to understand and cost effective for the farmer.
Farmers in Scotland are not known for spending their money unwisely. The practice recognises that the persistently infected and the naive animal both play a part in the epidemiology of the disease. One very important point is that breeding adults are tested and antigen positive animals culled.
Young stock are not tested, because the danger is that positives will be sold on to other farmers and the initiative would end up spreading the disease rather than controlling it.
Vaccination protects the naive animal and the approach is dependent on having no PIs and a protected herd before the start of the service season. The farmers notice the additional benefits of eradication with less general disease and greater production. Success breeds success and the farmers are now looking to control other diseases.
Controlling IBR
The problem of latency with IBR was one of the issues highlighted with controlling the disease by Bill May of Lambert, Leonard and May of Whitchurch. With over 200 dairy herds and an average herd size of over 200 cows, IBR outbreaks are common.
Latent infection of the Herpes virus recrudesces during travel, calving or following large doses of corticosteroid, and vaccination of an animal with latent infection does not prevent disease. Competitive ELISA (cELISA) is used for screening purchased stock with diagnosis of acute infection using paired serology with iELISA (Indirect).
To detect individuals vaccinated with marker vaccine and not exposed to field or non marker strains, gE antibody ELISA (gE ELISA) is available and the result will be gE negative. For individuals with raised temperature and a nasal discharge, FATs (Fluorescent Antibody Test) will demonstrate presence of the virus directly.
After infection, abortion can take place weeks or months later due to the pyrexia and calves damaged by bronchopneumonia usually end up being culled. Mortality and morbidity is very variable and the disease is highly contagious. With an outbreak, the intranasal vaccine provides a more rapid immunity but it is a hassle for farmers.
Bulls destined for AI should never be vaccinated. Eradication demands the use of marker vaccines, repeat testing, culling, biosecurity and monitoring. Experience indicates that routes of infection are poorly understood by farmers and one of the major pitfalls is failure to maintain vaccination.
Practical approach
Jonathan Statham of the Bishopston Veterinary Group at Ripon explained that the practice clients’ attitude to cost was similar to the Scot but with the generosity removed. A totally practical and cost effective approach has to be taken with leptospirosis.
Bulk milk screening with ELISA detects IgG but the test does not differentiate between infection and vaccination. Acute infections are detected with Lepto MAT (Microscopic Agglutination Test). Control on farm requires a reduction in exposure to the organism with antibiotic treatment and avoidance of waterways and co-grazing with sheep, plus vaccination to reduce the risk of reproductive pathology. There are pitfalls with vaccination and product sold is not the same as product injected. Unfortunately the vaccines are not fully effective, he said.
No effective treatment
It soon became clear why Keith Cutler from the Endell Veterinary Group in Salisbury is enthusiastic about controlling Johne’s Disease. His opening slide indicated that there was no effective treatment for Mycobacterium avium paratuberculosis (Map) and the disease was inevitably ultimately fatal.
The condition is chronic, causes enteritis with thickening of the gut wall and a reduction in nutrient absorption – and the organism can affect the TB test. Infection generally occurs during the first few weeks of life and calves can become infected in utero, by drinking infected colostrum and by ingesting faeces, especially from dirty teats.
Around 70% of animals culled for other reasons are found to be Map culture positive with direct effects on milk production and fertility with a susceptibility to other diseases.
Interpretation of tests is often at fault rather than the tests themselves. Testing (ELISA, Culture, PCR, ZN Smears) should include suspect cases, offspring of confirmed cases, cohorts of confirmed cases, cull cows and all animals over two years of age, at least on an annual basis.
Neospora problem
Neospora caninum testing using serum ELISA (aborting cows, identifying congenitally infected calves, herd investigation of dry cows and calves) requires care in interpretation. Following abortion the sample may be Neospora positive but the organism may not have caused the abortion.
Infection early on in gestation is likely to cause abortion whereas congenital infection is the outcome of later gestation infection.
Professor Diana Williams from Liverpool outlined the control and eradication of Neospora on the farm. Bulk milk ELISA is available to monitor negative herds.
All the presentations were supported by case histories, with pitfalls and errors openly reviewed as well as highly successful outcomes.
