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Infectious disease: diagnostic dilemmas and strategies for control

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01 August 2010, at 12:00am

Opening the latest meeting of the British Cattle Veterinary Association at Bromsgrove, with a line-up of speakers to deliver a real depth of knowledge, Keith Cutler, the president, introduced an element of concern. 

There have been a number of seminars and meetings involving the BCVA, arranged by various organisations with strong links to cattle production, where the perception is that vets are not engaging with BVD and Johne’s disease. Farmer groups are dissatisfied with the lack of progress and veterinary practices need to consider their own involvement with disease in clients’ herds. 

Mr Cutler stated that “veterinary surgeons cannot make money from sick cows”. There has to be an emphasis on health and the speakers were tasked with giving delegates positive and accurate information that will allow recognisable results with disease control.

Taking samples 

Guda van Burgt (VLA) admitted that the title of her presentation, The technical aspects of testing, what tests are available, when to use what tests and how to interpret the results, was a “dry subject” but she then carried the group by her enthusiasm. 

At the heart of any sampling was an understanding of what was being attempted, she said. Was the sample to detect the presence of disease, or confirm a diagnosis, or establish a diagnosis, and was a current infection to be identified, or a previous exposure, or a vaccination history? Was the herd being tested to establish freedom from disease or to monitor presence of disease? 

If in doubt about the type or number of samples to be taken, delegates were urged to ask the lab. Resampling the same animal, or other animals in contact, increases the chances of an accurate diagnosis. The advice is that if a result isn’t immediately forthcoming, don’t give up. 

Examples were shown of samples received at the lab that were contaminated in transit, of inadequate size, poorly preserved and generally showing a lack of appreciation by the vet of the difficulties being created to sort out a good outcome. 

It was pointed out that “there is no such thing as a perfect test” and sampling results have to be part of a bigger picture. Predictive values can be increased by using more than one type of test, targeting sampling for greater disease prevalence and using a test with a higher selectivity. 

It may pay the farmer to take a carcase to a VI centre rather than have post mortem samples taken on the farm. If a field post mortem is required, screen for the obvious. In discussion, practitioners highlighted the difficulties with taking tissue and other samples of the correct volume, in the correct containers, without contamination. 

Seven schemes 

There are seven schemes that are licensed within the Cattle Health Certification Standards (CHeCS) and several others that are in discussion about equivalence. David Graham (Agri-Food & Biosciences Institute, Belfast) discussed the assessment of herd health status with disease control and eradication strategies. The CHeCS initiative was established and is owned by the cattle industry and regulates the UK schemes. 

Because of the complications of the various tests and their interpretation it is necessary to develop a testing strategy. For example, it is more economic to test calves for BVD before too much growing money is put into the animal or the animal is found positive after the first calving. 

The ear notch sampling system is valuable for newborn calves. There was some concern in discussion about the sales of these tags direct to farmers. One danger is detecting transient infections and their use needs to be part of a control or eradication programme. 

Further information about the details of programmes is contained within the CHeCS technical document (www.checs.co.uk). Reading university has developed the disease cost calculator which is available at www.fhpmodels.reading.ac.uk/mo... m. The prevalence of persistently infected (PI) animals was shown in a study presented by Dr Richard Booth (RVC) to range from 0.3% to 3%, the highest being on non-vaccinated farms. Richard was congratulated on having his doctorate confirmed the evening before the meeting. 

Losses from BVD are associated with reproductive disorders, PI animals and other disease occurring simultaneously with BVD virus that suppresses the immune system. The national losses promoted are not seen by farmers as relevant to their herd. Cost calculators of potential loss are not impacting on herd owners and there is a need for a different approach to motivation.  

Poor youngstock health should ring alarm bells, particularly respiratory and enteric disease. If these observations are combined with reproductive disease, then BVD is a high cause for concern. Infection within the first 110 days of gestation leads to the cow and the calf becoming infected. The dam becomes immune but the calf is born persistently infected. 

The danger is to introduce a pregnant cow or heifer that tests negative. Persistently infected calves that are challenged with BVDV suffer fatal mucosal disease. The bull should always be tested. 

Bulk milk tank tests 

There is a danger in over-interpreting bulk milk tank tests as the result indicates what went on a few weeks ago, as historical infection yields high antibodies. Quarterly milk sampling is advised for monitoring herds. Antibody ELISA indicates exposure to disease but PIs show as antibody negative. Antigen ELISA at one month of age detects viraemic animals and PIs and positives should be retested 3-4 weeks later. 

PCR on blood will detect acute infection and can be used on all animals, but use on bulk milk can give unreliable results due to contaminated milk lines. The study has shown that double fencing the farm boundary can be impractical but avoiding co-grazing in fields next to neighbours’ stock is more
likely to be achievable. Attention to biosecurity has advantages for health and productivity in general and incoming stock should be quarantined and tested with paired serology.

Controlling BVD 

Frank Stephen (DS McGregor & Partners, Thurso) gave a presentation full of practical details about the control benefits and problems of introduction of BVD control in the 20,000 beef cows of Caithness. 

Many of the farms have rented grazing and the practice has developed its own strategy for local conditions. With a known infected herd, all adult breeding stock are tested, with low antibody or negative antibody also tested for antigen. All PIs are culled. Replacement breeding stock are tested before going to the bull and the herd is vaccinated. Youngstock are not tested because of the cost and interpretation of the findings. There is a fear that positives would be sold on. 

For a herd of unknown status, 10 youngstock are tested. If antibody negative, assume that the herd is free of BVD and try and get the farmer to vaccinate because of the potential for disease in naive animals. If there are antibody positives, test adults and cull PIs. 

Case studies showed successes towards BVD eradication but the vet needs to be alert to farmer attempts to frustrate control. Case examples are the introduction of disease from a new bull, possibly infected by intermixing in transit, and a pedigree cow sent to another farm to be served.  

The benefits of controlling BVD in beef herds are recognised by the farmer and success breeds further interest.

What do you know about Johne’s

It would appear that if you think you know all about Johne’s disease, then think again. That was the message that came through from two supporting presentations that highlighted issues with infected herds. 

George Caldow (SAC Veterinary Services) pointed out that although there was evidence about reduction in milk yield, clinical cases and an increased susceptibility to disease (70% of cows culled for other disease tested positive to Johne’s on faecal culture), farmers do not believe the modelling cost studies. If not controlled, the impact of Johne’s disease (Mycobacterium avium subspecies paratuberculosis) gets worse but the infection can be present without damaging the herd. 

In a UK study, 65% of the herds tested antibody positive. When visited, 37% of the farmers stated that they had Johne’s disease in the herd but the proportion of the herds that were positive after all tests were carried out and the analysis completed was 35%. This is comparable to investigations in other countries. 

Field experience indicates that adults can become infected but the calves are most susceptible in the first three months of life and calf to calf transmission has been demonstrated. In the preclinical phase there is low level shedding. 

Iceberg effect 

In a 100 cow herd, there is an iceberg effect with 10 to 14 animals less than two years old having silent infection, 4-8 with subclinical disease, 1-2 with clinical and one advanced case. This gives a total of 15 to 25 animals per 100 cows. 

Transmission is by the oral route through infected faeces, via colostrum (also mainly due to faecal contamination on the teats), via milk and in utero. Faeces contain very high numbers of organisms. Sheep have been shown to be infected on infected cattle farms and rabbits are a wildlife reservoir. Map has been isolated from bull semen. 

Management of the dry cow and calving areas is important with dung management, including addressing faecal contamination of water troughs and water systems. Waste milk should not be fed and greater hygienic precautions are necessary, particularly with the feeding of colostrum. A genetic susceptibility to the disease has been demonstrated. 

The diagnosis of clinical disease is not a problem but it is difficult to detect individual animals with subclinical infection. There is no gold standard for cattle diagnostic tests for Johne’s disease. Extraction of DNA from Map is difficult and inhibitors of PCR reaction are present in faeces. Commercially available PCR kits vary in their ability to overcome test inhibition. 

Serum samples are most likely to be positive at the end of lactation and milk samples at the beginning of lactation. However, there is no biological reason why an animal should have antibody in the milk and not in the serum. Milk antibody positive animals that are negative for serum antibody are false positives. Bulk milk antibody ELISA will test positive if at least 5% of the herd are antibody positive. 

By this time, clinical disease should have been observed and this approach can be used as a relatively insensitive early warning system. Details of the use of tests to monitor herds thought to be free of infection, for control programmes, and for screening purchased animals are detailed at www.johnesdisease.org.uk 

Keith Cutler (Endell Veterinary Group) introduced case studies and understanding from the work on Johne’s disease within the Wiltshire practice. He pointed out that if researchers did prove a link between Map in cattle and Crohn’s disease in man, there was potential for this to be the “next big food scare”. 

Johne’s disease has no effective treatment available and is therefore irreversible and ultimately fatal. The disease is long term with insidious onset and therefore chronic and the gut wall becomes thickened, so reducing absorption of nutrients and fluids and causes enteritis. Map is related to Tb and can affect the skin test. 

The trend for prevalence is up. Many animals are culled for other reasons before clinical Johne’s develops. 

More testing is required to establish herd status and prevalence and to guide a culling policy. The testing programme needs to include suspect cases, offspring of confirmed cases, cohorts of confirmed cases, cull cows and all animals over two years of age on an annual basis. 

Improvements in hygiene are essential and the theme is to keep baby calves and faeces from adult cows separate. Slurry management has to be applied. Examples of residual disease being spread on grazed pasture, from other farms, indicate an issue with contractors and cleanliness. 

If Johne’s disease is not controlled now, the incidence of this disease will rise unnoticed until it becomes impossible to do anything about it, he said. Eight case histories detailed the situations met, the results of testing, the actions taken and the outcomes. Copies of the farm examples will assist anyone facing similar situations.

On-farm programmes 

It was left to Peter Orpin (Park Veterinary Group) to consider the practical considerations when implementing disease control programmes on farm. His opening point was that “failing to plan is planning to fail – the only advantage to not planning is avoiding the period of worry before you fail”. 

There is a need to create a practice-based infectious disease control programme. For practice planning meetings he advises that you do not start with a blank piece of paper. Make a proposal for a plan beforehand and circulate it. It is worth creating an “infectious disease champion” within the practice.  

Knowledge and information will need to be collected and assessed in order to define the practice disease strategy, before arranging client meetings or other forms of communication. All farmers do not have the same aspirations with disease control so meetings need to allow the clients to express their concerns. 

“Let the farmers do the work,” he said. Various health planning tools are available and Peter demonstrated the benefits of a graphic presentation for each farm with the Myhealthyherd web-based health management programme.  

Charging options for practices include an interpretation fee for lab reporting, for extended time on the farm and a monthly fee for disease control incorporating a package of risk assessments, testing and reporting. 

The conclusion was that infectious disease management, managed well, is profitable and interesting.