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An online mathematical model has helped producers and their advisers to customise financial estimates for the cost and benefit of different vaccination strategies at the particular unit under review.
on May 7, 2008

The economics of health control — Section 1

Costings from field experience are now being included in advice to pig producers on the control of respiratory and circovirus infections.

Model measures respiratory protection 

Just a few weeks ago, a pig enterprise in northern Europe was going through an audit. On the basis of an evaluation by veterinary advisers it was told that it could achieve a substantial improvement in its current growth performance and grow-finish mortality rate by making sure it vaccinated against some major respiratory pathogens. In this case, though, the advice was both specific on the changes possible in technical results and on the likely cost-benefit.

A few details can be reported here without revealing the precise farm. For example, it has facilities for some 350 sows and grow-finish places for 3200 pigs. Out of the array of infections that make up the porcine respiratory disease complex or PRDC, neither the swine influenza virus nor the Haemophilus parasuis of Glässer's Disease is considered to play an important role in pig health on this site. It has officially-certified freedom from the causal agents of Aujeszky/pseudorabies or atropic rhinitis. Its problems in respiratory health seem to arise from PRRS, Mycoplasma hyopneumoniae, App and PCV2.

The audit differentiated between these infections in terms of their severity on the particular unit. While none was rated as severe, the circovirus effects received a rating of ‘moderate' and those of PRRS, M. hyo and App as ‘considerable'. Farm records indicated their combined impact to have elevated death losses during the grow-finish period to 3.0%. Daily weight gains were down around 720 grams. The feed conversion rate was no better than 2.8. Evidently the unit's results did not reach the standard that might be achieved and the presence and relative severity of the respiratory infections explained at least part of that performance gap.

The advisers also set out the options on vaccination for this unit as being between a trio of combinations. A PRRS virus vaccine would have to be represented, but it could be combined with M. hyo or App or put into an approach covering all 3 infections.

Without any vaccination at all, which means even stopping the M. hyo vaccine currently applied, the outcome calculated for the enterprise was a loss for disease reasons alone that came close to €31 000 per year. Vaccinating the PRRS + M. hyo route would change this around from a negative to a positive, bringing a margin over vaccination costs (including labour) of more than €28 000 per year. PRRS + App vaccination offered an annual €37 000, broadly similar to the advantage from using the full PRRS/M. hyo/App application.

Vaccination choices

Therefore the enterprise operator could make a more informed decision on the choice of vaccination schemes open to him. Put simply, PRRS + M. hyo was the most logical of the 3 options as addressing the primary problems for the site. The other extreme of triple vaccination was both the most expensive option and the one likely to yield the best technical results. But PRRS + App emerged as the one giving most added value between vaccination cost and improved performance.

Projections for the number of pigs sold annually also allowed a further comparison in terms of cost. This farm was reckoned unable to sell more than 7765 pigs per year if it practised no vaccination of any kind against respiratory threats (see Table 1) . Adopting a 2-vaccine regime was projected to raise this output to 8667 pigs/year in the case of PRRS + M. hyo or 8764 pigs/year with PRRS + App, with 8943 pigs produced annually where triple vaccinating was practised. The increases would change the loss of €2.87 per pig sold per year without vaccination into a financial benefit calculated at €2.39 on a per-pig-sold basis using a PRRS + M. hyo vaccine strategy, €3.11 per pig sold for PRRS + App and €2.75 for the triple vaccination.

While vaccinating on the basis of any of these schemes is clearly a good investment at this site, the operator must still consider the relative importance of the infections in his own pigs. For example, controlling PRRS and Mycoplasma hyopneumoniae on their own will not mean an end to all PRDC problems for the herd in question, because the disease audit showed App was also having a perceived role in its health profile. Good advice to all producers is that they should avoid too narrow a focus on the diseases apparently affecting their enterprise, not least when choosing a vaccination plan. Even if you decide to vaccinate against a single pathogen, keep in mind the others known to be present and the effects they can have.

Mathematical model

Costings to advise the European unit outlined above had been prepared with the help of an online auditing tool developed by Intervet/Schering-Plough Animal Health for the analysis of PRDC problems on pig units around the world. Fundamentally, it is a mathematical model. But this one differs by being directed solely at the respiratory health of an enterprise and the changes possible by vaccinating. Most of all for decision-making it takes the current cost of vaccinations for the unit under examination together with the financial implications of changes to its performance, so the enterprise operator is presented with real figures rather than theoretical data or benchmarks.

The idea behind the ResPig model is that determining the right control measures to adopt against PRDC has been made more difficult by the wide variety of pathogens that might be involved in each case. It therefore begins with an audit for the unit that includes a scoring system on the severity of each pathogen audited. In practice the score can be zero (meaning the infection is not present on this site), 1 (meaning moderately influential on farm results), 2 (indicating a considerable impact) or 3 (severe).

At the same time the auditor adds estimates for the effects of the pathogens and how these might be influenced by the use of an appropriate vaccine. The modelling tool then has an Economic Simulator function that converts the inputs into an assessment of the costs and likely financial results from each intervention strategy to control or reduce the respiratory infections causing trouble on this farm. Disease and vaccination effects within the economic calculation have been based on scientifically controlled field trials relating to single pathogens of the PRDC complex and on the published research of independent authors.

Other elements in the farm audit look at information on health aspects and risk factors for the enterprise in question. These range from clinical signs and data from post-mortem examinations to slaughterhouse reports. The answers also are given a score, although not at this stage with reference to any specific diseases. The next step of the disease audit takes it further by linking together the indicators of respiratory health status with the diseases and pathogens under consideration at this site. Additional remarks such as from laboratory tests can also be added.

The final assessment appears as a general audit in which the results are displayed not only in terms of their general score, but also according to each disease entity and each age group on the farm. The ground is therefore laid for an in-depth analysis that can give an overview of specific questions and provide the cost-benefit of different vaccination strategies against single pathogens or multiple agents.

We find in practice that the online model is being used by veterinary advisers to show to clients how the costs alter with different vaccination approaches for the control of respiratory problems. What is more, it has helped them demonstrate that PRDC is not a single disease with one solution, but is instead of a wide-ranging complex for which different control measures need to be according to the health status assessed for that enterprise. 

Although triple vaccination incurs a higher cost and gives the top technical results in this example, almost an equal annual farm income would be obtained by a dual respiratory vaccine application.
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