The Economics of Testing for Maedi-Visna
by Beth Kyle

Jim Fisher of Kemptville College and Dr. Paula Menzies of the University of Guelph recently released an analysis of the costs and returns of participating in the Maedi-Visna Flock Status Pilot Project (MVFSPP). The MVFSPP was started in 2000 as a joint project between the University of Guelph, the Canadian Food Inspection Agency and the Ontario Sheep Marketing Agency. Producers participating in the project have the option of following the ‘whole flock’ testing schedule to achieve a Maedi-Visna ‘free’ status or the ‘monitored’ schedule to screen for the prevalence of the disease in a flock.*

Maedi-Visna is a viral disease affecting sheep and goats and is frequently found in Ontario flocks. Although they may appear healthy, sheep often become infected as lambs and can spread the virus to their lambs or other adults in the flock for years before showing any signs of disease. There is no cure and the only way to eliminate Maedi-Visna is by repeatedly testing the flock and culling positive animals. Although the disease is ultimately fatal, a greater economic loss is due to decreased flock productivity through lower conception rates, decreased lamb birth weight and poorer lamb growth resulting from low milk production. One research study found that infected ewes wean on average 11lbs less per lamb crop than uninfected ewes. Not included in this assessment was the loss in productivity due to premature culling. Producers who sell breeding stock may also receive lower prices if their sheep have not been tested. Weighing against the benefits of eliminating Maedi-Visna from a flock are the costs involved in disease control. Some of these factors include the cost of replacing culled animals with equivalent stock, testing costs (e.g. veterinarian, lab analysis, labour), and the general requirements of the program (e.g. producer time, paperwork, biosecurity etc).

To help determine when the benefits of testing for the disease outweigh the costs, Mr Fisher conducted interviews with 15 producers participating in the MVFSPP. The average flock size for the producers interviewed was 149 ewes. The majority of these producers sold both breeding stock and market lambs, with the average sale of breeding stock equalling 26% of the lamb crop. With an average of seven positives on their first test, the cost of achieving ‘A’ status on the whole flock program for this group of producers was ~$5200 over five years. Although these producers would also benefit from an increase in flock productivity, the most apparent short-term advantage was an 11% increase in the price and/or quantity of breeding stock sales. Assuming there was no lag time between starting the program and observing this increase, producers could expect to breakeven with the costs of the program 1.5 years before reaching an ‘A’ status.

Using figures reported by the producers as a basis, Mr. Fisher developed a computer model that provides an educated guess for deciding when disease eradication is an economically sound investment given various flock sizes, lamb destination (breeding stock vs. market), flock infection levels, and testing costs. Based on various scenarios entered into the model, producers selling a portion of their lamb crop as breeding stock should always break even before or slightly after achieving an ‘A’ status on the whole flock program. The scenario is much different, however, for commercial producers raising 100% of their lamb crop for market. According to the model, the costs involved in eliminating the disease will always outweigh the benefits of improved ewe productivity for commercial producers with a low prevalence of Maedi-Visna in their flocks. Only when the flock infection rate reaches 10% or higher are the detrimental effects on productivity severe enough to warrant participation in the full flock test. Commercial producers may choose to use the monitored program to help determine the level of infection without testing the entire flock. If the level of infection is nil or very low, the improvements to productivity achieved by eliminating Maedi-Visna will be minimal while the costs remain high and the producer may decide not to test further. If the results from the monitored test indicate a heavy level of infection the producer has the option of joining the whole flock program to effectively control the disease.

Maedi-Visna has the potential to cause severe economic losses within a flock. As shown by this study, testing for the disease can be cost effective under certain circumstances, both for producers selling breeding stock and those raising commercial lambs. The computer model developed by Mr Fisher may provide a tool to help producers decide when eradicating Maedi-Visna is an economically viable option and how soon the benefits will outweigh the cost of testing for individual flocks.

For more information about this study contact Jim Fisher at jfisher@kemptvillec.uoguelph.ca

*For more information about the Maedi-Visna program contact OSMA at 519-836-0043.