Control and Eradication MV Programs are Based on Three Principles:

  1. Detect and remove all infected animals.
  2. Screen all incoming animals to prevent re-infection.
  3. Monitor the flock to assure that the status of the flock is unchanged.

Determining Flock Status

Because a test is being used to determine if a disease is present, and no test has perfect accuracy, we can never say that a flock is “free” of a particular disease.  However, using correct principles of testing and biosecurity, we can make statements about the flock status, i.e. the risk that the flock is infected with a particular disease.  Generally these statements say that the flock has either a low, moderate or high risk of infection – based on a pre-determined set of criteria.  These criteria usually involve whether or not there appears to be disease in the flock, the serological status of the flock, frequency of a negative serological test and the level of biosecurity of the flock (closed, quarantine period, etc).  At this point Ontario and Quebec have provincial programs that attempt to determine flock status with respect to MV infection.  Each are different, but are also very similar.

ButtonThe Ontario Maedi-Visna Flock Status Program

This program was developed in conjunction with the University of Guelph, the Ontario Sheep Marketing Agency (OSMA) and the Canadian Food Inspection Agency.  OSMA is responsible for administering the program.  The program is outlined below. 

A producer may enter either the Whole Flock Program – developed for purebred breeders that wish to have the highest status, or the Monitored Flock Program – developed for large flocks that are seeking a less rigorous and expensive way to reach a high health status.  The Whole Flock program requires annual testing with 2 whole flock negative tests of all sheep >= 180 days of age, followed by a negative flock test of a random sample of sheep >=365 days of age in order to receive the lowest risk status of "A". All test positive sheep must be removed, and if a ewe - her lambs < 180 days of age must also be removed. Status levels are ENROLLED; ENROLLED-NEGATIVE, "B" and "A".

The Monitored Program requires annual tests of random sample of sheep greater >= 365 days of age. The drawback to the Monitored Program is that if the flock is infected and tests positive, the test will not help to find and remove all the infected animals.  However, the Monitored Program is suitable for large flock producers that suspect that the flock likely is not infected.  If the flock is negative on three consecutive annual flocks and the producer does not introduce new sheep without testing and isolation, then the chance that an animal is infected in the flock is very low.  The producer may then elect to have one whole flock test. If that test is negative, then the flock would receive “B” status.  This gives an alternate route of reaching “A” status that takes longer but involves fewer animal tests.

For the Monitored Program or when the flock reaches “A” status, only a proportion of sheep are tested .  This proportion is not an exact percentage of the flock, but is based on detecting infection in a flock if it is present at 5% or better – 95 times out of 100.  If the flock size is small, almost all sheep in the flock would be tested, e.g 19 of 20. If the flock size is very large, the proportion of the flock needed for testing is relatively small, e.g. 56 of 500.    It is important that the sheep are all greater than 12 months of age and that they are randomly selected and not just the first sheep through the chute.  Given that infected sheep may be slower, this is particularly important.  For any test that only samples a portion of the flock, all sheep added to the flock in the last year should be tested as well as the random sample.  These programs and all protocols can be found here.

Summary

Maedi visna is a common infection in Ontario sheep flocks causing significant loss of productivity. It causes severe disease which most often results in the death or early culling of the sheep. The good news is that with appropriate testing and removing of infected animals and following biosecurity precautions, maedi visna virus can be eradicated from the flock.