BLOOD TESTING THE HEFTED FLOCKS

 

Concerned by the proposals to blood-test the hefted flocks, Dr Ruth Watkins (virologist & sheep farmer) has provided some very important guidelines on the handling of hefted flocks grazing on common land.

 

Gathering

Using the local men and their sheepdogs gather and sort into hefts.

A different team of vets and handlers should be allocated to each heft. I suggest this team does not come into contact with sheep before the local men, dogs and other personnel have sorted the flocks into hefts, and indeed into the individual pens. The tagging could begin after the sheep have been penned.

 

Penning

Each heft should be divided into pens of no more than 250 animals, preferably less i.e. 200. So a heft of 1000 animals would be divided into 5 pens. Every pen of every heft should be 200 metres apart. The penning could provide ideal conditions for spread of virus. If you had 1000 sheep in each pen and there was cross infection from the handling (contact) and ear tagging (inoculation) (a primary round of infections caused by cross infection from index case or cases) one could have several hundred animals with an acute infection simultaneously in 5 days time (a secondary round of infections) and risk airborne spread (small particle aerosol drifting downwind), see AI Donaldson et al Relative risks of the uncontrollable (airborne) spread of FMD by different species the Vet. Rec. May 12 2001 p602 - 604. This might happen in more than one pen of a 1000 animals and spread infection to previously uninfected pens or hefted sheep and farms downwind.

Individually number each sheep on the hill. Sequential numbering 1 to 10 000 this number to be put on each sample- if there is some mix up between hefts it does not matter. Sheep can be accurately matched with their results and re-sampled.

If their mouths are examined this will spread virus between members of the gathered heft unless the vet changes gloves between each sheep. I suggest it is irrelevant to examine their mouths but to rely entirely on the laboratory results.

The marking of sheep ought not to be a method that draws blood (ear marking by punching holes or slitting etc. is bloody) so a small number tag should be used instead. Again it would be good if the pliers to insert the ear tag are dipped in glutaraldehyde between each animal. The viraemia could be high enough to infect a number of animals by direct inoculation if the same bloody pliers are used without disinfection.

Once penned shearing and dipping etc. can be done.

 

Feeding and watering

 

It would be worth making the water PH5 to prevent cross-infection by sharing drinking receptacles. If 200 sheep per pen were to be fed on hay then long lines of hay could be laid allowing less overcrowding of sheep.

 

1st Sampling

 

I presume this is done on the day of penning or the next day.

Blood should be drawn from each animal for antibody testing with a sterile needle and syringe. This is because cross infection by inoculation of virus into the blood stream will occur from any viraemic animals if the same needle and syringe is used, the infection will amplify quickly in the pens. Also in sensitive antibody tests this could lead to a false positive in antibody negative animals following strongly antibody positive ones when the same syringe and needle is used. This blood can also be tested for antigen (defining a peak of viraemia when there will be no antibody present) and for viral RNA by PCR if desired. PCR is much more sensitive than tests for antigen.

The personnel should be the same as for the heft tagging.

 

2nd Sampling

 

Whatever the results of the first sampling, a second sampling 7 to 10 days later should be done for antibody and virus again. This time swabs should be taken from the throat as well. (if possible change gloves between throat sampling each sheep)

The personnel teams should be separate for each pen.

 

Interpreting results pen by pen: FIRST SAMPLING

 

A) 1st sampling negative for antibody and virus by antigen on all the blood samples.

Either no sheep are infected with FMD at all or one or more sheep are in the first few days of infection before the peak of viraemia or have been exposed and infected in the gathering. If the latter, then infection could have been transmitted from a different heft during mixing in the gathering. All the sheep are susceptible.

 

B) 1st sampling negative for antibody and positive for virus by antigen in the blood of one or more sheep.

One or more sheep are acutely infected and have been sampled at the peak of viraemia when they were gathered (they were infected before they were gathered). These sheep will be highly infectious to the others. All the others are susceptible.

 

C) 1st sampling positive for antibody in some sheep and positive for virus in blood of other sheep.

At the time of gathering antibody positive sheep either had past infection (of which some will have cleared the virus completely and some may be carriers) or were convalescent. The antigen positive sheep were acutely infected at the peak of viraemia and highly infectious at the time of gathering. There was an ongoing chain of acute infections in that heft at the time of gathering.

 

D) 1st sampling positive for antibody on some sheep and negative for virus by antigen in blood of all sheep.

At the time of gathering antibody positive sheep either had past infection (of which some will have cleared the virus completely and some may be carriers) or were convalescent. It is possible that none of these sheep were infectious at the time of gathering. However all antibody negative sheep are susceptible. These sheep could be incubating FMD infection or have been exposed and infected at the time of gathering.

 

SECOND SAMPLING IS ESSENTIAL WHATEVER THE RESULTS OF THE FIRST.

(unless all the sheep are vaccinated, see page 5 (3))

 

Interpreting results pen by pen: SECOND SAMPLING

(7 to 10 days after the first sampling)

 

2nd sampling negative for antibody and virus by antigen on all the blood samples and negative for virus in all throat swabs.

a) No sheep in this pen have been infected in the past or presently. The average incubation period is 5 days. It may be wise to take a third sampling in case there was one or a few exposures to a very low dose of virus at the time of gathering requiring the full incubation period of 14 days i.e. sample third time at 3 weeks after gathering. Little different from A above.

 

2nd sampling negative for antibody and positive for virus in the blood or and in the throat swabs of one or more sheep.

b) Acute infections have arisen either acquired by exposure during the gathering or shortly before the gathering, within a few days. All the other sheep in the pen are at risk. Rather as B above.

 

2nd sampling positive for antibody in some sheep and positive for virus in the blood or and throat of one or more other sheep.

c) There is acute ongoing FMD infection in this pen. The pens in C above would be expected to have these same results on the second sampling, when the virus positive sheep the first time round should now be antibody positive. It would be interesting to know the degree of amplification of the infection, the ratio of primary cases to the index cases in these pens.

If all had been negative for antibody and virus as in A above on the first sampling then this shows that infection had been spread shortly before the gathering from other sheep within this heft which had a low overall rate of acute infection e.g. about 1 in 200. This is because only already acutely infected at the time of gathering could have made antibody by 10 days. All antibody negative and virus negative sheep are susceptible or already exposed, infected and incubating the infection.

 

I would not expect any pens to show the appearance of antibody in some sheep that were negative on the first sampling when no virus is detected in any sheep at 7 to 10 days after penning by the second sampling. In other words sheep without antibody the first time round but acutely infected with FMD would have been highly infectious and would have spread the virus to others in the pen. These new infections in the pen would show as virus positive on the second sampling. The acutely infected sheep at the time of penning would meanwhile have developed antibodies, see c) 2nd paragraph.

 

Possible courses of action given results of the first sampling

 

1. Cull virus positive (antigen positive) pens only. No need to cull pens that are virus negative (antigen negative) regardless of whether the same heft or not. Also no need to cull any antibody positive / antigen negative pens nor antibody and antigen negative pens.

 

2. Vaccinate all antibody negative animals (also antigen negative) in any pen that has one or more virus positive (antigen positive) animals. (This will dampen infection, thus prevent intense amplification of infection.). This is an alternative to culling. Having put so much work into this experiment it would be worth vaccinating to examine effect with further monitoring.

 

3. Vaccinate all the pens, all animals that were antibody negative. No need to kill any sheep as virus positive sheep will probably have stopped shedding virus by the time the results arrive. You could release them all afterwards and re-gather in 3 weeks to give the second dose of vaccine. These would not require sorting or blood testing but simply vaccination and marking to show completion of the primary course of vaccination. You could sample again when the sheep return for winter and test for antibody to structural and non-structural proteins and for virus in a throat swab to document the success or otherwise of the vaccine program. They could also receive a booster dose of vaccine at that time.

 

Possible courses of action given results of second sampling

 

1. Following on from 1. above, cull the pens that are antigen positive on the second sampling. You should now have removed all acutely infected animals and those exposed to infection and incubating acute infection in the secondary round of infections in the pen. There is no need to cull any antibody positive / antigen negative on blood and throat swab pens- nor to cull antibody and antigen negative in blood and throat swab pens. You may find this conserves part of a heft of which some pens are culled. It may be necessary to test a third time to rule out incubation periods of infection that were 14 days.

 

2. Vaccinate as in 2. above.

Give a second dose of vaccine 3 weeks later (completion of course) to all the sheep vaccinated in the vaccinated pens. Then release all the sheep after one month in pens (which may require moving to prevent foot rot etc.) and sample later to check the success or otherwise of vaccination by looking for infection in unvaccinated sheep. One would have to test unvaccinated pens a third time before release in this experiment to exclude incubation times of 14 days.

 

 

OVERALL CONCLUSIONS.

 

IT IS ESSENTIAL THAT PENS ARE OF RELATIVELY SMALL NUMBERS (NO LARGER THAN 200 SHEEP PER PEN) and each separate from all others by at least 200 metres.

 

This would conserve at least parts of hefts and avoid creating the hazard of aerosol spread.

 

By culling virus positive pens only you get rid of acutely infected animals including those exposed and incubating the infection before any virus shedding can be detected.

 

Roving animals might spread the infection from one pen to another whilst the animals are penned for weeks. Persons going from pen to pen to lay down feed, hay, or bring water or treat animals might also spread infection from pen to pen.

 

IT IS CLEARLY COST EFFECTIVE TO VACCINATE THE WHOLE FLOCK RATHER THAN CONFINE THEM FOR 3 TO 4 WEEKS WITH THE EXPENSE OF FEEDING AND WATERING AND TEST THEM ALL ON 2 OCCASIONS.

 

Note: Vaccinating the whole flock (without killing any) would enable the hills to be opened to tourism. The sheep would not be kept penned for longer than one or two days and thus not amplify infection to as great an extent as if they were penned for weeks.