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This article was published in 1976
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Failure of Footrot Vaccines
D.G. Christie, B.V.Sc., H.D.A., Deputy Chief, Division of Animal Industry, Department of Agriculture.
When a veterinary student from 1937 to 1941, I looked forward each Monday morning to seeing the pithy saying displayed outside St. Barnabas Church at Broadway. Above the signboard was the caption 'FAILURE IS NOT FINAL'. Only a fortnight ago the signboard declared, 'Some people go into bars, optimistically and come out misty optically'.
The commercial vaccines failed in two vaccination trials in 1975. Keogh (1976) described the two trials and the accompanying tables taken from his paper summarise the results in terms of sheep affected and number of feet affected.
In the autumn trial no commercial vaccine showed any degree of protection in comparison with the controls, whereas the C.S.I.R.O. experimental vaccine group at week 13 after vaccination (7 weeks after donor sheep introduced) had 60% 'unaffected' (Scores of less than 3). Again, this group had less feet affected than the other groups. Because of breakdowns with the irrigation pump, conditions were not conducive to spread at the time the donors were introduced and it could be said that any immunity conferred by the commercial vaccines was past its peak when strong challenge occurred.
In the trial in the following spring only two firms fielded vaccines and one firm employed two vaccines, one of which was similar in strain content (no Australian strains) to the vaccine it used in the autumn trial. C.S.I.R.O. employed two vaccines, the second one on a group of 25. All vaccines were of oil adjuvant type with a two dose regime. On this occasion challenge took place 2 weeks after the second vaccination (one week in the case of the C.S.I.R.O. vaccine groups as they had received their second dose of vaccine a week later than the others.) As in the autumn trial, the donors were infected with strain 217 and 198.
The conditions were ideal for transmission and 28 days after the donors were introduced, 90% of the controls were affected. The vaccine without strains 217 and 198 had 72% affected. This group and the controls were then removed from the trial and fresh donors were introduced. The remaining groups scored 52% (A), 42% (B), 34% (C) and 4% (D) affected.
The conditions continued to favour transmission and the trial was continued on for a further 28 days. At this point the scores were 83% (A), 74% (B) 58% (C) and 32% (D) and controls (new) 68% affected.
The number of feet affected at the 28th day following the introduction of donors (excluding the group removed at that point) stood at 47 (A), 36 (B) 22 (C) and 1 (D). The control group had 105 feet affected.
At the end of the trial (56 days from introduction of donors) the number of feet affected stood at 108 (A), 79 (B), 42 (C) and 12 (D), while the new controls had 42 feet affected. The figures for group (D) and the new controls should be doubled to permit comparison with the other groups.
The reactions at the site of vaccination bordered on the unacceptable in some cases and it is certain that vaccines could not be employed on sheep intended for sale as prime lamb or hogget.
Following a meeting to consider the results of the spring trial both N.S.W. and Victorian departments decided to withdraw their approval for the distribution of vaccines on permit or prescription because the results showed that no worthwhile protection was given.
WHY DID THEY FAIL ?
Very heavy challenge occurred in each trial and whereas in practice footbathing would have been carried out or the sheep moved to higher drier areas, in the trials, since the aim was to assess immunogenicity, no other measures were employed. Yet despite this, the C.S.I.R.O. products gave a measure of protection. Possible explanations for the failure are: —
1. The vaccine did not include the strains which were employed for the challenge. The failure of the vaccine group pulled out at day 28 in the spring trial reflects this.
2. Serology was carried out in both trials and the results will be published later. This demonstrated that the commercial vaccines produced low antibody levels whereas the C.S.I.R.O. vaccines produced high levels. Serology gives a measure of the antigenicity of a vaccine and so the cell content but not necessarily the immunogenicity of the vaccine. However, a product which produces poor antibody levels is not likely to be effective.
It is considered that minimal cell levels must be present in the final vaccine and when more than one strain is employed, then the levels of cells of both strains must be optimal Evidently this calls for centrifugation to concentrate the cells and adds another cost to what is an expensive vaccine anyway.
3. Problems are experienced in culture of the organism in liquid medium and these increase in direct proportion to the size of the vessel. Yet it is necessary to employ large vessels if the cost of production is to be kept within bounds.
4. Two alum vaccines and an oil adjuvant vaccine were employed in the autumn trial. The response to an alum vaccine is both lower and shorter in duration than with an oil adjuvant product.
But failure is not final, and the degree of protection shown by the two commercial vaccines in the spring trial gives reason for hope. This is strengthened by the very encouraging result achieved by the C.S.I.R.O. vaccines in both trials. All firms are continuing to develop their products and the lessons learnt from the trials should result in vastly superior vaccines being offered for the trial projected for the spring of 1976. C.S.I.R.O. is continuing its research into vaccine production and proposes to establish a small irrigation area at Badgery's Creek so that challenge can be made under field conditions as well as pen conditions.
Considering the nature of footrot it is remarkable that affected sheep show an antibody response and equally remarkable that a vaccine can give protection against the disease. The nature of the protection appears to be more the limitation of an infection rather than absolute protection. On this premise its role should be viewed as an adjunct to control. A vaccine which reduces the incidence of clinical disease to manageable proportions and is acceptable price and local reaction wise has an assured future.
INCIDENCE OF FOOTROT - AUTUMN 1975 TRIAL
| Week After Vaccination | Commercial Blue 50 | Commercial Red 50 | Commercial Brown 50 | C.S.I.R.O. Dark Green 50 | Control White 50 | |
|---|---|---|---|---|---|---|
| (a) No. of sheep with 3+ or 4+ lesions on any foot | ||||||
| 9 | - | 2 | - | - | 1 | |
| 10 | 1 | 2 | 3 | 1 | 1 | |
| 11 | 28 | 32 | 28 | 15 | 27 | |
| 12 | 36 | 38 | 34 | 21 | 38 | |
| 13 | 34 | 31 | 29 | 20 | 36 | |
| 14 | 45 | 39 | 34 | 30 | 42 | |
| 15 | 45 | 41 | 40 | 36 | 44 | |
| (b) No. of feet with 3+ or 4+ lesions | ||||||
| 9 | - | 2 | - | - | 1 | |
| 10 | 1 | 2 | 3 | 1 | 1 | |
| 11 | 48 | 52 | 45 | 17 | 40 | |
| 12 | 79 | 74 | 62 | 32 | 70 | |
| 13 | 60 | 58 | 51 | 28 | 74 | |
| 14 | 65 | 68 | 66 | 41 | 88 | |
| 15 | 94 | 89 | 81 | 60 | 96 | |
DONOR SHEEP INTRODUCED WEEK 6.
INCIDENCE OF FOOTROT - SPRING 1975 TRIAL
| Week after 2nd Vaccination | Commercial A 50 | Commercial B 50 | C.S.I.R.O. C 50 | C.S.I.R.O. D 25 | Commercial 50 | Control 50 | New Control 25 |
|---|---|---|---|---|---|---|---|
| (A) No. of sheep with 3+ or 4+ lesion on any foot. | |||||||
| 4 | 6 | 3 | 3 | 0 | 13 | 17 | |
| 5 | 17 | 15 | 6 | 0 | 26 | 33 | |
| 6 | 26 | 21 | 17 | 1 | 36 | 45 | |
| 7 | 32 | 26 | 16 | 2 | 0 | ||
| 8 | 38 | 32 | 25 | 5 | 10 | ||
| 10 | 43 | 37 | 29 | 8 | 17 | ||
| (B) No. of feet with 3+ or 4+ lesion. | |||||||
| 4 | 10 | 3 | 4 | 0 | 21 | 21 | |
| 5 | 23 | 20 | 7 | 0 | 54 | 66 | |
| 6 | 47 | 36 | 22 | 1 | 74 | 105 | |
| 7 | 73 | 50 | 22 | 2 | 0 | ||
| 8 | 83 | 61 | 38 | 7 | 19 | ||
| 10 | 108 | 70 | 42 | 12 | 42 | ||
DONORS ADDED WEEK 2.
NEW CONTROLS ADDED WEEK 6.
References
KEOGH, J. (1976) Proceedings A.V.A. A.G.M., N.S.W. Division (In press)