Mares
3.5 Acknowledgements
Many thanks to:
1. The owners and staff of Camargue Stud and Rodney Clarkin and his staff for assistance with taking samples.
2. Kathy Devereaux for assistance in the laboratory.
3. Janusz Paweska and Sandra Croft for technical assistance with the virus neutralisation tests.
4. Gerry Weber for advice and suggestions.
5. Onderstepoort Veterinary Institute for financial assistance and sponsoring the virus neutralisation tests.
CHAPTER 4
GENERAL CONCLUSION
The literature review looked at the passive transfer of immunity from the mare to the foal.
The newborn foal is at risk from the development of infections as it adapts to its new environment. The risk is decreased by the passive transfer of immunity from the mare through colostrum . This immunity protects the foal until its own immune system is fully functional. Factors affecting the transfer of passive immunity and the consequences of failure of passive transfer of immunity were discussed. The importance of testing for the transfer of passive immunity and different methods of testing whether the transfer of passive immunity has occurred were reviewed. Each test method and its associated advantages and disadvantages were described. Passive immunity affects vaccination programmes, as the presence of maternal antibodies inhibits the active immune response. This means that passive immunity affects the timing of the firstvaccination in two ways. Firstly, the vaccination will only be effective when the passive immunity has faded sufficiently to allow an active response but, secondly, the vaccination should not be given after the passive immunity has declined to non-protective levels. The review concluded with a discussion of vaccination programmes using African horse sickness as an example.
Trial One looked at several different, locally available, methods of testing for the transfer of passive immunity. The tests investigated were the measurement of the colostral specific gravity, the measurement of total serum protein by the refractometer and the measurement of serum immunoglobulin G by the zinc sulphate turbidity (ZST) and glutaraldehyde coagulation (GC) tests. The results of these methods were compared to the results of the measurement of immunoglobulin G by the single radial immunodiffusion test (SRID).
The single radial immunodiffusion test is the benchmark test for immunoglobulin testing, and represents the ideal test to use as it gives a quantitative result and is not subjective. However,the test takes too long to complete for routine use in newborn foals,since the gastrointestinal becomes impermeable to immunoglobulins within approximately 18 to 24 hours of birth. This means that treatments for failure of transfer of passive immunity need to be given very soon after birth. The specific density of colostrum was shown not to be a good predictor of the transfer of passive immunity. However, the colostrum specific density test should still be used to test the colostrum that is going to be stored for use in treating failure of passive transfer. There were significant correlations in the values for IgG status between the three inexpensive tests and the SRID test (colostrum, p=O.1; refractometer, GC and ZST, p=O.01). The ZST and GC tests express IgG status
in categories , relative to the critical IgG levels of 400 and 800 mg IgG/dl serum. The refractometer yields a quantitative test result,allowing regression analysis to be used to give a prediction of the SRID result from refractometer readings.This analysis suggests that refractometer readings of 6.30 and 6.58 equate to SRID readings of 400 and 800 mg/dl, respectively. Chi-squared analysis of the GC, ZST and refractometer results showed there to be a significant relationship (p=0.1) between observed and expected IgG values for all three tests.
Over- and under-estimation of IgG status (% samples, compared to the SRID test results) were found to be, respectively, 7.04 and 2.82 (GC), 11.97 and 4.93 (ZST) and 12.68 and 0.7 (refractometer). The fina lchoice of the ZST, GC or refractometer tests as the foal-side test will have to be decided on by the stud manager and veterinarian,taking into account local conditions but the recommendation of the author would be to use the GC test. The current recommendation to test the foal six hours after birth and to treat before 18 hours of age with colostrum remain valid. The age of the mare, parity of the
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mare and the gestation length did not appear to affect the transfer of immunity from the mare to the foal.
Trial Two looked at the African Horse Sickness vaccination programme, focusing on when to vaccinate foals for the first time and what factors affect the response of the foals to the vaccine. A recommendation was made that management practices need to change in order to adhere more closely to the vaccination schedule recommended by the manufacturer. Suggested revisions to the existing vaccination schedule were also presented. The revised vaccination schedule is as follows: the mare should be vaccinated at three and two months prior to her expected foaling date with AHS 1 and AHS 2 respectively. The foal would then be vaccinated at four and five months of age with AHS 1 and AHS 2 and then again at six and seven months of age (AHS 1 and AHS 2).
The results of this work have raised many questions and there is little doubt that further .research is needed into the response of animals in the field to available vaccines.
Adjusting the current vaccination programme is only a start in the battle to protect South African horses as fully as possible against AHS.
Specific questionsthat further research should aim to answer are:
1) The effectiveness of any revised or proposed vaccination programmes (from foals to mature animals).
2) Should the vaccine be split further into monovalent vaccines or polyvalent vaccines with fewer serotypes?
3) How long before the expected foaling date should the mare be vaccinated in order for antibodies to be present in the colostrum?
4) How long does it take to transfer AHSV antibodies into the colostrum and does it differ for the different serotypes?
5) When should the third set of vaccinationstake place?
6) Is the virus from the vaccine being transferred to the foal at birth?
This thesis makes available valuablefield data on the effectiveness of the African horse sickness vaccination under practical conditions in the KwaZulu-Natal Midlands. It is hoped that,beyond the recommendations made above,other researchers will be able to make use of the data presented in the Appendices attached to this work to help design more efficient means of controlling African horse sickness in the Southern African region.