Attenuated heartwater vaccine (Ehrlichia ruminantium Welgevonden) : immunization of Angora goats using the intra-muscular route of administration

Abstract

Ehrlichia ruminantium, the causative organism of heartwater infections, places severe economic constraint on the livestock industry wherever Amblyomma tick vectors are present. Angora goats are particularly susceptible to this disease and the current live blood vaccine cannot safely be used to protect these animals. An attenuated E. ruminantium (Welgevonden) experimental vaccine has previously shown promising results in Merino sheep and Boer goats. The vaccine was administered by intravenous route (i/v). The general objective of this study was to test the efficacy and safety of the attenuated heartwater vaccine E. ruminantium (Welgevonden) in Angora goats. The specific objectives were, firstly to assess the intra-muscular route of administration of the attenuated vaccine as compared to the standard i/v route and, secondly, to study the haematological changes in Angora goats before, during and after vaccination under controlled conditions at the Onderstepoort Veterinary Institute tick-free stables. A total of 55 Angora goats were used in this trial. They were purchased from an area in South Africa which is known to be Amblyomma-free and heartwater-free. Furthermore, on arrival, the goats were screened for E. ruminantium infection by the immunofluorescent antibody (IFA) test to confirm their disease-free status. The Angora goats were divided into 3 groups: In Group 1, ten were vaccinated by the standard i/v route, in Group 2, 31 received the vaccine by i/m route and 10 served as untreated controls for Group 3. Five of the 10 i/v vaccinated group, 20/31 of the i/m vaccinated and 5 controls were challenged by feeding of known infected adult A hebreaum. The other remaining animals within the three groups were challenged using a known infected blood stabilate administered by the standard i/v route (dose 5xLD50). All animals were challenged 42 days after vaccination. The vaccine did not produce any inflammatory reactions at the site of injection. However, 3/31 (9.7%) of i/m and 7/10 (70%) of i/v vaccinated goats developed febrile reactions starting on Day 11 post-immunisation and were treated. All vaccinated goats were fully protected against either needle i/v or tick challenge, while the control non-vaccinated goats reacted severely to the challenge materials and required oxytetracycline treatment. Despite treatment, two of the unvaccinated goats died from the challenge material. 9 Haematological values (packed cell volume, differential blood cells count) were obtained on blood samples taken from the treatment and control groups at different times during the course of the trial. Wide within group variations as shown by the high standard deviation values were found. As no significant changes were found between vaccinated and control animals, it is likely that the attenuated vaccine does not cause significant clinical haematological changes. This study has demonstrated that the attenuated E. ruminantium (Welgevonden) vaccine is safe in 90.3% and efficacious (100% efficacy) for intramuscular administration in Angora goats. However, further laboratory and on-farms studies are needed in order to establish the lowest effective and safety dose, duration of immunity, and the vaccine’s safety in young and pregnant animals.