Vaccination of on-farm cattle against heartwater : safety and efficacy of Ehrlichia ruminantium (Welgevonden) vaccine

Abstract

Ehrlichia ruminantium (Rickettsiales, Rickettsiaceae) is the causative agent of heartwater disease transmitted to cattle, sheep, goats and wild ruminants e.g. springbok by Amblyomma hebraeum in South Africa (SA). The current live blood vaccine (Ball 3) used in SA has limitations; it does not show efficacy against most field strains, it is virulent and concurrent treatment with antibiotic is necessary and it is produced in life animals which hinders its quality control. Second generation vaccines have not yet been developed to commercial stages. Previous experiments using an attenuated E. ruminantium (Welgevonden) tissue culture experimental vaccine in Merino sheep, Boer goats and Angora goats, administered through intramuscular (I/M) and intravenous (I/V) routes, without the use of antibiotic, showed promising results in terms of safety and efficacy. The objective of the current study was to test the safety and efficacy of this attenuated tissue culture vaccine in cattle, administered by the I/M route. One sheep injected with 10 ml of the virulent Welgevonden heartwater strain was used to infect Amblyomma hebraeum nymphs. Twenty (8-24 months old) male Friesian Holstein cattle obtained from a heartwater and vector free area were used; ten were vaccinated with the attenuated Welgevonden tissue culture isolate intramuscularly and ten were untreated controls. The vaccine was prepared and inoculated with an estimated concentration of 1.11 X 10⁶ E. ruminantium organisms in 2 ml. Tick challenge of both cattle groups was performed with 17 infected A. hebraeum (7 males/10 females) 35 days following vaccination. Cattle were screened serologically by the indirect fluorescent antibody test (IFAT) and by molecular tools using pCS20 quantitative real-time TaqMan (qPCR) before and after vaccination and challenge. Cattle were weighed before vaccination, during vaccination reactions and tick challenge (Days 0-77). Animals were monitored for clinical signs of heartwater disease and treated according to a score sheet when appropriate. Samples of ticks that dropped from infected sheep on different days were highly infected (103) with E. ruminantium (Welgevonden) organisms as tested using qPCR and deemed suitable for animal challenge. The group of cattle (n=10) which were vaccinated intramuscularly, showed no clinical or local vaccine related reactions and no treatment was required. The vaccinated group was challenged 35 days post vaccination together with the untreated controls (n=10). The mean number of engorged female ticks that dropped from the vaccinated (n=7.7) and unvaccinated (n=8.3) groups showed no statistical difference. The vaccinated group did not show any clinical reactions, while 8/10 of the unvaccinated controls developed severe reactions and received treatment while one animal was euthanized following the score sheet. There was a statistical significant mean difference (p-value = 0.0003) in the final weight gain/loss between the vaccinated (mean 5.6 + 2.84 Kg) and unvaccinated (mean -0.33 + 2.78 Kg) groups. On Day 37 after tick challenge, 100% of the vaccinated and 80% of the unvaccinated cattle showed sero-conversion (1/180) in the IFAT. The results of the study have demonstrated the safety and efficacy of the attenuated E. ruminantium (Welgevonden) experimental vaccine against homologous challenge in cattle as judged by the severe clinical reactions in the unvaccinated group. The vaccinated group also had a gain in mean body weight compared to the unvaccinated group after challenge.