Development of inactivated heartwater (Ehrlichia ruminantium) vaccine in South Africa

Abstract

Heartwater, Ehrlichia ruminantium infection in cattle, sheep, goats, and some wild ruminants, is an economically important disease in Africa characterized by high mortality rates in susceptible populations. In South Africa, the current commercial heartwater vaccine is an infection and treatment type of immunization using virulent live E. ruminantium organisms generated from blood of infected sheep with subsequent treatment of the animals with antibiotics at specific times during the course of infection. This vaccine has several inherent problems preventing its wide use as the vaccine must be administered intravenously and it does not protect against all the South African field isolates. A vaccine based on inactivation of Zimbabwean E. ruminantium Mbizi strain organisms produced in endothelial cell cultures can be a sustainable option because it will not require antibiotic treatment and will be safe as there is no potential for reversion to virulence. Previous data generated in laboratory trials and under natural field setting provides support for this vaccine approach. Four inactivated vaccine formulations using the E. ruminantium Mbizi strain were tested for their efficacy in Merino sheep compared to an unvaccinated control group (11 sheep per group). Two vaccines were prepared by beta-propiolactone (BPL) inactivation, and two were inactivated with binary ethylenimine (BEI) while purification was done with both percoll and polyethylene glycol (PEG). The four vaccine preparations were formulated with Montanide ISA 50V2 adjuvant and administered twice subcutaneously (2 ml per dose) at an interval of 4 weeks. All groups were challenged with a virulent homologous cell-cultured E. ruminantium inoculated via the intra-venous route on day 56. The primary variable of efficacy was measured by the percentage survival rate or mortality between the Controls and Vaccine Groups. Three vaccine formulations (BEI/Percoll (Group 3), BEI/PEG (Group 4), BPL/Percoll, (Group 1) had a significantly higher percent of animal surviving challenge compared to the unvaccinated control (p-values 0.001, 0.035, 0.030, respectively). The highest number of survivors was obtained in Group 3 BEI/Percoll; 10/11 (91%). Groups 4 (BEI/PEG) and Group 1 (BPL/Percoll) produced similar percentage of survivals of 64%. In contrast, the lowest survival rate of 50% was observed in Group 2 (BPL/PEG) which was numerically different but not significantly different from the unvaccinated control which had an 18% survival rate (2/11). The inactivated vaccine using BEI or BPL as inactivating agents blended with ISA 50 adjuvant induced protective immunity against challenge. The BEI/Percoll (Group 3) vaccination regimen was most efficacious against a lethal heartwater challenge as it significantly protected sheep against mortality which is the most important aspect of heartwater infections. Future work should be directed towards improvement of this vaccine formulation especially from the down-stream processing point of view as the percoll method is not scalable for commercialization purposes.

Keywords: Cattle; Ehrlichia ruminantium; Goats; Heartwater; Inactivated vaccine; Sheep.