Safety and efficacy of an attenuated heartwater (Ehrlichia ruminantium) vaccine administered by the intramuscular route in cattle, sheep and Angora goats

Abstract

Heartwater is an economically important tick-borne disease of ruminants in Africa. The current commercial vaccine uses live Ehrlichia ruminantium from blood of infected sheep, requires antibiotic treatment during infection, needs to be administered intravenously and does not protect against all South African isolates. An attenuated tissue culture vaccine not requiring antibiotic treatment and effective against different field strains in small groups of goats and sheep was reported previously. The objective of the present study was to test safety and efficacy of this vaccine administered by intramuscular (i.m.) inoculation in larger groups of sheep, Angora goats and cattle. Animals were vaccinated via intravenous (i.v.) and i.m. routes and received E. ruminantium homologous challenge by feeding of infected ticks or by i.v. inoculation of infected blood. For vaccine titration in sheep and goats, the optimum safe and efficacious dose was determined using 2 ml equivalent of 10²-10⁵ culture-derived live elementary bodies (EBs). Similarly, the vaccine was titrated in cattle using 5 ml containing 10⁵-10⁷ EBs. Seventy percent of i.v. vaccinated and 9.7% of i.m. vaccinated Angora goats receiving 10⁵ EBs, developed severe reactions to vaccination and were treated. These treated animals and the remaining 90.3% of i.m.- vaccinated goats showed 100% protection against i.v. or tick challenge. Sheep and Angora goats vaccinated i.m. with 10⁴ EBs had no vaccination reactions and were fully protected against i.v. or tick challenge. Similarly, vaccinated cattle (dose 10⁶ EBs) did not react to vaccine inoculation and were fully protected against i.v. or tick homologous challenge. Control non-vaccinated animals reacted severely to challenge and required oxytetracycline treatment. This successfully demonstrated that Angora goats, sheep and cattle can be safely vaccinated with the attenuated E. ruminantium Welgevonden vaccine via the i.m. route, with no clinical reactions to vaccination and 100% protection against virulent i.v. and homologous tick challenge.

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