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

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.

Investigation into limiting dilution and tick transmissibility phenotypes associated with attenuation of the S24 vaccine strain.

BACKGROUND: Babesia bovis is the causal agent of Asiatic redwater, transmitted by the pandemic tick Rhipicephalus (Boophilus) microplus. Disease control may target the tick vector using acaricides or anti-tick vaccines, or the parasite using chemoprophylaxis or anti-parasite vaccines. Current anti-parasite vaccines comprise live blood vaccines using attenuated B. bovis strains. Attenuation is attained by rapid passage that may result in different phenotypes such as reduced virulence, non-transmissibility by the tick vector, inability to sequester in the host (lack of limiting dilution) and limited genetic diversity. Attenuation and phenotypes may be linked to selection of subpopulations during rapid passage. The South African B. bovis S24 vaccine strain comprise a subpopulation that present low virulence, non-transmissibility, lack of limiting dilution phenotype and the presence of a single A558 Bv80 allele. The S24 strain could be co-transmitted with a field strain (05-100) suggesting sexual recombination. The present study investigated the change in phenotype for the S24 vaccine strain during rapid passage and co-transmission. METHODS: Vaccine phenotype change during passage as well as co-transmissibility was monitored using Bv80 allele specific PCR, limiting dilution and Illumina-based genome sequencing. RESULTS: The S24 population could not be rescued from the S16 passage as previously attained suggesting that selection of the S24 vaccine strain was a serendipitous and stochastic event. Passage from S16 to S24 also resulted in loss of the limiting dilution phenotype. Genome sequencing indicated sexual recombination during co-transmission with the 05-100 field strain. Analysis of the recombinant strain indicate that VESA1, smORF and SBP2 family members are present and may be responsible for the limiting dilution phenotypes, while various regions may also be responsible for the tick transmission phenotype. CONCLUSIONS: The molecular basis for tick transmission and limiting dilution phenotypes may be defined in future using selection based on these traits in combination with sexual recombination.

Blocking Babesia bovis vaccine reactions of dairy cattle in milk.

The use of 1.16 mg/kg (one third) of the recommended dose of diminazene aceturate, administered indiscriminately to cattle on day seven of the unfrozen Babesia bovis and Babesia bigemina bivalent live blood vaccine reaction, was an infection and block treatment method of immunisation used successfully with no known adverse effect on the parasites or the development of protective immunity. Continuing with this practice after replacement of the unfrozen vaccine with deep-frozen monovalent B. bovis and B. bigemina live blood vaccines resulted in reports of vaccine failure. Laboratory investigation indicated the harmful effect of block treatment in preventing the development of durable immunity against B. bigemina as opposed to the much lesser effect it had on B. bovis. Consequently the practice was no longer recommended. A B. bovis vaccination attempt aimed at controlling the disease of dairy cows in milk (n = 30) resulted in 20% fatalities during the expected vaccine reaction period. The practice of block treating B. bovis was therefore reinvestigated, this time in a field trial using dairy cattle in milk (n = 11). Using 0.88 mg/kg (one quarter) of the recommended dose of diminazene administered on day 12 of the B. bovis vaccine reaction resulted in only two animals (n = 5) testing ≥ 1/80 positive with the indirect fluorescent antibody test (IFAT) although parasites could be demonstrated in three. In the untreated control group, by contrast, five of the vaccinated animals (n = 6) tested ≥ 1/80 positive with IFAT and parasites could be demonstrated in all. The unsatisfactory outcome obtained in this study, combined with that of the earlier investigation, indicated that there are more factors that influence successful vaccination than previously considered. It is therefore concluded that block treatment of the live frozen South African cattle babesiosis vaccines reactions is not recommended.