Efficiency of inactivated vaccines against heartwater in Burkina Faso: Impact of Ehrlichia ruminantium genetic diversity

The potential of Rhipicephalus microplus as a vector of Ehrlichia ruminantium in West Africa

Heartwater, or cowdriosis, is a virulent tick-borne rickettsial disease of ruminants caused by Ehrlichia ruminantium, biologically transmitted by Amblyomma species (A. variegatum in West Africa). In West Africa, this bacterium was recently reported to naturally infect the invasive cattle tick, Rhipicephalus microplus (Rm) through trans-ovarian transmission from replete adult females to offspring. A 'sheep-tick-sheep' cycle was set up to determine whether feeding the progeny of these ticks on naïve sheep could lead to infection, and to compare clinical outcomes resulting from this transmission with those observed following infection by the natural A. variegatum (Av) vector. Using local strains of ticks (KIMINI-Rm and KIMINI-Av) and of E. ruminantium (BK242), we recorded, using the PCR technique, the presence of bacterial DNA in ticks (larvae for Av and females for Rm) engorged on sheep inoculated by BK242-infected blood. The bacterial DNA was also detected in the next stages of the lifecycle of R. microplus (eggs and larvae), and in sheep infested either by those R. microplus larvae or by A. variegatum nymphs moulted from larvae engorged on blood-inoculated sheep. Bacterial infection in these sheep was demonstrated by detecting antibodies to E. ruminantium using the MAP1-B ELISA and by isolation of the bacterium on cell culture from blood. The sequences of PCS20 gene detected in ticks and sheep were identical to that of the BK242 strain. Our results confirm that R. microplus can acquire and transmit E. ruminantium to the next stage. However, this transmission resulted in a mild subclinical disease whereas severe clinical disease was observed in sheep infested by A. variegatum infected nymphs, suggesting differences in the tick/bacteria relationship. Future studies will focus on replicating these findings with ticks of different isolates and life stages to determine if R. microplus is playing a role in the epidemiology of heartwater in West Africa. Additionally, studies will investigate whether sheep that are seropositive due to infestation by E. ruminantium-infected R. microplus are subsequently protected against heartwater. Such data will add to our understanding of the possible impact of R. microplus in areas where it has become recently established.

Molecular Detection and Genetic Characterization of Ehrlichia ruminantium Harbored by Amblyomma hebraeum Ticks of Domestic Ruminants in North West Province, South Africa

Ehrlichia ruminantium (E. ruminantium) is the causative agent of heartwater disease and it is mainly transmitted to livestock by Amblyomma hebraeum (A. hebraeum) tick in South Africa. This study investigated the occurrence of E. ruminantium and its genetic diversity in ticks within Ngaka Modiri Molema district of North West Province in South Africa. Genomic DNA was extracted from whole A. hebraeum ticks totaling 876 and resulted in a total of 292 pooled DNA samples. Firstly, conventional PCR was used to detect Ehrlichia spp. targeting the dsbA gene, followed by nested PCR targeting the Map1 gene performed on DNA pool samples that were positive from the first PCR. One hundred and six tick DNA pool samples were positive by dsbA gene PCR for the presence of Ehrlichia spp. with minimum infection rate (MIR) of 121, while 13/106 were positive by Map1 PCR with MIR of 15. Different E. ruminantium Map1 genotypes (NWUe1, NWUe2, NWUe3, NWUe5, and NWUe6) were detected from tick samples and were closely related to more than 13 gene sequences of E. ruminantium from the NCBI GenBank database. These findings suggest that there is a significant diversity of E. ruminantium infecting ticks in the study area.

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.

Molecular detection of Ehrlichia ruminantium in engorged ablyomma variegatum and cattle in Ogun State, Nigeria

Early diagnosis of Ehrlichia ruminantium in cattle is a recipe for effective control of heartwater in ruminants. Hence, we assessed the presence of E. ruminantium in the blood of cattle and the engorged Amblyomma variegatum by nested PCR. The electrophoresed PCR products obtained after primary and secondary amplifications revealed amplicon sizes of about350 bp and 280 bp respectively, which corresponded with the partial region of pSC20 gene amplified. Sequences obtained had 95-99% homology with those sequences available in GenBank. The prevalence of the E. ruminantium in ticks (50%; 126/252) was significantly (p < 0.05) higher than that in cattle blood 23.55% (61/259). The prevalence was significantly (p < 0.05) higher in ticks from adult cattle 51.47% (133/259) than those from the young cattle 44.86% (116/259) and in tick from females 54.55% (141/259) than in ticks from the males 41.38% (107/259). Alignment of autochthonous sequences revealed that the three sequences were polymorphic with two sequences showing similar nucleotides deletion at points 87-91 and 107-108. The phylogenetic trees inferred by ML showed topologies with two autochthonous sequences, one each from cattle blood and tick, clustering together in one clade and the other clustering within those sequences from South Africa and Zimbabwe in another clade. In conclusion, this study revealed a higher prevalence of E. ruminantium in engorged A. variegatum than in the blood of infected cattle. Hence, it is suggested that the amplification that targets the pCS20 gene in engorged ticks may be more suitable to determine the E. ruminantium carrier status of cattle.

Tick-, Flea-, and Louse-Borne Diseases of Public Health and Veterinary Significance in Nigeria

Mosquito-borne diseases are common high-impact diseases in tropical and subtropical areas. However, other non-mosquito vector-borne pathogens (VBPs) may share their geographic distribution, seasonality, and clinical manifestations, thereby contributing their share to the morbidity and mortality caused by febrile illnesses in these regions. The purpose of this work was to collect and review existing information and identify knowledge gaps about tick, flea-, and louse-borne diseases of veterinary and public health significance in Nigeria. Full-length articles about VBPs were reviewed and relevant information about the vectors, their hosts, geographic distribution, seasonality, and association(s) with human or veterinary diseases was extracted. Specific laboratory tools used for detection and identification of VBPs in Nigeria were also identified. A total of 62 original publications were examined. Substantial information about the prevalence and impacts of ticks and fleas on pet and service dogs (18 articles), and livestock animals (23 articles) were available; however, information about their association with and potential for causing human illnesses was largely absent despite the zoonotic nature of many of these peri-domestic veterinary diseases. Recent publications that employed molecular methods of detection demonstrated the occurrence of several classic (Ehrlichia canis, Rickettsia africae, Bartonella sp.) and emerging human pathogens (R. aeschlimannii, Neoehrlichia mikurensis) in ticks and fleas. However, information about other pathogens often found in association with ticks (R. conorii) and fleas (R. typhi, R. felis) across the African continent was lacking. Records of louse-borne epidemic typhus in Nigeria date to 1947; however, its current status is not known. This review provides an essential baseline summary of the current knowledge in Nigeria of non-mosquito VBPs, and should stimulate improvements in the surveillance of the veterinary and human diseases they cause in Nigeria. Due to increasing recognition of these diseases in other African countries, veterinary and public health professionals in Nigeria should expand the list of possible diseases considered in patients presenting with fever of unknown etiology.

Efficient high-throughput molecular method to detect Ehrlichia ruminantium in ticks

BACKGROUND

Ehrlichia ruminantium is the causal agent of heartwater, a fatal tropical disease affecting ruminants with important economic impacts. This bacterium is transmitted by Amblyomma ticks and is present in sub-Saharan Africa, islands in the Indian Ocean and the Caribbean, where it represents a threat to the American mainland.

METHODS

An automated DNA extraction method was adapted for Amblyomma ticks and a new qPCR targeting the pCS20 region was developed to improve E. ruminantium screening capacity and diagnosis. The first step in the preparation of tick samples, before extraction, was not automated but was considerably improved by using a Tissue Lyser. The new pCS20 Sol1 qPCR and a previously published pCS20 Cow qPCR were evaluated with the OIE standard pCS20 nested PCR.

RESULTS

pCS20 Sol1 qPCR was found to be more specific than the nested PCR, with a 5-fold increase in sensitivity (3 copies/reaction vs 15 copies/reaction), was less prone to contamination and less time-consuming. As pCS20 Sol1 qPCR did not detect Rickettsia, Anasplasma and Babesia species or closely related species such as Panola Mountain Ehrlichia, E. chaffeensis and E. canis, its specificity was also better than Cow qPCR. In parallel, a tick 16S qPCR was developed for the quality control of DNA extraction that confirmed the good reproducibility of the automated extraction. The whole method, including the automated DNA extraction and pCS20 Sol1 qPCR, was shown to be sensitive, specific and highly reproducible with the same limit of detection as the combined manual DNA extraction and nested PCR, i.e. 6 copies/reaction. Finally, 96 samples can be tested in one day compared to the four days required for manual DNA extraction and nested PCR.

CONCLUSIONS

The adaptation of an automated DNA extraction using a DNA/RNA viral extraction kit for tick samples and the development of a new qPCR increased the accuracy of E. ruminantium epidemiological studies, as well as the diagnostic capabilities and turn-over time for surveillance of heartwater. This new method paves the way for large-scale screening of other bacteria and viruses in ticks as well as genetic characterization of ticks and tick-pathogen coevolution studies.

Innate immune transcriptomic evaluation of PBMC isolated from sheep after infection with E. ruminantium Welgevonden strain

Heartwater is a tick-borne non-infectious fatal disease of wild and domestic ruminants caused by the bacterium Ehrlichia ruminantium, transmitted by Amblyomma ticks. Although there is evidence that interferon-gamma (IFN-γ) controls E. ruminantium growth and that cellular immune responses could be protective, an effective recombinant vaccine for this disease is lacking. An overall analysis of which immune pathways are up- or down-regulated in sheep peripheral blood mononuclear cells is expected to lead to a better understanding of the global immune response of sheep to E. ruminantium infection. Therefore, a systems biology oriented approach following the infection with E. ruminantium was investigated from peripheral blood mononuclear cells to aid recombinant vaccine development. In this study, heartwater naïve sheep were infected and challenged by allowing E. ruminantium infected ticks to feed on them. After primary infection, all the animals were treated with antibiotic during the resulting febrile response. Blood was collected daily for E. ruminantium detection by qPCR (pCS20 assay). The pCS20 assay only detected the pathogen in the blood one day prior to and during the febrile stage of infection confirming infection of the sheep. IFN-γ real-time PCR indicated that this cytokine was expressed at specific time points: post infection, during the febrile stage of the disease and after challenge. These were used as a guide to select samples for transcriptome sequencing. This paper focuses on transcripts that are associated with innate activating pathways that were identified to be up- and down-regulated after primary infection and the subsequent challenge. These included the CD14 monocyte marker, toll-like receptor (TLR), nod-like receptor, chemokine, cytosolic and cytokine-cytokine interaction receptor pathways. In particular, TLR4, TLR9 and CD14 were activated together with DNA detection pathways, suggesting that vaccine formulations may be improved if CpG motifs and lipopolysaccharides are included. This data indicates that innate immune activation, perhaps by using adjuvants, should be an important component for consideration during future heartwater recombinant vaccine development.

A glycosylated recombinant subunit candidate vaccine consisting of Ehrlichia ruminantium major antigenic protein1 induces specific humoral and Th1 type cell responses in sheep

Heartwater, or cowdriosis, is a tick-borne disease of domestic and wild ruminants that is endemic in the Caribbean and sub-Saharan Africa. The disease is caused by an intracellular pathogen, Ehrlichia ruminantium and may be fatal within days of the onset of clinical signs with mortality rates of up to 90% in susceptible hosts. Due to the presence of competent tick vectors in North America, there is substantial risk of introduction of heartwater with potentially devastating consequences to the domestic livestock industry. There is currently no reliable or safe vaccine for use globally. To develop a protective DIVA (differentiate infected from vaccinated animals) subunit vaccine for heartwater, we targeted the E. ruminantium immunodominant major antigenic protein1 (MAP1) with the hypothesis that MAP1 is a glycosylated protein and glycans contained in the antigenic protein are important epitope determinants. Using a eukaryotic recombinant baculovirus expression system, we expressed and characterized, for the first time, a glycoform profile of MAP1 of two Caribbean E. ruminantium isolates, Antigua and Gardel. We have shown that the 37–38 kDa protein corresponded to a glycosylated form of the MAP1 protein, whereas the 31–32 kDa molecular weight band represented the non-glycosylated form of the protein frequently reported in scientific literature. Three groups of sheep (n = 3–6) were vaccinated with increasing doses of a bivalent (Antigua and Gardel MAP1) rMAP1 vaccine cocktail formulation with montanide ISA25 as an adjuvant. The glycosylated recombinant subunit vaccine induced E. ruminantium-specific humoral and Th1 type T cell responses, which are critical for controlling intracellular pathogens, including E. ruminantium, in infected hosts. These results provide an important basis for development of a subunit vaccine as a novel strategy to protect susceptible livestock against heartwater in non-endemic and endemic areas.

Recombination Is a Major Driving Force of Genetic Diversity in the Anaplasmataceae Ehrlichia ruminantium

The disease, Heartwater, caused by the Anaplasmataceae E. ruminantium, represents a major problem for tropical livestock and wild ruminants. Up to now, no effective vaccine has been available due to a limited cross protection of vaccinal strains on field strains and a high genetic diversity of Ehrlichia ruminantium within geographical locations. To address this issue, we inferred the genetic diversity and population structure of 194 E. ruminantium isolates circulating worldwide using Multilocus Sequence Typing based on lipA, lipB, secY, sodB, and sucA genes. Phylogenetic trees and networks were generated using BEAST and SplitsTree, respectively, and recombination between the different genetic groups was tested using the PHI test for recombination. Our study reveals the repeated occurrence of recombination between E. ruminantium strains, suggesting that it may occur frequently in the genome and has likely played an important role in the maintenance of genetic diversity and the evolution of E. ruminantium. Despite the unclear phylogeny and phylogeography, E. ruminantium isolates are clustered into two main groups: Group 1 (West Africa) and a Group 2 (worldwide) which is represented by West, East, and Southern Africa, Indian Ocean, and Caribbean strains. Some sequence types are common between West Africa and Caribbean and between Southern Africa and Indian Ocean strains. These common sequence types highlight two main introduction events due to the movement of cattle: from West Africa to Caribbean and from Southern Africa to the Indian Ocean islands. Due to the long branch lengths between Group 1 and Group 2, and the propensity for recombination between these groups, it seems that the West African clusters of Subgroup 2 arrived there more recently than the original divergence of the two groups, possibly with the original waves of domesticated ruminants that spread across the African continent several thousand years ago.

A user-friendly and scalable process to prepare a ready-to-use inactivated vaccine: the example of heartwater in ruminants under tropical conditions

The use of cheap and thermoresistant vaccines in poor tropical countries for the control of animal diseases is a key issue. Our work aimed at designing and validating a process for the large-scale production of a ready-to-use inactivated vaccine for ruminants. Our model was heartwater caused by the obligate intracellular bacterium Ehrlichia ruminantium (ER). The conventional inactivated vaccine against heartwater (based on whole bacteria inactivated with sodium azide) is prepared immediately before injection, using a syringe-extrusion method with Montanide ISA50. This is a fastidious time-consuming process and it limits the number of vaccine doses available. To overcome these issues, we tested three different techniques (syringe, vortex and homogenizer) and three Montanide ISA adjuvants (50, 70 and 70M). High-speed homogenizer was the optimal method to emulsify ER antigens with both ISA70 and 70M adjuvants. The emulsions displayed a good homogeneity (particle size below 1 μm and low phase separation), conductivity below 10 μS/cm and low antigen degradation at 4 °C for up to 1 year. The efficacy of the different formulations was then evaluated during vaccination trials on goats. The inactivated ER antigens emulsified with ISA70 and ISA70M in a homogenizer resulted in 80% and 100% survival rates, respectively. A cold-chain rupture assay using ISA70M+ER was performed to mimic possible field conditions exposing the vaccine at 37 °C for 4 days before delivery. Surprisingly, the animal survival rate was still high (80%). We also observed that the MAP-1B antibody response was very similar between animals vaccinated with ISA70+ER and ISA70M+ER emulsions, suggesting a more homogenous antigen distribution and presentation in these emulsions. Our work demonstrated that the combination of ISA70 or ISA70M and homogenizer is optimal for the production of an effective ready-to-use inactivated vaccine against heartwater, which could easily be produced on an industrial scale.

Pathogen prevalence in ticks collected from the vegetation and livestock in Nigeria

Ticks are important disease vectors that can cause considerable economic losses by affecting animal health and productivity, especially in tropical and subtropical regions. In this study, we investigated the prevalence and diversity of bacterial and protozoan tick-borne pathogens in ticks collected from the vegetation and cattle in Nigeria by PCR. The infection rates of questing ticks were 3.1% for Rickettsia species, 0.1% for Coxiella burnetii and 0.4% for Borrelia species. Other pathogens, such as Babesia, Theileria, Anaplasma, and Ehrlichia species, were not detected in ticks from the vegetation. Feeding ticks collected from cattle displayed infection rates of 12.5% for Rickettsia species, 14% for Coxiella burnetii, 5.9% for Anaplasma species, 5.1% for Ehrlichia species, and 2.9% for Theileria mutans. Babesia and Borrelia species were not detected in ticks collected from cattle. Mixed infections were found only in feeding ticks and mainly Rickettsia species and Coxiella burnetii were involved. The diversity of tick-borne pathogens in Nigeria was higher in feeding than in questing ticks, suggesting that cattle serve as reservoirs for at least some of the pathogens studied, in particular C. burnetii. The total estimated herd infection rates of 20.6% for a Rickettsia africae-like species, 27% for Coxiella burnetii, and 8.5% for Anaplasma marginale/centrale suggest that these pathogens may have considerable implications for human and animal health.

A new typing technique for the Rickettsiales Ehrlichia ruminantium: multiple-locus variable number tandem repeat analysis

Ehrlichia ruminantium (ER) is a member of the order Rickettsiales transmitted by Amblyomma ticks. This obligatory intracellular bacterium is the causative agent of a fatal disease in ruminants, named heartwater. It represents a constraint on breeding development in sub-Saharan Africa and in the Caribbean. The genetic diversity of the strains of ER, which could be a limiting factor to obtain effective vaccines, needs to be better characterized. For this purpose, we developed a molecular typing technique based on the polymorphism of variable number tandem repeat (VNTR) sequences, MLVA (multiple locus VNTR analysis). Eight (out of 21) VNTR candidates were validated using 17 samples representing a panel of ER strains from different geographical origins from West, South Africa, and Caribbean areas and in ER infected ticks and goat tissues. This result demonstrated the ability of these VNTRs to type a wide range of strains. The stability of the selected VNTR markers was very good, at the time scale needed for epidemiological purposes: in particular, no difference in the VNTR profiles was observed between virulent and attenuated strains (for Gardel and Senegal strains) and between strains (Gardel and Blonde strains) isolated in the same area 19years apart. We validated the strong discriminatory power of MLVA for ER and found a high level of polymorphism between the available strains, with 10 different profiles out of 13 ER strains. The MLVA scheme described in this study is a rapid and efficient molecular typing tool for ER, which allows rapid and direct typing of this intracellular pathogen without preliminary culture and gives reliable results that can be used for further epidemiological studies.

Development of multiple-locus variable-number tandem-repeat analysis for rapid genotyping ofEhrlichia ruminantiumand its application to infectedAmblyomma variegatumcollected in heartwater endemic areas in Uganda

The rickettsial bacteriumEhrlichia ruminantiumis the causative agent of heartwater, a serious tick-borne disease in ruminants. The genetic diversity of organisms in the field will have implications for cross-protective capacities of any vaccine developed, and for an effective vaccine design strategy proper genotyping and understanding of existing genetic diversity in the field is necessary. We searched for variable-number tandem-repeat (VNTR) loci for use in a multi-locus VNTR analysis (MLVA). Sequencing analysis of 30 potential VNTRs using a panel of 17 reference strains from geographically diverse origins identified 12 VNTRs with allelic profiles differing between strains. Application of MLVA to 38E. ruminantium-infectedAmblyomma variegatumcollected from indigenous cattle in 6 different districts of Uganda identified 21 MLVA types. The discriminatory power of MLVA was greater than that ofmap1PCR-restriction fragment length polymorphism analysis, with which only 6 genotypes were obtained. The high discriminatory power as well as cost- effective performance of MLVA provide the potential for this technique to be applied in the future with respect to optimizing vaccine trials by identifying local strain diversity, and also raise the possibility of exploring the association betweenE. ruminantiumgenotypes and phenotypes such as pathological outcome in the ruminant host.

Multi-locus sequence typing of Ehrlichia ruminantium strains from geographically diverse origins and collected in Amblyomma variegatum from Uganda

Background

The rickettsial bacterium Ehrlichia ruminantium is the causative agent of heartwater in ruminants. A better understanding of the population genetics of its different strains is, however, needed for the development of novel diagnostic tools, therapeutics and prevention strategies. Specifically, the development of effective vaccination policies relies on the proper genotyping and characterisation of field isolates. Although multi-locus sequence typing (MLST) has been recently developed, only strains from geographically restricted collections have been analysed so far. The expansion of the MLST database to include global strains with different geographic origins is therefore essential. In this study, we used a panel of reference strains from geographically diverse origins and field samples of E. ruminantium detected from its vector, Amblyomma variegatum, in heartwater-endemic areas in Uganda.

Results

A total of 31 novel alleles (six, four, six, three, two, five, three, and two for gltA, groEL, lepA, lipA, lipB, secY, sodB, and sucA loci, respectively) and 19 novel sequence types (STs) were identified. Both neighbour-joining and minimum spanning tree analyses indicated a high degree of genetic heterogeneity among these strains. No association was observed between genotypes and geographic origins, except for four STs from West African countries. When we performed six different tests for recombination (GeneConv, Bootscan, MaxChi, Chimaera, SiScan, and 3Seq) on concatenated sequences, four possible recombination events were identified in six different STs. All the recombination breakpoints were located near gene borders, indicating the occurrence of intergenic recombination. All four STs that localized to a distinct group in clustering analysis showed evidence of identical recombination events, suggesting that recombination may play a significant role in the diversification of E. ruminantium.

Conclusions

The compilation of MLST data set across the African continent will be particularly valuable for the understanding of the existing genetic diversity of field isolates in African countries. Comprehensive information on the degree of cross-protection between strains and further understanding of possible relationships between genotypes and phenotypes such as vaccine efficacy are expected to lead to the development of region-specific vaccination strategies.