Development and evaluation of PCR assay for detection of low levels of Cowdria ruminantium infection in Amblyomma ticks not detected by DNA probe

Molecular characterization of "Candidatus Anaplasma testudinis": An emerging pathogen in the threatened Florida gopher tortoise (Gopherus polyphemus)

Members of the family Anaplasmataceae are obligate intracellular bacteria that replicate within membrane bound vacuoles in the cytoplasm of cells in vertebrate and invertebrate hosts. This study reports a putative new Anaplasma species in gopher tortoises in Florida. Two Florida gopher tortoises (Gopherus polyphemus) presented at the University of Florida Veterinary Hospital with anemia and intracytoplasmic vacuoles filled with bacteria within erythrocytes. The bacteria within these parasitophorous vacuoles were morphologically similar to Anaplasma marginale. We inoculated ISE6 cells with blood from one tortoise and isolated bacterial colonies consistent with A. marginale. Molecular characterization targeting Anaplasmataceae 16S rRNA sequences indicated that the clinical isolate, named here provisionally as "Candidatus Anaplasma testudinis", grouped within the genus Anaplasma on a separate clade, most closely related to the A. marginale, Anaplasma ovis and Anaplasma centrale group. We next screened archived red blood cells from 38 wild gopher tortoises with documented clinical anemia. Fourteen of the 38 wild tortoises, representing 5 of 11 geographical locations were PCR-positive for Anaplasmataceae spp. Sequencing analysis revealed 16S rRNA sequence identical to "Ca. A. testudinis". The clinical presentation of significant anemia associated with "Ca. A. testudinis" in a threatened species could have conservation implications. Importantly, the availability of a clinical isolate will aid further studies to develop diagnostic tests and to investigate potential tick vectors and infectivity for other wildlife and domestic animal species.

Newly recognized Anaplasma sp. in erythrocytes from Gopher Tortoises (Gopherus polyphemus)

BACKGROUND

In 2015, a previously unrecognized intracytoplasmic erythrocytic inclusion was discovered in anemic wild-caught adult gopher tortoises (Gopherus polyphemus). Subsequently, molecular diagnostics revealed this inclusion to be a novel Anaplasma sp.

OBJECTIVES

The goal of this study was to morphologically characterize these erythrocytic inclusions by light and transmission electron microscopy (TEM).

METHODS

Blood samples were taken from two car-injured wild-caught gopher tortoises for the preparation of Wright-Giemsa stained smears and TEM specimens. CBC data were serially performed and morphologically examined during treatment periods.

RESULTS

Studies revealed a moderate to severe anemia with moderate regeneration as indicated by polychromasia and the presence of immature erythroid precursors. In addition, on light microscopy, one to two variably-sized round basophilic stippled paracentral erythrocytic inclusions were present per cell in both animals and involved 10%-25% of erythrocytes. TEM identified the intraerythrocytic inclusions as discrete membrane-bound cytoplasmic vacuoles (morulae) containing membrane-bound bacterial subunits that were of variable size, shape, and electron density. Serial hematologic data indicated complete remission of the infection in response to a single long-term course of doxycycline.

CONCLUSIONS

The presence of a regenerative anemia in gopher tortoises from Florida revealed a newly recognized bacterial species that has morphologic characteristics similar to members of the genus Anaplasma.

Tick-borne Rickettsiales: Molecular tools for the study of an emergent group of pathogens

The use of molecular techniques in recent years has enhanced the sensitivity and specificity of the diagnosis of Rickettsiales, a bacterial order which includes significant emerging and re-emerging pathogens of humans and animals. Molecular detection enables the accurate identification at the species level, providing additional information on the epidemiology and course of the clinical cases. Moreover, PCR and enzyme restriction analysis of the vector blood meal can be employed to study the tick feeding source and possibly identify pathogen's reservoir. Here, we review the molecular tools available for the identification and characterization of tick-borne bacteria from the genera Rickettsia, Ehrlichia and Anaplasma and for the study of ticks feeding behavior. We summarize the significant criteria for taxonomic identification of Rickettsiales species and propose a procedure algorithm for the classification of bacterial isolates as members of this order.

Microbial population analysis of the salivary glands of ticks; a possible strategy for the surveillance of bacterial pathogens

Ticks are one of the most important blood-sucking vectors for infectious microorganisms in humans and animals. When feeding they inject saliva, containing microbes, into the host to facilitate the uptake of blood. An understanding of the microbial populations within their salivary glands would provide a valuable insight when evaluating the vectorial capacity of ticks. Three tick species (Ixodes ovatus, I. persulcatus and Haemaphysalis flava) were collected in Shizuoka Prefecture of Japan between 2008 and 2011. Each tick was dissected and the salivary glands removed. Bacterial communities in each salivary gland were characterized by 16S amplicon pyrosequencing using a 454 GS-Junior Next Generation Sequencer. The Ribosomal Database Project (RDP) Classifier was used to classify sequence reads at the genus level. The composition of the microbial populations of each tick species were assessed by principal component analysis (PCA) using the Metagenomics RAST (MG-RAST) metagenomic analysis tool. Rickettsia-specific PCR was used for the characterization of rickettsial species. Almost full length of 16S rDNA was amplified in order to characterize unclassified bacterial sequences obtained in I. persulcatus female samples. The numbers of bacterial genera identified for the tick species were 71 (I. ovatus), 127 (I. persulcatus) and 59 (H. flava). Eighteen bacterial genera were commonly detected in all tick species. The predominant bacterial genus observed in all tick species was Coxiella. Spiroplasma was detected in Ixodes, and not in H. flava. PCA revealed that microbial populations in tick salivary glands were different between tick species, indicating that host specificities may play an important role in determining the microbial complement. Four female I. persulcatus samples contained a high abundance of several sequences belonging to Alphaproteobacteria symbionts. This study revealed the microbial populations within the salivary glands of three species of ticks, and the results will contribute to the knowledge and prediction of emerging tick-borne diseases.

High prevalence of spotted fever group rickettsiae in Amblyomma variegatum from Uganda and their identification using sizes of intergenic spacers

The spotted fever group (SFG) rickettsiae are obligate intracellular bacteria transmitted by ticks that cause several tick-borne rickettsioses in humans worldwide. This study was intended to determine the prevalence of SFG rickettsiae in Amblyomma variegatum from 7 districts across Uganda. In addition to sequencing of gltA and ompA genes, identification of Rickettsia species based on the sizes of highly variable intergenic spacers, namely, dksA-xerC, mppA-purC, and rpmE-tRNA(fMet) was carried out. Application of multiplex PCR for simultaneous amplification of 3 spacers combined with capillary electrophoresis separation allowed simple, accurate, and high-throughput fragment sizing with considerable time and cost savings. Rickettsia genus-specific real-time PCR detected 136 positives out of 140 samples, giving an overall prevalence of 97.1%. Most samples (n=113) had a size combination of 225, 195, and 341 bp for dksA-xerC, mppA-purC, and rpmE-tRNA(fMet), respectively, which was identical to that of R. africae, a causative agent of African tick bite fever. In addition, several samples had size variants in either dksA-xerC or rpmE-tRNA(fMet). Nonetheless, the partial sequences of gltA and ompA genes of samples of all size combinations showed the greatest similarity to R. africae (99.3-100% for gltA and 98.1-100% for ompA). Given these results, it is highly possible that the tested ticks were infected with R. africae or closely related species. This is a first report on molecular genetic detection of R. africae and its high endemicity in Uganda. Clinicians in this country should be aware of this pathogen as a cause of non-malarial febrile illness. This study provided a starting point for the development of Rickettsia species identification based on the sizes of intergenic spacers. The procedure is simple, rapid, and cost-effective to perform; hence it might be particularly well suited for preliminary species identification in epidemiological investigations. The results may be more detailed and reliable when simultaneous sequencing analysis is performed.

Development of loop-mediated isothermal amplification (LAMP) assays for rapid detection of Ehrlichia ruminantium

Background

The rickettsial bacterium Ehrlichia ruminantium is the causative agent of heartwater, a potential zoonotic disease of ruminants transmitted by ticks of the genus Amblyomma. The disease is distributed in nearly all of sub-Saharan Africa and some islands of the Caribbean, from where it threatens the American mainland. This report describes the development of two different loop-mediated isothermal amplification (LAMP) assays for sensitive and specific detection of E. ruminantium.

Results

Two sets of LAMP primers were designed from the pCS20 and sodB genes. The detection limits for each assay were 10 copies for pCS20 and 5 copies for sodB, which is at least 10 times higher than that of the conventional pCS20 PCR assay. DNA amplification was completed within 60 min. The assays detected 16 different isolates of E. ruminantium from geographically distinct countries as well as two attenuated vaccine isolates. No cross-reaction was observed with genetically related Rickettsiales, including zoonotic Ehrlichia species from the USA. LAMP detected more positive samples than conventional PCR but less than real-time PCR, when tested with field samples collected in sub-Saharan countries.

Conclusions

Due to its simplicity and specificity, LAMP has the potential for use in resource-poor settings and also for active screening of E. ruminantium in both heartwater-endemic areas and regions that are at risk of contracting the disease.

Natural history of Ehrlichia ruminantium

Ehrlichia ruminantium is an obligately intracellular proteobacterium which causes a disease known as heartwater or cowdriosis in some wild, and all domestic, ruminants. The organism is transmitted by ticks of the genus Amblyomma, and it is of serious economic importance wherever the natural vectors occur, an area which includes all of sub-Saharan Africa, and several islands in the Caribbean. The disease was first recognized in South Africa in the 19th century, where its tick-borne nature was determined in 1900, but the organism itself was not demonstrated until 1925, when it was recognized to be a rickettsia, initially named Rickettsia ruminantium. It was thus the first species of what are now known as Ehrlichia to be discovered, and most of the early work to elucidate the nature of the organisms, and its reservoirs and vectors, was performed in South Africa. The next milestone was the development, in 1945, of an infection and treatment regimen to immunize livestock, and this is still the only commercially available "vaccine" against the disease. Then in 1985, after fruitless attempts over many years, the organism was propagated reliably in tissue culture, opening the way for the first application of the newly developed techniques of molecular genetics. From 1990 onwards the pace of heartwater research accelerated rapidly, with notable advances in phylogeny, diagnosis, epidemiology, immunology, and vaccine development. The complete genome sequence was published in 2005, and during the last two years a new understanding has arisen of the remarkable genetic variability of the organism and new experimental vaccines have been developed. Despite all this the goal of producing an effective vaccine against the disease in the field still remains frustratingly just beyond reach. This article summarises our current understanding of the nature of E. ruminantium, at a time when the prospects for the development of an effective vaccine against the organism seem better than at any time since its discovery 83 years ago.

Differential strain-specific diagnosis of the heartwater agent: Ehrlichia ruminantium

Ehrlichia ruminantium is the causative agent of heartwater, a major tick-borne disease of livestock in Africa introduced in the Caribbean and threatening to emerge and spread in the American mainland. Complete genome sequencing was done for two isolates of E. ruminantium of differing phenotype, isolates Gardel (Erga) from Guadeloupe Island and Welgevonden (Erwe) originating from South Africa and maintained in Guadeloupe. The type strain of E. ruminantium (Erwo), previously isolated and sequenced in South Africa; is identical to Erwe with respect to target genes. They make the Erwe/Erwo complex. Comparative analysis of the genomes shows the presence of 49 unique CDS and 28 truncated CDS differentiating Erga from Erwe/Erwo. Three regions of accumulated differences (RAD) acting as mutational hot spots were identified in E. ruminantium. Ten CDS, six unique CDS and four truncated CDS corresponding to major genomic changes (deletions or extensive mutations) were considered as targets for differential diagnosis on four isolates of E. ruminantium: Erga, Erwe/Erwo, Senegal and Umpala. Pairs of PCR primers were developed for each target gene. PCR analysis of the target genes generated strain-specific patterns on Erga and Erwe/Erwo as predicted by comparative genomics, but also for isolates Senegal and Umpala. The target genes identified by bacterial comparative genomics are shown to be highly efficient for strain-specific PCR diagnosis of E. ruminantium and further vaccine management tools.

Immunisation of sheep against heartwater in The Gambia using inactivated and attenuated Ehrlichia ruminantium vaccines

Heartwater (cowdriosis) is a disease of ruminants caused by a rickettsial pathogen Ehrlichia ruminantium and transmitted by ticks of the genus Amblyomma. The purpose of this work was to evaluate the protective efficacies of inactivated and attenuated vaccines to protect sheep against heartwater in The Gambia. An inactivated vaccine, prepared from E. ruminantium (Gardel stock), and a live attenuated vaccine from E. ruminantium (Senegal stock), were evaluated in two independent on-station trials. A local stock of E. ruminantium (Kerr Seringe) was used as challenge material. Inactivated and live attenuated vaccines provided 43% and 100% protection, respectively, against virulent needle challenge. In a subsequent field trial, the attenuated vaccine protected 75% of sheep against virulent tick challenge, which was fatal for all control sheep. Quantification by real-time PCR showed that an immunising dose of approximately 23,000 attenuated E. ruminantium organisms was sufficient. Moreover, restriction fragment length polymorphism (RFLP) analysis indicated that the local Kerr Seringe genotype caused mortality amongst control sheep, whereas fatalities in the vaccinated group could be attributed to a different genotype.

Longitudinal monitoring of Ehrlichia ruminantium infection in Gambian lambs and kids by pCS20 PCR and MAP1-B ELISA

BACKGROUND

The epidemiology of E. ruminantium infection in extensively managed young animals is not adequately understood. Thus in this study, we monitored the onset (age at first infection) and kinetics of E. ruminantium infection and antibody response in extensively managed newborn lambs and kids at three sites in The Gambia.

METHODS

We used a nested pCS20 PCR and MAP1-B ELISA in a longitudinal study to monitor the onset (age at first infection) and kinetics of E. ruminantium infection and antibody response respectively, in 77 newborn lambs and kids under a traditional husbandry system at three sites (Kerr Seringe, Keneba, Bansang) in The Gambia where heartwater is known to occur. The animals were monitored for field tick infestation and the comparative performance of the two assays in detecting E. ruminantium infection was also assessed.

RESULTS

The infection rate detected by pCS20 PCR varied between 8.6% and 54.8% over the 162-day study period. Nineteen per cent of the animals in week 1 post-partum tested positive by pCS20 PCR with half of these infections (7/14) detected in the first 3 days after birth, suggesting that transmission other than by tick feeding had played a role. The earliest detectable A. variegatum infestation in the animals occurred in week 16 after birth. Antibodies detected by MAP1-B ELISA also varied, between 11.5% and 90%. Although there is considerable evidence that this assay can detect false positives and due to this and other reasons serology is not a reliable predictor of infection at least for heartwater. In contrast to the pCS20 PCR, the serological assay detected the highest proportion of positive animals in week 1 with a gradual decline in seropositivity with increasing age. The pCS20 PCR detected higher E. ruminantium prevalence in the animals with increasing age and both the Spearman's rank test (rs = -0.1512; P = 0.003) and kappa statistic (-0.091 to 0.223) showed a low degree of agreement between the two assays.

CONCLUSION

The use of pCS20 PCR supported by transmission studies and clinical data could provide more accurate information on heartwater epidemiology in endemic areas and single-occasion testing of an animal may not reveal its true infection status. The view is supported because both the vector and vertical transmission may play a vital role in the epidemiology of heartwater in young small ruminants; the age range of 4 and 12 weeks corresponds to the period of increased susceptibility to heartwater in traditionally managed small ruminants.

Detection of Salmonella sp. in Dermanyssus gallinae using an FTA filter-based polymerase chain reaction

Salmonella spp. bacteria are responsible for some of the most important zoonoses worldwide. Because Dermanyssus gallinae (DeGeer) (Acari: Dermanyssidae) has been recently reported to be an experimental vector of Salmonella Enteritidis, it would be of benefit to evaluate the presence of this bacterium in mites. A molecular detection tool associating a simple filter-based DNA preparation with a specific 16S rDNA Salmonella sp. polymerase chain reaction (PCR) amplification was described. The limit of detection with this method was 2 x 10(4) bacteria per mite. To adapt this technique for large-scale studies, two sizes of mite pools were tested and a preliminary investigation was carried out on mites from 16 currently or previously contaminated farms. Mites sampled from one farm of each type were positive for Salmonella, suggesting that Dermanyssus could act as a reservoir between flocks. In further investigations, it will be necessary to carry out a large-scale study to assess the role of D. gallinae in the epidemiology of avian salmonellosis.

Comparative genomics of three strains of Ehrlichia ruminantium: a review

The tick-borne Rickettsiale Ehrlichia ruminantium (E. ruminantium) is the causative agent of heartwater in Africa and the Caribbean. Heartwater, responsible for major losses on livestock in Africa represents also a threat for the American mainland. Three complete genomes corresponding to two different groups of differing phenotypes, Gardel and Welgevonden, have been recently described. One genome (Erga) represents the Gardel group from Guadeloupe Island and two genomes (Erwo and Erwe) belong to the Welgevonden group. Erwo, isolated in South Africa, is the parental strain of Erwe, which was maintained for 18 years in Guadeloupe under different culture conditions than Erwo. The three strains display genomes of differing sizes with 1,499,920 bp, 1,512,977 bp, and 1,516,355 bp for Erga, Erwe, and Erwo, respectively. Gene sequences and order are highly conserved between the three strains, although several gene truncations could be pinpointed, most of them occurring within three regions of accumulated differences (RAD). E. ruminantium displays a strong leading/lagging compositional bias inducing a strand-specific codon usage. Finally, a striking feature of E. ruminantium is the presence of long intergenic regions containing tandem repeats. These repeats are at the origin of an active process, specific to E. ruminantium, of genome expansion/contraction based on the addition or removal of tandem units.

Molecular detection of Ehrlichia ruminantium infection in Amblyomma variegatum ticks in The Gambia

In West Africa, losses due to heartwater disease are not known because the incidence/prevalence has not been well studied or documented. To develop a diagnostic tool for molecular epidemiology, three PCR-based diagnostic assays, a nested pCS20 PCR, a nested map1 PCR and a nested reverse line blot (RLB) hybridization assay, were evaluated to determine their ability to detect infection in vector ticks, by applying them simultaneously to A. variegatum field ticks to detect Ehrlichia ruminantium, the causative agent of heartwater. The nested pCS20 PCR assay which amplified the pCS20 gene fragment showed the highest detection performance with a detection rate of 16.6%; the nested map1 PCR, which amplified the gene encoding the major antigenic protein1 (map1 gene) showed a detection rate of 11% and the RLB, based on the 16S rDNA sequence of anaplasma and ehrlichial species, detected 6.2%. The RLB, in addition, demonstrated molecular evidence of Ehrlichia ovina, Anaplasma marginale and Anaplasma ovis infections in The Gambia. Subsequently, the pCS20 assay was applied to study the prevalence and distribution of E. ruminantium tick infection rates at different sites in five divisions of The Gambia. The rates of infection in the country ranged from 1.6% to 15.1% with higher prevalences detected at sites in the westerly divisions (Western, Lower River and North Bank; range 8.3-15.1%) than in the easterly divisions (Central River and Upper River; range 1.6-7.5%). This study demonstrated a gradient in the distribution of heartwater disease risk for susceptible livestock in The Gambia which factor must be considered in the overall design of future upgrading programmes.

Emerging Tick-Borne Disease in African Vipers Caused by aCowdria-Like Organism

Heartwater is a tick-borne infectious disease caused by the rickettsial organism Cowdria ruminantium, currently Ehrlichia ruminantium. It poses an imminent threat to the Western Hemisphere, where it could cause mortality in cattle and other ruminant livestock in excess of 70%. It has been reported in the Caribbean; and its vector, Amblyomma sparsum, has been found on imported African spurred tortoises (Geochelone sulcata) and leopard tortoises (Geochelone pardalis) in southern Florida in the United States, leading to an importation ban on these reptiles. Symptoms have not been previously reported in reptiles. Here, we report peracute and acute deaths in African vipers imported from Africa through Florida. Signs included vomiting mucoid fluid, diarrhea, emaciation, convulsions, and death. Postmortem showed few gross lesions. The most consistent peracute and acute lesions were the pulmonary lesions and pericarditis with considerable bloody fluid in the pericardial sac (hydropericardium). These lesions strongly resembled the lesions of heartwater and a coccobacillus of less than 1-micron diameter was isolated in viper cell culture. The outbreak was brought to a halt by tick control and treatment of all exposed snakes with tetracycline. This isolation, tetracycline sensitivity, clinical signs, preliminary results with polymerase chain reaction of pCS20 ORF, and the viper preference of the disease may indicate a Cowdria-related attenuated species that has adapted to infect reptiles or an emerging new form of this group of microbes.

The epidemiology of heartwater: Establishment and maintenance of endemic stability

Although heartwater (Cowdria ruminantium infection) is one of the most economically important tick-borne diseases of sub-Saharan Africa, its epidemiology he's remained poorly understood until recently. New data, suggesting that heartwater is present in an endemically stable state in much of sub-Saharan Africa and demonstrating vertical transmission of Cowdria ruminantium in the field, have altered previously accepted views on heartwater epidemiology. In this paper, Sharon Deem and colleagues present an overview of the epidemiology of heartwater based on recent studies, discuss the factors that make endemic stability possible, make recommendations for future directions in research, and provide a foundation for the development of epidemiological models.

Nested PCR for detection and genotyping of Ehrlichia ruminantium: use in genetic diversity analysis

Ehrlichia ruminantium, the agent of cowdriosis transmitted by Amblyomma ticks, presents an extensive genetic and antigenic diversity of key importance for vaccine formulation. Two means of nested polymerase chain reaction (PCR) targeting were developed to conduct molecular epidemiology studies in the Caribbean and Africa. The first used a conserved DNA fragment for detection of the pathogen in animals and vectors, and the second relied on the polymorphic map1 gene for genotyping. As compared to a PCR, the nested PCR showed a 2-Log10 improvement of sensitivity and allowed amplification from ticks, blood, brain, and lungs from infected animals, providing a more accurate picture of the tick infection rate. In Guadeloupe, this rate reached 36% (N = 212) instead of 1.7% (N = 224), as previously estimated. Genetic typing was done by restriction fragment length polymorphism or sequencing of map1 amplification products. Molecular epidemiology studies conducted in field sites selected for vaccination trials with inactivated vaccine, revealed the circulation of genetically divergent strains in limited geographical areas. It is known, then, that genetic clustering based on map1 has no predictive value regarding the protective value of a given strain against a new strain. However, tracing the strains by this technique revealed the extent of E. ruminantium diversity that one can expect in a given region, and the method allows differentiation between an inadequate immune response and the challenge by a breakthrough strain on animals dying despite vaccination. Up to now, genetic typing does not avoid cross-protection studies, which were conducted in parallel, although on a more limited scale. The importance of pathogen diversity studies for optimization of vaccine design is discussed as well as the research for new polymorphic genes. These genes may allow better predictions on cross-protection, given the recent completion of the sequence of the full genome of two E. ruminantium strains.

Development of a p28-based PCR assay for Ehrlichia chaffeensis

Detection of Ehrlichia chaffeensis is necessary to study interactions between the parasite and its vertebrate and invertebrate hosts. The purpose of this study was to develop a sensitive, specific PCR assay for E. chaffeensis based on the outer membrane protein gene, p28. Candidate primer sets were identified and ranked based on annealing scores, similarities to three major p28 sequence clusters, dissimilarity to E. canis p30, an ortholog of p28, and the proximities of flanking primer sequences for nested PCR. The relative sensitivities of five optimized single-step and two nested PCR assays were determined, and the most sensitive assay was found to be a single-step PCR that was as much as 1000-fold more sensitive than a standard 16S rDNA-based nested PCR assay. This p28-based PCR assay amplified the target amplicon from isolates representative of all three major clusters of known p28 sequences, and this assay did not amplify template prepared from either of the two species most closely related to E. chaffeensis, E. canis and E. muris. These results indicate that this sensitive, specific and isolate-universal single-step PCR assay will be a useful tool in characterizing the transmission of this important zoonotic pathogen.

Characterization of the pCS20 region of different Ehrlichia ruminantium isolates

Heartwater is a serious tick-borne disease of ruminants caused by the rickettsial organism Ehrlichia (Cowdria) ruminantium. A diagnostic test, targeting the pCS20 genomic region and using PCR amplification and probe hybridization, detects E. ruminantium infection in ticks and animals. However, only the pCS20 sequence of the Crystal Springs E. ruminantium isolate is available and the existence of sequence variation amongst different E. ruminantium isolates has not been determined. Primers were designed from the published pCS20 sequence to obtain sequences of the pCS20 region of various E. ruminantium isolates. These primers were unable to amplify the pCS20 region from genomic Welgevonden DNA and genome walking was used to characterize the pCS20 region. This technique showed that the published pCS20 sequence is from a chimeric clone. Sequences of the pCS20 region of 14 different E. ruminantium isolates were determined after amplification with newly designed primers. Sequencing data indicated that West African E. ruminantium isolates are highly conserved, whereas more variation occurs amongst the southern African isolates. These results facilitated the design of a short pCS20 probe and a large PCR target that improved the sensitivity of the E. ruminantium detection assay.

Simultaneous detection of Anaplasma and Ehrlichia species in ruminants and detection of Ehrlichia ruminantium in Amblyomma variegatum ticks by reverse line blot hybridization

The detection of Anaplasma and Ehrlichia species is usually based on species-specific PCR assays, since no assay is yet available which can detect and identify these species simultaneously. To this end, we developed a reverse line blot (RLB) assay for simultaneous detection and identification of Anaplasma and Ehrlichia species in domestic ruminants and ticks. In a PCR the hypervariable V1 region of the 16S ribosomal RNA (rRNA) gene was amplified with a set of primers unique for members of the genera Anaplasma and Ehrlichia [Int. J. Syst. Evol. Microbiol. 51 (2001) 2145]. Amplified PCR products from blood of domestic ruminants or Amblyomma variegatum tick samples were hybridized onto a membrane to which eight species-specific oligonucleotide probes and one Ehrlichia and Anaplasma catch-all oligonucleotide probe were covalently linked. No DNA was amplified from uninfected blood, nor from other hemoparasites such as Theileria annulata, or Babesia bigemina. The species-specific probes did not cross-react with DNA amplified from other species. E. ruminantium, A. ovis and another Ehrlichia were identified by RLB in blood samples collected from small ruminants in Mozambique. Finally, A. variegatum ticks were tested after feeding on E. ruminantium infected sheep. E. ruminantium could be detected in adult ticks even if feeding of nymphs was carried out 3.5 years post-infection. In conclusion, the developed species-specific oligonucleotide probes used in an RLB assay can simultaneously detect and identify several Ehrlichia and Anaplasma species. However, as no quantitative data for the detection limit are available yet, only positive results are interpretable at this stage.

Resistance of leopard tortoises and helmeted guineafowl to Cowdria ruminantium infection (heartwater)

Experimental infection trials were conducted to investigate susceptibility of leopard tortoises (Geochelone pardalis) and helmeted guineafowl (Numida meleagris) to infection with Cowdria ruminantium, the causative agent of heartwater, a tickborne disease of domestic and wild ruminants. Ten guineafowl were inoculated intravenously with a virulent dose of C. ruminantium derived from bovine endothelial cell cultures, and four leopard tortoises were exposed to C. ruminantium infection by the feeding of infected Amblyomma hebraeum ticks. Uninfected A. hebraeum ticks (on both tortoises and guineafowl) and Amblyomma marmoreum ticks (on tortoises only) were fed on the animals during weeks 2 and 3 post-exposure in an attempt to detect infection. These ticks were analyzed for C. ruminantium infection by xenodiagnosis and with the C. ruminantium-specific pCS20 polymerase chain reaction (PCR) assay. Attempts to detect infection in ticks fed on either species were negative by both tests. These results suggest that leopard tortoises and helmeted guineafowl are refractory to C. ruminantium infection and, therefore, are unlikely to be capable of introducing heartwater directly into new areas. However, leopard tortoises are efficient hosts of A. marmoreum and A. hebraeum and are likely to be important epidemiologically in the transport and maintenance of these tick vector species.

Antibody responses to MAP 1B and other Cowdria ruminantium antigens are down regulated in cattle challenged with tick-transmitted heartwater

Serological diagnosis of heartwater or Cowdria ruminantium infection has been hampered by severe cross-reactions with antibody responses to related ehrlichial agents. A MAP 1B indirect enzyme-linked immunosorbent assay that has an improved specificity and sensitivity for detection of immunoglobulin G (IgG) antibodies has been developed to overcome this constraint (A. H. M. van Vliet, B. A. M. Van der Zeijst, E. Camus, S. M. Mahan, D. Martinez, and F. Jongejan, J. Clin. Microbiol. 33:2405-2410, 1995). When sera were tested from cattle in areas of endemic heartwater infection in Zimbabwe, only 33% of the samples tested positive in this assay despite a high infection pressure (S. M. Mahan, S. M. Samu, T. F. Peter, and F. Jongejan, Ann. N.Y. Acad. Sci 849:85-87, 1998). To determine underlying causes for this observation, the kinetics of MAP 1B-specific IgG antibodies in cattle after tick-transmitted C. ruminantium infection and following recovery were investigated. Sera collected weekly over a period of 52 weeks from 37 cattle, which were naturally or experimentally infected with C. ruminantium via Amblyomma hebraeum ticks, were analyzed. MAP 1B-specific IgG antibody responses developed with similar kinetics in both field- and laboratory-infected cattle. IgG levels peaked at 4 to 9 weeks after tick infestation and declined to baseline levels between 14 and 33 weeks, despite repeated exposure to infected ticks and the establishment of a carrier state as demonstrated by PCR and xenodiagnosis. Some of the serum samples from laboratory, and field-infected cattle were also analyzed by immunoblotting and an indirect fluorescent-antibody test (IFAT) to determine whether this observed seroreversion was specific to the MAP 1B antigen. Reciprocal IFAT and immunoblot MAP 1-specific antibody titres peaked at 5 to 9 weeks after tick infestation but also declined between 30 and 45 weeks. This suggests that MAP 1B-specific IgG antibody responses and antibody responses to other C. ruminantium antigens are down regulated in cattle despite repeated exposure to C. ruminantium via ticks. Significantly, serological responses to the MAP 1B antigen may not be a reliable indicator of C. ruminantium exposure in cattle in areas of endemic heartwater infection.

Evidence of Cowdria ruminantium infection (heartwater) in Amblyomma sparsum ticks found on tortoises imported into Florida

Amblyomma marmoreum and A. sparsum ticks were collected from tortoises imported into Florida from Africa and were tested for Cowdria ruminantium infection using a C. ruminantium-specific pCS20 polymerase chain reaction assay. In I shipment imported from Zambia, 15 of the 38 A. sparsum male ticks collected from the leopard tortoises (Geochelone pardalis) were found to be positive for infection with C. ruminantium. In contrast, all 148 A. marmoreum tested were negative for C. ruminantium infection. This is the first reported evidence of the introduction of heartwater-infected ticks into the United States, but there were no opportunities to confirm isolation of C. ruminantium from the ticks by either culture or transmission studies.

Detection of the agent of heartwater, Cowdria ruminantium, in Amblyomma ticks by PCR: validation and application of the assay to field ticks

We have previously reported that the pCS20 PCR detection assay for Cowdria ruminantium, the causative agent of heartwater disease of ruminants, is more sensitive than xenodiagnosis and the pCS20 DNA probe for the detection of infection in the vector Amblyomma ticks. Here, we further assessed the reliability of the PCR assay and applied it to field ticks. The assay detected DNA of 37 isolates of C. ruminantium originating from sites throughout the distribution of heartwater and had a specificity of 98% when infected ticks were processed concurrently with uninfected ticks. The assay did not detect DNA of Ehrlichia chaffeensis, which is closely related to C. ruminantium. PCR sensitivity varied with tick infection intensity and was high (97 to 88%) with ticks bearing 10(7) to 10(4) organisms but dropped to 61 and 28%, respectively, with ticks bearing 10(3) and 10(2) organisms. The assay also detected C. ruminantium in collections of Amblyomma hebraeum and Amblyomma variegatum field ticks from 17 heartwater-endemic sites in four southern African countries. Attempts at tick transmission of infection to small ruminants failed with four of these collections. The pCS20 PCR assay is presently the most characterized and reliable test for C. ruminantium in ticks and thus is highly useful for field and laboratory epidemiological investigations of heartwater.

Molecular detection of pathogen DNA in ticks (Acari: Ixodidae): a review

Ticks play an important role in human and veterinary medicine, in particular due to their ability to transmit a wide spectrum of pathogenic micro-organisms of protozoal, rickettsial, bacterial and viral origin. Pathogens in ticks can be identified by conventional methods such as indirect immunofluorescence, isolation in cell culture or by using histological staining techniques. However, the advent of the polymerase chain reaction (PCR) has resulted in tremendous improvements in the specific and sensitive detection of pathogen DNA in ticks. In this paper, literature on DNA extraction methods, PCR protocols, primers and probes, which are in use for the successful detection and identification of pathogens in ticks, are critically reviewed. Some recommendations are also given towards the end of this review.

Cowdria ruminantium infection in ticks in the Kruger National Park

Adult Amblyomma hebraeum ticks, the principle vector of heartwater (cowdriosis) of domestic ruminants in southern Africa, were collected in pheromone traps placed in Kruger National Park, an exclusively wildlife sanctuary in South Africa. These ticks transmitted Cowdria ruminantium, the rickettsial agent causing heartwater, to a susceptible goat, resulting in acute, fatal disease. C ruminantium was isolated in bovine endothelial cell culture from the plasma of this animal during the febrile stage of the disease and transmitted to susceptible goats, causing fatal heartwater. The prevalence of C ruminantium infection in 292 ticks was determined by polymerase chain reaction (PCR) analysis to be 1.7 per cent (95 per cent confidence interval 0.71 to 4.0 per cent). A DNA probe analysis, which is less sensitive than PCR, detected infection in three of the five PCR-positive ticks. The remaining infections were below the detection limit of the DNA probe, which is approximately 70,000 organisms. This is the first evidence that a vector-wildlife cycle of transmission of C ruminantium can be maintained independently of domestic ruminants.

The effect of subclinical experimental Cowdria ruminantium infection on the health and reproductive performance of breeding ewes

This study documented the effect on the productivity of Dorper-Merino cross ewes when they became infected with Cowdria ruminantium as would occur in an endemically stable state. A flock of 152 breeding ewes was randomly divided into two matched groups. One group was infected on multiple occasions with C. ruminantium; the other group remained uninfected. The ewes were bred and monitored for one breeding/lambing/weaning cycle. Hematological, reproductive, and health parameters were recorded at regular intervals. Statistically significant differences in hematological values between the two groups were infrequent and transient. No significant differences in weights, mortality rates or reproductive parameters were detected between the two groups. Subclinical C. ruminantium infection did not negatively affect health and reproductive performance of breeding ewes; endemic stability would be a suitable alternative method of controlling heartwater.

Development and evaluation of a polymerase chain reaction assay using the 16S rRNA gene for detection of Eperythrozoon suis infection

The 16S ribosomal RNA (rRNA) gene of Eperythrozoon suis was amplified using gene-specific primers developed from GenBank sequence accession U88565. The gene was subsequently cloned and sequenced. Based on these sequence data, 3 sets of E. suis-specific primers were designed. These primers selectively amplified 1394, 690, and 839 base-pair (bp) fragments of the 16S rRNA gene from DNA of E. suis extracted from the blood of an experimentally infected pig during a parasitemic episode. No polymerase chain reaction (PCR) products were amplified from purified DNA of Haemobartonella felis, Mycoplasma genitalium, or Bartonella bacilliformis using 2 of these primer sets. When the primer set amplifying the 690-bp fragment was used, faint bands were observed with H. felis as the target DNA. No PCR products were amplified from DNA that had been extracted from the blood of a noninfected pig or using PCR reagents without target DNA. The detection limits for E. suis by competitive quantitative PCR were estimated to range from 57 and 800 organisms/assay. This is the first report of the utility of PCR-facilitated diagnosis and quantitation of E. suis based on the 16S rRNA gene. The PCR method developed will be useful in monitoring the progression and significance of E. suis in the disease process in the pig.

Body lice as tools for diagnosis and surveillance of reemerging diseases

Body lice are vectors of three bacteria which cause human disease: Rickettsia prowazekii, the agent of epidemic typhus; Bartonella quintana, the agent of trench fever; and Borrelia recurrentis, the agent of relapsing fever. A recrudescence of body lice is being observed as the numbers of individuals living under social conditions which predispose individuals to infestation have increased. Because this phenomenon may lead to the reemergence of infections transmitted by body lice, we aimed to assess the occurrence and prevalence of the three agents described above in more than 600 body lice collected from infested individuals in the African countries of Congo, Zimbabwe, and Burundi, in France, in Russia, and in Peru. The presence of the three bacteria in each louse was determined by specific PCR amplification, and the identities of the organisms detected were confirmed by determination of the nucleotide base sequences of the amplification products. Using this approach, we were able to confirm the presence of R. prowazekii in lice collected from refugees in Burundi, among whom typhus was epidemic, and the presence of B. quintana in lice collected from all locations except the Congo. B. recurrentis was never found. Molecular approaches are convenient tools for the detection and identification of bacterial DNA in body lice and for the epidemiological study of louse-borne bacteria from countries where no medical and biological laboratory facilities are available.

5' nuclease PCR assay to detect Yersinia pestis

The 5' nuclease PCR assay uses a fluorescently labeled oligonucleotide probe (TaqMan) to rapidly detect and quantitate DNA templates in clinical samples. We developed a 5' nuclease PCR assay targeting the plasminogen activator gene (pla) of Yersinia pestis. The assay is species specific, with a detection threshold of 2.1 x 10(5) copies of the pla target or 1.6 pg of total cell DNA. The assay detected Y. pestis in experimentally infected Xenopsylla cheopis fleas and in experimentally infected monkey blood and oropharyngeal swabs. The TaqMan assay is simple to perform and rapid and shows promise as a future field-adaptable technique.

The inactivated Cowdria ruminantium vaccine for heartwater protects against heterologous strains and against laboratory and field tick challenge

We previously described that an inactivated vaccine against heartwater prepared from Cowdria ruminantium (Crystal Springs strain) organisms and administered in complete Freund's adjuvant (cFA) protected sheep against homologous needle challenge. Further studies, described herein, demonstrated that this vaccine protected 100% of sheep against death on challenge with laboratory-infected ticks and with field ticks in a heartwater endemic farm, whereas a mortality rate of 44% and 62%, respectively, was recorded in the control sheep. Subsequently, the Mbizi strain of C. ruminantium was incorporated into the vaccine because of its wider cross-protective capacity, and trial data suggested that protection may be achieved against challenge with diverse geographical strains using this strain. The efficacy of five adjuvants with acceptable safety was compared with that of cFA. Against a homologous intravenous challenge, highest survival rates were observed in sheep vaccinated with inactivated C. ruminantium in either cFA, Montanide ISA 50 or Quil A. The vaccine prepared in Montanide ISA 50 protected six of seven sheep against natural challenge from field ticks on a farm in Zimbabwe where heartwater is endemic, whereas six of seven control sheep died (P = 0.029). These data support optimization of the vaccine prepared in Montanide ISA 50, followed by evaluation of its efficacy in all target domestic ruminant species and in other geographical regions where heartwater constrains livestock production.

Comparative evaluation of 16S, map1 and pCS20 probes for the detection of Cowdria and Ehrlichia species in ticks

We have developed a panel of 16S ribosomal RNA gene probes for heartwater epidemiology; five of these detect different Cowdria genotypes (Ball3, Senegal, Omatjenne, Crystal Springs, and Mara 87/7); one detects all five of these genotypes; one detects any Group III Ehrlichia species other than Cowdria; one detects any Group II Ehrlichia species. We have used these probes on PCR-amplified rickettsial 16S rRNA genes from over 200 Amblyomma ticks. Control ticks were laboratory-reared and either uninfected or fed on sheep experimentally infected with different Cowdria isolates, field ticks were harvested from animals in heartwater-endemic and heartwater-free areas. All the samples were also examined by PCR amplification and probing for two other Cowdria genes (map1 and pCS20) which have been used for heartwater epidemiology. This paper describes the first direct comparison of all the currently available DNA probes for heartwater-associated organisms.

PCR detection of Cowdria ruminantium infection in ticks and animals from heartwater-endemic regions of Zimbabwe

The development of a PCR assay for the detection of Cowdria ruminantium infection in ticks has been previously described. Here we report a further evaluation of this assay by comparison with a DNA probe and with the mouse inoculation assay (MIA). Application of the PCR assay in determining the prevalence of infection in ruminants and ticks from heartwater-endemic areas of Zimbabwe is also described. One hundred uninfected and 120 infected Amblyomma hebraeum ticks were analyzed by PCR, DNA probe and the MIA. These tests detected infection in 92%, 77% and 8% of the infected ticks, respectively, showing PCR to be the most sensitive assay. None of the uninfected ticks were positive by any of the 3 tests. The PCR assay detected infection rates of 10.5%, 12.5% and 3.2% in 200 male, 241 female and 95 nymphal A. hebraeum ticks, respectively, which were collected from a heartwater-endemic farm. The PCR test was also applied to cattle of different age groups and goats from the same heartwater-endemic farm. In a cross-sectional survey, the assay detected infection in 3.3% to 26.7% of the cattle and in 23.3% of the goats. The implications of these findings and the potential for the application of PCR in heartwater diagnosis is discussed.

Development and evaluation of a PCR-based assay for detection of Haemobartonella felis in cats and differentiation of H. felis from related bacteria by restriction fragment length polymorphism analysis

The 16S rRNA gene of Haemobartonella felis was amplified by using universal eubacterial primers and was subsequently cloned and sequenced. Based on this sequence data, we designed a set of H. felis-specific primers. These primers selectively amplified a 1,316-bp DNA fragment of the 16S rRNA gene of H. felis from each of four experimentally infected cats at peak parasitemia. No PCR product was amplified from purified DNA of Eperythrozoon suis, Mycoplasma genitalium, and Bartonella bacilliformis. Blood from the experimental cats prior to infection was negative for PCR products and was greatly diminished or absent 1 month after doxycycline treatment. The overall sequence identity of this fragment varied by less than 1.0% among experimentally infected cats. By taking into consideration the secondary structure of the 16S rRNA molecule, we were able to further verify the alignment of nucleotides and quality of our sequence data. In this PCR assay, the minimum detectable number of H. felis organisms was determined to be between 50 and 704. The potential usefulness of restriction enzymes DdeI and MnlI for distinguishing H. felis from closely related bacteria was examined. This is the first report of the utility of PCR-facilitated diagnosis and discrimination of H. felis infection in cats.

Evidence to show that an agent that cross-reacts serologically with Cowdria ruminantium in Zimbabwe is transmitted by ticks

The serological diagnosis of heartwater based on reactions to the immunodominant Cowdria ruminantium major antigen protein-1 (MAP-1) is impaired by the detection of false-positive reactions. In this study, the prevalence of false-positive reactions on seven heartwater-free farms in Zimbabwe was determined to be 8-94% by immunoblotting against C. ruminantium antigens. The highest prevalence of false-positives on Spring Valley Farm correlated with the presence of Rhipicephalus evertsi evertsi ticks. The other tick species found on these seven farms were Hyalomma truncatum and Hyalomma marginatum rufipes. Rhipicephalus evertsi evertsi ticks collected from Spring Valley Farm and fed on seronegative sheep caused seroconversion in one of two sheep. This sheep developed a mild febrile reaction and C. ruminantium MAP-1 antigen reactive antibodies 3 weeks after the ticks started feeding. Polymerase chain reactions (PCRs), conducted using C. ruminantium-specific primers on ticks collected from the seven farms and on some of the R. e. evertsi ticks that had caused seroconversion in one sheep, were negative. However, some of these ticks gave positive PCRs with DNA primers which amplify a 350 bp DNA fragment of the 16s rRNA gene from all ehrlichial agents indicating the presence of infection with one or more Ehrlichia species. Although attempts to isolate the cross-reacting agent from the sheep were unsuccessful, this study demonstrates that false-positive reactions with the MAP-1 C. ruminantium antigen are associated with agents transmitted by ticks.

Construction and initial analysis of a representative lambda ZAPII expression library of the intracellular rickettsia Cowdria ruminantium: cloning of map1 and three other Cowdria genes

The causative agent of heartwater, the rickettsia Cowdria ruminantium, is very poorly understood at the molecular level owing to a profound lack of suitable tools. We have developed an immunoaffinity chromatographic method to purify C. ruminantium from host cell components and the purified rickettsial cells have been used to prepare substantially pure Cowdria DNA. This DNA has been used to construct what we believe to be the first fully representative C. ruminantium expression library. A clone containing the complete Cowdria map1 gene has been isolated and sequenced. This gene has been expressed in E. coli cells from the native Cowdria promoter, suggesting that the mechanisms for gene transcription and translation are similar between these two organisms. Parts of three other Cowdria genes have also been isolated and sequenced.

Different organisms associated with heartwater as shown by analysis of 16S ribosomal RNA gene sequences

Cowdria ruminantium is a rickettsial parasite which causes heartwater, a economically important disease of domestic and wild ruminants in tropical and subtropical Africa and parts of the Caribbean. Because existing diagnostic methods are unreliable, we investigated the small-subunit ribosomal RNA (srRNA) gene from heartwater-infected material to characterise the organisms present and to develop specific oligonucleotide probes for polymerase chain reaction (PCR) based diagnosis. DNA was obtained from ticks and ruminants from heartwater-free and heartwater-endemic areas from Cowdria in tissue culture. PCR was carried out using primers designed to amplify only rickettsial srRNA genes, the target region being the highly variable V1 loop. Amplicons were cloned and sequenced; 51% were C. ruminantium sequences corresponding to four genotypes, two of which were identical to previously reported C. ruminantium sequences while the other two were new. The four different Cowdria genotypes can be correlated with different phenotypes. Tissue-culture samples yielded only Cowdria genotype sequences, but an extraordinary heterogeneity of 16S sequences was obtained from field samples. In addition to Cowdria genotypes we found sequences from previously unknown Ehrlichia spp., sequences showing homology to other Rickettsiales and a variety of Pseudomonadaceae. One Ehrlichia sequence was phylogenetically closely related to Ehrlichia platys (Group II Ehrlichia) and one to Ehrlichia canis (Group III Ehrlichia). This latter sequence was from an isolate (Germishuys) made from a naturally infected sheep which, from brain smear examination and pathology, appeared to be suffering from heartwater; nevertheless no Cowdria genotype sequences were found in this isolate. In addition no Cowdria sequences were obtained from uninfected ticks. Complete 16S rRNA gene sequences were determined for two C. ruminantium genotypes and for two previously uncharacterised heartwater-associated Ehrlichia spp. Sequenced difference within the V1 loop were sufficient for the derivation of four Cowdria genotype-specific oligonucleotide probes. Four further probes were designed; one for the detection of any Cowdria genotype, one for the detection of any Group II Ehrlichia sp., one for any Group III Ehrlichia sp. and one for all Pseudomonadaceae. All the probes were specific except that for the Cowdria (Ball 3) genotype. The high prevalence (96%) in field samples of pseudomonad-like 16S sequences was the result of environmental contamination. The probes were used to screen DNA from goats in an area free of both Amblyomma ticks and clinical heartwater. A substantial proportion (42%) gave positive reactions for the apparently apathogenic Cowdria (Omatjenne), indicating that this genotype is relatively common.

Ehrlichia phagocytophila genogroup rickettsiae in ixodid ticks from California collected in 1995 and 1996

A total of 1,246 ixodid ticks collected in 1995 and 1996 from seven California counties were examined for the presence of Ehrlichia phagocytophila genogroup rickettsiae by using a nested PCR technique. Of 1,112 adult Ixodes pacificus Cooley and Kohls ticks tested, nine pools, each containing five ticks, were positive (minimum percentage of ticks harboring detectable ehrlichiae, 0.8%). Positive ticks were limited to four of the seven counties (Sonoma, El Dorado, Santa Cruz, and Orange). In Santa Cruz County, three positive pools were identified at the home of an individual with prior confirmed human granulocytic ehrlichiosis. In El Dorado County, positive ticks were found at sites where cases of granulocytic ehrlichiosis in a horse and a llama had recently occurred. Among 47 nymphal I. pacificus ticks collected in Sonoma County, one positive pool was identified. Fifty-seven adult Dermacentor occidentalis Marx and 30 adult D. variabilis Say ticks, collected chiefly in southern California, were negative. These data, although preliminary, suggest that the prevalence of E. phagocytophila genogroup rickettsiae in ixodid ticks of California may be lower than in cognate vector populations (i.e., I. scapularis Say = I. dammini Spielman, Clifford, Piesman, and Corwin) in the eastern and midwestern United States.

Detection of Theileria annulata by the PCR in ticks (Acari:Ixodidae) collected from cattle in Mauritania

We report on the detection of Theileria annulata in infected Hyalomma ticks by the PCR using primers derived from the gene encoding the 30 kDa major merozoite surface antigen (TamsI-1). No inhibition of the PCR was observed and as little as 0.1 pg of parasite DNA, corresponding to 12 sporozoites, could be detected in non-infected tick DNA samples, spiked with T. annulata genomic DNA. Hyalomma dromedarii ticks, fed on a calf experimentally infected with T. annulata, were used to validate the PCR further. The infection rate in the adult ticks, fed as nymphs during the febrile reaction, was high (62%), dropped to zero for 1 day in tick batches that engorged after treatment with Butalex and increased to 30% 2 days later and 38% of the ticks acquired the infection after feeding as nymphs during a carrier state piroplasm parasitaemia of less than 0.1%. As an internal control, 16S tick rDNA sequences could be amplified from T. annulata-negative tick samples. Finally, 202 adult ticks from Mauritania, collected from zebu cattle carrying low levels of Theileria piroplasms, were tested by the PCR. Thirty-eight out of 52 (73%) and 17 out of 30 (57%) H. dromedarii from the Gorgol and Trarza regions, respectively and two out of 30 (7%) Hyalomma marginatum rufipes from the Gorgol region were positive. Hyalomma marginatum rufipes, Rhipicephalus evertsi evertsi and Rhipicephalus guilhoni from the Trarza region were negative. These findings confirm that H. dromedarii is the main vector of T. annulata in Mauritania and that the PCR is a useful method of determining the infection rates in ticks collected from cattle carrying low levels of T. annulata piroplasms.

Demonstration of vertical transmission of Cowdria ruminantium, the causative agent of heartwater, from cows to their calves

One of the most important questions about the epidemiology of heartwater in the field is how Cowdria ruminatium is transmitted within vertebrate host populations. In this study vertical transmission of C. ruminantium from cows to their calves was demonstrated. Twelve mixed-breed calves, born to dams living in a heartwater-endemic area of Zimbabwe, were tested post-natally for the presence of C. ruminantium. Vertical transmission was demonstrated to occur under natural field conditions using tests in which uninfected laboratory-reared Amblyomma ticks were fed on neonatal calves and subsequently either fed on, or inoculated into, susceptible small ruminants or tested by a polymerase chain reaction (PCR) assay. Prior to natural Amblyomma tick infestation, C. ruminantium infection in 5 of the 12 calves was confirmed by tick transmission to small ruminants and 11 of the 12 calves tested positive based on PCR analysis of ticks fed on them. The role of colostral cells, as one mode of infection, was demonstrated by the transmission of C. ruminantium to three out of five goats inoculated intravenously with viable colostral cells collected from dams living in a heartwater-endemic area. The significance of vertical transmission is presented in relation to the epidemiology and control of heartwater.

Review of the molecular biology of Cowdria ruminantium

Cowdria ruminantium is a rickettsial agent which causes heartwater, an economically important disease of livestock in the tropics and the Caribbean. Significant advances have been made in the molecular biology of C. ruminantium since its reproducible propagation in vitro in bovine endothelial cells. These advances have been targeted towards the development of improved vaccines and diagnostic tests. Several immunogenic proteins of C. ruminantium have been identified and monoclonal antibodies have been developed to some. The gene for the 21 kDa C. ruminantium protein has been cloned, characterized, sequenced and expressed to high levels to produce a recombinant analogue. This gene is conserved amongst all C. ruminantium isolates tested. The gene for the immunodominant 32 kDa protein has also been cloned recently. Analysis shows that this protein varies structurally between different C. ruminantium isolates. Recombinant protein analogues will have application in vaccine studies and subunit enzyme-linked immunosorbent assay (ELISA) developments. In the field of serology, a 32 kDa protein specific competitive ELISA (cELISA) is available and a 21 kDa protein specific direct and a cELISA is being developed. Detection of C. ruminantium in Amblyomma ticks and in animals is now possible using DNA and RNA probes and the sensitivity of these nucleic acid based assays is being maximized using polymerase chain reaction (PCR). Sequencing of the 16s rRNA gene has revealed the close phylogenetic relationship of C. ruminantium to Rickettsia, Anaplasma and Ehrlichia species. The implications of these studies will be discussed.