A cloned DNA probe identifies Cowdria ruminantium in Amblyomma variegatum ticks

Development of a Quantitative PCR Assay for Differentiating the Agent of Heartwater Disease, Ehrlichia ruminantium, from the Panola Mountain Ehrlichia

Panola Mountain Ehrlichia (PME) is an emerging Ehrlichia sp. reported in ten US states. Based on the sequence homology of all known genes, PME is closely related to Ehrlichia ruminantium (ER), the causative agent of heartwater. Heartwater is an economically important tick-borne disease of cattle, sheep and goats responsible for stock losses in sub-Saharan Africa. Unfortunately, ER was imported to the Caribbean islands in the 19th century, and the presence of this foreign animal disease in the Caribbean poses a threat to the US mainland. If introduced, a heartwater outbreak would cause massive losses of naïve livestock. The serologic assay of choice to diagnose heartwater is cross-reactive with Ehrlichia spp., including PME, as we demonstrate here, which would confound disease surveillance in the event of a heartwater outbreak. The purpose of this study was to develop a diagnostic assay capable of rapidly distinguishing between these pathogens. Using synthetic MAP-1B peptides for ER and PME, we tested the cross-reactivity of this assay using sera from infected livestock. The MAP-1B ELISA cannot distinguish between animals infected with PME and ER. Therefore, a dual-plex Taqman^™ qPCR assay targeting the groEL gene of PME and ER was developed and validated. Primers were designed that are conserved among all known strains of ER, allowing for the amplification of strains from the Caribbean and Africa. The assay is highly sensitive (10 copies of DNA) and specific. This assay distinguishes between infection with PME and ER and will be a valuable tool in the event of heartwater outbreak on the US mainland, or for epidemiological studies involving either disease-causing organism.

The role of nuclear technologies in the diagnosis and control of livestock diseases--a review

Nuclear and nuclear-related technologies have played an important role in animal health, particularly in relation to disease diagnosis and characterization of pathogenic organisms. This review focuses primarily on how and where nuclear technologies, both non-isotopic and isotopic methods, have made their impact in the past and where it might be expected they could have an impact in the future. The review outlines the extensive use of radiation attenuation in attempts to create vaccines for a multiplicity of pathogenic organisms and how the technology is being re-examined in the light of recent advances in irradiation techniques and cryopreservation/lyophilization that might obviate some of the problems of maintenance of viable, attenuate vaccines and their transport and use in the field. This approach could be used for a number of parasitic diseases where vaccination has been problematic and where investigations into the development of molecular vaccines have still failed to deliver satisfactory candidates for generating protective immune responses. Irradiation of antigens or serum samples also has its uses in diagnosis, especially when the samples need to be transported across international boundaries, or when handling the pathogens in question when carrying out a test presents serious health hazards to laboratory personnel. The present-day extensive use of enzyme immunoassays and molecular methods (e.g., polymerase chain reaction) for diagnosis and characterization of animal pathogens has its origins in the use of isotope-labeled antigens and antibodies. These isotopic techniques that included the use of 75Se, 32P, 125I, and 35S isotopes enabled a level of sensitivity and specificity that was hitherto unrealized, and it is prescient to remind ourselves of just how successful these technologies were, in spite of their infrequent use nowadays. Finally, the review looks at the potential for stable isotope analysis for a variety of applications--in the tracking of animal migrations, where the migrant are potential carriers of transboundary animal diseases, and where it would be useful to determine the origins of the carrier, e.g., Highly Pathogenic Avian Influenza and its dissemination by wild water fowl. Other applications could be in monitoring sequestered microbial culture (e.g., rinderpest virus) where in the case of accidental or deliberate release of infective culture it would be possible to identify the laboratory from which the isolate originated.

Ehrlichia species, probable emerging human pathogens in sub-Saharan Africa: environmental exacerbation

Ehrlichiae are obligate intracellular Gram-negative tick-borne bacteria that are responsible for life-threatening emerging human zoonoses and diseases of veterinary importance worldwide, collectively called ehrlichioses. The genus Ehrlichia consists of five recognized species, including E. canis, E. chaffeensis, E. ewingii, E. muris, and E. ruminantium. The recent discoveries of Ehrlichia species in new areas and of tick species that were previously thought to be uninfected by these agents have suggested that these agents may have wider distribution than originally thought. Environmental factors like temperature, migration, control failure, and host population have been known to exacerbate the spread of Ehrlichia species. Human cases of moderate to severe disease caused by E. chaffeensis have been reported mainly in North America. In this article, we present an overview of ehrlichiae as emerging pathogens in sub-Saharan Africa, where E. ruminantium, the causative agent of heartwater, a disease of domestic and wild ruminants, is most established. Molecular evidence indicates that E. ruminantium may be an emerging pathogen of a life-threatening human disease. Ehrlichia ruminantium is considered an agricultural biothreat, with several strains reported throughout sub-Saharan Africa, where the infection is considered endemic. Understanding the diversity of E. ruminantium and other Ehrlichia species from all geographically distinct areas of sub-Saharan Africa may enhance our knowledge of the pathogenesis and epidemiology of these pathogens.

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.

A quantitative real-time PCR assay for Ehrlichia ruminantium using pCS20

Heartwater is a tick borne disease that affects ruminants and wild animals in Africa south of the Sahara. It is caused by Ehrlichia ruminantium and transmitted by the tick Amblyomma hebraeum. The protocols currently used to detect heartwater take several days to complete. Here, we describe the development of a pCS20 quantitative real-time PCR TaqMan probe assay to detect E. ruminantium in livestock blood and ticks from the field. The assay is based on the conserved pCS20 gene region of E. ruminantium that contains two overlapping genes, rnc and ctaG [Collins, N.E., Liebenberg, J., De Villiers, E.P., Brayton, K.A., Louw, E., Pretorius, A., Faber, F.E., Van Heerden, H., Josemans, A., Van Kleef, M., Steyn, H.C., Van Strijp, M.F., Zweygarth, E., Jongejan, F., Maillard, J.C., Berthier, D., Botha, M., Joubert, F., Corton, C.H., Thomson, N.R., Allsopp, M.T., Allsopp, B.A., 2005. The genome of the heartwater agent Ehrlichia ruminantium contains multiple tandem repeats of actively variable copy number. PNAS 102, 838-843]. The pCS20 quantitative real-time PCR TaqMan probe was compared to the currently used pCS20 PCR and PCR/32P-probe test with regards to sensitivity, specificity and the ability to detect DNA in field samples and in blood from experimentally infected sheep. This investigation showed that the pCS20 quantitative real-time PCR TaqMan probe was the most sensitive assay detecting seven copies of DNA/mul of cell culture. All three assays, however, cross react with Ehrlichia canis and Ehrlichia chaffeensis. The pCS20 real-time PCR detected significantly more positive field samples. Both the PCR and pCS20 real-time PCR could only detect E. ruminantium parasites in the blood of experimentally infected sheep during the febrile reaction. The PCR/32P-probe assay, however, detected the parasite DNA 1 day before and during the febrile reaction. Thus, because this new quantitative pCS20 real-time PCR TaqMan probe assay was the most sensitive and can be performed within 2h it is an effective assay for epidemiological surveillance and monitoring of infected animals.

Extensive genetic recombination occurs in the field between different genotypes of Ehrlichia ruminantium

The intracellular bacterium Ehrlichia ruminantium is the causative agent of heartwater throughout sub-Saharan Africa, Madagascar, and some islands of the Caribbean. The disease is tick-borne and causes substantial livestock losses, threatening food security and productivity in both the commercial and small-scale farming sectors in endemic areas. Immunization by infection and treatment is currently practised in South Africa, and it is known that a variety of immunotypes of the organism occur in the field, and that cross-protection between them varies widely from total to minimal. Future vaccines may therefore need to incorporate components from different genotypes so it is essential to have information on the extent of genetic variation among isolates. To obtain this information we amplified and sequenced a panel of eight core function genes from 12 different cultured stocks originally isolated in different areas of Africa and the Caribbean. Phylogenetic trees inferred from the sequences yielded different branching orders for different genes, and the reason for this inconsistency appears to be that extensive recombination takes place between different genotypes in the field. It is possible that recombination occurs during the period when the organisms are extracellular within the tick, immediately after feeding and before intracellular infection is established, although detection of more than one genotype in DNA from single ticks is encountered infrequently. The results of the analysis show that the phylogenetic variation is greatest among the isolates of southern African origin, suggesting that this is the region where the parasite first evolved. It also appears likely that the Gardel genotype, isolated in the Caribbean, originally came from west central Africa, not from west Africa as had long been assumed.

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.

Differential transcription of the major antigenic protein 1 multigene family of Ehrlichia ruminantium in Amblyomma variegatum ticks

The rickettsial pathogen Ehrlichia ruminantium causes heartwater in ruminants and is transmitted by ticks of the genus Amblyomma. The map1 gene, encoding the major surface protein MAP1, is a member of a multigene family containing 16 paralogs. In order to investigate differential transcription of genes of the map1 multigene family in vivo in unfed and feeding ticks, RNA was extracted from midguts and salivary glands of E. ruminantium-infected adult female Amblyomma variegatum ticks and analysed by RT-PCR using MAP1 paralog-specific primers. In unfed ticks, only transcripts from the map1-1 gene were observed in midguts and no transcripts were detected in salivary glands. In feeding ticks, map1-1 transcripts were more abundant in midguts whereas high levels of map1 transcripts were observed in salivary glands. Our results show that differential transcription of genes of the E. ruminantium map1 cluster occurs in vivo in different tissues of infected ticks before and during transmission feeding, indicating that this multigene family may be involved in functions of biological relevance in different stages of the life cycle of E. ruminantium.

Restriction and expansion of Ehrlichia strain diversity

Ehrlichia are tick-borne gram negative, obligately intracellular bacteria. The 16S rRNA gene DNA sequences are highly conserved among strains of each Ehrlichia species. The 28-kDa/Map-1 outer membrane protein genes are highly diversified among strains of Ehrlichia chaffeensis and E. ruminantium, but are highly conserved among E. canis isolates. The diversity of the immunodominant proteins of E. chaffeensis and E. ruminantium in contrast with the conservation of the immunodominant proteins of E. canis suggests that E. chaffeensis and E. ruminantium face more host immune pressure than E. canis or that E. chaffeensis and E. ruminantium evolved earlier than E. canis and have diverged.

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.

An attenuated Ehrlichia ruminantium (Welgevonden stock) vaccine protects small ruminants against virulent heartwater challenge

Heartwater is a tick-borne disease of ruminants caused by the intracellular rickettsia Ehrlichia ruminantium. The only commercially available immunization procedure involves infecting animals with cryopreserved sheep blood containing virulent E. ruminantium organisms, followed by treatment with tetracyclines when fever develops. The virulent Welgevonden stock of E. ruminantium was attenuated by continuous propagation of the organisms in a canine macrophage-monocyte cell line (DH82), followed by re-adaptation to grow in a bovine endothelial cell line (BA 886). The material used for the present experiments consisted of the attenuated stock between passages 43 and 64 after re-adaptation. When inoculated into sheep or goats the attenuated organisms did not produce disease, and the only symptom observed was a rise in body temperature in most, but not all, animals. All sheep injected with 2 ml of culture suspension were subsequently found to be fully protected against a lethal needle challenge with the virulent homologous stock or with one of four different heterologous stocks (Ball 3, Gardel, Mara 87/7, Blaauwkrans). Titrations of elementary body suspensions showed that 2ml of a 1:10,000 dilution of culture suspension injected into sheep or goats was still sufficient to trigger an immune response which resisted a lethal needle challenge with the virulent Welgevonden stock. Adult Amblyomma hebraeum ticks, fed as nymphs on sheep immunized with DH82-derived organisms of passage 111, were able to transmit the attenuated stock to a naive sheep, which was found to be protected against a subsequent lethal homologous needle challenge.

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.

Comparative genomics of Rickettsia prowazekii Madrid E and Breinl strains

Rickettsia prowazekii, the causative agent of epidemic typhus, has been responsible for millions of human deaths. Madrid E is an attenuated strain of R. prowazekii, while Breinl is a virulent strain. The genomic DNA sequence of Madrid E has recently been published. To study the genomic variations between Madrid E (reference) and Breinl (test) DNAs, cohybridization experiments were performed on a DNA microarray containing all 834 protein-coding genes of Madrid E. Of the 834 genes assessed, 24 genes showed 1.5- to 2.0-fold increases in hybridization signals in Breinl DNA compared to Madrid E DNA, indicating the presence of genomic variations in approximately 3% of the total genes. Eighteen of these 24 genes are predicted to be involved in different functions. Southern blot analysis of five genes, virB4, ftsK, rfbE, lpxA, and rpoH, suggested the presence of an additional paralog(s) in Breinl, which might be related to the observed increase in hybridization signals. Studies by real-time reverse transcription-PCR revealed an increase in expression of the above-mentioned five genes and five other genes. In addition to the elevated hybridization signals of 24 genes observed in the Breinl strain, one gene (rp084) showed only 1/10 the hybridization signal of Madrid E. Further analysis of this gene by PCR and sequencing revealed a large deletion flanking the whole rp084 gene and part of the rp083 gene in the virulent Breinl strain. The results of this first rickettsial DNA microarray may provide some important information for the elucidation of pathogenic mechanisms of R. prowazekii.

Phylogenetic relationships among Ehrlichia ruminantium isolates

Ehrlichia ruminantium, the causative agent of heartwater, is a tick-borne pathogen infecting ruminants throughout sub-Saharan Africa and on some Caribbean islands. The most reliable test for E. ruminantium is PCR-based, but this gives positive results in some areas free of clinical heartwater and of the known Amblyomma spp. tick vectors. To investigate the molecular basis for this finding we have sequenced and carried out phylogenetic analysis of a range of genes from a number of E. ruminantium isolates. The genes include ribonuclease III and cytochrome c oxidase assembly protein genes (the pCS20 region), groESL, citrate synthase (gltA), and 16S ribosomal RNA. Relationships among major antigenic protein (map1) genes have been exhaustively investigated in a previous study that showed that the genes are variable in length, have non-synonymous mutations, and show no geographical specificity among isolates. The 16S sequences are highly conserved, except in the V1 loop region. The pCS20, groESL, and gltA genes show only single nucleotide polymorphisms (SNPs) dispersed throughout the sequenced regions. Phylogenetic analysis using pCS20 data differentiates the western African isolates into a single clade, which also includes a southern African isolate. All other southern African isolates and a Caribbean isolate fall into a further clade, which is subdivided into two groups. Sequence variation within this clade is greater than that within the western African clade, suggesting that E. ruminantium originated in southern Africa.

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.

Molecular diagnosis of theileriosis and heartwater in bovines in Africa

The advent of the polymerase chain reaction (PCR) coupled with the specificity of deoxyribocucleic acid (DNA)-DNA hybridization has led to the development of specific and sensitive molecular diagnostic tests to detect and characterize the organisms that cause theileriosis and heartwater. Theileriosis is a widespread disease of wild and domestic ruminants caused by apicomplexan parasites of the genus Theileria. Species-specific variations in small subunit ribosomal ribonucleic acid genes (SSUrRNA) have been used to develop probes that can distinguish between Theileria species such as T. parva, T. annulata, T. mutans, T. buffeli and T. taurotragi. Routine application of this test has led to the discovery of previously unknown species, such as Theileria sp. (buffalo) which is apparently apathogenic to both buffalo and cattle, and Theileria sp. (sable) which is pathogenic to sable and possibly also to roan antelope. In addition, characterization probes located in the internal transcribed spacer (ITS) can be used to distinguish between most isolates of the causative agents of East Coast fever (T. p. parva) and Corridor disease (T. p. lawrencei). Heartwater is an economically important disease of livestock and some wild ruminants, caused by the intracellular rickettsial parasite Ehrlichia (ex Cowdria) ruminantium. DNA probes used to detect and characterize E. ruminantium isolates include SSUrRNA (16S) probes, the pCS20 probe and map1 probes. A panel of eight 16S probes has been developed for the detection of E. ruminantium and related Ehrlichia species. There are probes for 5 different E. ruminantium genotypes, one which will detect all 5 of these genotypes, one to detect any Ehrlichia species other than E. ruminantium, and one for any Anaplasma species. The pCS20 probe is specific for E. ruminantium and is the most sensitive of the probes for E. ruminantium detection, but it is not able to distinguish among the different genotypes. The map1 gene has also been used for diagnosis, but the extensive polymorphism of this gene means that it is most useful for characterization of different genotypes of the parasite. Routine application of these tests has led to the discovery of new genotypes that are probably not E. ruminantium but are probably new species of Ehrlichia.

Novel Ehrlichia genotype detected in dogs in South Africa

DNA samples from dogs presenting with symptoms suggestive of canine ehrlichiosis, but with no morulae detected on blood smears, frequently failed to give a positive reaction with a North American Ehrlichia canis-specific PCR assay targeting the 16S rRNA gene. We suspected the presence of a pathogen genetically different from North American E. canis, and we performed experiments to test this hypothesis. DNA from one canine blood sample was subjected to PCR with primers designed to amplify Ehrlichia (Cowdria) ruminantium ruminantium 16S and map1 genes. Amplicon sequencing yielded 16S and map1 sequences which were more closely related to other E. ruminantium sequences than to those of any other Ehrlichia species. Fifty canine DNA samples were subjected to a PCR assay, previously found to be Cowdria-specific, which targets the pCS20 gene. Thirty-seven (74%) gave a positive signal, and 16 (32%) also gave visible amplicons after gel electrophoresis, suggesting that this E. ruminantium organism is common in the Pretoria-Johannesburg area. The organism has not been isolated in culture, so we cannot definitively state that it was responsible for the canine ehrlichiosis symptoms, although the occurrence of several similar cases suggests this to be so. Most importantly, we also do not yet know whether the organism is infective for, or causes heartwater in, ruminants.

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.

Genome size and genetic map of Cowdria ruminantium

Cowdria ruminantium is the cause of a serious tick-borne disease of domestic ruminants, known as heartwater or cowdriosis. The organism belongs to the tribe Ehrlichieae:, which contains obligate intracellular pathogens, causing several important animal and human diseases. Although a few C. ruminantium genes have been cloned and sequenced, very little is known about the size, gross structure and organization of the genome. This paper presents a complete physical map and a preliminary genetic map for C. ruminantium. Chromosomal C. ruminantium DNA was examined by PFGE and Southern hybridization. PFGE analysis revealed that C. ruminantium has a circular chromosome approximately 1576 kb in size. A physical map was derived by combining the results of PFGE analysis of DNA fragments resulting from digestion of the whole genome with KSP:I, RSR:II and SMA:I and Southern hybridization analysis with a series of gene probes and isolated macrorestriction fragments. A genetic map for C. ruminantium with a mean resolution of 290 kb was established, the first for a member of the Ehrlichieae: A total of nine genes or cloned C. ruminantium DNA fragments were mapped to specific KSP:I, RSR:II and SMA:I fragments, including the major antigenic protein gene, map-1.

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.

Cross-reactive epitope mimics in a fragmented-genome phage display library derived from the rickettsia, Cowdria ruminantium

BACKGROUND

Epitopes can be mapped by comparing immunoaffinity-selected peptides from fragmented-gene display libraries with the target gene. With larger libraries derived from unsequenced genomes, this is not possible. Spurious epitope mimics may be created by expressing DNA in a variety of meaningless reading frames and orientations.

OBJECTIVES

To determine empirically whether panning a large fragmented-genome phage display library with antibodies to MAP1, the major antigenic protein of the rickettsial parasite Cowdria ruminantium, would result in the selection of irrelevant, cross-reactive mimotopes.

STUDY DESIGN

A gene III phage library displaying peptides derived from C. ruminantium was constructed using cloned DNA from a bacteriophage lambda genomic library. After in vivo excision, plasmids were cleaved with PvuII followed by PCR. Genes with a PvuII site, including MAP1 were therefore not amplified. DNA was sonicated, partially digested with DNase and cloned into the display vector fUSE2. Affinity-purified MAP1 antibodies were used for panning. Peptides expressed by panned phages were tested for recognition in Western blot and ELISA. Oligonucleotides representing antigenic sequences were used to locate their encoding DNA sequences in the original lambda library. The phage display library was also panned with two monoclonal antibodies (Mabs) against bluetongue virus (BTV).

RESULTS

Five different peptide sequences were selected from the MAP1-deficient phage display library. None was identical to MAP1, but four peptides had regions that were similar, both to each other, and to the parasite protein. They produced strong signals in ELISA and Western blot. None could be located to any C. ruminantium open reading frame. Two BTV Mabs recognised a sequence similar to their authentic epitope.

CONCLUSION

Large genome-targeted phage display libraries may be sufficiently diverse to allow the selection of peptides that mimic actual antigenic determinants. This diversity may be exploited in the search for useful epitopes.

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.

Purification of Cowdria ruminantium organisms for use in genome analysis by pulsed-field gel electrophoresis

Cowdria ruminantium is an obligate intracellular rickettsial pathogen which is responsible for a tick-borne disease of domestic and wild ruminants called heartwater or cowdriosis. Although several genes have been cloned and partially sequenced, the genome size, gross structure, and organization of the C. ruminantium genome is unknown. Genome analysis of the organism has been hindered because it is difficult to obtain C. ruminantium DNA free from contaminating host cell DNA, and this probably accounts for the lack of genome size data for this organism. In this study we investigated several methods for purifying C. ruminantium from bovine cellular contaminants and organisms of a relatively high purity were obtained. These were used to prepare Cowdria DNA which was analyzed by pulsed-field gel electrophoresis (PFGE) and which revealed a genome approximately 1900 kbp in length plus an additional extra-chromosomal fragment migrating with an apparent size of 815 kbp. This is the first time that the genome size of C. ruminantium has been determined and the first demonstration of an extrachromosomal element.

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.

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.

In vitro cultivation of Anaplasma marginale in bovine erythrocytes co-cultured with endothelial cells

Primary cultures of Anaplasma marginale infected erythrocytes were used to determine conditions for in vitro cultivation of the rickettsia. The infected erythrocytes that were maintained by regular addition of Glasgow's MEM with fetal calf serum and uninfected erythrocytes showed a 1-5% increase in percent infected erythrocytes on the evaluation of Giemsa stained smears. This increase in parasitemia resulted in up to 70% change in the number of infected erythrocytes. Co-culture of the infected erythrocytes with endothelial cell monolayers allowed for longer maintenance with the parasitemia ranging from 5-13% through four passages over 16 weeks. Examination of cultures using transmission electron microscopy showed initial bodies within the erythrocytes at 10 days after the initial passage of the primary culture. The endothelial cell monolayers in the co-cultures contained multiple initial bodies. We have demonstrated that A. marginale can be grown for a limited number of passages in the co-culture system, which will facilitate the development of a continuous culture of the organism.

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.

Isolation of the causative agent of heartwater (Cowdria ruminantium) from three Amblyomma species in eight Districts of Kenya

Thirteen isolates of Cowdria ruminantium were made from eight different Districts of Kenya by four different isolation methods. Feeding adult Amblyomma species ticks derived from nymphs collected in the field and the inoculation of homogenates prepared from adult field ticks had the highest success rate. The reattachment of adult ticks collected in the field was successful on only one of five attempts, and the subinoculation of blood from suspected heartwater carriers was unsuccessful. Seven of the isolates were derived from A variegatum ticks, four from A gemma, one from A lepidum and one from a mixed pool of the last two species. This is the first report of the isolation of C ruminantium from A gemma ticks, and the first report of its transtadial transmission from nymphal to adult A gemma.

Amino acid and protein depletion in medium of cell cultures infected with Cowdria ruminantium

Cowdria ruminantium (Rickettsiales) causes heartwater in ruminants of Africa, and some islands off Africa and in the Caribbean Sea. The in vitro culture method for the organism devised in 1985, which provided for the first time a means for production of adequate quantities of live organisms and their products, is erratic and requires improvement. We studied depletion of amino acids (AAs) and major proteins in culture medium taken daily from infected and uninfected ovine and bovine vascular endothelial cell cultures. AAs of these samples were analyzed by Pico Tag reversed phase HPLC precolumn derivatization, and major proteins determined by capillary electrophoresis using a 57 cm x 75 microns fused silica tube at high pH. In both ovine and bovine cell cultures, significant depletion of arginine and glutamine occurred over a 5-day observation period regardless of whether they were infected or uninfected. This indicates that supplementation of nutrient media with these AAs might improve conditions for growth of the organism. Both AAs are essential for survival of cultured cells, and probably for the rickettsia (although the metabolism of C. ruminantium is poorly understood). Concentrations of several AAs increased in infected cultures, implying de novo synthesis and/or proteolysis on the part of the organism. In fact, several protein fractions did decrease in culture medium throughout the course of infection, while increasing or remaining unchanged in uninfected control cultures. Proteolytic activity by C. ruminantium may be essential for nitrogen metabolism by the organism. It is suggested that studies such as these will facilitate the development of a specific medium for optimal in vitro growth of the heartwater organism, and may also lead to an understanding of the metabolic stratagem of C. ruminantium. This knowledge, in turn, could reveal the mechanism for pathogenesis of heartwater, with implications for control.

Purification and characterization of the Pac1 ribonuclease of Schizosaccharomyces pombe

The pac1+ gene of the fission yeast Schizosaccharomyces pombe is essential for viability and its overexpression induces sterility and suppresses mutations in the pat1+ and snm1+ genes. The pac1+ gene encodes a protein that is structurally similar to RNase III from Escherichia coli, but its normal function is unknown. We report here the purification and characterization of the Pac1 protein after overexpression in E. coli. The purified protein is a highly active, double-strand-specific endoribonuclease that converts long double-stranded RNAs into short oligonucleotides and also cleaves a small hairpin RNA substrate. The Pac1 RNase is inhibited by a variety of double- and single-stranded polynucleotides, but polycytidylic acid greatly enhances activity and also promotes cleavage specificity. The Pac1 RNase produces 5'-phosphate termini and requires Mg2+; Mn2+ supports activity but causes a loss of cleavage specificity. Optimal activity was obtained at pH 8.5, at low ionic strength, in the presence of a reducing agent. The enzyme is relatively insensitive to N-ethylmaleimide but is strongly inhibited by ethidium bromide and vanadyl ribonucleoside complexes. The properties of the Pac1 RNase support the hypothesis that it is a eukaryotic homolog of RNase III.

Colostrum from dams living in a heartwater-endemic area influences calfhood immunity to Cowdria ruminantium

Two studies were carried out to determine whether colostrum from dams living in a heartwater-endemic area has an influence on calfhood immunity to Cowdria ruminantium infection. The initial study was conducted using Friesian calves originating from a heartwater-free herd. Experimental groups consisted of calves receiving colostrum from dams living in a heartwater-endemic area and known to be exposed to C. ruminantium (as determined by the indirect fluorescent antibody test) and calves receiving colostrum from dams located in a heartwater-free area. All calves were challenged at 3 days of age with a homologous C. ruminantium blood stabilate originating from the same area as the endemic colostrum. A significant difference in both the intensity of clinical reactions and percentage of fatal heartwater cases was noted between the two groups of calves. Three of the five calves receiving the colostrum free of C. ruminantium-specific antibodies succumbed to challenge, whereas none of the five calves receiving the colostrum from the heartwater-endemic area showed any clinical reactions. A second similar study was carried out using 12 mixed breed calves born in a heartwater-endemic area of Zimbabwe. The results from this study supported the findings of the first. The significant role of colostrum in calfhood immunity to C. ruminantium infection is discussed in relation to the epidemiology and control of heartwater.

Laboratory reared Amblyomma hebraeum and Amblyomma variegatum ticks differ in their susceptibility to infection with Cowdria ruminantium

The susceptibility of laboratory reared Zimbabwean Amblyomma hebraeum and A. variegatum ticks to infection with geographically distinct Cowdria ruminantium strains was investigated by feeding both species simultaneously on individual sheep infected with one of the four strains (Crystal Springs [Zimbabwe], Ball 3 [South Africa], Gardel [Guadeloupe] and Nigeria [Nigeria]). A. hebraeum ticks demonstrated a high susceptibility to infection with all four C. ruminantium strains. In comparison, A. variegatum were less susceptible to infection with the Crystal Springs and Ball 3 strains (P < 0.001), but showed a similar susceptibility to the Gardel and Nigeria strains. The differences in susceptibility of A. variegatum to infection with the four strains of C. ruminantium correlated with the origin of these strains. The consistently higher susceptibility of A. hebraeum ticks to infection with geographically different C. ruminantium strains may be one explanation for the observation that heartwater is a more serious problem where A. hebraeum is the vector of the disease.

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.

Nucleic acid probes as a diagnostic method for tick-borne hemoparasites of veterinary importance

An increased number of articles on the use of nucleic acid-based hybridization techniques for diagnostic purposes have been recently published. This article reviews nucleic acid-based hybridization as an assay to detect hemoparasite infections of economic relevance in veterinary medicine. By using recombinant DNA techniques, selected clones containing inserts of Anaplasma, Babesia, Cowdria or Theileria genomic DNA sequences have been obtained, and they are now available to be utilized as specific, highly sensitive DNA or RNA probes to detect the presence of the hemoparasite DNA in an infected animal. Either in an isotopic or non-isotopic detection system, probes have allowed scientists to test for--originally in samples collected from experimentally infected animals and later in samples collected in the field--the presence of hemoparasites during the prepatent, patent, convalescent, and chronic periods of the infection in the host. Nucleic acid probes have given researchers the opportunity to carry out genomic analysis of parasite DNA to differentiate hemoparasite species and to identify genetically distinct populations among and within isolates, strains and clonal populations. Prevalence of parasite infection in the tick vector can now be accomplished more specifically with the nucleic acid probes. Lately, with the advent of the polymerase chain reaction technique, small numbers of hemoparasites can be positively identified in the vertebrate host and tick vector. These techniques can be used to assess the veterinary epidemiological situation in a particular geographical region for the planning of control measures.

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

The sensitivities of a PCR assay and a DNA probe assay were compared for the detection of Cowdria ruminantium in Amblyomma ticks that were fed on C. ruminantium-infected, clinically reacting, and recovered carrier animals. The PCR assay and DNA probe detected infection in 86.0 and 37.0%, respectively, of 100 ticks fed on a febrile animal. In 75 ticks fed on carrier animals, PCR and the DNA probe detected infection in 28.0 and 1.33% of ticks, respectively. This demonstrates that the DNA probe has poor sensitivity for the detection of low levels of infection in ticks and that PCR is necessary for this purpose. The PCR assay had a detection limit of between 1 and 10 C. ruminantium organisms and did not amplify DNA from Ehrlichia canis, which is phylogenetically closely related to C. ruminantium, Theileria parva, or uninfected Amblyomma hebraeum or A. variegatum. PCR detected infection in A. hebraeum and A. variegatum adult ticks infected with one of six geographically different C. ruminantium strains. Amplification was also possible from desiccated ticks and ticks fixed in 70% ethanol, 10% buffered formalin, or 2% glutaraldehyde. The PCR assay supersedes the DNA probe and older detection methods for the detection of C. ruminantium in ticks, particularly those fed on carrier animals, and is suitable for both prospective and retrospective studies which require accurate detection of C. ruminantium in individual ticks. Application of the PCR assay should significantly improve the understanding of heartwater epidemiology, particularly through the determination of field tick infection rates.

Molecular cloning, sequence analysis, and expression of the gene encoding the immunodominant 32-kilodalton protein of Cowdria ruminantium

Cowdria ruminatium, the causative agent of heartwater disease, expresses an immunodominant and conserved 32-kilodalton protein (MAP1; formerly called Cr32), which is currently in use for serodiagnosis of the disease. The gene encoding this protein, designated map1, was detected, cloned, and characterized. The gene is conserved between four different stocks of C. ruminantium originating from Senegal, Sudan, South Africa, and Zimbabwe. Homology searches revealed MAP1 to be homologous to the Anaplasma marginale surface protein MSP4, a potential protective antigen. The MAP1 protein, expressed in Escherichia coli fused with glutathione S-transferase, is specifically recognized by sera from animals infected with seven different stocks of C. ruminantium.

An immunoblotting diagnostic assay for heartwater based on the immunodominant 32-kilodalton protein of Cowdria ruminantium detects false positives in field sera

Heartwater, a major constraint to improved livestock production in Zimbabwe, threatens to invade areas which have been previously unaffected. To monitor its spread in Zimbabwe, an immunoblotting diagnostic assay based on the responses of animals to the immunodominant, conserved 32-kDa protein of Cowdria ruminantium was evaluated. In this assay, no false reactions were detected with sera known to be positive and negative, but sera from some cattle, sheep, and goats from heartwater-free areas of Zimbabwe reacted strongly with the 32-kDa protein, suggesting that either these animals had previous exposure to heartwater or they were false positives. To investigate the possibility of previous exposure to heartwater, 11 immunoblot-positive and 6 immunoblot-negative sheep from heartwater-free areas of Zimbabwe were compared regarding their susceptibilities to challenge with C. ruminantium. Prior to challenge, C. ruminantium could not be detected in any sheep by transmission to Amblyomma hebraeum ticks or by the polymerase chain reaction (PCR) conducted with plasma samples. All sheep were equally susceptible to the challenge, and infection was confirmed by brain biopsy, necropsy, PCR, and transmission of C. ruminantium to ticks. Our data suggest that the immunoblot-positive reactions of sera from heartwater-free areas were due not to previous C. ruminantium infection but rather to antigenic cross-reactivity between C. ruminantium and another agent(s) such as Ehrlichia species. In conclusion, the immunodominant 32-kDa protein is not antigenically specific to C. ruminantium and its use in serological diagnosis of heartwater requires reevaluation.

Progress in research on tick-borne diseases: theileriosis and heartwater

The rapid population growth in subsaharan Africa necessitates a great increase in animal production in the more humid zones. Vector-borne diseases occurring in these zones will assume more importance, but are difficult to control. They include theileriosis and heartwater. Recent developments in research on these diseases are presented. Indigenous animal populations in endemic areas, subjected to natural selection, are far less susceptible than exotic stock. Heartwater, caused by the rickettsia Cowdria ruminantium, transmitted by Amblyomma ticks, causes high mortality in exotic ruminants. It has received much attention in recent years, partly because the disease has been introduced from Africa into the Caribbean and threatens the American mainland. Since the recent success of in vitro culture, much progress in research has been made, but so far prevention still relies mainly on acaricidal tick control; an infection and treatment method is used on a limited scale. Antigenic diversity is a complication for immunization procedures. Theileria parva (East Coast fever, Corridor disease and January disease) and T.annulata (Mediterranean or tropical theileriosis) are the most pathogenic of the 6 species of this protozoan genus that infect cattle. Great progress has been made in recent years in knowledge on the immunology, the epidemiology, the taxonomy and the chemotherapy of theileriosis. Intensive acaricidal tick control can now be supplemented by an attenuated schizont vaccine against T.annulata, while immunization against East Coast fever is carried out on a limited scale using virulent sporozoite infection and treatment. Research on recombinant vaccines is promising. Antigenic diversity in T.parva is a serious complication.

Detection of Cowdria ruminantium by means of a DNA probe, pCS20 in infected bont ticks, Amblyomma hebraeum, the major vector of heartwater in southern Africa

A DNA probe, pCS20, previously described for use in detection of Cowdria ruminantium infections in Amblyomma variegatum (the principal vector of heartwater) hybridized with C. ruminantium DNA in organs of laboratory-infected A. hebraeum adult ticks (the major southern African vector of heartwater). The probe hybridized with C. ruminantium DNA in 46/49 midguts from male ticks and 26/29 from females, thus indicating infection. Corresponding salivary glands were less heavily infected, but infections were more numerous in glands from males. Infection in ticks was confirmed by transmission of the disease to susceptible goats. The probe did not hybridize with DNA from uninfected ticks or with DNA from a spotted fever group rickettsia commonly associated with A. hebraeum in Zimbabwe. The C. ruminantium specific pCS20 DNA probe can be applied to determine accurately the infection rates in the two major vectors of heartwater and the risk of exposure of ruminants in endemic areas.