Anaplasma marginale superinfection attributable to pathogen strains with distinct genomic backgrounds

Transovarial Transmission of Anaplasma marginale in Rhipicephalus (Boophilus) microplus Ticks Results in a Bottleneck for Strain Diversity

Anaplasma marginale is an obligate intraerythrocytic bacterium of bovines, responsible for large economic losses worldwide. It is mainly transmitted by Rhipicephalus (Boophilus) microplus ticks and, despite mounting evidence suggesting transovarial transmission, the occurrence of this phenomenon remains controversial. We evaluated the vector competence of R. microplus larvae vertically infected with A. marginale to transmit the bacterium to a naïve bovine. A subgroup of engorged female ticks collected from an A. marginale-positive animal was dissected and the presence of the pathogen in its tissues was confirmed. A second subgroup of ticks was placed under controlled conditions for oviposition. After confirming the presence of A. marginale in the hatched larvae, an experimental infestation assay was conducted. Larvae were placed on an A. marginale-free splenectomized calf. The bacterium was detected in the experimentally infested bovine 22 days post-infestation. We analyzed the A. marginale diversity throughout the transmission cycle using the molecular marker MSP1a. Different genotypes were detected in the mammalian and arthropod hosts showing a reduction of strain diversity along the transmission process. Our results demonstrate the vertical transmission of A. marginale from R. microplus females to its larvae, their vector competence to transmit the pathogen, and a bottleneck in A. marginale strain diversity.

Both co-infection and superinfection drive complex Anaplasma marginale strain structure in a natural transmission setting

Vector-borne pathogens commonly establish multi-strain infections, also called complex infections. How complex infections are established, either prior to or after the development of an adaptive immune response, termed co-infection or superinfection, respectively, has broad implications for the maintenance of genetic diversity, pathogen phenotype, epidemiology, and disease control strategies. Anaplasma marginale, a genetically diverse, obligate, intracellular tick-borne bacterial pathogen of cattle commonly establishes complex infections, particularly in regions with high transmission rates. Both co-infection and superinfection can be established experimentally, however it is unknown how complex infections develop in a natural transmission setting. To address this question, we introduced naïve animals into a herd in southern Ghana with high infection prevalence and high transmission pressure and tracked strain acquisition of A. marginale through time using multi-locus sequence typing. As expected, genetic diversity among strains was high and 97% of animals in the herd harboured multiple strains. All the introduced, naïve animals became infected, and three to four strains were typically detected in an individual animal prior to seroconversion, while one to two new strains were detected in an individual animal following seroconversion. On average, the number of strains acquired via superinfection was 16% less than those acquired via co-infection. Thus, while complex infections develop via both co-infection and superinfection, co-infection predominates in this setting. These findings have broad implications for the development of control strategies in high transmission settings.

Sequence and immunologic conservation of Anaplasma marginale OmpA within strains from Ghana as compared to the predominant OmpA variant

A primary challenge in developing effective vaccines against obligate, intracellular, bacterial tick-borne pathogens that establish persistent infection is the identification of antigens that cross protect against multiple strains. In the case of Anaplasma marginale, the most prevalent tick-borne pathogen of cattle found worldwide, OmpA is an adhesin and thus a promising vaccine candidate. We sequenced ompA from cattle throughout Ghana naturally infected with A. marginale in order to determine the degree of variation in this gene in an area of suspected high genetic diversity. We compared the Ghanaian sequences with those available from N. America, Mexico, Australia and Puerto Rico. When considering only amino acid changes, three unique Ghanaian OmpA variants were identified. In comparison, strains from all other geographic regions, except one, shared a single OmpA variant, Variant 1, which differed from the Ghanaian variants. Next, using recombinant OmpA based on Variant 1, we determined that amino acid differences in OmpA in Ghanaian cattle as compared to OmpA Variant 1 did not alter the binding capacity of antibody directed against OmpA Variant 1, supporting the value of OmpA as a highly conserved vaccine candidate.

Segmental Variation in a Duplicated msp2 Pseudogene Generates Anaplasma marginale Antigenic Variants

Anaplasma marginale is a prototypical highly antigenically variant bacterial pathogen dependent on the sequential generation of major surface protein 2 (Msp2) outer membrane variants to establish persistent infection. Msp2 is encoded by a single expression site, and diversity is achieved by gene conversion of chromosomally encoded msp2 pseudogenes. Analysis of the full complement of msp2 pseudogenes in the St. Maries strain revealed identical sequences in different loci. The Florida strain shared the same locus structure, but in the loci where the St. Maries strain had two identical pseudogenes, the Florida strain had one whose sequence was identical to the St. Maries sequences, while the sequence of the second pseudogene differed. Consequently, we hypothesized that the msp2 pseudogene repertoire arose via gene duplication, allowing structural variation to occur in one copy but the utility of the other to be retained. Using comparative genomics, we first established that duplication of msp2 pseudogenes is common among A. marginale strains: all seven examined strains had at least one duplicate pair in which either the genes in the pair were maintained as identical copies or the genes contained segmental changes. We then demonstrated that a minimal segmental change in a duplicated pseudogene locus is sufficient for immune escape from the broad antibody response generated in a natural host, as is a completely divergent pseudogene sequence in an otherwise conserved locus. The results support a model in which a locus first duplicates, resulting in a second identical copy, and then progressively incorporates changes to generate an msp2 repertoire capable of generating sufficient antigenic variants to escape immunity and establish persistent infection.

Co-infections with multiple genotypes of Anaplasma marginale in cattle indicate pathogen diversity

BACKGROUND

Only a few studies have examined the presence of Anaplasma marginale and Anaplasma centrale in South Africa, and no studies have comprehensively examined these species across the whole country. To undertake this country-wide study we adapted a duplex quantitative real-time PCR (qPCR) assay for use in South Africa but found that one of the genes on which the assay was based was variable. Therefore, we sequenced a variety of field samples and tested the assay on the variants detected. We used the assay to screen 517 cattle samples sourced from all nine provinces of South Africa, and subsequently examined A. marginale positive samples for msp1α genotype to gauge strain diversity.

RESULTS

Although the A. marginale msp1β gene is variable, the qPCR functions at an acceptable efficiency. The A. centrale groEL gene was not variable within the qPCR assay region. Of the cattle samples screened using the assay, 57% and 17% were found to be positive for A. marginale and A. centrale, respectively. Approximately 15% of the cattle were co-infected. Msp1α genotyping revealed 36 novel repeat sequences. Together with data from previous studies, we analysed the Msp1a repeats from South Africa where a total of 99 repeats have been described that can be attributed to 190 msp1α genotypes. While 22% of these repeats are also found in other countries, only two South African genotypes are also found in other countries; otherwise, the genotypes are unique to South Africa.

CONCLUSIONS

Anaplasma marginale was prevalent in the Western Cape, KwaZulu-Natal and Mpumalanga and absent in the Northern Cape. Anaplasma centrale was prevalent in the Western Cape and KwaZulu-Natal and absent in the Northern Cape and Eastern Cape. None of the cattle in the study were known to be vaccinated with A. centrale, so finding positive cattle indicates that this organism appears to be naturally circulating in cattle. A diverse population of A. marginale strains are found in South Africa, with some msp1α genotypes widely distributed across the country, and others appearing only once in one province. This diversity should be taken into account in future vaccine development studies.

Insight into the genetic diversity of Anaplasma marginale in cattle from ten provinces of China

BACKGROUND

Anaplasma marginale is an important tick-transmitted rickettsial pathogen of cattle, with worldwide distribution and an important economic impact. The genetic diversity of A. marginale strains has been extensively characterized in different geographical regions throughout the world, while information is limited on studies in China. This study was carried out to determine the prevalence and genetic diversity of A. marginale strains in cattle from ten provinces of China.

METHODS

A total of 557 blood samples from cattle were collected and screened for the occurrence of A. marginale by PCR based on the msp4 gene. The partial msp1a gene containing tandem repeat sequences was further amplified from msp4 positive samples. The Msp1a amino acid repeats were identified, and genetic variation of A. marginale strains was characterized based on the variation in the repeated portion of Msp1a.

RESULTS

Our results showed that 31.6% of 557 cattle were positive for A. marginale. The infection rates of A. marginale varied considerably from 0 to 96.9% in different sampling regions. Sequence analysis revealed that two msp4 sequence variants of A. marginale exist in cattle. One hundred and three msp1a sequences were obtained and permitted to identify 42 Msp1a tandem repeats, 21 of which were not previously published for A. marginale. Moreover, 61 A. marginale genotypes were identified based on the structure of Msp1a tandem repeats.

CONCLUSIONS

Anaplasma marginale is widely distributed in China and a high prevalence of infection was observed in cattle. The geographical strains of A. marginale were molecularly characterized based on the structure of Msp1a tandem repeats. Forty-two Msp1a tandem repeats and 61 genotypes of A. marginale were identified. This study, for the first time, revealed the genetic diversity of A. marginale strains in cattle in China.

Antigenic Variation in Bacterial Pathogens

Antigenic variation is a strategy used by a broad diversity of microbial pathogens to persist within the mammalian host. Whereas viruses make use of a minimal proofreading capacity combined with large amounts of progeny to use random mutation for variant generation, antigenically variant bacteria have evolved mechanisms which use a stable genome, which aids in protecting the fitness of the progeny. Here, three well-characterized and highly antigenically variant bacterial pathogens are discussed: Anaplasma, Borrelia, and Neisseria. These three pathogens display a variety of mechanisms used to create the structural and antigenic variation needed for immune escape and long-term persistence. Intrahost antigenic variation is the focus; however, the role of these immune escape mechanisms at the population level is also presented.

Superinfection Exclusion of the Ruminant Pathogen Anaplasma marginale in Its Tick Vector Is Dependent on the Time between Exposures to the Strains

The remarkable genetic diversity of vector-borne pathogens allows for the establishment of superinfection in the mammalian host. To have a long-term impact on population strain structure, the introduced strains must also be transmitted by a vector population that has been exposed to the existing primary strain. The sequential exposure of the vector to multiple strains frequently prevents establishment of the second strain, a phenomenon termed superinfection exclusion. As a consequence, superinfection exclusion may greatly limit genetic diversity in the host population, which is difficult to reconcile with the high degree of genetic diversity maintained among vector-borne pathogens. Using Anaplasma marginale, a tick-borne bacterial pathogen of ruminants, we hypothesized that superinfection exclusion is temporally dependent and that longer intervals between strain exposures allow successful acquisition and transmission of a superinfecting strain. To test this hypothesis, we sequentially exposed Dermacentor andersoni ticks to two readily tick-transmissible strains of A. marginale The tick feedings were either immediately sequential or 28 days apart. Ticks were allowed to transmission feed and were individually assessed to determine if they were infected with one or both strains. The second strain was excluded from the tick when the exposure interval was brief but not when it was prolonged. Midguts and salivary glands of individual ticks were superinfected and transmission of both strains occurred only when the exposure interval was prolonged. These findings indicate that superinfection exclusion is temporally dependent, which helps to account for the differences in pathogen strain structure in tropical compared to temperate regions.

IMPORTANCE

Many vector-borne pathogens have marked genetic diversity, which influences pathogen traits such as transmissibility and virulence. The most successful strains are those that are preferentially transmitted by the vector. However, the factors that determine successful transmission of a particular strain are unknown. In the case of intracellular, bacterial, tick-borne pathogens, one potential factor is superinfection exclusion, in which colonization of ticks by the first strain of a pathogen it encounters prevents the transmission of a second strain. Using A. marginale, the most prevalent tick-borne pathogen of cattle worldwide, and its natural tick vector, we determined that superinfection exclusion occurs when the time between exposures to two strains is brief but not when it is prolonged. These findings suggest that superinfection exclusion may influence strain transmission in temperate regions, where tick activity is limited by season, but not in tropical regions, where ticks are active for long periods.

Anaplasma marginale and Anaplasma phagocytophilum: Rickettsiales pathogens of veterinary and public health significance

Anaplasma marginale and Anaplasma phagocytophilum are the most important tick-borne bacteria of veterinary and public health significance in the family Anaplasmataceae. The objective of current review is to provide knowledge on ecology and epidemiology of A. phagocytophilum and compare major similarities and differences of A. marginale and A. phagocytophilum. Bovine anaplasmosis is globally distributed tick-borne disease of livestock with great economic importance in cattle industry. A. phagocytophilum, a cosmopolitan zoonotic tick transmitted pathogen of wide mammalian hosts. The infection in domestic animals is generally referred as tick-borne fever. Concurrent infections exist in ticks, domestic and wild animals in same geographic area. All age groups are susceptible, but the prevalence increases with age. Movement of susceptible domestic animals from tick free non-endemic regions to disease endemic regions is the major risk factor of bovine anaplasmosis and tick-borne fever. Recreational activities or any other high-risk tick exposure habits as well as blood transfusion are important risk factors of human granulocytic anaplasmosis. After infection, individuals remain life-long carriers. Clinical anaplasmosis is usually diagnosed upon examination of stained blood smears. Generally, detection of serum antibodies followed by molecular diagnosis is usually recommended. There are problems of sensitivity and cross-reactivity with both the Anaplasma species during serological tests. Tetracyclines are the drugs of choice for treatment and elimination of anaplasmosis in animals and humans. Universal vaccine is not available for either A. marginale or A. phagocytophilum, effective against geographically diverse strains. Major control measures for bovine anaplasmosis and tick-borne fever include rearing of tick-resistant breeds, endemic stability, breeding Anaplasma-free herds, identification of regional vectors, domestic/wild reservoirs and control, habitat modification, biological control, chemotherapy, and vaccinations (anaplasmosis and/or tick vaccination). Minimizing the tick exposure activities, identification and control of reservoirs are important control measures for human granulocytic anaplasmosis.

Primary Structural Variation in Anaplasma marginale Msp2 Efficiently Generates Immune Escape Variants

Antigenic variation allows microbial pathogens to evade immune clearance and establish persistent infection. Anaplasma marginale utilizes gene conversion of a repertoire of silent msp2 alleles into a single active expression site to encode unique Msp2 variants. As the genomic complement of msp2 alleles alone is insufficient to generate the number of variants required for persistence, A. marginale uses segmental gene conversion, in which oligonucleotide segments from multiple alleles are recombined into the expression site to generate a novel msp2 mosaic not represented elsewhere in the genome. Whether these segmental changes are sufficient to evade a broad antibody response is unknown. We addressed this question by identifying Msp2 variants that differed in primary structure within the immunogenic hypervariable region microdomains and tested whether they represented true antigenic variants. The minimal primary structural difference between variants was a single amino acid resulting from a codon insertion, and overall, the amino acid identity among paired microdomains ranged from 18 to 92%. Collectively, 89% of the expressed structural variants were also antigenic variants across all biological replicates, independent of a specific host major histocompatibility complex haplotype. Biological relevance is supported by the following: (i) all structural variants were expressed during infection of a natural host, (ii) the structural variation observed in the microdomains corresponded to the mean length of variants generated by segmental gene conversion, and (iii) antigenic variants were identified using a broad antibody response that developed during infection of a natural host. The findings demonstrate that segmental gene conversion efficiently generates Msp2 antigenic variants.

Association of Anaplasma marginale strain superinfection with infection prevalence within tropical regions

Strain superinfection occurs when a second strain infects a host already infected with and having mounted an immune response to a primary strain. The incidence of superinfection with Anaplasma marginale, a tick-borne rickettsial pathogen of domestic and wild ruminants, has been shown to be higher in tropical versus temperate regions. This has been attributed to the higher prevalence of infection, with consequent immunity against primary strains and thus greater selective pressure for superinfection with antigenically distinct strains. However an alternative explanation would be the differences in the transmitting vector, Dermacentor andersoni in the studied temperate regions and Rhipicephalus microplus in the studied tropical regions. To address this question, we examined two tropical populations sharing the same vector, R. microplus, but with significantly different infection prevalence. Using two separate markers, msp1α (one allele per genome) and msp2 (multiple alleles per genome), there were higher levels of multiple strain infections in the high infection prevalence as compared to the low prevalence population. The association of higher strain diversity with infection prevalence supports the hypothesis that high levels of infection prevalence and consequent population immunity is the predominant driver of strain superinfection.