Extensive genetic diversity of Anaplasma bovis in ruminants in Xi'an, China

Genomic characters of Anaplasma bovis and genetic diversity in China

The emergence of Anaplasma bovis or A. bovis-like infection in humans from China and the United States of America has raised concern about the public health importance of this pathogen. Although A. bovis has been detected in a wide range of ticks and mammals in the world, no genome of the pathogen is available up to now, which has prohibited us from better understanding the genetic basis for its pathogenicity. Here we describe an A. bovis genome from metagenomic sequencing of an infected goat in China. Anaplasma bovis had the smallest genome of the genus Anaplasma, and relatively lower GC content. Phylogenetic analysis of single-copy orthologue sequence showed that A. bovis was closely related to A. platys and A. phagocytophilum, but relatively far from intraerythrocytic Anaplasma species. Anaplasma bovis had 116 unique orthogroups and lacked 51 orthogroups in comparison to other Anaplasma species. The virulence factors of A. bovis were significantly less than those of A. phagocytophilum, suggesting less pathogenicity of A. bovis. When tested by specific PCR assays, A. bovis was detected in 23 of 29 goats, with an infection rate up to 79.3% (95% CI: 64.6% ∼94.1%). The phylogenetic analyses based on partial 16S rRNA, gltA and groEL genes indicated that A. bovis had high genetic diversity. The findings of this study lay a foundation for further understanding of the biological characteristics and genetic diversity of A. bovis, and will facilitate the formulation of prevention and control strategies.

Molecular Epidemiology of Ticks and Tick-Borne Pathogens in the Ta-Pa Mountain Area of Chongqing, China

To validate the prevalence and biodiversity of ticks and tick-borne pathogens in Chongqing, a total of 601 ticks were collected from dogs, cattle, and goats within the Ta-pa Mountain range in Chongqing, China. Five distinct tick species were identified, including Ixodes ovatus (1.66%, 10/601), I. acutitarsus (0.50%, 3/601), Haemaphysalis flava (10.32%, 62/601), Ha. hystricis (9.82%, 59/601), and Ha. longicornis (77.70%, 467/601). A suit of semi-nest PCR and nest PCR primers were custom-synthesized for the detection of tick-borne pathogens. The analysis yielded positive results for 7.15% Rickettsia (Candidatus R. principis, R. japonica, and R. raoultii), 3.49% Anaplasma (A. bovis and A. capra), 1.16% Ehrlichia, 1.83% Coxiella burnetii, and 3.49% protozoa (Theileria. capreoli, T. orientalis, T. luwenshuni, and Babesia sp.) in ticks. Notably, Ca. R. principis was identified for the first time in I. ovatus and Ha. longicornis. These findings underscore the significant prevalence and diversity of ticks and their associated pathogens within the Chongqing Ta-pa Mountain region. This study accordingly provides an extensive dataset that contributes to the epidemiological understanding and disease prevention strategies for tick-borne illnesses in the local area.

Molecular detection of Anaplasma bovis, Candidatus Anaplasma boleense and Rickettsia spp. in ticks infesting small ruminants

Anaplasma spp. and Rickettsia spp. are intracellular vector-borne pathogens and harbored by a wide range of ticks and vertebrate hosts. Aim of this study was to molecularly characterize Anaplasma spp. and Rickettsia spp. in different ticks collected from livestock hosts in nine districts of Khyber Pakhtunkhwa (KP), Pakistan. In total, 862 ticks were collected from cattle, goats and sheep. Highest tick's infestation was observed on cattle 56.14% (32/57), followed by goats 45.45% (40/88), and sheep 42.05% (45/107). Rhipicephalus microplus (305/862, 35.38%) was predominant species, followed by Haemaphysalis sulcata (243/862, 28.19%), Hyalomma anatolicum (133/862, 15.42%), Haemaphysalis bispinosa (120/862, 13.92%), and Hyalomma kumari (61/862, 7.07%). A subset of 135 ticks were screened for Anaplasma spp. and Rickettsia spp. based on the amplification of partial 16 S rDNA and outer-membrane protein A (ompA) fragments, respectively. In total, 16 ticks (11.85%) were positive for Anaplasma spp. and Rickettsia spp. Obtained 16 S rDNA sequences for Anaplasma spp. detected in Ha. bispinosa and Ha. sulcata showed 99.98% identity with Anaplasma bovis, while other detected in Rh. microplus showed 99.84% identity with Candidatus Anaplasma boleense. Similarly, detected ompA sequence in Ha. sulcata showed 100% identity with Rickettsia sp. and 97.93% with Rickettsia slovaca, and another sequence detected in Rh. microplus showed 100% identity with Candidatus Rickettsia shennongii. In phylogenetic trees, these sequences clustered with corresponding species from Pakistan, China, Turkey, South Korea, South Africa, and Herzegovina. This is the first study reporting detection of A. bovis in Ha. bispinosa and Ha. sulcata, Ca. A. boleense in Rh. microplus collected from goats, and R. slovaca-like in Ha. sulcata. Our results enforce the need for regular surveillance of Rickettsiales in hard ticks infesting livestock in the region.

Great genetic diversity of vector-borne bacteria and protozoan in wild rodents from Guangxi, China

BACKGROUND

Rodents are recognized as the hosts of many vector-borne bacteria and protozoan parasites and play an important role in their transmission and maintenance. Intensive studies have focused on their infections in vectors, especially in ticks, however, vector-borne bacterial and protozoan infections in rodents are poorly understood although human cases presenting with fever may due to their infection have been found.

METHODS

From May to October 2019, 192 wild rodents were trapped in wild environment of Guangxi Province, and the spleen samples were collected to reveal the presence of vector-borne bacterial and protozoan infections in them. The microorganisms in rodents were identified by detecting their DNA using (semi-)nested PCR. All the PCR products of the expected size were subjected to sequencing, and then analyzed by BLASTn. Furthermore, all the recovered sequences were subjected to nucleotide identity and phylogenetic analyses.

RESULTS

As a result, 192 rodents representing seven species were captured, and Bandicota indica were the dominant species, followed by Rattus andamanensis. Based on the (semi-)nested PCR, our results suggested that Anaplasma bovis, Anaplasma capra, Anaplasma ovis, Anaplasma phagocytophilum, "Candidatus Neoehrlichia mikurensis", "Candidatus E. hainanensis", "Candidatus E. zunyiensis", three uncultured Ehrlichia spp., Bartonella coopersplainsensis, Bartonella tribocorum, Bartonella rattimassiliensis, Bartonella silvatica, two uncultured Bartonella spp., Babesia microti and diverse Hepatozoon were identified in six rodent species. More importantly, six species (including two Anaplasma, two Bartonella, "Ca. N. mikurensis" and Bab. microti) are zoonotic pathogens except Anaplasma bovis and Anaplasma ovis with zoonotic potential. Furthermore, dual infection was observed between different microorganisms, and the most common type of co-infection is between "Ca. N. mikurensis" and other microorganisms. Additionally, potential novel Bartonella species and Hepatozoon species demonstrated the presence of more diverse rodent-associated Bartonella and Hepatozoon.

CONCLUSIONS

The results in this work indicated great genetic diversity of vector-borne infections in wild rodents, and highlighted the potential risk of human pathogens transmitted from rodents to humans through vectors.

Anaplasma, Bartonella, and Rickettsia infections in Daurian ground squirrels (Spermophilus dauricus), Hebei, China

Rodents have been confirmed as hosts of various vector-borne zoonotic pathogens and are important for the maintenance of these microbes in nature. However, surveillance for zoonotic pathogens is limited for many wild rodent species in China, so our knowledge of pathogen ecology, genetic diversity, and the risk of cross-species transmission to humans is limited. In this study, 165 spleen samples of Daurian ground squirrels (Spermophilus dauricus) were collected from Weichang Manchu and the Mongolian Autonomous County of Hebei Province, China, and Rickettsia, Bartonella, and Anaplasma were identified by DNA detection using polymerase chain reaction (PCR). Sequence analysis identified eight bacterial pathogens: R. raoultii, R. sibirica, Candidatus R. longicornii, B. washoensis, B. grahamii, B. jaculi, A. capra, and Candidatus Anaplasma cinensis. Co-infection of B. grahamii and R. raoultii in one sample was observed. Our results demonstrated the genetic diversity of bacteria in Daurian ground squirrels and contributed to the distribution of these pathogens. Six species, A. capra, R. raoultii, R. sibirica, Candidatus R. longicornii, B. washoensis, and B. grahamii, are known to be pathogenic to humans, indicating a potential public health risk to the local human population, especially to herders who frequently have close contact with Daurian ground squirrels and are thus exposed to their ectoparasites.

First report on the molecular detection and genetic diversity of Anaplasma marginale in healthy dairy cattle in Khon Kaen province, Thailand

Background and Aim

Bovine anaplasmosis (BA) is one of the most important diseases of ruminants worldwide, causing significant economic losses in the livestock industry due to the high morbidity and mortality in susceptible cattle herds. Anaplasma marginale is the main causative agent of BA occurring worldwide in tropical and subtropical regions. This study aimed to investigate the first molecular detection and genetic diversity of A. marginale in dairy cattle in Khon Kaen Province, Thailand.

Materials and Methods

Blood samples were collected from 385 lactating cows from 40 dairy farms in five districts of Khon Kaen, regardless of age and health status. To detect A. marginale, all DNA preparations were used for molecular diagnosis using a single polymerase chain reaction with the msp4 gene target. A phylogenetic tree was constructed from the msp4 gene sequences using molecular genetic characterization. Genetic diversity was calculated as haplotype diversity, haplotype number, number of nucleotide differences, nucleotide diversity, and average number of nucleotide differences.

Results

The overall prevalence of A. marginale was 12.72% (49/385). The highest prevalence (17.19%) was found in Ubolratana district, followed by Muang, Kranuan, Khao Suan Kwang, and Nam Phong districts (14.94%, 14.74%, 13.79%, and 3.70%, respectively). Phylogenetic analysis showed that A. marginale was closely related to isolates from Australia (98.96%), China (99.68%), Spain (99.74%), and the USA (99.63%).

Conclusion

The molecular prevalence of BA in dairy cattle is the first to be observed in this area, and the genetic variability with separated clusters shown in the msp4 gene of A. marginale revealed species variation in dairy cattle. This significant genetic diversity contributes to the understanding of the diversity of A. marginale and will be important for the control and prevention of A. marginale in dairy cattle.

Occurrence of Anaplasma spp. in wild lagomorphs from Southern Spain: Molecular detection of new Anaplasma bovis lineages

Wild lagomorphs can act as reservoirs of several pathogens of public and animal health concern. However, the number of studies assessing the presence of Anaplasma spp. in these species is scarce. The aim of the present study was to molecularly identify Anaplasma spp. in wild rabbits (Oryctolagus cuniculus) and Iberian hares (Lepus granatensis) from Southern Spain and assess their epidemiological role in the maintenance of the bacterium. During 2017-2021, spleen samples of 394 wild rabbits and 145 Iberian hares were collected. Anaplasma DNA was detected using different PCR assays (16S rRNA and groEL) and phylogenetic analyses were carried out by Bayesian approach. The possible influence of lagomorph species, age and sex on the prevalence of Anaplasma spp. was evaluated by a multiple logistic regression model. The 9.4% of the rabbits were positive to Anaplasma bovis, but all the hares were negative. No significant differences were found in Anaplasma spp. prevalence regarding to age or sex. This is the first report of A. bovis in lagomorphs from Europe. The phylogenetic analysis of A. bovis confirms the existence of different clusters suggesting the existence of several lineages. In addition, a high divergence of nucleotide identity was observed within the lineage 4, which could result in the under-detection of some strains when using A. bovis-specific PCR, hindering its detection and characterization. Since this analysis is based on a limited number of nucleotide bases and sequences, more studies are needed for further characterize A. bovis, as well as its relationship with other Anaplasma spp.

Molecular surveillance and genetic diversity of Anaplasma spp. in cattle (Bos taurus) and goat (Capra aegagrus hircus) from Hainan island/province, China

Anaplasmosis is a highly prevalent tick-borne intracellular bacterial disease that affects various host species globally, particularly ruminants in tropical and subtropical regions. However, information regarding the distribution and epidemiology of anaplasmosis in small and large ruminants on Hainan Isalnd is limited. To address this knowledge gap, the present study aimed to assess the occurrence of Anaplasma spp. infections in goats (N = 731) and cattle (N = 176) blood samples using nested PCR and conventional PCR based assays. The results revealed an overall prevalence of 30.1% in goats and 14.8% in cattle. The infection rates of A. bovis, A. phagocytophilum, A. ovis and A. capra in goat samples were 22.7%, 13.8%, 2.0% and 3.4%, respectively, while the infection rates of A. bovis, A. phagocytophilum and A. marginale in cattle samples were 11.4%, 6.3% and 5.7%, respectively. A. bovis exhibited the highest prevalence among the Anaplasma spp. in both goat and cattle samples. In addition, the most frequent co-infection was the one with A. phagocytophilum and A. bovis. It was found that the age, sex and feeding habits of cattle and goats were considered to be important risk factors. Evaluation of the risk factor relating to the rearing system showed that the infection rate for the free-range goats and cattle was significantly higher when compared with stall-feeding system.This study represents one of the largest investigations on the distribution, prevalence, and risk factors associated with Anaplasma infection in ruminants on Hainan Island, highlighting a higher circulation of the infection in the region than previously anticipated. Further reasesrch is necessary to investigate tick vectors, reservoir animals, and the zoonotic potential of the Anaplasma spp. in this endemic region of Hainan Island.

Molecular evidence of Anaplasma spp. in blood-sucking flies from China and identification of a putative novel Anaplasma species

Tabanids and stomoxes are important mechanical vectors for the transmission of pathogens. Although the agents they transmitted have been well studied, bacteria of the genus Anaplasma harbored by these flies have never been reported in China. In this study, 262 blood-sucking flies (128 Stomoxys calcitrans, 45 Tabanus birmanicus, 69 Tabanus hypomacros, and 20 Tabanus taiwanus) were collected from the Wuhan and Nanping cities of China. Anaplasma marginale, Anaplasma bovis, and Candidatus Anaplasma cinensis are detected in S. calcitrans from Wuhan City, with positive rates of 15.63%, 1.56%, and 7.81%, respectively. Out of our expectations, a putative novel Anaplasma species was identified in all three tabanid species (40.00% in T. birmanicus, 15.94% in T. hypomacros, and 10.00% in T. taiwanus) from Nanping City. The 16 S rRNA and groEL gene sequences have highest 99.37-99.75% and 91.46% identities to A. marginale, while the gltA gene sequences have highest 88.34% identity to Anaplasma centrale. In the phylogenetic trees, these strains form a distinct clade. Herein we name it "Candidatus Anaplasma nanpingensis". The present study shows the existence of multiple Anaplasma species in blood-sucking flies in China. This may be the first report that blood-sucking flies harbor Anaplasma in China.

Molecular Detection and Phylogenetic Characterization of Anaplasma spp. in Dogs from Hainan Province/Island, China

Anaplasmosis is a serious infection which is transmitted by ticks and mosquitos. There are very few reports and studies that have been carried out to understand the prevalence, distribution, and epidemiological profile of Anaplasma spp. infection in dogs in Hainan province/island. In the present study, we have tried to understand the prevalence, distribution, and occurrence of Anaplasma spp. infections in dogs (n = 1051) in Hainan Island/Province to establish a surveillance-based study. The confirmed positive samples by Polymerase chain reaction (PCR) were subjected to capillary sequencing for further strain-specific confirmation, followed by the construction of phylogenetic trees to determine their genetic relations. Various statistical tools were used to analyze related risk factors. There were three species of Anaplasma detected from the Hainan region; namely, A. phagocytophilum, A. bovis, and A. platys. The overall prevalence of Anaplasma is 9.7% (102/1051). A. phagocytopihum was prevalent in 1.0% of dogs (11/1051), A. bovis was found in 2.7% of dogs (28/1051), and A. platys in 6.0% of dogs (63/1051). Our surveillance-based study conducted to understand the occurrence and distribution pattern of Anaplasma spp. in Hainan will help in designing effective control measures along with management strategies so as to treat and control the infection in the area.

Diversity of Rickettsiales bacteria in five species of ticks collected from Jinzhai County, Anhui Province, China in 2021-2022

The order Rickettsiales in the class Alphaproteobacteria comprises vector-borne pathogens of both medical and veterinary importance. Ticks, as a group, are second only to mosquitoes as vectors of pathogens to humans, playing a critical role in the transmission of rickettsiosis. In the present study, 880 ticks collected from Jinzhai County, Lu'an City, Anhui Province, China in 2021-2022 were identified as belonging to five species from three genera. DNA extracted from individual ticks was examined using nested polymerase chain reaction targeting the 16S rRNA gene (rrs), and the gene fragments amplified were sequenced to detect and identify Rickettsiales bacteria in the ticks. For further identification, the rrs-positive tick samples were further amplified by PCR targeting the gltA and groEL gene and sequenced. As a result, 13 Rickettsiales species belonging to the genera Rickettsia, Anaplasma, and Ehrlichia were detected, including three tentative species of Ehrlichia. Our results reveal the extensive diversity of Rickettsiales bacteria in ticks from Jinzhai County, Anhui Province. There, emerging rickettsial species may be pathogenic and cause under-recognized diseases. Detection of several pathogens in ticks that are closely related to human diseases may indicate a potential risk of infection in humans. Therefore, additional studies to assess the potential public health risks of the Rickettsiales pathogens identified in the present study are warranted.

Rickettsia sp. and Anaplasma spp. in Haemaphysalis longicornis from Shandong province of China, with evidence of a novel species "Candidatus Anaplasma Shandongensis"

Haemaphysalis longicornis is one of the most dominant and widespread tick species in China. This species mainly infests wild animals and occasionally attacks humans, and has been associated with the transmission of a variety of zoonotic pathogens including spotted fever group Rickettsia (SFGR), severe fever with thrombocytopenia syndrome virus (SFTSV), Anaplasma phagocytophilum, Babesia spp. and Theileria spp.. Although there are increasing reports of various pathogens associated with H. longicornis, some neglected pathogens in certain areas still need to be studied. In this study, a total of 171 H. longicornis ticks were collected from goats in three locations of Shandong Province, Eastern China (Zibo, Linyi, and Qingdao cities), and subsequently screened for the presence of Rickettsia, Anaplasma, and Ehrlichia bacteria. A total of four bacterial species were identified and characterized. "Candidatus Rickettsia jingxinensis" was detected in one tick specimen from Zibo city. Of 98 ticks from Linyi city, 63.27% (62/98) were tested positive for Anaplasma capra and 5.10% (5/98) were positive for Anaplasma bovis. Interestingly, a novel Anaplasma species was detected and characterized in one tick specimen from Zibo and one other from Linyi, respectively. Genetic and phylogenetic analysis based on the 16S, gltA, groEL, and msp4 genes indicated that it was divergent from all known Anaplasma species but mostly related to A. phagocytophilum and "Cadidatus Anaplasma boleense". Based on where it was first detected, we named it "Candidatus Anaplasma shandongensis".

Molecular Characterization of Anaplasma spp. among Dairy, Cashmere, and Meat Goats in Shaanxi Province, Northwestern China

Anaplasma spp. are important tick-borne pathogens endangering the health of humans and various animals. Although several studies have reported Anaplasma infection in livestock in China, little is known about the impact of production categories on the occurrence of Anaplasma species. In the present study, PCR tools targeting the 16S rRNA and msp4 genes were applied to investigate the prevalence of Anaplasma spp. in 509 blood samples of dairy (n = 249), cashmere (n = 139), and meat (n = 121) goats from Shaanxi province. The prevalence of Anaplasma spp. was 58.5% (298/509) in goats, and significant differences (p < 0.001) were identified in the prevalence among production categories, with the highest in meat goats (84.3%, 102/121), followed by cashmere goats (58.3%, 81/139) and dairy goats (46.2%, 115/249). Significant differences (p < 0.001) in prevalence were also found among sampling sites and age groups. Meanwhile, the prevalence was 36.9% (188/509) for A. phagocytophilum, 36.1% (184/509) for A. bovis, and 11.0% (56/509) for A. ovis, and significant differences (p < 0.001) in prevalence of A. phagocytophilum, A. bovis and A. ovis were recognized among production categories and sampling sites. A. phagocytophilum, A. bovis and A. ovis were dominant species in meat, dairy, and cashmere goats, respectively, and A. ovis was absent in meat goats. Co-infections were found in 124 (24.4%) investigated samples. Goats aged < 2, 3−6, and 7−12 months, and goats from Qingjian and Zhenba were risk factors associated with the occurrence of Anaplasma. Phylogenetic analysis indicated separate clades for the distribution of A. phagocytophilum from different ruminant, reflecting potential host adaption within this species. This study reported the colonization occurrence of Anaplasma spp. among production categories in goats in Shaanxi province and enriched our knowledge on the transmission of Anaplasma spp. in goats in China. Considering the existence of zoonotic A. phagocytophilum in goats in this study and previous reports, interventions based on One Health are needed to be developed to control the transmission of Anaplasma spp. between humans and animals.

Molecular Survey of Vector-Borne Pathogens in Ticks, Sheep Keds, and Domestic Animals from Ngawa, Southwest China

Vector-borne pathogens are mainly transmitted by blood-feeding arthropods such as ticks, mosquitoes, fleas, lice, mites, etc. They pose a significant threat to animal and human health due to their worldwide distribution. Although much work has been performed on these pathogens, some neglected areas and undiscovered pathogens are still to be further researched. In this study, ticks (Haemaphysalis qinghaiensis), sheep keds (Melophagus ovinus), and blood samples from yaks and goats were collected in Ngawa Tibetan and Qiang Autonomous Prefecture located on the eastern edge of the Qinghai–Tibet Plateau, Southwest China. Several vector-borne bacterial pathogens were screened and studied. Anaplasma bovis strains representing novel genotypes were detected in ticks (8.83%, 37/419), yak blood samples (45.71%, 64/140), and goat blood samples (58.93%, 33/56). Two spotted fever group (SFG) Rickettsiae, Candidatus Rickettsia jingxinensis, and a novel Rickettsia species named Candidatus Rickettsia hongyuanensis were identified in ticks. Another Rickettsia species closely related to the Rickettsia endosymbiont of Polydesmus complanatus was also detected in ticks. Furthermore, a Coxiella species was detected in ticks (3.34%, 14/419), keds (1.89%, 2/106), and yak blood (0.71%, 1/140). Interestingly, another Coxiella species and a Coxiella-like bacterium were detected in a tick and a goat blood sample, respectively. These results indicate the remarkable diversity of vector-borne pathogens circulating in this area. Further investigations on their pathogenicity to humans and domestic animals are still needed.

Epizootic Situation on Anaplasmosis of Small Ruminants in the Irkutsk Region

Anaplasmosis of ruminants is a group of natural focal infections caused by bacteria from the genus Anaplasma of the Anaplasmataceae family. The main etiological agent of anaplasmosis in sheep, goats, and wild ruminants is Anaplasma ovis, which parasitizes in the erythrocytes of these animals. The purpose of this study was the finding and identification of Anaplasma spp. in the blood of small ruminants using genetic methods and obtaining data on the distribution of anaplasmosis in the Irkutsk region. 20 goat blood samples, 611 sheep blood samples and 209 Dermacentor nuttalli ticks from 12 districts of the Irkutsk region were examined for the presence of Anaplasma spp. Only one type of anaplasma, A. ovis, was found among the genotyped samples. A. ovis was found in the blood of sheep and goats in all of the studied districts of the Irkutsk region. The proportion of sheep blood samples containing anaplasma DNA varied from 30 % to 85 %, in goats – from 10 % to 100 % in different districts, and averaged 57.8 % in sheep and 55,0 % in goats. Frequency of infection of D. nuttalli ticks with A. ovis was 5.7 %. The nucleotide sequences of the samples detected in the blood of small ruminants on the territory of the Irkutsk region differed from each other by a single nucleotide substitution and were identical to the sequences of the type strain Haibei, as well as the sequences of A. ovis previously found in the blood of sheep from Mongolia, deer from China, and Dermacentor niveus and Dermacentor nuttalli ticks from China. These sequences were also identical to the sequences previously found in the blood of sheep from Altai and in Dermacentor nuttalli ticks from Tuva, which indicates the wide distribution of these A. ovis genovariants in Siberia and the probable role of D. nuttalli as a carrier of the agent of anaplasmosis of small ruminants in the Irkutsk region.

Genetic diversity of Anaplasma bacteria: Twenty years later

The genus Anaplasma (family Anaplasmataceae, order Rickettsiales) includes obligate intracellular alphaproteobacteria that multiply within membrane-bound vacuoles and are transmitted by Ixodidae ticks to vertebrate hosts. Since the last reclassification of Anaplasmataceae twenty years ago, two new Anaplasma species have been identified. To date, the genus includes eight Anaplasma species (A. phagocytophilum, A. marginale, A. centrale, A. ovis, A. bovis, A. platys, A. odocoilei, and A. capra) and a large number of unclassified genovariants that cannot be assigned to known species. Members of the genus can cause infection in humans and a wide range of domestic animals with different degrees of severity. Long-term persistence which, in some cases, is manifested as cyclic bacteremia has been demonstrated for several Anaplasma species. Zoonotic potential has been shown for A. phagocytophilum, the agent of human granulocytic anaplasmosis, and for some other Anaplasma spp. that suggests a broader medical relevance of this genus. Genetic diversity of Anaplasma spp. has been intensively studied in recent years, and it has been shown that some Anaplasma spp. can be considered as a complex of genetically distinct lineages differing by geography, vectors, and host tropism. The aim of this review was to summarize the current knowledge concerning the natural history, pathogenic properties, and genetic diversity of Anaplasma spp. and some unclassified genovariants with particular attention to their genetic characteristics. The high genetic variability of Anaplasma spp. prompted us to conduct a detailed phylogenetic analysis for different Anaplasma species and unclassified genovariants, which were included in this review. The genotyping of unclassified genovariants has led to the identification of at least four distinct clades that might be considered in future as new candidate species.