Anaplasma phagocytophilum: A Comprehensive Veterinary Reference on Canine Granulocytic Anaplasmosis, Vector Ecology, Molecular Diagnostics, and Pathogenesis

Introduction

Anaplasma phagocytophilum is an obligate intracellular, Gram-negative bacterium belonging to the family Anaplasmataceae, order Rickettsiales [1, 2]. The organism is the etiologic agent of granulocytic anaplasmosis in domestic animals, including dogs, horses, cats, and ruminants, as well as in humans [1, 3, 2]. In dogs, the disease is termed canine granulocytic anaplasmosis (CGA) and is characterized by an acute febrile illness with hematologic and biochemical abnormalities [1, 2]. The pathogen is transmitted primarily by ticks of the Ixodes persulcatus complex, with Ixodes ricinus serving as the principal vector in Europe and Ixodes scapularis in North America [1, 4, 5]. The global distribution of A. phagocytophilum is expanding, and retrospective studies have documented an increasing prevalence of positive test results in canine populations over the past decade [1, 2]. This article provides a detailed, citation-grounded overview of the biology, ecology, pathogenesis, clinical presentation, and diagnostic management of A. phagocytophilum infection in dogs, with comparative references to other host species where relevant.

Taxonomy and Genetic Diversity

Anaplasma phagocytophilum is classified within the genus Anaplasma, which also includes A. marginale, A. centrale, A. ovis, A. platys, and the recently described A. capra [6, 7, 23]. The species exhibits considerable genetic heterogeneity, and molecular analyses based on the groEL, 16S rRNA, msp2, and msp4 genes have revealed multiple ecotypes and genetic variants [8, 9, 10, 11]. Ecotype I is considered the zoonotic variant and is associated with human granulocytic anaplasmosis, while ecotype II is predominantly found in roe deer and other wild ruminants [10, 11]. Additional variants, including A. phagocytophilum‑like 1 and A. phagocytophilum‑like 2, have been described in small ruminants and ticks in Asia and Europe [6, 12, 22]. The presence of a novel ecotype, designated strain “Patagonia,” was recently reported in pudu deer from Chile, expanding the known geographic range of the species complex [8]. Phylogenetic analyses of the groEL gene have defined at least four ecotypes in Europe, with ecotype I circulating in multiple ungulate hosts and ticks [9, 10, 11]. The genetic diversity of A. phagocytophilum has implications for host tropism, pathogenicity, and diagnostic test sensitivity [2, 9].

Vector Transmission and Ecology

Anaplasma phagocytophilum is maintained in enzootic cycles involving ixodid ticks and vertebrate reservoir hosts. The primary vectors are members of the Ixodes ricinus complex in Europe, Ixodes scapularis in North America, and Ixodes persulcatus in Asia [1, 4, 5, 9]. Transstadial transmission is the principal route of maintenance within tick populations, as larvae and nymphs acquire the bacterium from infected reservoir hosts and subsequently transmit it to susceptible hosts during later blood meals [13, 29]. Transovarial transmission in I. ricinus has been documented under field conditions, with detection of A. phagocytophilum DNA in questing larvae, but the efficiency appears low [13]. In a systematic review and meta-analysis of 126 studies from 33 countries, the global infection rate in questing and host‑attached ticks was estimated at 4.76% (95% CI: 3.96–5.71) [4]. Infection rates vary significantly by geographic region, tick species, and habitat. In England and Wales, the mean prevalence in questing I. ricinus nymphs was 3.6%, with higher rates in northern England (4.7%) and in areas with sheep presence (8.4%) [5]. In Western Ukraine, A. phagocytophilum prevalence was 27.4% in I. ricinus and 15.9% in Dermacentor reticulatus [32]. The invasive Asian longhorned tick (Haemaphysalis longicornis) in Pennsylvania has been found to harbor the human pathogenic variant of A. phagocytophilum, indicating potential for novel vector–pathogen associations [14].

Vertebrate reservoir hosts include wild ungulates such as roe deer, red deer, moose, and wild boar, as well as small mammals and birds [5, 10, 11, 15, 21, 26]. In Norway, moose were found to carry both ecotype I and ecotype II, suggesting that cervids can serve as reservoirs for multiple ecotypes [10]. Peridomestic animals, including rodents, hedgehogs, and birds, may contribute to local transmission cycles [15]. The role of dogs in the enzootic cycle is primarily as incidental hosts, although they can serve as sentinels for human exposure [2]. Transmission of A. phagocytophilum from infected ticks to dogs can occur within hours of attachment; in an experimental study, a minimum attachment period of 48 hours was required to establish infection in dogs, although bacterial DNA was detected in artificial feeding chambers as early as 6 hours post‑attachment [29].

Pathogenesis and Host Cell Interactions

Anaplasma phagocytophilum exhibits a marked tropism for neutrophils and, to a lesser extent, eosinophils [2, 16]. The bacterium invades host cells through a novel mechanism involving the outer membrane adhesin Asp14, which binds to host cell surface protein disulfide isomerase (PDI) [28]. This interaction promotes reduction of bacterial surface disulfide bonds, a critical step for cellular invasion [28]. Once internalized, the bacterium resides within a host cell‑derived vacuole, termed the Anaplasma‑containing vacuole (ApV), which is acidified and enriched in lysobisphosphatidic acid [16]. The ApV interfaces with the endolysosomal pathway and exploits multivesicular body (MVB) biogenesis for proliferation and dissemination [16]. Components of the endosomal sorting complexes required for transport (ESCRT) machinery, including ESCRT‑0, ESCRT‑III, and the accessory protein ALIX, localize to the ApV membrane [16]. Inhibition of ESCRT‑III function reduces intralumenal vesicle formation and arrests bacterial growth, while blockade of Rab27a‑dependent MVB exocytosis with a small molecule inhibitor abrogates release of infectious progeny [16]. This exploitation of the host MVB pathway is essential for all major stages of the intracellular infection cycle: intravacuolar growth, conversion to the infectious form, and exit from the host cell [16].

The bacterium modulates host cell functions to evade immune clearance. It downregulates tick microRNA‑133, leading to increased expression of the organic anion transporting polypeptide (isoatp4056) in the vector, which is critical for bacterial survival in the tick and transmission to the vertebrate host [27]. Anaplasma phagocytophilum also secretes effector proteins via a type IV secretion system (T4SS); computational prediction using the OPT4e software identified 48 candidate effectors in strain HZ, including three validated effectors [35]. These effectors manipulate host cell signaling, apoptosis, and cytoskeletal dynamics to facilitate infection [35].

The pathological consequences of infection arise from both direct bacterial damage and host inflammatory responses. Infected neutrophils exhibit altered function, including impaired phagocytosis, reduced oxidative burst, and delayed apoptosis, which prolongs the intracellular niche for bacterial replication [2]. The release of proinflammatory cytokines contributes to the clinical signs of fever, lethargy, and musculoskeletal pain [2].

Clinical Manifestations in Dogs

Canine granulocytic anaplasmosis typically presents as an acute febrile illness. Common clinical signs include lethargy, inappetence, weight loss, and musculoskeletal pain [1, 2]. In a retrospective study of 27,368 dogs tested by PCR in Germany, 4.9% were positive, and seropositivity was 27.4% among 90,376 dogs tested by immunofluorescence antibody test (IFAT) or enzyme‑linked immunosorbent assay (ELISA) [1]. Male dogs and senior dogs had higher rates of seropositivity [1]. Seasonal peaks in PCR positivity coincided with periods of high tick activity [1]. Hematologic abnormalities commonly include thrombocytopenia, anemia, and the presence of morulae (intracytoplasmic inclusions) within neutrophils [2]. Biochemical alterations may include elevated liver enzyme activities [2]. Coinfections with other tick‑borne pathogens, particularly Borrelia burgdorferi sensu lato, are frequently reported and can complicate the clinical presentation and diagnosis [2, 25]. Neurologic manifestations, such as encephalitis, have been described in human cases but are rarely reported in dogs [3]. In cats, the clinical signs are similar to those in dogs, including fever, lethargy, and anorexia, and the diagnosis is based on serology and PCR [24].

Diagnostic Approaches

The diagnosis of A. phagocytophilum infection in dogs relies on a combination of direct and indirect detection methods. Direct detection is achieved through polymerase chain reaction (PCR) targeting the msp2 or 16S rRNA genes, which can identify active infection [1, 17, 23, 30]. Quantitative real‑time PCR (qPCR) provides high sensitivity and allows quantification of bacterial DNA [13, 23]. In a study of stray dogs in Bosnia and Herzegovina, 24% of seropositive dogs were positive by real‑time PCR for A. phagocytophilum [30]. Indirect detection involves serological assays, including the immunofluorescence antibody test (IFAT) and enzyme‑linked immunosorbent assay (ELISA) for antibodies against A. phagocytophilum [1, 31]. The kinetics of seroconversion show that in dogs with positive PCR results, antibody peaks are observed approximately four weeks after initial testing [1]. Serological cross‑reactivity occurs between A. phagocytophilum and A. platys; therefore, confirmatory molecular testing is recommended [30].

The table below summarizes the main diagnostic methods.

Restriction fragment length polymorphism (RFL) analysis of the 16S rRNA and groEL genes allows discrimination between A. phagocytophilum and related variants, such as A. phagocytophilum‑like 1 and 2 [6, 12, 22]. Phylogenetic analysis of groEL sequences is used for ecotype assignment [8, 9, 10, 11]. In a study of small ruminants in Turkey, PCR‑RFLP and sequencing revealed the presence of A. phagocytophilum‑like 1 in 26.5% of samples [12]. The duplex PCR assay targeting msp2 and msp4 has been developed for simultaneous detection of A. phagocytophilum and A. ovis in small ruminants [17].

Treatment and Prevention

The recommended therapeutic agent for canine granulocytic anaplasmosis is doxycycline, administered at a dosage of 10 mg/kg orally every 24 hours for 14 to 28 days [2]. Clinical improvement is typically observed within 24 to 48 hours of treatment initiation [2]. In human cases of anaplasmosis, intravenous doxycycline has also been effective [3]. No licensed vaccine is currently available for A. phagocytophilum in dogs [2]. Prevention relies on the use of acaricidal products that exert a rapid killing effect on ticks to prevent transmission [29]. The time required for transmission of A. phagocytophilum by I. ricinus ticks in dogs is greater than 48 hours, but acaricides that repel or kill ticks within hours can reduce the risk [29]. Environmental control measures, such as habitat management and avoidance of tick‑infested areas, are also recommended [4].

Mermaid Diagram: Diagnostic Decision Tree for CGA

flowchart TD
    A[Clinical signs: fever, lethargy, thrombocytopenia], > B{History of tick exposure?}
    B, >|Yes| C[Perform PCR on whole blood]
    B, >|No| D[Consider alternative diagnoses]
    C, > E{PCR positive?}
    E, >|Yes| F[Confirm active infection; treat with doxycycline]
    E, >|No| G[Perform serology (IFAT/ELISA)]
    G, > H{Serology positive?}
    H, >|Yes| I[Prior exposure or recent infection; consider convalescent serology]
    H, >|No| J[Infection unlikely; reassess for other tick-borne pathogens]
    F, > K[Monitor clinical response and repeat PCR if needed]

FAQ: Clinical and Diagnostic Queries

What is the incubation period for canine granulocytic anaplasmosis?

The incubation period in dogs is typically 1 to 2 weeks after tick attachment, based on experimental transmission studies [29]. Clinical signs appear within 7 to 14 days post‑infestation when infected ticks are allowed to feed to repletion [29].

How is Anaplasma phagocytophilum transmitted in nature?

The bacterium is transmitted by ixodid ticks, primarily Ixodes ricinus in Europe and Ixodes scapularis in North America [1, 4, 5]. Transstadial transmission is the main mechanism, with transovarial transmission occurring at low efficiency [13]. Reservoir hosts include wild cervids, rodents, and birds [5, 10, 15, 21].

Which diagnostic test is most appropriate for acute infection?

Real‑time PCR targeting the msp2 gene is the most sensitive method for detecting active infection during the acute phase [1, 23]. Serology may be negative in the first week of infection, so convalescent testing is recommended for retrospective confirmation [1].

Can dogs be coinfected with other tick‑borne pathogens?

Yes, coinfections with Borrelia burgdorferi sensu lato, Babesia spp., and Anaplasma platys are frequently reported [2, 25, 26, 30]. Coinfection may exacerbate clinical signs and complicate diagnostic interpretation [2].

What is the recommended treatment for canine granulocytic anaplasmosis?

Doxycycline at 10 mg/kg orally every 24 hours for 14 to 28 days is the standard therapy [2]. Clinical improvement is usually rapid, and complete recovery is expected with appropriate treatment [2].

Are there zoonotic risks associated with canine A. phagocytophilum infection?

Anaplasma phagocytophilum is a zoonotic pathogen, and dogs may serve as sentinels for human exposure [2]. The same ecotypes that infect dogs can cause human granulocytic anaplasmosis [11]. However, direct transmission from dogs to humans has not been documented; infection occurs through tick bites [2].

What is the role of ecotyping in understanding A. phagocytophilum epidemiology?

Ecotyping based on groEL sequences distinguishes between zoonotic (ecotype I) and non‑zoonotic (ecotype II) strains [9, 10, 11]. This information is valuable for assessing public health risk and understanding local transmission cycles [11].

References

[1] Schäfer I, Kohn B, Silaghi C, et al. Molecular and Serological Detection of Anaplasma phagocytophilum in Dogs from Germany (2008–2020). Animals. 2023. URL: https://www.semanticscholar.org/paper/276d428f380547d6cadb78858f4c4e159d93965d

[2] El Hamiani Khatat S, Daminet S, Duchateau L, et al. Epidemiological and Clinicopathological Features of Anaplasma phagocytophilum Infection in Dogs: A Systematic Review. Frontiers in Veterinary Science. 2021. URL: https://www.semanticscholar.org/paper/36a35115630b5fa90a09d91eb82402547c6175dc

[3] Cosiquien RJS, Stojiljkovic N, Nordstrom CW, et al. Anaplasma phagocytophilum Encephalitis: A Case Report and Literature Review of Neurologic Manifestations of Anaplasmosis. Infectious Disease Reports. 2023. URL: https://www.semanticscholar.org/paper/d6bf8ac2ac575616e29c44163f9f8ba8ae828204

[4] Karshima S, Ahmed M, Kogi C, et al. Anaplasma phagocytophilum infection rates in questing and host-attached ticks: a global systematic review and meta-analysis. Acta Tropica. 2022. URL: https://www.semanticscholar.org/paper/fb71baa80c030d4596dc00e3c4bb8ca8f891f03d

[5] Gandy S, Hansford K, McGinley L, et al. Prevalence of Anaplasma phagocytophilum in questing Ixodes ricinus nymphs across twenty recreational areas in England and Wales. Ticks and Tick-borne Diseases. 2022. URL: https://www.semanticscholar.org/paper/577001a07c702eacc0bfc79000d17f6e27fae5cd

[6] Altay K, Erol U, Sahın OF, et al. The detection and phylogenetic analysis of Anaplasma phagocytophilum-like 1, A. ovis and A. capra in sheep: A. capra divides into two genogroups. Veterinary Research Communications. 2022. URL: https://www.semanticscholar.org/paper/068d707507a2548593920177f27bbb2f327f612f

[7] Altay K, Erol U, Sahın OF, et al. First molecular detection of Anaplasma species in cattle from Kyrgyzstan; molecular identification of human pathogenic novel genotype Anaplasma capra and Anaplasma phagocytophilum related strain. Ticks and Tick-borne Diseases. 2021. URL: https://www.semanticscholar.org/paper/f96f448f6a84dfba1d98d77f2f8151a7150ddeac

[8] Santodomingo A, Thomas RS, Robbiano S, et al. Wild deer (Pudu puda) from Chile harbor a novel ecotype of Anaplasma phagocytophilum. Parasites & Vectors. 2023. URL: https://www.semanticscholar.org/paper/8a25a81c6b12692bc9b319ba072f6983a8b6481a

[9] Jaarsma RI, Sprong H, Takumi K, et al. Anaplasma phagocytophilum evolves in geographical and biotic niches of vertebrates and ticks. Parasites & Vectors. 2019. URL: https://www.semanticscholar.org/paper/05c28c9e27bfb82ed8ca088f887f984c3f6541b8

[10] Stigum VM, Jaarsma RI, Sprong H, et al. Infection prevalence and ecotypes of Anaplasma phagocytophilum in moose Alces alces, red deer Cervus elaphus, roe deer Capreolus capreolus and Ixodes ricinus ticks from Norway. Parasites & Vectors. 2019. URL: https://www.semanticscholar.org/paper/e2e6863fd20051becb7f5d85122c176731741066

[11] Grassi L, Franzo G, Martini M, et al. Ecotyping of Anaplasma phagocytophilum from Wild Ungulates and Ticks Shows Circulation of Zoonotic Strains in Northeastern Italy. Animals. 2021. URL: https://www.semanticscholar.org/paper/d5204e76092103ed94030504f52d4717d2821086

[12] Aktas M, Özübek S, Ulucesme M. Molecular Detection and Phylogeny of Anaplasma phagocytophilum and Related Variants in Small Ruminants from Turkey. Animals. 2021

[13] Hauck D, Jordan D, Springer A, et al. Transovarial transmission of Borrelia spp., Rickettsia spp. and Anaplasma phagocytophilum in Ixodes ricinus under field conditions extrapolated from DNA detection in questing larvae. Parasites & Vectors. 2020. URL: https://www.semanticscholar.org/paper/98c0418fcb935f05c567234013876b9f5aa15582

[14] Price KJ, Ayres BN, Maes SE, et al. First detection of human pathogenic variant of Anaplasma phagocytophilum in field‐collected Haemaphysalis longicornis, Pennsylvania, USA. Zoonoses and Public Health. 2021. URL: https://www.semanticscholar.org/paper/15703898e1744c7eed20aafd99131536f57dde18

[15] Lesiczka P, Hrazdilová K, Majerová K, et al. The Role of Peridomestic Animals in the Eco-Epidemiology of Anaplasma phagocytophilum. Microbial Ecology. 2021. URL: https://www.semanticscholar.org/paper/d67a69ebbf73c4279c6d5e5e7bb461f00df65166

[16] Read CB, Lind MCH, Chiarelli T, et al. The Obligate Intracellular Bacterial Pathogen Anaplasma phagocytophilum Exploits Host Cell Multivesicular Body Biogenesis for Proliferation and Dissemination. mBio. 2022. URL: https://www.semanticscholar.org/paper/74f0a3f0f5c97785558f84b7fce8055036de2834

[17] M’ghirbi Y, Oporto B, Hurtado A, et al. First Molecular Evidence for the Presence of Anaplasma phagocytophilum in Naturally Infected Small Ruminants in Tunisia, and Confirmation of Anaplasma ovis Endemicity. Pathogens. 2022. URL: https://www.semanticscholar.org/paper/033e60b8f3149b96b501258ec7b74ba35c858ea5

[18] del Cerro A, Oleaga Á, Somoano A, et al. Molecular identification of tick-borne pathogens (Rickettsia spp., Anaplasma phagocytophilum, Borrelia burgdorferi sensu lato, Coxiella burnetii and piroplasms) in questing and feeding hard ticks from North-Western Spain. Ticks and Tick-borne Diseases. 2022. URL: https://www.semanticscholar.org/paper/0c10b8e90f932fc3899d7847402699f753a8900e

[19] Bauer B, Răileanu C, Tauchmann O, et al. Anaplasma phagocytophilum and Anaplasma ovis–Emerging Pathogens in the German Sheep Population. Pathogens. 2021. URL: https://www.semanticscholar.org/paper/36068206d02fbc5c6a2e439bfa80e53b36bc1042

[20] Duan R, Lv D, Fan R, et al. Anaplasma phagocytophilum in Marmota himalayana. BMC Genomics. 2022. URL: https://www.semanticscholar.org/paper/c415f84c3406192eb218f1daa75fe2ee18deb452