Introduction

Anaplasma marginale and Anaplasma phagocytophilum are the most important tick-borne bacteria of veterinary and public health significance. Bovine anaplasmosis (BA) caused by A. marginale is a globally distributed tick-borne disease with great economic importance in cattle industry including Asia, Africa, Australia, Southern Europe, and Central and South America (Jongejan and Uilenberg 2004), biologically transmitted by Rhipicephalus ticks and mechanically by biting flies, blood-contaminated needles, and farm equipments. Anaplasma phagocytophilum is an emerging globally distributed, zoonotic tick-borne pathogen of wide mammalian hosts, transmitted mainly by Ixodes ticks (de la Fuente et al. 2005). The infection in domestic animals is generally referred as tick-borne fever, responsible for important economic loss to cattle and sheep industry (Stuen 2007; Grøva et al. 2011). Anaplasma phagocytophilum is known since 200 years, but it became real research focus after first case of human granulocytic anaplasmosis (HGA) in 1986 (Maeda et al. 1987). These closely related bacteria share some common features such as coexistence during concurrent infection in ticks, domestic, wild ruminant reservoir hosts in same geographic region (de la Fuente et al. 2005). In terms of developing diagnostic assays, vaccines, and future research, similarities and differences of these organisms have to be considered. Limited reviews are available in this perspective. The current review focus on updated knowledge on epidemiology, ecology of A. phagocytophilum and broad comparison of A. marginale and A. phagocytophilum for effective prevention and control of anaplasmosis in humans and animals.

Anaplasma phagocytophilum

Morphology

Anaplasma phagocytophilum is alpha pleomorphic gram-negative bacterium measuring 0.4 to 1.3 or 2 μm in size. The bacterial outer membrane is often coarse with irregular periplasmic spaces without capsule. Bacterium lacks lipopolysaccharide and peptidoglycan. This obligate intracellular bacterium, after staining with Romanowsky stain, show purple color mulberry-like microcolonies called Morulae with 1.5 to 2.5 or 6 μm in diameter (Foggie 1951; Woldehiwet and Scott 1982; Rikihisa et al. 1997; Popov et al. 1998). The bacteria can also be stained with May-Grünwald or Write-Giemsa stains. Three species previously belong to Ehrlichia were now included in genus Anaplasma namely Anaplasma phagocytophilum, earlier known as Ehrlichia phagocytophila, the causative agent of human granulocytic anaplasmosis; Anaplasma bovis (formerly known as Ehrlichia bovis); and Anaplasma platys (formerly Ehrlichia platys; Dumler et al. 2001; Table 1). All these aforementioned pathogens infect blood cells of their respective hosts.

Table 1 Classification of genus Anaplasma, Ehrlichia, and Neorickettsia of family Anaplasmataceae
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Life cycle

So far, no transovarial transmission (from adult ticks to eggs) has been reported (Woldehiwet 2010), except for moose tick (Dermacentor albipictus). Transovarial transmission ability of moose tick is due to atypical feeding systems as compared to normal Ixodes infection cycle (Baldridge et al. 2009). A reservoir host is required to keep A. phagocytophilum in nature. Life cycle starts with blood meal after tick bite to infected mammals. A. phagocytophilum survives and maintain in larva/nymph to adult developmental stages of ticks and transmit this to mammals during the next blood meal (Telford et al. 1996; Ogden et al. 1998; Zhi et al. 2002).

Clinical signs

Tick-borne fever is mostly seen in sheep and cattle, but it can also be demonstrated in goats, reindeer, and deer. In sheep, clinical signs include high fever, inappetence, dullness, sudden drop in milk yield, reduced weight gain, coughing, abortion, stillbirth and low fertility in sheep, and reduced semen quality in rams (CFSPH 2013). Dairy cattle upon its return from pasture usually becomes infected with variable severity of illness including dullness, anorexia, reduced milk production, respiratory distress, coughing, abortions, and stillbirth are common. The important finding that mild cases recover within 14 days and death is unusual outcome (Tuomi 1967; Taylor and Kenny 1980; Stuen et al. 1992; Grøva et al. 2011; CFSPH 2013). The Anaplasma phagocytophilum-suspected cases are usually subject to secondary infection with tick pyemia, pasteurellosis, and septicemic listeriosis. Anaplasmosis in equines is called as equine granulocytic anaplasmosis. Horses with more than 3 years of age develop severe disease including fever, anorexia, depression, petechial hemorrhages, icterus, ataxia, and distal limb edema and may have severe myopathy. Fever and lethargy is most commonly seen in canine granulocytic anaplasmosis. Infection in cats is called as feline granulocytic anaplasmosis, and cats show generalized nonspecific signs include fever, dullness, and anorexia (CFSPH 2013). In humans (human granulocytic anaplasmosis (HGA)), the major clinical signs include fever, headache, myalgias, and chills. Leucopenia, thrombocytopenia and/or anemia and elevated liver enzymes are usual hematological and biochemical finding in humans and all animals species (common in dog, cat, and humans) (Bakken et al. 1994; Aguero-Rosenfeld et al. 1996). Clinical signs 2–3 weeks after tick bite is suggestive of HGA. The HGA cannot be diagnosed only on the basis of clinical signs. Severe clinical signs include prolonged fever, septic shock-like illness, respiratory distress, acute renal failure, gastrointestinal tract bleeding, rhabdomyolysis, and secondary infections (AABB 2009).

The pathogenesis of HGA is inadequately understood. Little amount of bacteria in infected animal and human’s peripheral blood indicate the presence of proinflammatory cytokines. In human patients, increased concentrations of serum gamma interferon and interleukin-10 protein have been detected during acute infection as compared to restorative or patients with no clinical signs (Dumler et al. 2000). This suggests that human monocytes, rather human neutrophils, are responsible for proinflammatory cytokine production.

Epidemiology

Geographic distribution

Anaplasma phagocytophilum is endemic or potentially endemic in 42 countries of the world with an overall case fatality of 5 % (Berger 2014). This has been detected throughout Europe, America (North and South), Asia (Pakistan, India, Korea, and Japan) and Africa (Kawahara et al. 2006; Kang et al. 2013; M’ghirbi et al. 2012; Djiba et al. 2013; Stuen et al. 2013a; Borthakur et al. 2014; Razzaq et al. 2015; Pantchev et al. 2015). Human seroprevalence in disease endemic area of Wisconsin and New York (USA) is 15–36 %, whereas seroprevalence in Europe range from 1 and 20 % depending upon immunity, tick exposure, and age of the patients (CDC 2013). Majority of the human cases of infection in USA occur in June–July.

Hosts

Sheep and cattle are the main hosts but infection has been detected in goat, horse, donkey, dog, cat, and wild ruminants. Definite reservoir hosts for A. phagocytophilum in animals and humans are not known to date. Identification of reservoirs is important for epidemiological standpoint. However, humans are the dead end host. Anaplasma phagocytophilum is most frequently been found in roe deer, red deer, and fallow deer, and the highest prevalence A. phagocytophilum is reported in roe deer and red deer ranging from 12 to 85 % (Hulínská et al. 2004; Zeman and Pecha 2008; Scharf et al. 2011; Overzier et al. 2013).

The pathogen has been detected in white tailed deer, sika deer, Korean water deer, wild boar (Sus scrofa), Alpine ibex (Capra ibex), chamois (Rupicapra rupicapra), mouflon (Ovis musimon), European bison (Bison bonasus), mule deer (Odocoileus hemonius hemonius), reindeer (Rangifer tarandus), elk (Cervus elaphus nannodes), llama (Lama glama), alpaca (Vicugna pacos), Suri alpaca (Vicugna pacos), Swedish moose (Alces alces), and birds (Tinkler et al. 2012; Stuen et al. 2013a; Malmsten et al. 2014); small mammals: dusky-footed wood rats (Neotoma fuscipes) (Rikihisa 2003), white-footed mice (Peromyscus leucopus) (Keesing et al. 2012), vole (Clethrionomys gapperi), Eastern chipmunk (Tamias striatus), squirrel (Spermophilus lateralis), Virginia opossum (Didelphis virginiana), striped skunk (Mephitis mephitis); insectivorous mammals: hedgehog (Erinaceus europaeus), shrew; reptiles and snakes: northern alligator lizard (Elgaria coeruleus), Pacific gopher snake (Pituophis catenifer); others: cotton tail rabbit (Sylvilagus floridanus), gray fox (Urocyon cinereoargenteus), raccoon (Procyon lotor), timber wolf (Canis lupus occidentalis), American black bears (Ursus americanus) (Drazenovich et al. 2006), European brown bear (Ursus arctos arctos) (Víchová et al. 2010). In cattle, A. phagocytophilum is usually associated during concurrent infection with Borrelia burgdorferi and/or A. marginale (Hofmann-Lehmann et al. 2004; Berger 2014). Individuals recovered from acute disease develop persistent infection. This is a complex process having cyclic episodes of lowering and peak bacteremia phases, under the influence of host immunity. Persistently infected individuals serve as reservoir for maintenance and further spread of infection.

Breed resistance

Little information is available on the breed resistance of tick-borne fever and granulocytic fevers of domestic and wild animals. As for bovine anaplasmosis, there is individual variance in susceptibility to ticks and tick-borne fever. Likewise, Old Norse sheep is naturally resistant to tick-borne infections than other Norwegian breeds (Stuen 2003; Stuen et al. 2011; Granquist et al. 2010b).

Risk factors

Young domestic animals purchased from tick-free area and moved to tick-infested areas is the major risk factor of tick-borne fever (Tuomi 1967). It has been seen in some regions that the higher prevalence of roe deer and white tailed deer resulted in higher prevalence of anaplasmosis (Stuen et al. 2013a). Risk factors for human anaplasmosis include high-risk outdoor activities (such as hiking and gardening) and immunocompromised individuals (cancer treatments, prior organ transplants, HIV infection), and people after blood transfusion are at higher risk.

Transmission

Ticks play a key role in multiplication, persistence, and pathogen transmission to mammalian hosts (Hodzic et al. 1998; Katavolos et al. 1998). The RNA interference (RNAi) technique identified that Salp16, a salivary gland protein of I. scapularis tick, is required for infectivity of salivary glands and further transmission (Ramakrishnan et al. 2005; Sukumaran et al. 2006). Ixodes ricinus is the main vector of A. phagocytophilum throughout Europe. Additionally, the pathogen has been detected with molecular methods in I. persulcatus from Latvia, Russia, and Estonia, as well as in Dermacentor reticulatus, Haemaphysalis concinna, and I. ventalloi ticks (Santos et al. 2004; Masuzawa et al. 2008; Paulauskas et al. 2012; Tomanovic et al. 2013).

Bacterium is usually transmitted by I. pacificus (Western black-legged tick) in Western USA, but Dermacentor variabilis and D. occidentalis has also been reported from California (Holden et al. 2003; Lane et al. 2010; Rejmanek et al. 2011); by I. scapularis (deer tick or black-legged tick) (Lovrich et al. 2011; Roellig and Fang 2012) in Eastern USA; by I. scapularis and Amblyomma americanum in Florida (USA); by I. spinipalpis in North Colorado (USA) (Zeidner et al. 2000); by I. scapularis and D. albipictus in Canada (Baldridge et al. 2009; Krakowetz et al. 2014); by I. persulcatus, I. nipponensis, I. ovatus, Dermacentor silvarum, Haemaphysalis megaspinosa, H. douglasii, H. longicornis, and H. japonica in Asia (China, Japan, Korea, Russia); by I. ricinus and Hyalomma (Hy.) marginatum, Hy. detritum in North Africa (Algeria, Tunisia, and Morocco) (Sarih et al. 2005; M’ghirbi et al. 2012); and by Hyalomma marginatum, Rhipicephalus turanicus, and Boophilus kohlsi in Israel (Keysary et al. 2007).

As mentioned above, various studies indicate presence of A. phagocytophilum in different ticks, but the vector competence of only few American and European ticks have to date been proved yet for I. ricinus, I. scapularis, I. pacificus, and I. spinipalpis (Woldehiwet 2010). There is a lot more research scope for the identification of competent vectors and reservoir hosts. The prevalence of A. phagocytophilum in I. scapularis (<1–50 %) and I. pacificus ticks (~1–10 %) in the USA has been reported, whereas I. persulcatus had <1 to 21.6 % in Asia (Stuen et al. 2013a). Prevalence in questing I. ricinus ticks in Europe range from 0.7 and 14.5 % (Hartelt et al. 2008; Rizzoli et al. 2014). The DNA of A. phagocytophilum has also been detected in Asia in I. ovatus, I. nipponensis, D. silvarum, Haemaphysalis (H.) megaspinosa, H. douglasii, H. longicornis, and H. japonica tick species. Furthermore, this pathogen has also been demonstrated in questing I. dentatus, Amblyomma americanum, Dermacentor variabilis, and D. occidentalis ticks (Goethert and Telford 2003).

Mechanical transmission by blood-sucking deer ked (Lipoptena cervi) from red deer (Cervus elaphus), roe deer (Capreolus capreolus), and fallow deer (Dama dama) have been reported using PCR (Víchová et al. 2011). Similarly, there are reports of transplacental (lambs and calves), perinatal, blood transfusions, and nosocomial associated transmissions (Bakken et al. 1996; Horowitz et al. 1998; Dhand et al. 2007; Zhang et al. 2008; Annen et al. 2012; Henniger et al. 2013; Reppert et al. 2013). These modes of transmissions are further complicating the epidemiology of TBF and HGA.

Strains

Anaplasma phagocytophilum have higher degree of genetic diversity, variation in pathogenicity, and host tropisms (Baráková et al. 2014). Higher degree of disparity exists in the prevalence of variants within and among hosts as well as between variants of different regions (Foley et al. 2008; Morissette et al. 2009). Genetic variability have been studied using 16S rRNA, major surface protein coding genes (msp4), groEL heat-shock protein, msp2/p44, and ankA genes (Granquist et al. 2010c; Silaghi et al. 2011a, b).

Strains isolated from human patients can cause clinical disease in horses. This may be considered as valuable animal model for HGA (Madigan et al. 1995). American strain from horse was not infectious for ruminants (Stannard et al. 1969), whereas a European strain isolated from cattle did not show any clinical signs in horses (Pusterla et al. 1998). Rodents do not play a major role in the Europe whereas the white-footed mouse (Peromyscus leucopus) is the important reservoir of human pathogenic strain (Ap-ha) (Massung et al. 2003). The Ap-ha is pathogenic to humans; ruminants and mice can be experimentally infected, whereas the Ap-variant 1 is nonpathogenic to humans and mice, conversely infectious to deer and goats (Massung et al. 2003, 2005, 2006; Tate et al. 2005).

Several distinct ecological clusters have been established. While the latest reports based on multilocus sequencing mentioned that roe deer do not contribute in Europe for human infection but suspected to be the reservoir of Norwegian sheep strain belonging to different enzootic cycle (Stuen et al. 2010; Huhn et al. 2014). However, another study established link of human pathogenic strains to ungulates (Baráková et al. 2014). A potential human pathogenic strain of A. phagocytophilum in Europe has been connected to wild boars (Silaghi et al. 2014).

Four major ecotypes were identified. Ecotype-I has widest host range. Up till now, all human cases grouped in ecotype-I have the widest host range (including domesticated animals, red deer, wild boar, and urban hedgehogs) and further expanded incorporating I. ricinus ticks or urban vertebrates. Ecotype-II was associated with roe deer and some rodents, and ecotype-III included only rodents. Birds seem to have a different enzootic cycle and grouped in ecotype-IV. The study based on population genetic parameters; revealed that ecotype-I expressed the major expansion due to either increase in the population of I. ricinus ticks or in the domestic vertebrate hosts or both (Jahfari et al. 2014).

Diagnosis

Clinical signs rarely aid in diagnosis. Usually, laboratory tests are required for confirmatory diagnosis. Blue intra-cytoplasmic inclusion microcolonies (morulae) can be usually seen in granulocytes, especially monocytes and neutrophils in peripheral blood-stained smear during acute phase of the infection (Figs. 1, 2, and 3). Hematological and biochemical parameters show anemia, leucopenia, thrombocytopenia, and increase of aspartate aminotransferase and alanine aminotransferase enzymes, 5–21 days after infected tick bite (Bakken and Dumler 2008). Confirmation of Anaplasma can be done either by electron microscopy of blood/organs smears, cell culture or immunohistochemistry. Common serological techniques for sero-diagnosis of A. phagocytophilum include indirect immunofluorescent antibody (IFA) test, ELISA, complement fixation test, and counter-current immunoelectrophoresis test. Commercially available “SNAP®4Dx®” ELISA test is good for rapid in-house detection of A. phagocytophilum antibodies in dog serum. The kit has been used successfully on sheep and horse sera (Granquist et al. 2010a; Hansen et al. 2010). Moreover, “MegaFLUO® ANAPLASMA phagocytophilum” is an indirect semiquantitative immunofluorescent test commercially available for the detection of A. phagocytophilum IgG antibodies in horse and dog serum or plasma. Fourfold rise of IgG antibodies in paired human samples within 2–4-week interval for immunofluorescence assay (IFA) is the gold standard test for HGA (CDC 2013). Various PCR techniques including conventional, nested, and real-time have been developed for the detection of A. phagocytophilum infection in blood and tissue samples targeting 16S rRNA, msp4, groEL, ankA, and p44 genes (Chen et al. 1994; Courtney et al. 2004; Alberti et al. 2005). Four to five times enlarged spleen with subscapular bleeding is the most important post-mortem finding of sheep, roe deer, and reindeer (Gordon et al. 1932; Øverås et al. 1993; Stuen 2003).

Fig. 1
figure 1

Anaplasma phagocytophilum (Morulae) in neutrophil of Giemsa-stained blood smear of new born calf (Henniger et al. 2013). Magnification 10 × 100

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Fig. 2
figure 2

Anaplasma phagocytophilum (Morulae) in lymphocyte of Giemsa-stained blood smear of new born calf (Henniger et al. 2013). Magnification 10 × 100

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Fig. 3
figure 3

Anaplasma marginale (arrow) in Giemsa-stained blood smears of dairy cow (Bos indicus). Magnification 10 × 100

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Treatment

Tetracycline is the drug of choice although levofloxacin, a fluoroquinolone, also showed vivo activity in cell culture in a human patient with history of chronic obstructive disease at the dose rate of 500 mg daily intravenous every 6 h for the first 24 h and oral 500 mg for 6 days (a total of 13 days treatment) but failed to control relapse of infection (Wormser et al. 2006).

Doxycycline, oral at the dose rate of 100 mg twice daily for 7–14 days, proved effective for adults in treating clinical human granulocytic anaplasmosis, and clinical recovery was noticed within 24 h using tetracycline at the dose rate of 500 mg/kg four times per day orally for 14 days (Goodman et al. 1996). Similarly, doxycycline hyclate 4.2 mg sustained release proved 100 % effective in preventing anaplasmosis as well as B. burgdorferi infection in mice (Zeidner et al. 2008). In pregnant women and patients with intolerant or allergy to tetracycline, rifampin at 10 mg/kg/day oral or chloramphenicol may be given (Goodman et al. 1996).

Long-acting oxytetracycline had proved effective for treatment and elimination of Anaplasma phagocytophilum in lambs at the dose rate of 20 and 10 mg/kg body weight intramuscular (Stuen and Bergstrom 2001). Similarly, this drug is also effective against other ruminants and horses. Doxycycline showed efficacy against canine and feline anaplasmosis in cat, dog, and captive timber wolf (CFSPH 2013).

Prevention

Various antigens have been recommended as vaccine candidates, but the main problem associated with development of effective vaccine is the existence of different variants, selection of suitable conserved antigen, lack of cross-protection studies, and antigenic variation against diverse genotypes (Stuen et al. 2013a). Recently, an important development in vaccine development is the identification of three invasin proteins OmpA, Asp14, and Aip A that are involved in infection process. The antibodies against these proteins most effectively blocked the A. phagocytophilum infection of host cells. The binding domains of these proteins could be used to develop vaccine (Seidman et al. 2015).

Control

Vaccine for A. phagocytophilum is not yet available. But, anti-tick vaccines would be a good option to control infection. Use of tick vaccines are environment-friendly, reduce tick load, decrease the incidence of tick-borne diseases (bovine anaplasmosis, babesiosis, and theileriosis), and minimize acaricide use (Graf et al. 2004; de la Fuente et al. 2006, 2011). Understanding vector-pathogen interactions would be an important tool for the control of tick and tick-borne pathogens. Immunization of animal reservoirs, high-risk animals, and human population would help in the control of anaplasmosis. Tick vaccines based on tick proteins interfere with tick vector competence such as SILK (Hajdušek et al. 2013; Zivkovic et al. 2010) and TROSPA (Hajdušek et al. 2013; Antunes et al. 2012). Rhipicephalus (R.) microplus tick proteins (BM86/BM95) have proved effective in reducing cattle tick infestations (Willadsen 2006; de la Fuente et al. 2007). Labuda et al. (2006) found that tick vaccine containing tick cement protein 64P of Rhipicephalus appendiculatus protected mice against Ixodes ricinus tick infestation and tick-associated encephalitis virus (TBEV). Subolesin (SUB) protein have resulted in lower R. microplus infestations and reduced levels of A. marginale and B. bigemina blood pathogens (de la Fuente et al. 2011; Merino et al. 2013). Tick proteins such as Q38, SILK, and SUB when used as vaccine reduced tick infestation and oviposition of R. microplus ticks (Merino et al. 2013).

Subolesin and akirin proteins are conserved among different vector species which might serve as a candidate for universal vaccine against various vector species, vector-borne diseases, and pathogen infection (de la Fuente et al. 2013). The vaccine that target both pathogen and vector for the prevention of ticks and anaplasmosis is a good option as in case of bovine anaplasmosis. Recently, Torina and associates (2014), developed vector-pathogen vaccine using both tick subolesin and Anaplasma marginale MSP1a proteins together. This resulted in lower tick infestation percentage and lower weight of female Boophilus microplus ticks and reduced A. marginale infection in cattle (Torina et al. 2014).

Similarities and differences

Differences

Anaplasma marginale and A. phagocytophilum are closely related bacteria that invade different host cell types. Only ruminants are prone to A. marginale infection, whereas A. phagocytophilum is a hetero-genetic, zoonotic pathogen with diverse host range including domestic and wild animals, rodents, reptiles, birds, and humans infecting neutrophils, monocytes, or endothelial cells (Foggie 1951; Rikihisa 2011). Bovine anaplasmosis caused by A. marginale characterized by fever, severe anemia, jaundice, pale mucous membranes, brownish urine, abortion, decreased milk production, hyperexcitability, weight loss, and mortality without hemoglobinemia and hemoglobinuria during acute phase of the infection (Richey and Palmer 1990). Hemolytic anemia is the major hematological finding. Conversely, A. phagocytophilum infections generate sudden onset of fever accompanied by secondary infection which is the common sequel of tick-borne fever and HGA in contrast to bovine anaplasmosis. Characteristic microcolonies of the bacterium called Morulae develop in peripheral blood of granulocytes (especially neutrophils) and monocytes and their precursors in the bone marrow (Foggie 1951).

Anaplasma phagocytophilum can be cultured in Ixodes scapularis tick cells ISE6 and IDE8, as well as human promyelocytic cell line HL-60, and where A. marginale cannot propagate in continuous mammalian culture system owing to unknown cell surface receptors. The coinfection between these pathogens cannot be achieved in same tissue culture system or cell line (Munderloh et al. 2004). These organisms show different gene expression on I. scapularis tick cell lines (ISE6) (Zivkovic et al. 2009). Moreover, there are cellular and humoral complexities in A. phagocytophilum as compared to A. marginale.

Similarities

A. marginale and A. phagocytophilum are closely related organisms on the basis of amino acid sequences (Aubry and Geale 2011). Concurrent infections, biological, mechanical, transplacental, and blood-contaminated/blood transfusion-associated transmission potentials are the major similarities for both Anaplasma species. Common treatment, prevention, elimination/chemosterilization, vaccination, tick control, breed resistance/susceptibility, selected common hosts, persistent infection, cross-reactivity, antigenic variation, super-infection, re-infection, strain diversity, host tropism or 16SrRNA gene base classification (family Anaplasmataceae), major surface proteins, pseudogenes, and hypothetical proteins are the common attributes of these tick-associated pathogens.

Concurrent infection with other tick-borne pathogens (marginale and/or Borrelia burgdorferi) in animals and tick vectors have been reported (Berger 2014; de la Fuente et al. 2005). Both pathogens have the ability to produce novel antigens in the presence of other strains (super-infection). Antigenic variants develop specific antibodies after each rickettsemic peak by msp2/p44. Re-infection is also the attribute of these organisms by their respective heterologous strains (Stuen et al. 2009; Futse et al. 2008; Vallejo Esquerra et al. 2014).

Molecular analysis of 16SrRNA gene of A. marginale and A. phagocytophilum showed that both the organisms transform major surface proteins variants during persistent infection in host and tick vectors. During each rickettsemic peak, msp2 antigenic gene variants are formed that have specific host antibody responses. Extensive antigenic variations of msp2 gene of outer surface membrane of A. marginale influenced by host immune system during persistence in host and tick vectors during developmental cycle. Antigenic variations and cross-protection are the challenges for vaccine development (Kocan et al. 2010b).

The P44 protein of A. phagocytophilum is homologous to msp2 belong to multigene family of A. marginale evolved due to combinatorial gene conversion with the establishment of condensed pseudogenes (Brayton et al. 2002). These two organisms are similar on the basis of diverse antigenic surface proteins in their small genome and generate specific antibody response. There are six major surface antigenic proteins including msp1α, msp1β, msp2, msp3, msp4, and msp5, whereas msp1α and msp1β are distinctive in A. marginale.

A dominant antibody response is established against 40-kDa outer membrane protein (MSP2/P44) of Anaplasma phagocytophilum, same as expressed in A. marginale infections (Ijdo et al. 1997). Expression of immunodominant antigenic major surface proteins (MSPs) share homology with Anaplasma marginale MSP2 and MSP4 (Lin et al. 2004; Vidotto et al. 2006). The MSP4 exhibits host-specific properties and involved in host-pathogen interaction, and because of this property, MSP4 bear selective pressure by the host immune system. Thus, a high degree of genetic heterogeneity is seen among A. phagocytophilum strains (Massung et al. 2003; de la Fuente et al. 2005). The msp4 gene sequences may not give phylo-geographic information but can be used for A. phagocytophilum strain differentiation from humans, ruminants, and nonruminant domestic animals.

The msp5 gene is conserve in all the Rickettsiales, this is highly stable among all Anaplasma species as well as isolates from the USA (Palmer et al. 2004). This is the cause of cross reactivity of Anaplasma marginale with A. phagocytophilum during indirect and competitive ELISA (Strik et al. 2007)

Detailed knowledge about epidemiology, ecology, vector biology and competence, risk factors and longitudinal studies for clinical manifestations, pathogenesis, and cellular and humoral responses of virulent pathogenic strains is lacking. Cell culture techniques (animal and tick) with targeted knockdown genes, transformation, multilocus sequence typing, blood meal genetic analyses, pulse field gel electrophoresis, high-throughput genome sequencing, and microbiomic and metagenomic analyses are currently available methodologies to explain population genetic structures and the evolutionary mechanism. Upcoming studies should therefore focus on the association between diverse genetic strains in different reservoir hosts and ticks by blood meal analysis and genetic fingerprinting to unstitch the biology, phylo-geographic distribution for better estimation of risk factors, and disease management.

Conclusion

All age groups are susceptible to bovine anaplasmosis and tick-borne fever, but prevalence increases with age. Movement of susceptible domestic animals from tick-free non-endemic regions to disease endemic regions is the major risk factor. Recreational or high-risk tick exposure activities and blood transfusion is an important risk factor of human granulocytic anaplasmosis. Efforts are being focused for the identification of novel antigenic parts for a universal vaccine, effective against diverse geographic stains.

It is difficult to bridge the similarities and differences between two important tick-transmitted pathogens of family Anaplasmataceae (Table 2). A. phagocytophilum is relatively a new pathogen which got international research focus after the first human case in 1986. Both the organisms share a reasonable degree of commonality. The advances in epidemiological, molecular, and genetic engineering approaches in cell culture, vector ecology, clinical, experimental, immunological, and longitudinal studies may be utilized for both organisms being the member of family Anaplasmataceae keeping in view the differences in pathogenicity, host tropism, and strain diversity for ticks, humans, and domestic and wild animals. Moreover, the whole plasmid (4.5 kb) was successfully inserted in the chromosome of Anaplasma marginale by single homologous crossover (Felsheim et al. 2010). Similarly, this technique should be applied to transform Anaplasma phagocytophilum as earlier performed for A. marginale for better understanding the infection biology for effective prevention and control of this pathogen as well.

Table 2 Comparison of Anaplasma marginale and Anaplasma phagocytophilum bacteria on epidemiology, diagnosis, treatment, prevention, and control
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