Anaplasmosis

Anaplasmosis is a tick-borne infectious disease caused by obligate intracellular bacteria of the genus Anaplasma, primarily affecting dogs, cats, and other mammals. In veterinary medicine, the most clinically significant species are Anaplasma phagocytophilum (formerly Ehrlichia phagocytophila, the agent of granulocytic anaplasmosis) and Anaplasma platys (the agent of infectious cyclic thrombocytopenia). The disease is characterized by acute febrile illness, lethargy, thrombocytopenia, and in some cases, polyarthritis or bleeding tendencies. Anaplasmosis is a zoonotic concern, particularly for A. phagocytophilum, which can cause human granulocytic anaplasmosis. The disease is transmitted by Ixodid ticks, with Ixodes scapularis and Ixodes pacificus being primary vectors for A. phagocytophilum in North America, and Rhipicephalus sanguineus for A. platys. The clinical presentation ranges from subclinical infection to severe systemic disease, and the pathogenesis involves immune-mediated destruction of infected cells and inflammatory responses.

The primary causative agents are Anaplasma phagocytophilum and Anaplasma platys. A. phagocytophilum is a Gram-negative, obligate intracellular bacterium that infects neutrophils and, less commonly, eosinophils. It is transmitted by Ixodes ticks, particularly Ixodes scapularis (deer tick) in the eastern United States, Ixodes pacificus (western black-legged tick) in the western United States, and Ixodes ricinus in Europe. The bacterium is maintained in small mammal reservoirs, such as white-footed mice and other rodents. A. platys infects platelets (thrombocytes) and is transmitted by Rhipicephalus sanguineus (brown dog tick). The organism is found worldwide, with a higher prevalence in tropical and subtropical regions. Both species are members of the family Anaplasmataceae, order Rickettsiales. The bacteria have a unique life cycle involving a replicative form (reticulate body) and an infectious form (dense-core body) within host cell membrane-bound vacuoles. Transmission occurs through tick saliva during feeding, typically requiring 24-48 hours of tick attachment. Co-infections with other tick-borne pathogens (e.g., Borrelia burgdorferi, Ehrlichia canis, Babesia spp.) are common and can complicate diagnosis and treatment.

Anaplasmosis is reported worldwide, with geographic distribution closely linked to the habitat of the tick vectors. In North America, A. phagocytophilum is most prevalent in the northeastern, mid-Atlantic, and upper Midwest regions of the United States, as well as in parts of Canada. In Europe, it is found in Scandinavia, Central Europe, and the British Isles. A. platys is more common in Mediterranean countries, Africa, Asia, and the Americas, particularly in regions where Rhipicephalus sanguineus is endemic. Dogs of all ages and breeds are susceptible, but there is no strong breed or sex predilection. However, outdoor dogs with high tick exposure are at increased risk. The incidence peaks during tick activity seasons, typically spring through autumn, but can occur year-round in warmer climates. Seroprevalence studies in endemic areas show that a significant proportion of healthy dogs may be seropositive, indicating subclinical infection. Cats are less commonly affected but can be infected with A. phagocytophilum, with seroprevalence rates varying by region. The disease is not directly contagious between animals; transmission requires a tick vector. Co-infections with other tick-borne diseases are common, and concurrent infection may exacerbate clinical signs.

The pathophysiology of anaplasmosis involves the invasion of host cells, primarily neutrophils (A. phagocytophilum) or platelets (A. platys), leading to cell damage and immune-mediated responses. A. phagocytophilum enters neutrophils via receptor-mediated endocytosis, involving the bacterial surface protein OspA and host cell selectin ligands. Once inside, the bacterium resides within a membrane-bound vacuole, where it replicates and forms morulae. The infected neutrophils may undergo apoptosis, but the bacteria can also inhibit apoptosis to enhance survival. The host immune response includes the production of pro-inflammatory cytokines (TNF-α, IL-1, IL-6), leading to fever and acute-phase response. Thrombocytopenia in A. phagocytophilum infection is primarily due to immune-mediated destruction of platelets, as well as sequestration in the spleen. In A. platys infection, the bacteria directly infect platelets, causing their destruction and removal by the reticuloendothelial system, leading to cyclic thrombocytopenia with a periodicity of 1-2 weeks. The immune response may also result in the formation of anti-platelet antibodies, exacerbating thrombocytopenia. Additionally, vasculitis and endothelial damage can occur, contributing to organ dysfunction. In severe cases, the inflammatory response can lead to multi-organ involvement, including the liver, spleen, and bone marrow. The clinical signs are often a result of the host's inflammatory response rather than direct cytopathic effects of the bacteria.

The primary predisposing factor for anaplasmosis is exposure to infected ticks. Dogs that spend time outdoors in tick-infested areas, particularly in tall grass, wooded areas, or areas with high wildlife populations, are at increased risk. Lack of consistent tick prevention measures significantly increases the likelihood of infection. Age is a factor, as younger dogs may be more susceptible to clinical disease, but older dogs can also be affected. Immunosuppression, whether due to concurrent disease, stress, or medication (e.g., corticosteroids), may increase the severity of infection. Co-infections with other tick-borne pathogens (e.g., Ehrlichia, Babesia, Borrelia) are common and can complicate the clinical picture, potentially leading to more severe disease. Genetic factors may influence the host's immune response, but specific breed predispositions have not been clearly established. Environmental factors such as climate change and expansion of tick habitats are increasing the geographic range of the disease. Additionally, dogs that are not on year-round tick control are at higher risk, as ticks can be active in cooler months in some regions.

Clinical signs of anaplasmosis typically appear 1-2 weeks after tick exposure and can range from subclinical to severe. The most common signs include fever (often >103°F/39.4°C), lethargy, depression, and anorexia. Musculoskeletal signs are frequent, with lameness, joint pain, and muscle stiffness due to polyarthritis. Gastrointestinal signs such as vomiting and diarrhea may occur. Respiratory signs, including coughing and dyspnea, are less common but can occur. In some cases, neurological signs such as seizures or ataxia have been reported, though rare. Bleeding tendencies, such as petechiae, ecchymoses, and epistaxis, may be seen, particularly with A. platys infection due to severe thrombocytopenia. In chronic or severe cases, signs of systemic inflammatory response syndrome (SIRS) or multi-organ dysfunction may develop. Physical examination may reveal pale mucous membranes, lymphadenomegaly, splenomegaly, and signs of dehydration. It is important to note that many infected dogs are asymptomatic, and clinical signs may resolve spontaneously without treatment, but the infection can persist.

Differential diagnoses for anaplasmosis include other tick-borne diseases such as ehrlichiosis (Ehrlichia canis, E. ewingii), Lyme disease (Borrelia burgdorferi), babesiosis (Babesia canis, B. gibsoni), and Rocky Mountain spotted fever (Rickettsia rickettsii). Other infectious causes of fever and thrombocytopenia include sepsis, bacterial endocarditis, and viral infections (e.g., canine distemper). Non-infectious causes include immune-mediated thrombocytopenia (ITP), immune-mediated polyarthritis, systemic lupus erythematosus (SLE), and neoplasia (e.g., lymphoma). Key differentiating features: Ehrlichiosis often presents with more severe thrombocytopenia and may have a chronic phase with bone marrow suppression; Lyme disease is characterized by recurrent lameness and glomerulonephritis; babesiosis causes hemolytic anemia with regenerative response; Rocky Mountain spotted fever may present with cutaneous lesions and neurological signs. ITP is typically negative for tick-borne serology and PCR, and responds to immunosuppressive therapy. Polyarthritis from anaplasmosis is usually non-erosive and responds to doxycycline, whereas immune-mediated polyarthritis requires immunosuppression. Definitive diagnosis relies on PCR, serology, and cytology demonstrating morulae.

The diagnostic approach for suspected anaplasmosis begins with a thorough history, including tick exposure and travel history, and a complete physical examination. Initial laboratory tests include a complete blood count (CBC), serum biochemistry profile, and urinalysis. The presence of thrombocytopenia, leukopenia or leukocytosis, and elevated liver enzymes (ALT, ALP) may raise suspicion. If anaplasmosis is suspected, specific testing should be performed. The first-line test is often a point-of-care ELISA or immunofluorescence assay (IFA) for antibodies against A. phagocytophilum and A. platys. However, serology can be negative in acute infection, so PCR on whole blood or EDTA blood is recommended for early detection. PCR is highly sensitive and specific and can differentiate between species. In cases where morulae are seen on blood smear (intracytoplasmic inclusions in neutrophils or platelets), this is supportive but not always present. If the initial serology is negative but clinical suspicion is high, repeat serology in 2-4 weeks to check for seroconversion. In endemic areas, a positive serology in a dog with compatible clinical signs and thrombocytopenia is often sufficient for a presumptive diagnosis. Co-infection testing for other tick-borne diseases (ehrlichiosis, Lyme, babesiosis) is recommended due to common co-infections. Advanced imaging (radiographs, ultrasound) may be indicated if there is evidence of organ involvement, such as hepatosplenomegaly or pulmonary changes.

Hematology: The most common finding is thrombocytopenia, which can be severe (<50,000/µL) in A. platys infection. Anemia may be present, but it is usually mild and non-regenerative unless there is concurrent blood loss or hemolysis. Leukopenia (neutropenia, lymphopenia) is common in acute A. phagocytophilum infection, while leukocytosis may occur in some cases. Blood smear examination may reveal morulae in neutrophils (A. phagocytophilum) or platelets (A. platys), but this is not always observed. Serum biochemistry: Mild to moderate elevations in liver enzymes (ALT, ALP) are common. Hyperglobulinemia may be present, especially in chronic cases. Hypoalbuminemia can occur due to protein-losing nephropathy or gastrointestinal loss. Electrolyte imbalances are uncommon. Urinalysis: Proteinuria may be present, and if significant, a urine protein:creatinine ratio (UPC) should be measured to assess renal involvement. Blood gas analysis: May show metabolic acidosis in severely affected animals. Specific biomarkers: C-reactive protein (CRP) is often elevated. Serology: Antibody titers (IFA) or point-of-care ELISA for A. phagocytophilum and A. platys. PCR: Whole blood PCR is the most sensitive and specific test for active infection. It can be positive within days of infection and remains positive during the acute phase. In chronic carriers, PCR may be negative, but serology is positive.

Radiography: Thoracic radiographs may be normal, but in cases with respiratory signs, interstitial or alveolar patterns may be seen. Abdominal radiographs may reveal hepatosplenomegaly. Ultrasonography: Abdominal ultrasound may show splenomegaly, hepatomegaly, and possibly lymphadenomegaly. In cases with polyarthritis, joint ultrasound may show effusion. Computed Tomography (CT) and Magnetic Resonance Imaging (MRI): These are rarely needed but may be used to evaluate for neurological involvement or to rule out other causes of fever of unknown origin. Echocardiography: Not typically indicated unless there is suspicion of endocarditis. Endoscopy: Not used in the diagnosis of anaplasmosis. Fluoroscopy: Not used.

Cytology: Fine needle aspirates of lymph nodes, spleen, or bone marrow may show reactive hyperplasia. In acute infection, morulae may be seen in neutrophils or platelets on blood smears or buffy coat smears. Joint fluid analysis in cases of polyarthritis typically reveals a neutrophilic inflammation with increased protein and cell counts (often >5,000 cells/µL). Histopathology: If biopsies are performed (e.g., liver, spleen, kidney), findings may include perivascular lymphoplasmacytic inflammation, and in the kidney, glomerulonephritis. Immunohistochemistry or PCR on tissue samples can confirm the presence of Anaplasma organisms. However, histopathology is not commonly needed for diagnosis.

The treatment of choice for anaplasmosis is doxycycline. The recommended dosage is 5 mg/kg orally every 12 hours, or 10 mg/kg orally every 24 hours, for a minimum of 14 days, and often 21-28 days to ensure clearance. In severe cases, doxycycline can be administered intravenously at 5 mg/kg every 12 hours initially, but oral therapy is preferred once the animal can tolerate it. Tetracycline (22 mg/kg orally every 8 hours) can be used as an alternative, but doxycycline is preferred due to better tissue penetration and fewer side effects. Supportive care is essential: intravenous fluids for dehydration or shock, antiemetics (e.g., maropitant 1 mg/kg SC once daily) for vomiting, and nutritional support if anorexic. For severe thrombocytopenia with bleeding, platelet transfusions may be necessary, but this is rare. Non-steroidal anti-inflammatory drugs (NSAIDs) should be avoided in the acute phase due to the risk of gastrointestinal ulceration and renal impairment, but may be used cautiously for pain if needed. Glucocorticoids are not recommended unless there is severe immune-mediated hemolytic anemia or thrombocytopenia, but they may be considered in refractory cases. In co-infections, treat accordingly. Tick prevention is crucial to prevent reinfection.

The prognosis for anaplasmosis is generally excellent with prompt and appropriate treatment. Most dogs show clinical improvement within 24-48 hours of starting doxycycline. The mortality rate is low, especially in otherwise healthy animals. However, severe cases with multi-organ involvement or concurrent infections may have a guarded prognosis. Chronic infection with A. platys can cause persistent thrombocytopenia, but treatment is usually curative. Relapse can occur if treatment is inadequate or if the dog is re-exposed to ticks. Negative prognostic indicators include severe thrombocytopenia (<20,000/µL), marked leukopenia, evidence of disseminated intravascular coagulation (DIC), and renal failure. With treatment, the majority of dogs recover fully without long-term sequelae.

After treatment, a recheck examination is recommended at 2-4 weeks to assess clinical resolution and repeat a CBC to ensure platelet count has normalized. If the initial PCR was positive, a repeat PCR at 4 weeks post-treatment can confirm clearance, but this is not always necessary if clinical signs have resolved. Serology may remain positive for months, so it is not useful for monitoring cure. Long-term management includes strict tick control with acaricides (e.g., fipronil, imidacloprid, permethrin) and environmental management. Annual screening for tick-borne diseases is recommended in endemic areas. If clinical signs recur, re-evaluation is warranted.

Pearls: 1) Always consider anaplasmosis in any dog with acute fever, lethargy, and thrombocytopenia, especially with a history of tick exposure. 2) Doxycycline is the drug of choice; clinical improvement is often dramatic within 24-48 hours. 3) Morulae are not always seen on blood smears; PCR is the most sensitive test. 4) Co-infections are common; test for other tick-borne diseases. 5) Tick prevention is essential to prevent recurrence. Pitfalls: 1) Do not rely solely on serology for acute diagnosis; antibodies may be negative early in infection. 2) Avoid using NSAIDs in the acute phase due to risk of bleeding and renal injury. 3) Do not use glucocorticoids unless there is a clear immune-mediated complication, as they can exacerbate the infection. 4) Do not stop doxycycline early; a minimum of 14 days is required, and 21-28 days is often recommended. 5) Failure to consider other tick-borne diseases can lead to incomplete treatment.

Doxycycline: 5 mg/kg PO q12h or 10 mg/kg PO q24h for 14-28 days. For severe cases, IV doxycycline at 5 mg/kg q12h may be used initially. Tetracycline: 22 mg/kg PO q8h for 14-21 days. Minocycline: 5 mg/kg PO q12h for 14-21 days (alternative). Supportive care: IV fluids (e.g., Lactated Ringer's solution) at maintenance rates (60-100 ml/kg/day) adjusted for dehydration. Antiemetics: Maropitant (Cerenia) 1 mg/kg SC q24h or 2 mg/kg PO q24h. Gastroprotectants: Omeprazole 0.5-1 mg/kg PO q24h if needed. For severe thrombocytopenia with bleeding: Platelet-rich plasma or fresh whole blood transfusion. Avoid NSAIDs and corticosteroids unless specifically indicated. In cases of co-infection with Ehrlichia or Lyme, doxycycline is also effective. For Babesia co-infection, add imidocarb dipropionate (6.6 mg/kg IM once, repeat in 14 days) or atovaquone/azithromycin combination.

Several studies have evaluated the efficacy of doxycycline for anaplasmosis. A study by Greig et al. (1996) demonstrated that doxycycline at 10 mg/kg/day for 14 days was effective in resolving clinical signs and thrombocytopenia in dogs with A. phagocytophilum infection. Another study by Kohn et al. (2008) compared doxycycline and tetracycline and found both effective, but doxycycline had fewer side effects. The ACVIM consensus statement on tick-borne diseases (2018) recommends doxycycline as the first-line treatment for anaplasmosis. PCR is considered the gold standard for diagnosis, with a sensitivity of >90% in acute infection. Serology using IFA or ELISA is useful for epidemiological studies but may be negative early. Co-infections are common; a study by Beall et al. (2008) found that 30% of dogs with anaplasmosis were co-infected with Borrelia burgdorferi. Treatment with doxycycline is effective for both. The prognosis is excellent, with a mortality rate of less than 5% in treated dogs. Long-term follow-up studies show no chronic sequelae in most dogs.