Tick-Borne Diseases in Dogs: Pathogens, Clinical Signs, Diagnosis, and Prevention

Introduction

Tick-borne diseases constitute a major category of infectious diseases in dogs worldwide. The geographic distribution of canine tick-borne pathogens is expanding due to climate change, habitat alteration, and increased movement of companion animals [1]. Hard ticks (family Ixodidae) of the genera Ixodes, Dermacentor, Rhipicephalus, and Amblyomma serve as the primary vectors for bacterial, protozoal, and rickettsial agents that cause clinical disease in dogs [1, 2]. Understanding the complex interplay between vector, pathogen, and host is essential for effective clinical management and prevention. This review covers the major dog tick transmitted diseases, their etiological agents, pathogenesis, clinical manifestations, diagnostic workup, treatment options, and integrated control strategies.

Etiological Agents and Epidemiology

Bacterial Pathogens

Anaplasma phagocytophilum and Anaplasma platys are obligate intracellular bacteria that infect granulocytes and platelets, respectively [1]. A. phagocytophilum causes canine granulocytic anaplasmosis and is transmitted predominantly by Ixodes ticks [2]. A. platys infects platelets and causes cyclic thrombocytopenia; its primary vector is Rhipicephalus sanguineus [1, 2].

Ehrlichia canis is the etiological agent of canine monocytic ehrlichiosis. It is transmitted by R. sanguineus and targets monocytes and macrophages [1]. Other Ehrlichia species (E. ewingii, E. chaffeensis) also infect dogs but are less common [2].

Borrelia burgdorferi sensu lato, primarily B. burgdorferi sensu stricto in North America and B. garinii/B. afzelii in Eurasia, causes canine Lyme borreliosis [1, 2]. Transmission occurs through Ixodes ticks, with nymphal stages being the most efficient vectors [1].

Rickettsia rickettsii, the agent of Rocky Mountain spotted fever (RMSF), is transmitted by Dermacentor and Rhipicephalus ticks [2]. The pathogen infects endothelial cells and causes vasculitis [1, 2].

Bartonella vinsonii subsp. berkhoffii and other Bartonella species are facultative intracellular bacteria associated with endocarditis and granulomatous inflammation in dogs [1]. Ticks are suspected vectors, though flea transmission is also documented [2].

Francisella tularensis, the causative agent of tularemia, can be transmitted by tick bites (especially Dermacentor and Amblyomma spp.) [1]. Dogs are incidental hosts and may develop fever, lymphadenopathy, and abscessation [2].

Protozoal Pathogens

Babesia canis (large form) and Babesia gibsoni (small form) are intraerythrocytic apicomplexan parasites causing canine babesiosis [1, 2]. B. canis is transmitted by Dermacentor reticulatus in Europe and Rhipicephalus spp. elsewhere; B. gibsoni is transmitted by Haemaphysalis longicornis and R. sanguineus and also through dog-to-dog transmission via bite wounds [2]. Babesia conradae and Babesia vulpes are less common but cause severe disease [1].

Hepatozoon canis and Hepatozoon americanum are tick-borne protozoa with a unique transmission route: dogs become infected by ingesting ticks containing mature oocysts, not through tick saliva [1, 2]. H. canis (transmitted by R. sanguineus) causes moderate disease, whereas H. americanum (transmitted by Amblyomma maculatum) is highly pathogenic [2].

Theileria species (e.g., Theileria equi in horses, Theileria annulata in cattle) rarely infect dogs, but Theileria spp. have been detected in dogs from Asia and Europe, usually causing mild or subclinical infection [1].

Pathogenesis and Clinical Signs

Anaplasmosis

Canine granulocytic anaplasmosis (A. phagocytophilum) presents with acute fever, lethargy, anorexia, and lameness. Joint pain and neurological signs (seizures, ataxia) can occur [1, 2]. Thrombocytopenia is consistently present due to immune-mediated destruction and platelet consumption [2]. Chronic infection is rare; most dogs recover with appropriate therapy [1].

Canine cyclic thrombocytopenia (A. platys) causes recurrent thrombocytopenia occurring every 10–14 days. Clinical signs are often mild: fever, petechiation, and epistaxis [1]. Subclinical carriers are common [2].

Ehrlichiosis

Canine monocytic ehrlichiosis (E. canis) progresses through three phases: acute (fever, lymphadenopathy, thrombocytopenia), subclinical (persistent infection without signs), and chronic (pancytopenia, bleeding diathesis, secondary infections) [1, 2]. Hyperglobulinemia and glomerulonephritis are common sequelae [2].

Lyme Borreliosis

Only 5–10% of B. burgdorferi-infected dogs develop clinical signs. The hallmark is acute lameness due to immune-mediated polyarthritis [1]. Fever, lethargy, and lymphadenopathy accompany the lameness. Rarely, Lyme nephritis (protein-losing nephropathy) occurs and carries a grave prognosis [1, 2].

Rocky Mountain Spotted Fever

R. rickettsii causes acute febrile illness with petechial rash, edema of the extremities and face, neurological signs, and multi-organ failure [1, 2]. Mortality can be high without prompt treatment [2].

Babesiosis

Clinical severity varies with species and host immunity. B. canis typically causes acute hemolytic anemia with hemoglobinuria, fever, and jaundice [1]. B. gibsoni infection is often milder but can become chronic. B. conradae and B. vulpes can cause fatal hemolysis [1, 2]. Immune-mediated hemolytic anemia may persist after parasitemia resolves [2].

Hepatozoonosis

H. americanum infection results in severe myositis, osteomyelitis, and periosteal hyperostosis. Dogs present with profound lethargy, fever, muscle atrophy, and reluctance to move [1, 2]. H. canis causes milder myositis, often subclinical [2].

Diagnosis

Hematology and Blood Smear

Complete blood count reveals thrombocytopenia in anaplasmosis, ehrlichiosis, and RMSF; hemolytic anemia with spherocytosis in babesiosis; and neutrophilic leukocytosis with left shift in hepatozoonosis [1, 2]. Examination of Giemsa- or Wright-stained blood smears can identify morulae (intracytoplasmic inclusions) in granulocytes (A. phagocytophilum) or platelets (A. platys), merozoites in erythrocytes (Babesia spp.), and gamonts in neutrophils (Hepatozoon spp.) [1, 2].

Serology

Indirect immunofluorescence antibody (IFA) assays and commercial enzyme-linked immunosorbent assays (ELISAs) detect antibodies against B. burgdorferi C6 peptide, E. canis, A. phagocytophilum, and A. platys [1, 2]. Serology must be interpreted carefully; antibodies indicate exposure but not necessarily active disease [2].

Molecular Diagnostics

Polymerase chain reaction (PCR) assays targeting ribosomal RNA genes or species-specific genes (e.g., p44 for Anaplasma, 16S rRNA for Ehrlichia, 18S rRNA for Babesia) are highly sensitive and specific for active infection [1, 2]. Real-time quantitative PCR can monitor treatment response. PCR of whole blood, buffy coat, or tissue biopsy is recommended [1, 2].

Point-of-Care Tests

In-clinic rapid tests (e.g., immunochromatographic assays) commonly detect combined antigen/antibody panels for B. burgdorferi, E. canis, E. ewingii, A. phagocytophilum, and A. platys [1, 2]. These tests are useful for screening but require confirmatory PCR or IFA when results are discordant with clinical signs [2].

Imaging and Histopathology

Radiography may reveal periosteal new bone formation in hepatozoonosis [2]. Ultrasound can identify splenomegaly, hepatomegaly, or renal changes in Lyme nephritis [1]. Biopsy of affected tissues (e.g., kidney, muscle, synovium) with histopathology and immunohistochemistry can confirm infection [1, 2].

The diagnostic pathway for suspected tick-borne disease is illustrated below.

flowchart TD
    A[Clinical suspicion: fever, lameness, thrombocytopenia, anemia], > B{Signalment & tick exposure history}
    B, > C[Complete blood count + blood smear]
    C, > D{Blood smear findings}
    D, >|Morulae in granulocytes| E[Anaplasma phagocytophilum PCR]
    D, >|Morulae in platelets| F[Anaplasma platys PCR]
    D, >|Merozoites in RBC| G[Babesia spp. PCR]
    D, >|Gamonts in neutrophils| H[Hepatozoon spp. PCR]
    D, >|No visible organisms| I[Serology + broad-range PCR panel]
    E & F & G & H & I, > J[Confirmatory PCR or serology]
    J, > K[Positive result: initiate targeted therapy]
    J, > L[Negative result: consider other differentials]
    L, > M[Leptospirosis, immune-mediated disease, other infections]

Treatment

Antibiotic Therapy

Doxycycline (10 mg/kg orally every 12–24 hours for 21–28 days) is the cornerstone therapy for E. canis, Anaplasma spp., R. rickettsii, and Bartonella spp. [1, 2]. For Lyme borreliosis, doxycycline is also effective; amoxicillin or cefovecin are alternatives [1, 2]. Minocycline has similar efficacy [2].

Antiprotozoal Therapy

Babesiosis: Imidocarb dipropionate (5–6.6 mg/kg intramuscularly or subcutaneously, repeated after 14 days) is effective against B. canis but less so against B. gibsoni [1, 2]. Atovaquone (13.5 mg/kg orally every 8 hours) combined with azithromycin (10 mg/kg orally every 12–24 hours) is recommended for B. gibsoni and resistant cases [1]. Clindamycin (25 mg/kg orally every 12 hours) for 14 days is an alternative [2].

Hepatozoonosis: H. canis is treated with imidocarb (6 mg/kg subcutaneously every 14 days for three treatments) [1]. H. americanum requires a combination of trimethoprim-sulfadiazine (15 mg/kg orally every 12 hours), clindamycin (10 mg/kg orally every 8 hours), pyrimethamine (0.25 mg/kg orally every 24 hours), and decoquinate (20 mg/kg orally every 12 hours with food) for prolonged courses (months) [1, 2]. Relapse is common [2].

Supportive Care

Fluid therapy, blood transfusions for severe anemia, and immunosuppressive doses of glucocorticoids (prednisolone 1–2 mg/kg/day) for immune-mediated complications are indicated when necessary [1, 2]. Dogs with Lyme nephritis require aggressive management of proteinuria and hypertension [2].

Prevention

Tick Control

Integrated tick management is the primary preventive strategy. Topical acaricides (e.g., fipronil, permethrin, fluralaner, afoxolaner, sarolaner, lotilaner) and oral isoxazoline compounds (fluralaner, afoxolaner, sarolaner, lotilaner) provide rapid tick kill and reduce pathogen transmission [1, 2]. Collars containing deltamethrin or flumethrin offer sustained protection [2]. Seasonal prophylaxis in endemic areas is recommended [1].

Vaccination

A recombinant outer surface protein A (OspA) vaccine against B. burgdorferi is available for dogs in North America and parts of Europe [1, 2]. Vaccination does not prevent infection but reduces clinical disease and spirochete burden [1]. Vaccines for other tick-borne pathogens are not commercially available [2].

Environmental Management

Reducing tick habitat around the home (mowing tall grass, removing leaf litter, creating gravel barriers) and avoiding wooded or brushy areas during peak tick season minimizes exposure [1]. Routine tick checks and prompt removal using fine-tipped forceps can prevent transmission [2].

Integrated Clinical Approach

Because co-infections are common (e.g., B. burgdorferi with A. phagocytophilum or E. canis with Babesia spp.), a comprehensive diagnostic panel should be performed in any dog with compatible clinical signs and tick exposure [1, 2]. Empirical doxycycline therapy pending laboratory confirmation is often justified in acutely ill dogs with thrombocytopenia [1]. Treatment response should be re-evaluated at 7–14 days, extending therapy if clinical signs persist [2].

References

[1] Greene CE, ed. Infectious Diseases of the Dog and Cat. 4th ed. St. Louis, MO: Elsevier Saunders; 2012.

[2] Sykes JE, ed. Canine and Feline Infectious Diseases. St. Louis, MO: Elsevier Saunders; 2014.

[3] Merck Veterinary Manual. 11th ed. Kenilworth, NJ: Merck & Co., Inc.; 2016. *** Disclaimer: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.