Tick-Borne Illnesses in Dogs: A Comprehensive Clinical Review of Pathogens and Treatment Protocols

1. Introduction

Tick-borne diseases (TBDs) represent a significant and growing threat to canine health worldwide, driven by the expansion of tick vector habitats, increased global movement of animals, and climatic changes that favor tick reproduction and survival [1, 2]. Ticks are obligate hematophagous ectoparasites that serve as biological vectors for a diverse array of pathogenic bacteria, protozoa, and viruses [3, 4]. In dogs, the clinical consequences of tick-borne infections range from subclinical carrier states to acute, life-threatening multisystemic illness [5, 6]. The complexity of these diseases is compounded by the frequent occurrence of co-infections, where a single tick bite may transmit multiple pathogens simultaneously, leading to overlapping clinical syndromes and diagnostic challenges [1, 7].

This review provides a comprehensive clinical reference for veterinary practitioners and diagnosticians, focusing on the major bacterial and protozoal tick-borne pathogens affecting dogs. The epidemiology, vector biology, pathogenesis, clinical presentation, diagnostic approaches, and evidence-based treatment protocols for each pathogen group are discussed in detail. Emphasis is placed on the molecular mechanisms of host-pathogen interaction and the biophysical principles underlying modern diagnostic assays.

2. Major Bacterial Pathogens

2.1 Ehrlichia canis and Canine Monocytic Ehrlichiosis

Ehrlichia canis is an obligate intracellular Gram-negative bacterium belonging to the family Anaplasmataceae [8]. It is the primary etiological agent of canine monocytic ehrlichiosis (CME), a globally distributed disease transmitted predominantly by the brown dog tick, Rhipicephalus sanguineus sensu lato [5]. The pathogen exhibits tropism for mononuclear phagocytes, specifically monocytes and macrophages, within which it replicates within membrane-bound vacuoles called morulae [8].

The pathogenesis of CME proceeds through three clinical phases: acute, subclinical, and chronic [28]. During the acute phase, which occurs 1 to 3 weeks post-infection, E. canis disseminates via infected monocytes to the spleen, liver, and lymph nodes, triggering a systemic inflammatory response [28]. Thrombocytopenia is a hallmark hematological abnormality, resulting from immune-mediated platelet destruction, increased platelet consumption, and bone marrow suppression [28, 33]. The subclinical phase may persist for months to years, during which the dog remains seropositive but clinically normal, although persistent infection can lead to chronic disease characterized by bone marrow hypoplasia, pancytopenia, and secondary infections [28].

Molecular detection via polymerase chain reaction (PCR) targeting the 16S rRNA gene or species-specific genes such as p30 or dsb is considered the gold standard for confirming active infection [5]. Serological assays, including indirect immunofluorescence antibody (IFA) tests and enzyme-linked immunosorbent assays (ELISAs), detect antibodies against E. canis but cannot distinguish between active and past infection [9]. A study in Egypt reported a molecular prevalence of 23.33% (42/180) in dogs, with significant associations between infection and age, season, tick infestation, and hemorrhage [28]. Co-infections with Hepatozoon canis have been documented in Serbia, highlighting the need for comprehensive molecular screening in endemic regions [5].

2.2 Anaplasma Species

Two major Anaplasma species infect dogs: Anaplasma phagocytophilum (causing canine granulocytic anaplasmosis) and Anaplasma platys (causing infectious cyclic thrombocytopenia) [25, 27]. A. phagocytophilum is transmitted by Ixodid ticks, primarily Ixodes scapularis in North America and Ixodes ricinus in Europe [27]. The bacterium targets neutrophils, surviving within phagosomes by inhibiting phagolysosomal fusion and modulating host immune responses [27]. Clinical signs include acute onset fever, lethargy, anorexia, and musculoskeletal pain, often accompanied by thrombocytopenia and elevated liver enzyme activities [27]. Co-infections with Borrelia burgdorferi are common due to shared vector ecology, complicating clinical diagnosis and treatment [27].

A. platys is a platelet-specific pathogen transmitted by R. sanguineus s.l. [2]. Infection results in cyclical thrombocytopenia with a periodicity of 10 to 14 days, corresponding to rickettsemic peaks [2]. Most infections are subclinical, but severe thrombocytopenia can lead to petechiation and epistaxis [2]. A molecular survey in Turkey detected A. platys in 8.57% of dogs, with hemoplasma co-infections frequently observed [2].

2.3 Borrelia burgdorferi and Lyme Borreliosis

Borrelia burgdorferi sensu lato is a spirochete bacterium transmitted by Ixodid ticks, causing Lyme borreliosis in dogs [10]. The pathogen disseminates from the tick bite site through the bloodstream to joints, kidneys, and other tissues [34]. Clinical signs in dogs include acute lameness, fever, lymphadenopathy, and lethargy [34]. A subset of infected dogs develops Lyme nephritis, a severe immune-complex glomerulonephritis with a poor prognosis [34].

Seroprevalence studies using commercial ELISA kits have demonstrated widespread exposure in endemic areas [10]. A study in Saipan reported a seroprevalence of 58.0% for Ehrlichia spp. and 43.1% for Anaplasma spp., but B. burgdorferi seroprevalence was notably lower, reflecting regional differences in vector distribution [10]. Vaccination against B. burgdorferi is available and has been shown to reduce the odds of developing clinical signs after exposure, although efficacy varies [34].

2.4 Rickettsia Species

Rickettsia species are obligate intracellular Gram-negative bacteria belonging to the spotted fever group (SFG) [11]. Rickettsia conorii, the agent of Mediterranean spotted fever, and Rickettsia rickettsii, the agent of Rocky Mountain spotted fever, are the most clinically relevant species in dogs [11]. Rickettsia parkeri strain Atlantic rainforest has been identified in Amblyomma ovale ticks from dogs in El Salvador, representing a potential zoonotic risk [11]. The non-pathogenic Candidatus Rickettsia andeanae has been proposed to competitively exclude R. parkeri in Amblyomma tigrinum tick populations in Brazil [30].

Dogs serve as sentinel hosts for SFG rickettsiosis [11]. Clinical signs in dogs are often mild or subclinical, but fever, lymphadenopathy, and rash may occur [25]. Diagnosis relies on serology (IFA) and PCR of blood or tick tissue [12].

2.5 Other Bacterial Pathogens

Coxiella burnetii, the agent of Q fever, has been detected in ticks collected from dogs in Egypt and Ghana [1, 4]. The zoonotic potential of C. burnetii is significant, but its role in canine clinical disease is poorly defined [1, 4]. Bartonella species, including B. henselae, have been identified in dog ticks and fleas, and may cause endocarditis and granulomatous disease in dogs [13]. Brucella canis and Chlamydia spp. have been detected in ticks, but conclusive evidence of tick-borne transmission to dogs remains lacking [14].

3. Major Protozoal Pathogens

3.1 Babesia Species

Canine babesiosis is caused by intra-erythrocytic apicomplexan parasites of the genus Babesia [15, 16]. The two major species are Babesia canis (large piroplasm, 4-5 µm) and Babesia gibsoni (small piroplasm, 1-3 µm) [17]. Babesia vogeli is a subspecies of B. canis transmitted by R. sanguineus s.l. and is prevalent in tropical and subtropical regions [16, 6]. Babesia rossi is a highly virulent species found in southern Africa [23]. Babesia conradae is a small piroplasm identified in coyote-hunting Greyhound dogs in the United States [15].

The pathogenesis of babesiosis involves hemolytic anemia, hemoglobinuria, and systemic inflammatory response syndrome [23, 26]. Clinical signs range from mild fever and lethargy to severe hemolytic crisis, jaundice, and acute kidney injury [15]. Co-infections with Ehrlichia canis or Theileria equi have been reported, although a study in South Africa found a low co-infection prevalence of 2.0% in dogs with B. rossi infection, suggesting a possible protective effect [23].

Diagnosis is achieved through light microscopy of Giemsa-stained blood smears and PCR targeting the 18S rRNA gene [16]. A recombinant B. gibsoni thrombospondin-related adhesive protein (BgTRAP)-based indirect ELISA has been developed for serological surveillance, demonstrating 84% sensitivity and 73.33% specificity [17].

3.2 Hepatozoon canis

Hepatozoon canis is a protozoan parasite transmitted by ingestion of infected R. sanguineus s.l. ticks, rather than through tick saliva [2, 5, 16]. The parasite undergoes sexual reproduction in the tick gut, and infectious oocysts are released into the tick hemocoel. Dogs become infected by ingesting ticks containing mature oocysts [2]. The parasite then migrates to target organs, primarily the spleen, bone marrow, and lymph nodes, where merogony occurs [5].

Clinical signs of hepatozoonosis include fever, lethargy, muscle atrophy, and periosteal bone proliferation [5, 16]. Co-infections with E. canis are common [5]. Molecular surveys have reported H. canis prevalence rates of 22.85% in Turkey and 13.6% in ticks from dogs in Egypt [1, 2].

4. Diagnostic Approaches

The diagnosis of tick-borne illnesses in dogs requires a multimodal approach integrating clinical assessment, hematological and biochemical profiling, direct pathogen detection, and serological testing [9]. The following table summarizes the key diagnostic modalities for major canine tick-borne pathogens.

The diagnostic workflow for a dog presenting with suspected tick-borne illness is illustrated in the following decision tree.

graph TD
    A["Clinical Suspicion: Fever, Lethargy, Thrombocytopenia, Anemia"] --> B{History of Tick Exposure?}
    B -->|Yes| C[Perform CBC, Biochemistry, Blood Smear]
    B -->|No| D[Consider Other Etiologies]
    C --> E{Morulae or Piroplasms Detected?}
    E -->|Yes| F[Targeted PCR for Suspected Pathogen]
    E -->|No| G[Perform Broad-Spectrum PCR Panel]
    F --> H[Species Confirmation via Sequencing]
    G --> H
    H --> I{Pathogen Identified?}
    I -->|Yes| J[Initiate Pathogen-Specific Treatment]
    I -->|No| K[Consider Serology for Antibody Detection]
    K --> L{Seropositive?}
    L -->|Yes| M["Interpret as Exposure; Consider Acute vs. Chronic"]
    L -->|No| N["Re-evaluate Clinical Case; Consider Non-Tick Etiologies"]
    J --> O[Monitor Clinical Response and Hematological Recovery]
    O --> P[Repeat PCR at 30 Days Post-Treatment to Confirm Clearance]

5. Treatment Protocols

Treatment of tick-borne illnesses in dogs is pathogen-specific and should be guided by confirmed diagnosis and clinical severity. The following protocols are based on current evidence and clinical guidelines.

5.1 Ehrlichiosis and Anaplasmosis

The treatment of choice for both E. canis and Anaplasma spp. infections is doxycycline hyclate administered at 5 mg/kg orally twice daily (or 10 mg/kg once daily) for 28 days [28, 33]. Doxycycline is a bacteriostatic tetracycline antibiotic that inhibits protein synthesis by binding to the 30S ribosomal subunit. Clinical improvement is typically observed within 24 to 48 hours, but the full course is necessary to eliminate the pathogen [28]. For dogs that cannot tolerate doxycycline, minocycline or chloramphenicol may be considered, although efficacy data are limited [33]. Supportive care, including fluid therapy, blood transfusions for severe anemia, and immunomodulatory doses of corticosteroids for immune-mediated thrombocytopenia, may be required in severe cases [28, 33].

5.2 Lyme Borreliosis

Doxycycline at 10 mg/kg orally once daily for 30 days is the recommended treatment for Lyme borreliosis [34]. Doxycycline is preferred over amoxicillin due to its additional activity against co-infecting Anaplasma and Ehrlichia species [34]. Clinical signs of lameness and fever typically resolve within 24 to 72 hours. Dogs with Lyme nephritis require aggressive supportive care, including immunosuppressive doses of corticosteroids, angiotensin-converting enzyme inhibitors, and dietary protein restriction [34].

5.3 Babesiosis

Treatment of canine babesiosis depends on the species involved. For B. canis and B. vogeli, the standard therapy is imidocarb dipropionate administered at 5 to 6.6 mg/kg intramuscularly or subcutaneously, repeated once after 14 days [16]. Imidocarb is a carbanilide derivative that interferes with parasite nucleic acid synthesis. For B. gibsoni and B. conradae, which are often resistant to imidocarb, a combination of atovaquone (13.3 mg/kg orally three times daily) and azithromycin (10 mg/kg orally once daily) for 10 days is recommended [15, 17]. Supportive care includes intravenous fluids, blood transfusions for severe anemia, and hepatoprotectants [26].

5.4 Hepatozoonosis

Treatment of hepatozoonosis is challenging and often requires a combination of antiprotozoal and anti-inflammatory agents. Imidocarb dipropionate at 5 to 6.6 mg/kg every 14 days for three treatments, combined with doxycycline at 5 mg/kg twice daily for 28 days, has been used with variable success [5, 16]. Clindamycin and sulfonamide-trimethoprim combinations have also been reported [5]. Long-term management is often necessary due to the persistence of tissue cysts.

5.5 Rickettsiosis

Doxycycline at 5 mg/kg twice daily for 7 to 14 days is effective against SFG rickettsiae [11]. Clinical response is typically rapid, with fever resolution within 24 to 48 hours.

6. Prevention and Control

Prevention of tick-borne illnesses in dogs relies on effective tick control through the use of acaricidal products, including topical spot-on formulations, oral isoxazoline compounds, and tick collars [24]. Regular environmental management, such as mowing grass and removing leaf litter, reduces tick habitat [24]. Vaccination is available for B. burgdorferi and has been shown to reduce clinical disease, but it does not prevent infection [34]. No commercial vaccines are currently available for E. canis, Anaplasma spp., or Babesia spp., although experimental vaccines targeting Ehrlichia surface proteins have shown promise in animal models [22].

7. Conclusion

Tick-borne illnesses in dogs represent a complex and evolving clinical challenge. The diversity of pathogens, frequent co-infections, and overlapping clinical presentations necessitate a systematic diagnostic approach integrating molecular and serological methods. Early and accurate diagnosis, followed by pathogen-specific treatment, is essential for favorable outcomes. Continued surveillance, vector control, and vaccine development are critical components of a comprehensive One Health strategy to mitigate the impact of these diseases on canine and human health.

References

[1] Senbill, H., Karawia, D., Zeb, J., et al. Molecular screening and genetic diversity of tick-borne pathogens associated with dogs and livestock ticks in Egypt. PLoS Neglected Tropical Diseases, 2024. https://www.semanticscholar.org/paper/b41f8a000561bdcd88ead0fe29fb3461066c70b8

[2] Coşkun, A., Aydın, M. F. Molecular investigation of dogs' tick-borne infections in the southern region of Central Anatolia. Etlik Veteriner Mikrobiyoloji Dergisi, 2026. https://www.semanticscholar.org/paper/0eb14305a27cc321139ff6f23a2b71ada200a031

[3] Truong, A., Noh, J., Park, Y., et al. Molecular Detection and Phylogeny of Tick-Borne Pathogens in Ticks Collected from Dogs in the Republic of Korea. Pathogens, 2021. https://www.semanticscholar.org/paper/57cf4bd95e0a2ad03e0d33208917d80b6d512512

[4] Nimo-Paintsil, S. C., Mosore, M., Addo, S. O., et al. Ticks and prevalence of tick-borne pathogens from domestic animals in Ghana. Parasites & Vectors, 2022. https://www.semanticscholar.org/paper/a49384d63ea55c9d1fc4f8cfe64834b764aef31a

[5] Sukara, R., Andrić, N., Francuski Andrić, J., et al. Autochthonous infection with Ehrlichia Canis and Hepatozoon Canis in dogs from Serbia. Veterinary Medicine and Science, 2022. https://www.semanticscholar.org/paper/8b6b69162009d764054f0b0cce3c057593227903

[6] Charles, R. A., Pow-Brown, P., Gordon-Dillon, A., et al. Completing the Puzzle: A Cluster of Hunting Dogs with Tick-Borne Illness from a Fishing Community in Tobago, West Indies. Pathogens, 2024. https://www.semanticscholar.org/paper/8d73e108b25f53e2d46bec228077c366c9a3d0ac

[7] Cvek, M., Pustijanac, E., Vucelja, M., et al. Spatial Epidemiology and Ecological Determinants of Ticks and Tick-Borne Pathogens Co-Circulation in Brijuni National Park, Croatia. International Journal of Environmental Research and Public Health, 2026. https://www.semanticscholar.org/paper/486d6ab3e62fcea20c232ceeca89b

[8] Cheng, C., Ganta, R. Laboratory Maintenance of Ehrlichia chaffeensis and Ehrlichia canis and Recovery of Organisms for Molecular Biology and Proteomics Studies. Current Protocols in Microbiology, 2008. https://www.semanticscholar.org/paper/c47903804e5ccf6e3a5f38deeaeb50dfe5840a83

[9] Alcon-Chino, M. E. T., De-Simone, S. Recent Advances in the Immunologic Method Applied to Tick-Borne Diseases in Brazil. Pathogens, 2022. https://www.semanticscholar.org/paper/0007eccf3244c61e76f05e4d5b93321a84ef56d1

[10] Kelly, M. A., Anderson, K., Saleh, M. N., et al. High seroprevalence of selected vector-borne pathogens in dogs from Saipan, Northern Mariana Islands. Parasites & Vectors, 2025. https://www.semanticscholar.org/paper/5e7edd2d3c7e4835c89b170824c95d1cbc02b874

[11] Romero, L., Binder, L. C., Marcili, A., et al. Ticks and tick-borne rickettsiae from dogs in El Salvador, with report of the human pathogen Rickettsia parkeri. Ticks and Tick-borne Diseases, 2023. https://www.semanticscholar.org/paper/0b6a52ce4693fb46b3d621ba45a863db7423667c

[12] Masu, G., Sechi, S., Cocco, R., et al. Validation of a serological test for the diagnosis of canine rickettsial disease. Ticks and Tick-borne Diseases, 2012. https://www.semanticscholar.org/paper/2931b2ec0c7a90f218eaccef796ff1a421bfd2b3

[13] Tuten, H., Glowacki, M. N., Hefley, C., et al. Presence of Bartonella and Rickettsia spp. in cat fleas and brown dog ticks collected from dogs in American Samoa. Journal, 2013. https://www.semanticscholar.org/paper/ffb66c51e34145a906f68d161c765af21ffc8b6e

[14] Petrauskaitė, V., Zamokas, G., Stadalienė, I., et al. Brucella canis and Chlamydia spp.: insights into canine infections, diagnostics, and potential tick-borne transmission. Biologija, 2024. https://www.semanticscholar.org/paper/9cad51cc5c7257edd1747c4c8642468d2c350f7e

[15] Furman, H., Scimeca, R. Detection of Babesia conradae in Coyotes (Canis latrans) and Coyote-Hunting Greyhound Dogs (Canis familiaris). Pathogens, 2023. https://www.semanticscholar.org/paper/cdd4e2c5eb68edb781b7d9b3b8ee69925f2e77db

[16] Bouattour, A., Chabchoub, A., Hajjaji, I., et al. Hepatozoon canis and Babesia vogeli infections of dogs in Tunisia. Veterinary Parasitology: Regional Studies and Reports, 2021. https://www.semanticscholar.org/paper/ab43cbf8c82a0b3d2afa51fdb51e49ff393fa108

[17] Deepa, C. K., Varghese, A., Ajith Kumar, K. G., et al. Evaluation of recombinant Babesia gibsoni thrombospondin-related adhesive protein (BgTRAP) for the sero-diagnosis of canine babesiosis. Experimental Parasitology, 2023. https://www.semanticscholar.org/paper/aef0a2e4209299f39aef80adb6fbc04d274e696c

[18] Esteve-Gasent, M. D., Snell, C. B., Adetunji, S., et al. Serological detection of Tick-Borne Relapsing Fever in Texan domestic dogs. PLoS ONE, 2017. https://www.semanticscholar.org/paper/51e2c11510a36cf54baef4fa7bcd24df16d5a1f1

[19] Oguntomole, O., Nwaeze, U., Eremeeva, M. Tick-, Flea-, and Louse-Borne Diseases of Public Health and Veterinary Significance in Nigeria. Tropical Medicine and Infectious Disease, 2018. https://www.semanticscholar.org/paper/65f0eb77fac2164be42df25f87512c3cc17f5481

[20] Wang, S., Hua, X., Cui, L. Characterization of microbiota diversity of engorged ticks collected from dogs in China. Journal of Veterinary Sciences, 2021. https://www.semanticscholar.org/paper/f8c6169ff681e771dec8ad74e9a0b5755e46af47

[21] Cocco, R., Sechi, S., Rizzo, M., et al. Haematochemical Profile of Healthy Dogs Seropositive for Single or Multiple Vector-Borne Pathogens. Veterinary Sciences, 2024. https://www.semanticscholar.org/paper/77f09c24e86772bbf49bdaf4a30e8c7080c46eb8