Borrelia anserina: Avian Spirochetosis – Tick-Borne Pathogenesis, Diagnosis, and Control
Etiology and Taxonomy
Borrelia anserina is a helical, motile, Gram-negative spirochete belonging to the family Spirochaetaceae, genus Borrelia. It is the etiologic agent of avian spirochetosis, an acute, septicemic disease of domestic and wild birds. The organism measures approximately 8 to 20 micrometers in length and 0.2 to 0.3 micrometers in width, with 5 to 8 irregular, loose coils. Its periplasmic flagella (endoflagella) enable a characteristic corkscrew motility in liquid media. The outer membrane contains lipoproteins and lipopolysaccharide-like glycolipids that contribute to its antigenic profile and pathogenicity.
Unlike the Lyme disease agent Borrelia burgdorferi, which is maintained in enzootic cycles involving Ixodes ticks and mammalian reservoirs, B. anserina is strictly an avian pathogen with a primary vector relationship with argasid (soft) ticks, particularly Argas persicus, the fowl tick. The organism does not infect mammals, including humans, under natural conditions. This host restriction is a critical distinction for diagnostic and epidemiological purposes.
Epidemiology and Transmission
Avian spirochetosis occurs worldwide in tropical and subtropical regions where its tick vectors are endemic. The disease is most commonly reported in chickens, turkeys, ducks, geese, and pheasants. Wild birds, including sparrows and other passerines, can serve as reservoir hosts, maintaining the spirochete in the environment.
The primary vector is Argas persicus, the fowl tick, which is discussed in detail in the article Ectoparasites of Poultry: Dermanyssus gallinae, Ornithonyssus sylviarum, Knemidocoptes mutans, Knemidocoptes gallinae, and Argas persicus – Identification, Life Cycles, and Control. Other argasid ticks, including Argas radiatus and Argas miniatus, have also been implicated in transmission. The spirochete multiplies within the tick's hemolymph and salivary glands. Transmission to birds occurs via the tick's bite during blood feeding. Ticks remain infected for life and can transmit B. anserina transstadially (from nymph to adult) and transovarially to their offspring, making them both vectors and reservoirs.
Mechanical transmission by blood-feeding insects such as mosquitoes and mites has been suggested but is considered epidemiologically insignificant compared to tick-borne transmission. Ingestion of infected ticks or contaminated water may also contribute to flock outbreaks, though this route is less efficient.
Outbreaks typically occur during warm months when tick activity peaks. Introduction of infected birds or infested premises facilitates rapid spread within a flock. Mortality rates in naive flocks can exceed 50 percent, with morbidity approaching 100 percent.
Pathogenesis
Following inoculation by an infected tick, B. anserina enters the bloodstream and undergoes rapid multiplication, resulting in a spirochetemia that can exceed 10^8 organisms per milliliter of blood. The spirochetes adhere to erythrocytes and endothelial cells via outer surface proteins, leading to hemolysis, anemia, and vascular damage. The organism's motility and ability to penetrate intercellular junctions facilitate dissemination to the liver, spleen, bone marrow, and other organs.
The host inflammatory response is characterized by activation of macrophages and release of pro-inflammatory cytokines, including tumor necrosis factor-alpha and interleukins. This systemic inflammatory response contributes to fever, depression, and multi-organ dysfunction. The liver and spleen are primary sites of spirochete sequestration and clearance, and their enlargement reflects the intensity of the immune response.
In surviving birds, antibody-mediated opsonization and complement activation lead to spirochete clearance within 7 to 14 days. However, chronic carriers may harbor the organism in the spleen or bone marrow, serving as a source for future outbreaks under stress or immunosuppression.
Clinical Signs
The incubation period ranges from 3 to 12 days, depending on the infectious dose and host susceptibility. The disease course is acute to peracute.
Clinical signs include:
- Sudden onset of fever (body temperature elevation of 1 to 2 degrees Celsius)
- Profound depression, lethargy, and anorexia
- Ruffled feathers and drooping wings
- Greenish-yellow diarrhea due to biliverdinuria from hemolysis
- Anemia evidenced by pale comb and wattles
- Neurologic signs in some cases: ataxia, torticollis, and paralysis
- Decreased egg production in laying hens
- Sudden death, often without premonitory signs in peracute cases
Mortality typically peaks 5 to 7 days after the onset of clinical signs. Recovered birds may exhibit stunted growth and prolonged convalescence.
Pathology
Gross lesions are consistent with a septicemic and hemolytic disease. Key findings include:
- Enlarged, friable, and congested liver (hepatomegaly) with petechial or ecchymotic hemorrhages
- Splenomegaly with a mottled or marbled appearance
- Pale, icteric mucous membranes and subcutaneous tissues
- Petechiae on the epicardium, serosal surfaces, and skeletal muscle
- Catarrhal enteritis with greenish intestinal contents
- Pulmonary congestion and edema
Histopathologic examination reveals:
- Hepatic necrosis, sinusoidal congestion, and infiltration of heterophils and macrophages
- Splenic lymphoid depletion, reticuloendothelial hyperplasia, and fibrinoid necrosis
- Erythrophagocytosis in the spleen and bone marrow
- Perivascular cuffing and edema in the brain in cases with neurologic involvement
Silver stains (e.g., Warthin-Starry) or immunohistochemistry can demonstrate spirochetes in tissue sections, particularly in the liver, spleen, and blood vessels.
Diagnosis
A definitive diagnosis of avian spirochetosis requires laboratory confirmation. The differential diagnosis includes other acute septicemic diseases of poultry such as Fowl Cholera in Poultry: Pasteurella multocida Pathogenesis, Clinical Signs, Prevention, Control, and WOAH Classification, Salmonella in Chickens: Clinical Signs, Zoonotic Risks, and Diagnostic Differentiation from Other Enteric Pathogens, and Highly Pathogenic Avian Influenza (H5N1) in Poultry and Wild Birds: Clinical Signs, Transmission Dynamics, and Surveillance Maps.
Direct Detection
Dark-field microscopy of fresh blood smears from febrile birds reveals motile, coiled spirochetes. This method is rapid but requires expertise and is not species-specific.
Giemsa-stained blood smears show the spirochetes as thin, wavy, blue-purple organisms against the pink background of erythrocytes. Sensitivity is lower than dark-field examination.
Polymerase chain reaction (PCR) targeting the 16S rRNA gene or the flagellin (flaB) gene is the most sensitive and specific diagnostic method. PCR can detect B. anserina DNA in whole blood, tissues, or ticks. Real-time PCR assays provide quantitative data and are suitable for surveillance. PCR is particularly useful for detecting chronic carriers with low-level spirochetemia.
Serology
Enzyme-linked immunosorbent assay (ELISA) using whole-cell or recombinant antigen preparations can detect anti-B. anserina antibodies. The Enzyme-Linked Immunosorbent Assay (ELISA) for Feline Leukemia Virus article provides a general framework for ELISA principles, though the antigen and target species differ. Paired acute and convalescent sera are recommended for seroconversion confirmation.
Indirect fluorescent antibody (IFA) tests are also available but are less standardized than ELISA. Cross-reactivity with other Borrelia species is a potential limitation.
Culture
Isolation of B. anserina requires specialized media such as Barbour-Stoenner-Kelly (BSK) medium supplemented with rabbit serum. Incubation at 33 to 35 degrees Celsius under microaerophilic conditions for 5 to 14 days is typical. Culture is technically demanding and not routinely performed in diagnostic laboratories.
Diagnostic Algorithm
flowchart TD
A[Clinical suspicion: fever, anemia, green diarrhea, high mortality], > B{Blood smear or dark-field microscopy}
B, >|Positive| C[Presumptive diagnosis: Avian spirochetosis]
B, >|Negative| D[Collect whole blood and tissue samples]
D, > E[PCR for Borrelia anserina 16S rRNA or flaB]
E, >|Positive| C
E, >|Negative| F[Consider differentials: Fowl cholera, Salmonellosis, Avian influenza]
C, > G[Confirm with culture or serology if needed]
G, > H[Implement treatment and control measures]
Treatment
Antimicrobial therapy is most effective when initiated early in the course of disease. The spirochete is susceptible to several classes of antibiotics.
Tetracyclines are the drugs of choice. Oxytetracycline or chlortetracycline administered in feed or drinking water at therapeutic doses (e.g., 200 to 400 grams per ton of feed or 10 to 20 milligrams per kilogram body weight) for 7 to 10 days is standard. Injectable oxytetracycline is used for individual birds.
Penicillin G and ampicillin are also effective. Procaine penicillin G given intramuscularly at 20,000 to 40,000 IU per kilogram body weight daily for 3 to 5 days can reduce mortality.
Tylosin and erythromycin have demonstrated in vitro activity but are considered second-line agents.
Supportive care includes provision of clean water, electrolyte solutions, and vitamin supplementation to aid recovery. Severely affected birds may require isolation and nursing care.
Antimicrobial resistance in B. anserina has been reported but is not widespread. Susceptibility testing by broth microdilution is recommended when treatment failure is suspected.
Control and Prevention
Control of avian spirochetosis relies on integrated vector management, biosecurity, and vaccination.
Vector Control
Elimination of argasid ticks is the cornerstone of prevention. The article Ectoparasites of Poultry: Dermanyssus gallinae, Ornithonyssus sylviarum, Knemidocoptes mutans, Knemidocoptes gallinae, and Argas persicus – Identification, Life Cycles, and Control provides detailed guidance on tick control. Key measures include:
- Application of acaricides (e.g., permethrin, cyfluthrin, or amitraz) to poultry housing, perches, and nesting areas
- Removal and replacement of infested litter and bedding
- Sealing cracks and crevices in poultry houses to eliminate tick hiding places
- Rotating pasture or range areas to break the tick life cycle
- Treating birds with approved topical acaricides
Biosecurity
- Quarantine new birds for at least 14 days before introduction to the flock
- Prevent contact between domestic poultry and wild birds, which may carry infected ticks
- Restrict visitor access and implement boot and equipment disinfection protocols
- Maintain all-in/all-out flock management to allow thorough cleaning and disinfection between cycles
Vaccination
Inactivated bacterins and live attenuated vaccines have been developed. Vaccination of breeder flocks can provide passive immunity to progeny via maternal antibodies. Killed vaccines are administered subcutaneously or intramuscularly in two doses, 2 to 4 weeks apart. Protection is strain-specific, and autogenous vaccines may be necessary in regions with unique circulating strains.
Flock Management
- Monitor flocks for signs of tick infestation and clinical disease, especially during warm months
- Promptly isolate and treat affected birds
- Cull chronically infected or non-responsive birds to reduce the reservoir
- Maintain accurate records of morbidity, mortality, and treatment outcomes
Public Health Considerations
Borrelia anserina is not considered a zoonotic pathogen. There are no documented cases of human infection. This host specificity distinguishes it from other Borrelia species such as B. burgdorferi and B. recurrentis. However, the tick vectors, particularly Argas persicus, can bite humans and cause local inflammatory reactions. The primary public health concern is the economic impact on poultry production and food security in affected regions.
Conclusion
Borrelia anserina remains a significant cause of morbidity and mortality in poultry flocks in tropical and subtropical regions. The spirochete's reliance on argasid tick vectors for transmission makes vector control the most effective prevention strategy. Rapid diagnosis using PCR or dark-field microscopy, combined with early antimicrobial therapy, can reduce flock losses. Integrated control programs that combine acaricide use, biosecurity, and vaccination are essential for long-term management. Continued surveillance and research into vaccine efficacy and antimicrobial resistance patterns are needed to sustain control efforts.
References
Ataliba, A. C., Resende, J. S., Yoshinari, N., & Labruna, M. B. (2007). Isolation and molecular characterization of Borrelia anserina from Argas miniatus in Brazil. Journal of Medical Entomology, 44(4), 655-660.
DaMassa, A. J., & Adler, H. E. (1979). Avian spirochetosis: natural transmission by Argas persicus and experimental transmission by mosquitoes. Avian Diseases, 23(3), 667-675.
McNeil, E., Hinshaw, W. R., & Kissling, R. E. (1949). A study of Borrelia anserina infection (spirochetosis) in turkeys. Journal of the American Veterinary Medical Association, 115(873), 436-440.
Saik, J. E., & Hinshaw, W. R. (1955). Avian spirochetosis: a review. Veterinary Medicine, 50(8), 365-370.
Smith, T. (1918). Spirochetosis of fowls. Journal of Experimental Medicine, 28(3), 333-346.