Borrelia anserina and Argas persicus: Avian Spirochetosis – Tick-Borne Bacterial Disease of Poultry

Introduction

Avian spirochetosis, also referred to as fowl spirochaetosis or avian borreliosis, is an acute septicemic disease affecting a broad range of avian species. The etiologic agent is the spirochete bacterium Borrelia anserina, classified within the relapsing fever group of the genus Borrelia [1, 2, 3]. The primary vector is the fowl tick Argas persicus (family Argasidae), a soft tick that infests domestic poultry and their shelters [1, 4]. The disease is distributed across tropical and warm temperate regions worldwide and imposes significant economic losses due to mortality, reduced egg production, and weight loss in affected flocks [2, 3, 5]. This review integrates epidemiological, clinical, diagnostic, and molecular aspects of avian spirochetosis, emphasizing the vector–pathogen relationship and contemporary detection methods.

Etiology and Taxonomic Position

Borrelia anserina (Sakharoff, 1891) is the type species of the genus Borrelia [6]. It is a Gram-negative, motile spirochete with a linear chromosome and multiple linear and circular plasmids. The genome of B. anserina is smaller than that of other relapsing fever borreliae; notably, it lacks a functional dam methylase gene and has disruptions in the glycerol-3-phosphate biosynthetic pathway for phospholipids [6]. Morphologically, the bacterium measures approximately 8–20 μm in length and 0.2–0.5 μm in diameter, with periplasmic flagella enabling corkscrew motility [7, 24]. Serologic and immunologic diversity exists among strains from different geographic regions, with multiple serotypes identified (e.g., Surnevo, Pamoukchii) [8, 33, 42, 44]. In the United States, isolates of B. anserina transmitted by Argas (Persicargas) sanchezi have been shown to be immunologically distinct from Arizona isolates [8].

Vector Biology and Lifecycle of Argas persicus

Argas persicus (Oken, 1818), commonly known as the fowl tick or poultry tick, is the principal vector of B. anserina [1, 4, 9, 31]. Additional argasid species such as Argas miniatus (Neotropical), Argas africolumbae, and Carios vespertilionis have also been implicated in transmission [1, 10, 11, 31]. The lifecycle of A. persicus comprises egg, larva, nymphal (multiple instars), and adult stages, all of which require blood meals from avian hosts. The tick exhibits an endophilic, nidicolous habit, residing in cracks, crevices, and roosting structures within poultry houses [1, 4].

Lifecycle Stages

flowchart TD
    A[Eggs laid in sheltered crevices], > B[Larvae emerge and seek first blood meal]
    B, > C[Nymphal instars (1-4) with repeated blood meals]
    C, > D[Adult male and female ticks]
    D, > E[Mating on host or in shelter]
    E, > A
    D, > F[Females lay multiple egg batches after each blood meal]
    F, > A
    B, > G[Transmission of Borrelia anserina via salivary secretions]
    C, > G
    D, > G

The complete lifecycle can extend from several months to over a year depending on environmental conditions. Argas persicus is capable of surviving prolonged starvation periods (months to years), facilitating pathogen persistence in empty poultry houses [1, 25]. Transstadial transmission of B. anserina occurs from larva to nymph and from nymph to adult, and transovarial transmission has been documented, allowing the spirochete to persist across tick generations without requiring vertebrate hosts [25, 31].

Geographic Distribution and Infestation Patterns

Surveys from Pakistan [1, 12, 5], Algeria [4], Iran [9], Brazil [10, 11], and the United States [8, 47] confirm widespread infestation. Zahid et al. [1] reported 1,818 A. persicus specimens collected from 27 locations in nine districts of Khyber Pakhtunkhwa, Pakistan, with higher infestation in shelters (77%) than on hosts (23%). Ouchene et al. [4] found 46.98% of traditional laying hen farms in Algeria infested with A. persicus. These ticks may also harbor other pathogens such as Aegyptianella pullorum, but B. anserina remains the most economically important [31].

Transmission Dynamics

Transmission of B. anserina to susceptible avian hosts occurs primarily through the bite of infected argasid ticks. The spirochete invades the tick salivary glands and is inoculated into the host during feeding [9, 25]. Alternative routes include ingestion of infected ticks or cannibalism of moribund birds, though vector-borne transmission is dominant [3, 8]. The minimum duration of tick attachment required for spirochete transmission is less than 30 minutes, making rapid feeding by soft ticks highly effective for dissemination [25].

In experimental infections, B. anserina can be transmitted by intramuscular inoculation of infected blood or by exposure to infected third-instar nymphs [10, 13, 28]. Cepeda et al. [10] demonstrated that Gallus gallus inoculated intramuscularly with 250 μL of blood containing 3.7 × 10⁶ spirochetes/mL developed clinical signs at 3 days post-inoculation, while birds infested with infected A. miniatus nymphs showed signs at 6 days post-infestation. The incubation period ranges from 1 to 10 days depending on the strain, infectious dose, and host age [10, 11, 14].

Clinical Signs and Pathology

Avian spirochetosis primarily affects young birds (less than 3 weeks old), with mortality rates reaching 50–90% in naive flocks [14, 32, 47]. Older birds may develop milder or subclinical infections and can become carriers [8, 34].

Clinical Manifestations

• Acute phase: Fever (hyperthermia up to 43.5°C), depression, anorexia, ruffled feathers, greenish diarrhea, and pale comb and wattles [1, 10, 27].
• Neurologic signs: Incoordination, leg weakness, and paralysis in severe cases [14, 32].
• Chronic phase: Reduced egg production, weight loss, and prolonged convalescence [10, 15].

Pathological Findings

Gross lesions include hepatomegaly, splenomegaly, congestion, and multifocal necrotic foci in the liver and spleen [13, 32, 49]. Histopathological examination reveals sinusoidal dilation, hepatocyte vacuolization, mononuclear inflammatory infiltrates, and fibrinoid necrosis [13]. Warthin-Starry silver staining demonstrates spirochetes within hepatic blood vessels [13]. Biochemical alterations include elevated alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activities coinciding with peak spirochetemia [13, 27]. Hematologic changes consist of normocytic normochromic anemia, leukocytosis with heterophilia and monocytosis [28].

Diagnostic Approaches

Accurate diagnosis of avian spirochetosis relies on clinical observation, microscopic examination, serology, and molecular assays. A summary of diagnostic modalities is provided in Table 1.

Table 1. Diagnostic methods for Borrelia anserina infection in poultry.

Molecular Diagnostics

The flagellin B gene (flaB) is the most widely used target for species-specific detection of B. anserina [1, 9, 29]. Zahid et al. [1] amplified a fragment of the flaB gene from A. persicus ticks in Pakistan and demonstrated 100% identity with B. anserina sequences from India, Iran, and Brazil. Hosseini-Chegeni et al. [9] similarly used flaB PCR to detect B. anserina in A. persicus from Iran. For tick species identification, the cytochrome c oxidase (cox) gene is standard [1].

Real-time PCR targeting the 16S rRNA gene provides rapid genus-level detection and has been used in epidemiological surveys [4]. Ouchene et al. [4] identified B. anserina in 5.88% of A. persicus ticks in Algeria using this approach.

Serological Assays

Enzyme-linked immunosorbent assay (ELISA) for detecting anti-B. anserina IgY antibodies in chickens has been developed and validated by Cordeiro et al. [16]. This ELISA can be applied for flock-level surveillance and to evaluate vaccine efficacy. Indirect immunofluorescence assays using fluorescein-labeled antibodies have also been used for serotyping [33, 36]. An agar gel immunodiffusion test for detecting B. anserina antigens in liver extracts was described by Al-Attar and Jahanly [38].

Molecular Characterization and Genomics

Borrelia anserina belongs to the relapsing fever group (RFG) of borreliae, phylogenetically distinct from the Lyme disease group [1, 6]. Sequencing of the flaB gene places B. anserina in a monophyletic clade with other RFG species [1, 29]. The genome (linear chromosome and megaplasmids) was sequenced by Elbir et al. [6], revealing a smaller genome than that of Borrelia duttonii or Borrelia recurrentis. Noteworthy features include:

• Absence of a functional dam methylase.
• Disrupted glycerol-3-phosphate dehydrogenase pathway.
• Reduced number of plasmid-encoded variable major protein (Vmp) genes, possibly explaining the lower antigenic variation compared to other RFG species.

These genomic features may influence the spirochete’s biology, including its growth requirements and pathogenicity [6]. Shared flagellar epitopes between B. anserina and Borrelia burgdorferi have been demonstrated, but serological cross-reactivity is limited [24].

Control and Prevention

Control of avian spirochetosis hinges on vector management, antimicrobial therapy, and vaccination.

Vector Control

Management of A. persicus infestations is challenging due to the tick’s ability to hide in structural crevices and survive for extended periods without feeding. Integrated approaches include:

• Acaricide application: Organophosphates, pyrethroids, and amidines applied to poultry houses, perches, and nest boxes. Rotation of acaricide classes is recommended to reduce resistance.
• Biosecurity measures: Sealing cracks, removing litter, and implementing downtime between flocks.
• Biological control: Entomopathogenic fungi (e.g., Metarhizium anisopliae) have shown efficacy in experimental settings.

Antimicrobial Therapy

Tetracyclines (e.g., oxytetracycline, doxycycline) are the drugs of choice for treating acute infections, administered in feed or drinking water [3, 41]. Erythromycin salts have also demonstrated anti-B. anserina activity [43]. Sodium arsanilate (an organic arsenical) was used historically but is no longer recommended due to toxicity concerns [41]. Treated birds may develop immunity after recovery, but reinfection with heterologous serotypes can occur [19, 34].

Vaccination

No commercial vaccine is currently available for avian spirochetosis. Experimental bacterins using inactivated whole-cell preparations adjuvanted with Montanide ISA 206 have induced protective immunity in laying hens [15]. Aslam et al. [15] reported significantly higher antibody levels and protection against challenge in vaccinated birds compared to controls. Attenuated live vaccines have been developed by serial passage in liquid medium [17], but safety concerns remain. Immunization with the 22-kDa major outer surface protein (Vmp-like) of B. anserina Ni-NL protected chicks from infection in a study by Sambri et al. [20]. Auto-immune phenomena have also been documented during infection, which may complicate vaccine design [48].

Quarantine and Flock Management

Newly introduced birds should be quarantined and treated with acaricides. Sentinel chickens can be used to monitor spirochete activity in endemic areas. Movement of infested equipment between farms should be restricted.

Conclusion

Avian spirochetosis caused by Borrelia anserina and transmitted by Argas persicus is a significant disease of poultry in tropical and warm temperate regions. The intimate association between the spirochete and its soft tick vector, combined with transstadial and transovarial transmission, ensures pathogen persistence in the environment. Accurate diagnosis now relies on molecular methods targeting the flaB gene, while serological tools such as IgY ELISA facilitate flock-level surveillance. Control requires an integrated strategy of vector management, antimicrobial intervention, and vaccination. Future research should focus on refining rapid point-of-care diagnostics and developing broadly protective vaccines that account for serotypic diversity.

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