Avian Spirochetosis in Chickens: Symptoms, Causes and Treatment

Borrelia anserina is a spirochete bacterium transmitted exclusively by Argas persicus (the fowl tick), a soft-bodied tick that feeds at night and hides in crevices during the day.

• Tick vector: Argas persicus ticks are the sole transmission route. The tick feeds on an infected bird, acquires the spirochete, and transmits it to the next bird it feeds on. The spirochete does not spread directly between birds.
• Geographic distribution: Most common in tropical, subtropical, and semi-arid regions. In the US, primarily found in the southwestern states, Gulf Coast, and Hawaii. Uncommon in northern or cold-climate regions.
• Tick biology: Argas persicus is a soft tick that hides in cracks, crevices, and litter during the day and feeds on roosting birds at night. Ticks survive for years without a blood meal, maintaining infection in a coop long after birds are removed.

Avian spirochetosis is diagnosed by detecting Borrelia anserina spirochetes in the blood of a febrile, acutely ill bird. The diagnosis should be confirmed before initiating penicillin treatment, as blood smear results are rapid and readily available.

• Blood smear (primary test): A thin blood smear from a febrile bird, stained with Giemsa or Wright's stain, reveals helical spirochetes among red blood cells during the acute bacteremic phase. Spirochetemia is typically detectable within 24-48 hours of onset of clinical signs and before mortality begins. Sample at least 3 birds to increase detection probability.
• Dark-field microscopy: A drop of fresh blood examined under dark-field illumination shows the characteristic corkscrew motility of B. anserina spirochetes in real time. Results in minutes without staining.
• Tick identification: Finding Argas persicus ticks in the coop confirms the vector and supports the diagnosis. Inspect after dark with a flashlight: ticks feed at night on roosting birds and retreat to crevices at dawn. Flat, oval, brownish-gray, 5-10mm unfed ticks in wall cracks adjacent to roost bars are characteristic.
• Differentials: Fowl cholera (bipolar-staining rods on smear, no spirochetes, swollen wattles); Newcastle disease (neurological signs without fever pattern); heat stress (no systemic bacteremia).

Penicillin is highly effective against B. anserina if administered before paralysis becomes severe. Begin treatment as soon as spirochetemia is confirmed on blood smear rather than waiting for laboratory culture.

Procaine penicillin G (drug of choice): 100,000 IU/kg body weight IM once daily for 2-3 consecutive days. In a 2 kg hen, this is approximately 0.6 ml of procaine penicillin G (300,000 IU/ml) injected into the breast muscle. Most birds show dramatic improvement within 24-48 hours and recover fully within 72 hours if treated before paralysis is established. Birds that have been paralyzed for more than 24 hours have a much poorer prognosis.

Oxytetracycline (flock-wide treatment): When IM injection of every bird is not practical, oxytetracycline at 10 mg/kg/day in drinking water for 5-7 days provides reasonable flock coverage. Less effective than penicillin for individual birds but achieves therapeutic blood levels through the water line simultaneously across the entire flock. Ensure the only water source contains the medicated water.

Supportive care: Provide electrolyte solution (Sav-A-Chick or equivalent) in water during treatment to support hydration in birds with diarrhea. Move paralyzed birds to a separate pen with food and water within easy reach to prevent death from starvation or dehydration while they recover motor function.

Concurrent tick elimination (essential): Antibiotic treatment stops the current outbreak but does not prevent reinfection from Argas ticks still in the coop. Apply permethrin-based acaricide spray thoroughly to all crevices, roost boards, floor joints, and wall cracks. Repeat every 7 days for 3 weeks to kill newly hatching larvae. Replace heavily infested wooden roost bars with new material, as ticks embedded deep in wood grain survive surface spraying.

• Inspect coops for Argas ticks during warm months, especially in endemic regions. Check roost boards and wall crevices after dark with a flashlight when ticks are feeding.
• Apply permethrin-based acaricide to all coop surfaces, focusing on crevices where Argas ticks hide. Repeat monthly during tick season.
• Seal cracks and crevices in wooden structures where ticks overwinter. Replace heavily infested wood rather than treating in place.
• Quarantine new birds from endemic areas. Argas ticks may be transported on birds from tick-endemic regions.