Proventricular Dilatation Disease and Avian Bornavirus (PaBV) in Psittacines: Why Infection Is Not Disease, and the Diagnosis, Treatment, and Biosecurity Reality

Bottom line

Proventricular dilatation disease (PDD) is an immune-mediated lymphoplasmacytic ganglioneuritis of psittacine birds whose principal infectious trigger is parrot bornavirus (PaBV, avian bornavirus) [1]. The single most important interpretive point is that infection is not disease: many parrots test PaBV-positive, shed the virus, and remain clinically healthy for years, so a positive bornavirus test in a well bird does not by itself diagnose PDD [1]. There is no cure and no effective antiviral; anti-inflammatory therapy is used but contested, and prognosis once clinical signs appear is guarded to poor [2].

Etiology: parrot bornavirus, and why infection is not disease

PaBV, a neurotropic orthobornavirus, is the principal infectious agent of PDD, also called avian bornaviral ganglioneuritis [1]. Multiple genotypes circulate. The review by Boatright-Horowitz counts fifteen bornavirus genotypes overall, with eight found in parrot populations worldwide, and identifies PaBV-2 and PaBV-4 as the genotypes most virulent for parrots [1]. One reported clinical association is that PaBV-4 trends toward neurologic signs while PaBV-2 mainly affects the gastrointestinal tract with more severe disease progression, and co-infection with both appears to produce more severe signs [1]; treat that as a tendency, not a rule.

The causation story is genuinely contested, and you should present it as such. Experimental support exists: Gray and colleagues inoculated two Patagonian conures with a cultured avian bornavirus (ABV4) isolate and reproduced clinical PDD by day 66, with lymphoplasmacytic ganglioneuritis in the crop, proventriculus, gizzard, and intestine, which the authors stated fulfills Koch's postulates [3]. But reproduction has been inconsistent across studies and routes: attempts to infect birds across mucosal (nasal and oral) surfaces have given inconsistent results, and reliable experimental disease has generally required parenteral (intramuscular, intracerebral, or intravenous) inoculation rather than natural routes [1]. That gap between infected and diseased is the crux; strong evidence indicates birds can carry bornavirus infection and stay clinically healthy for decades [1].

The leading explanation for why only some infected birds develop PDD is immune-mediated injury. The lesion is driven by the host response: CD8+ T cells directly damage ganglia and neurons with CD4 T-cell recruitment of macrophages, and an autoimmune arm has been proposed in which host antibodies attack gangliosides in neural cell membranes. One study reported elevated anti-ganglioside antibodies in 98% of clinically symptomatic, histologically positive birds among 650 avian serum samples [1]. The autoimmune model is not settled: inoculating chickens and Quaker parrots with gangliosides did not reproduce disease after 114 days, yet ganglioside-inoculated cockatiels did develop clinical signs and typical histological lesions — the evidence cuts both ways [1]. The practical synthesis is that PDD behaves as an immune-mediated ganglioneuritis triggered by, but not synonymous with, PaBV infection.

Pathophysiology: the GI form and the CNS form

The unifying lesion is lymphoplasmacytic inflammation of autonomic ganglia and the CNS [3]. When the myenteric (autonomic) ganglia of the upper GI tract are targeted, functional denervation impairs proventricular and ventricular motility: the proventriculus loses tone and dilates, food is not ground or moved distally, and the bird effectively starves with a full gut — the classic GI form [1]. When inflammation involves the brain, spinal cord, and peripheral nerves as a lymphocytic-plasmacytic infiltrate, the neurologic form results [4]. Both forms share one pathology and frequently co-occur, so a patient may present with GI signs, CNS signs, or both [2].

Clinical signs and species predisposition

The GI picture is chronic weight loss with a maintained or even ravenous appetite, regurgitation, and passage of undigested food — most easily recognized when whole seeds appear in the droppings — together with proventricular dilatation on imaging [2]. Crop stasis and a distended, food-filled crop are common. The neurologic picture includes ataxia, tremors, weakness, abnormal head movements, seizures, proprioceptive deficits, and blindness, and it may occur with or without concurrent GI signs [2].

Merck states that the disease primarily affects macaws, conures, and African grey parrots, although all parrots are considered susceptible [2]; cockatoos and Amazons are also over-represented in the broader avian-medicine literature (own synthesis).

Diagnosis: why antemortem diagnosis is hard, and the interpretive trap

Lead with the caveat: antemortem diagnosis of PDD is difficult, and no single test confirms it.

Imaging. Survey and contrast radiography (barium series) or fluoroscopy can show proventricular dilatation and delayed GI transit; Merck notes that a dilated proventriculus may be seen radiographically [2]. Dilatation is a sign, not an etiology; work the differentials below before anchoring on PDD.

PaBV testing and its limits. RT-PCR of a cloacal or fecal swab (also crop swab or blood) is the most common assay, but the virus is shed intermittently in feces and urates [4], so a single negative does not exclude infection; Merck advises testing at least three times at monthly intervals before calling a bird negative [2]. Serology (anti-PaBV antibodies) may not detect early infections [2]. The decisive limitation is interpretive, not analytic: because an estimated 10% to 45% of captive birds — at least one in three clinically healthy captive parrots — test positive for bornavirus, a positive PaBV result does not by itself confirm PDD [1]. You are diagnosing a disease, not an infection.

Biopsy and histopathology. Crop biopsy that captures ganglia was used historically but is insensitive; Merck puts its diagnostic yield at only about 30% to 35% of cases [2]. Definitive diagnosis remains histopathologic demonstration of lymphoplasmacytic ganglioneuritis, frequently at necropsy [3].

Differential diagnosis for proventricular dilatation and neurologic signs

Because proventricular dilatation and neurologic signs are nonspecific, work through the mimics before committing to PDD (own synthesis):

• Heavy-metal (lead, zinc) toxicosis — a key mimic producing GI stasis and CNS signs; screen with radiographs and blood metals. See avian heavy metal toxicosis.
• GI foreign body or obstruction, and proventricular or ventricular neoplasia — mechanical and structural causes of dilatation.
• Macrorhabdus ornithogaster ("megabacteria") and other primary GI disease.
• Hypocalcemia — a neurologic differential, classically in African greys; see avian hypocalcemia in African greys.
• Hepatic disease and other causes of chronic weight loss; see avian hepatic lipidosis.
• Systemic or CNS infectious disease including aspergillosis; see avian aspergillosis.

Treatment: no cure, and a contested anti-inflammatory strategy

There is no cure and no effective antiviral for PaBV/PDD; management is supportive plus attempts to blunt the immune-mediated ganglioneuritis [2].

The COX-2 case, and the counter-evidence. Because the lesion is inflammatory, COX-2-selective NSAIDs have been used. The Boatright-Horowitz review reports that Dahlhausen used celecoxib at 10 mg/kg PO once daily with reported clinical improvement and good tolerance, and that celecoxib and meloxicam remain widely recommended [1]. That practice does not hold up in controlled work: Escandon and colleagues gave PaBV-2-inoculated cockatiels meloxicam 1.0 mg/kg PO once daily or celecoxib 10.0 mg/kg PO once daily for 150 days and found no difference in clinical presentation, viral shedding, gross lesions, viral distribution, or histopathology; four NSAID-treated birds developed black intestinal material, raising a GI-irritation concern, and the authors concluded there is no justification for continuing to administer NSAIDs to birds with clinical PDD [5]. Present both sides. Merck lists meloxicam, robenacoxib, and celecoxib among options [2]; where no formulary or paper states a robenacoxib dose for this indication, do not approximate one — consult Carpenter's Exotic Animal Formulary.

Immunosuppression. Consistent with a T-cell-mediated mechanism, a 2025 case series used cyclosporine at 10 mg/kg PO every 12 hours together with itraconazole at 2.5 mg/kg PO once daily (as antifungal prophylaxis) for palliation; outcomes were mixed — several birds continued to thrive, while one died of gout attributed to nephrotoxicity — so this is palliative, not curative [6].

Supportive care. An easily digestible diet, control of secondary bacterial and fungal infections, and GI-supportive measures are standard [2]. All of the drug uses above are extra-label in psittacines (own synthesis).

Prognosis. Guarded to poor. PDD is progressive and is generally fatal once clinical signs develop [2].

Biosecurity, flock management, and the zoonosis question

Transmission. The fecal-oral route is considered most likely [2], with horizontal spread through contact and ingestion of urofecal matter; PaBV is not efficiently transmitted, and laboratory contagion studies have been inconsistent [1]. Vertical (egg) transmission has tested positive in some work but is not established [1].

Managing carriers and new birds. Quarantine and PaBV-test incoming birds, remembering that intermittent shedding means serial testing — Merck's three monthly PCRs — is needed before trusting a negative [2]. In a collection, the hard reality is that subclinically infected, shedding birds are common and most never develop PDD [1]; culling of infected birds is not recommended, and isolation of infected birds is preferred [4].

Zoonosis. Address this precisely. Avian bornaviruses are regarded as not harmful to humans and do not grow in mammalian tissue in the laboratory [1]. This is distinct from mammalian bornaviruses such as variegated squirrel bornavirus (VSBV-1), which caused fatal human encephalitis in three people in Germany bitten by variegated squirrels [1]; do not extrapolate that risk to the avian (parrot) bornaviruses.

Frequently Asked Questions

Does a positive bornavirus test mean my patient has PDD?

No. This is the most misunderstood point in the disease. The Boatright-Horowitz 2020 review in Veterinary Medicine International states that an estimated 10% to 45% of captive birds are infected — at least one in three clinically healthy captive parrots tests positive — and that birds can carry bornavirus infection and remain clinically healthy for decades. A positive PaBV RT-PCR or serology therefore documents infection, not disease. PDD is a clinical and histopathologic diagnosis (lymphoplasmacytic ganglioneuritis), so a positive test in a well bird does not confirm it.

Which bornavirus genotypes cause PDD?

Parrot bornavirus (PaBV, avian bornavirus) is the principal agent. The Boatright-Horowitz review counts fifteen bornavirus genotypes overall with eight found in parrot populations worldwide, and names PaBV-2 and PaBV-4 as the most virulent for parrots. The review notes a reported tendency for PaBV-4 to produce more neurologic signs and PaBV-2 to affect mainly the GI tract with more severe progression, with co-infection associated with more severe disease — a tendency rather than a rule.

How is PDD diagnosed antemortem?

With difficulty, and never on one test. Contrast radiography or fluoroscopy can show proventricular dilatation and delayed transit; the Merck Veterinary Manual notes a dilated proventriculus may be seen radiographically. RT-PCR of cloaca, feces, crop, or blood is the common test, but shedding is intermittent, so Merck advises testing at least three times at monthly intervals before calling a bird negative, and serology may miss early infections. Crop biopsy is diagnostic in only about 30% to 35% of cases per Merck. Definitive diagnosis is histopathologic — lymphoplasmacytic ganglioneuritis — often at necropsy.

What are the celecoxib and meloxicam doses for PDD, and do NSAIDs work?

The Boatright-Horowitz review reports that Dahlhausen used celecoxib at 10 mg/kg PO once daily with reported clinical improvement, and celecoxib and meloxicam are widely recommended. However, in a controlled study (Escandon and colleagues, 2019), PaBV-2-inoculated cockatiels given meloxicam 1.0 mg/kg PO once daily or celecoxib 10.0 mg/kg PO once daily for 150 days showed no difference in clinical signs, viral shedding, gross lesions, viral distribution, or histopathology, four NSAID-treated birds developed black intestinal material, and the authors concluded there is no justification for continuing NSAIDs in clinical PDD. Discuss both sides with the client; all use is extra-label.

Is there a cure, and what about immunosuppressive therapy?

There is no cure and no effective antiviral. Because the ganglioneuritis is T-cell-mediated, a 2025 case series (Kleinschmidt, Hoppes, and Musser in Veterinary Sciences) used cyclosporine at 10 mg/kg PO every 12 hours with itraconazole at 2.5 mg/kg PO once daily for palliation, with mixed outcomes — some birds thrived while one died of gout attributed to nephrotoxicity. Treat this as palliative, not curative, and monitor renal function; use is extra-label.

Is avian bornavirus zoonotic?

The Boatright-Horowitz review states that avian bornaviruses are regarded as not harmful to humans and do not grow in mammalian tissue in the laboratory. This must be kept separate from mammalian bornaviruses such as variegated squirrel bornavirus (VSBV-1), which the review links to fatal encephalitis in three people in Germany bitten by variegated squirrels. That mammalian risk should not be extrapolated to parrot bornaviruses.

How should I manage a PaBV-positive bird within a collection?

Quarantine and test incoming birds, and because shedding is intermittent, use serial PCR — the Merck Veterinary Manual advises three tests at monthly intervals — before trusting a negative. The fecal-oral route is considered most likely, though PaBV is not efficiently transmitted. Most positive birds are subclinical carriers that never develop PDD, so per LafeberVet, culling of infected birds is not recommended; isolation of infected birds is preferred. Manage the collection expecting persistent subclinical carriers rather than eradication.