Proventricular Dilation Disease (Avian Bornavirus Infection)

Proventricular Dilation Disease (PDD) is a fatal, progressive, inflammatory neurologic and gastrointestinal disease of psittacine birds (parrots, macaws, cockatoos, conures, African grey parrots, budgerigars) and occasionally non-psittacine species (canaries, finches, waterfowl, raptors). It is caused by infection with avian bornavirus (ABV), a negative-sense, single-stranded RNA virus of the family Bornaviridae. The disease is characterized by lymphoplasmacytic infiltration of the central and peripheral nervous systems, particularly the autonomic nerves supplying the gastrointestinal tract, leading to dilation and dysfunction of the proventriculus, ventriculus, and sometimes the esophagus and small intestine. Clinically, PDD presents with weight loss, regurgitation, passage of undigested seeds in feces, and neurologic signs such as ataxia, tremors, and seizures. The disease is also known as macaw wasting disease, proventricular dilatation syndrome, and lymphoplasmacytic ganglioneuritis. Diagnosis is based on clinical signs, imaging (radiography, fluoroscopy, ultrasound), endoscopy, histopathology, and molecular testing (RT-PCR) for ABV. There is no cure; treatment is supportive and aims to manage inflammation and secondary infections. Prognosis is poor to grave, with most affected birds dying within months to years.

The primary causative agent of PDD is avian bornavirus (ABV), a non-segmented, negative-sense RNA virus belonging to the genus Orthobornavirus, family Bornaviridae. Multiple genotypes have been identified, including ABV-1 through ABV-8, with ABV-1, ABV-2, and ABV-4 commonly associated with PDD in psittacines. The virus has a broad host range, infecting various psittacine species, as well as some passerines (canaries, finches) and waterfowl. ABV is neurotropic, targeting the central and peripheral nervous systems, particularly the autonomic ganglia and nerves of the gastrointestinal tract. The virus is shed in feces and possibly oral secretions, and transmission is believed to be fecal-oral, with environmental contamination playing a role. Vertical transmission has been suggested but not definitively proven. The virus can establish persistent infection, and not all infected birds develop clinical disease; some become asymptomatic carriers. The exact mechanisms of viral pathogenesis are not fully understood, but it is thought that the host immune response, particularly cell-mediated immunity, contributes to the inflammatory lesions seen in PDD. Other potential cofactors, such as stress, immunosuppression, or concurrent infections, may influence disease expression.

PDD is reported worldwide, with a higher prevalence in captive psittacine populations, especially in breeding facilities, aviaries, and pet bird collections. The disease affects a wide range of psittacine species, including macaws (Ara spp.), African grey parrots (Psittacus erithacus), cockatoos (Cacatua spp.), Amazon parrots (Amazona spp.), conures (Aratinga, Pyrrhura spp.), and budgerigars (Melopsittacus undulatus). Non-psittacine species such as canaries (Serinus canaria), finches, and waterfowl (geese, swans) have also been reported to be infected. There is no apparent age or sex predilection, but young birds (under 3 years) are more commonly diagnosed, possibly due to increased susceptibility or higher viral loads. The incidence in captive populations varies, with some studies reporting seroprevalence rates of 20-40% in apparently healthy birds. Risk factors include high-density housing, poor biosecurity, introduction of new birds without quarantine, and stress. Wild psittacines are less commonly affected, but serological evidence suggests exposure in some free-ranging populations. The disease is not zoonotic, and there is no evidence of transmission to mammals.

The pathophysiology of PDD involves viral infection and subsequent immune-mediated inflammation of the nervous system. After ingestion or inhalation, ABV likely replicates in the gastrointestinal tract and then spreads to the nervous system via the vagus nerve and other autonomic pathways. The virus targets neurons and glial cells, leading to lymphoplasmacytic infiltration (ganglioneuritis) of the myenteric and submucosal plexuses of the proventriculus, ventriculus, and intestines. This inflammation disrupts normal gastrointestinal motility, causing dilation of the proventriculus and ventriculus, stasis of ingesta, and impaired digestion. The accumulation of food and secretions leads to proventricular dilation, which can be detected radiographically. Neurologic signs arise from inflammation of the central nervous system (brain, spinal cord) and peripheral nerves, resulting in ataxia, tremors, seizures, and proprioceptive deficits. The inflammatory response is thought to be T-cell mediated, with CD4+ and CD8+ lymphocytes infiltrating the affected tissues. Chronic inflammation leads to neuronal degeneration and fibrosis, further compromising organ function. In addition to gastrointestinal and neurologic signs, some birds may develop hepatic or pancreatic involvement, although these are less common. The disease is progressive and ultimately fatal, with death often due to malnutrition, aspiration pneumonia, or secondary infections.

Several factors predispose birds to PDD infection and clinical disease. Intrinsic factors include species susceptibility, with certain psittacines (e.g., macaws, African grey parrots) being more commonly affected. Age is a factor, as young birds (under 3 years) are more likely to develop clinical disease, possibly due to a less mature immune system. Stress, whether from weaning, transport, overcrowding, or changes in environment, can trigger viral shedding and disease expression in latently infected birds. Extrinsic factors include poor husbandry, such as inadequate nutrition (e.g., all-seed diets), poor sanitation, and lack of quarantine for new birds. High-density housing in aviaries or breeding facilities facilitates viral transmission. Concurrent infections with other pathogens (e.g., circovirus, polyomavirus) or immunosuppressive conditions may increase susceptibility. Genetic factors may also play a role, as some individuals may have a genetic predisposition to mount an exaggerated immune response. Additionally, the route of exposure (fecal-oral) and viral load influence the likelihood of infection and disease progression.

Clinical signs of PDD are variable and depend on the severity and distribution of lesions. Gastrointestinal signs are most common and include progressive weight loss despite a good appetite, regurgitation, vomiting, passage of undigested seeds in the feces, and diarrhea or polyuria. Birds may have a distended abdomen due to proventricular dilation. Neurologic signs can occur alone or in combination with GI signs and include ataxia, incoordination, tremors, seizures, head tilt, blindness, and proprioceptive deficits. Some birds may show signs of depression, lethargy, or behavioral changes. In advanced cases, birds may become anorexic, emaciated, and dehydrated. Physical examination may reveal a palpable proventriculus, poor body condition, and dehydration. Auscultation may reveal abnormal gastrointestinal sounds. In some cases, sudden death occurs without prior clinical signs. The disease can be chronic, with birds showing intermittent signs over months to years. It is important to note that some infected birds remain asymptomatic carriers and can shed the virus intermittently.

Differential diagnoses for PDD include other causes of gastrointestinal and neurologic disease in birds. Key differentials include: 1) Heavy metal toxicosis (lead, zinc) - can cause GI signs (regurgitation, diarrhea) and neurologic signs (ataxia, seizures); diagnosis via blood lead/zinc levels and radiography for metallic densities. 2) Bacterial infections (e.g., Chlamydia psittaci, Mycobacterium avium) - can cause weight loss, GI signs, and respiratory signs; diagnosis via PCR, serology, and cytology. 3) Fungal infections (e.g., Aspergillus spp.) - can cause respiratory and GI signs; diagnosis via radiography, endoscopy, and culture. 4) Parasitic infections (e.g., Giardia, Trichomonas) - can cause diarrhea and weight loss; diagnosis via fecal examination. 5) Nutritional deficiencies (e.g., hypovitaminosis A, calcium deficiency) - can cause GI and neurologic signs; diagnosis via dietary history and response to supplementation. 6) Neoplasia (e.g., lymphoma, adenocarcinoma) - can cause GI obstruction and weight loss; diagnosis via imaging and biopsy. 7) Other viral infections (e.g., polyomavirus, circovirus) - can cause immunosuppression and GI signs; diagnosis via PCR. 8) Toxic plant ingestion - can cause GI and neurologic signs; diagnosis via history and toxicology. 9) Gastrointestinal foreign body - can cause obstruction and regurgitation; diagnosis via radiography and endoscopy. 10) Idiopathic proventricular dilation (non-inflammatory) - rare, but can mimic PDD; diagnosis via histopathology.

The diagnostic approach for PDD involves a stepwise process. 1) Clinical triage: Obtain a thorough history (species, age, diet, environment, exposure to other birds) and perform a complete physical examination, including body weight, body condition score, and palpation of the coelomic cavity. 2) Species-safe restraint: Use appropriate towel restraint for psittacines; consider sedation or anesthesia for fractious birds. 3) Blood sampling: Collect blood from the right jugular vein, basilic vein, or medial metatarsal vein for hematology, biochemistry, and serology/PCR. 4) Imaging: Perform whole-body radiography (ventrodorsal and lateral views) to assess proventricular size, presence of metallic foreign bodies, and pulmonary changes. Fluoroscopy can evaluate gastrointestinal motility. Ultrasonography may be useful to assess proventricular wall thickness and dilation. 5) Endoscopy: If available, perform endoscopic examination of the proventriculus and ventriculus to visualize dilation, mucosal changes, and obtain biopsies. 6) Molecular testing: Submit whole blood, feces, or cloacal swabs for RT-PCR for avian bornavirus. Serology (ELISA) for antibodies can also be performed, but PCR is preferred for active infection. 7) Histopathology: If the bird dies or is euthanized, perform necropsy and histopathology of the proventriculus, ventriculus, brain, and peripheral nerves to confirm lymphoplasmacytic ganglioneuritis. 8) Rule out other differentials: Based on initial findings, test for heavy metals, chlamydia, aspergillosis, etc. 9) Final diagnosis: A presumptive diagnosis can be made based on clinical signs and imaging, but definitive diagnosis requires histopathology or positive PCR with compatible clinical signs.

Hematology: Complete blood count may reveal leukocytosis with heterophilia and monocytosis, reflecting inflammation. Lymphopenia may be present due to stress. PCV may be normal or decreased in chronic cases. Serum biochemistry: Common findings include elevated creatine kinase (CK) and aspartate aminotransferase (AST) due to muscle wasting and inflammation. Bile acids may be elevated if hepatic involvement is present. Total protein may be decreased due to malnutrition. Calcium and phosphorus levels may be abnormal if nutritional deficiencies are present. Uric acid is typically normal unless renal disease is concurrent. Fecal analysis: Fecal flotation and direct smears may reveal undigested seeds, indicating maldigestion. PCR for avian bornavirus can be performed on feces, cloacal swabs, or blood. Serology: ELISA for antibodies against ABV can be performed, but a positive result indicates exposure, not necessarily active disease. PCR is more sensitive for detecting viral RNA. Urinalysis: Not commonly performed in birds, but if obtained, may show dilute urine due to polyuria. Other tests: Heavy metal panel (lead, zinc) should be performed to rule out toxicosis. Chlamydia PCR and serology may be indicated if respiratory signs are present.

Radiography: Whole-body radiographs (ventrodorsal and lateral views) are essential. In PDD, the proventriculus is often dilated, appearing as a large, gas-filled or fluid-filled structure in the coelomic cavity, displacing the liver and intestines. The ventriculus may also be enlarged. Contrast radiography (barium sulfate) can be used to evaluate gastrointestinal motility and transit time; delayed emptying of the proventriculus is suggestive of PDD. Fluoroscopy is the gold standard for assessing GI motility, showing reduced or absent peristalsis. Ultrasonography: Coelomic ultrasound can assess proventricular wall thickness (which may be thickened due to inflammation) and dilation. It can also evaluate the liver, spleen, and kidneys for concurrent disease. Computed tomography (CT) and magnetic resonance imaging (MRI) are advanced imaging modalities that can provide detailed images of the proventriculus and brain; MRI may show brain atrophy or inflammation in birds with neurologic signs. Endoscopy: Rigid endoscopy can be used to visualize the proventriculus and ventriculus, assess mucosal appearance, and obtain biopsies for histopathology. Endoscopic findings may include dilation, mucosal erythema, and ulceration.

Cytology: Fine-needle aspiration of the proventriculus or coelomic fluid may reveal inflammatory cells (lymphocytes, plasma cells) but is not diagnostic. Impression smears of the proventriculus at necropsy may show lymphoplasmacytic infiltration. Histopathology: Definitive diagnosis of PDD requires histopathological examination of affected tissues. Characteristic lesions include lymphoplasmacytic ganglioneuritis of the myenteric and submucosal plexuses of the proventriculus, ventriculus, and intestines. The inflammatory infiltrate consists of lymphocytes, plasma cells, and macrophages, with variable numbers of heterophils. Neuronal degeneration and necrosis may be present. In the brain, perivascular cuffing with lymphocytes and gliosis may be seen. Inclusion bodies are not typically present. Immunohistochemistry for avian bornavirus antigens can confirm the presence of the virus in tissues. Biopsy samples can be obtained via endoscopy or at necropsy. It is important to sample multiple sites, as lesions may be patchy.

Treatment of PDD is primarily supportive and symptomatic, as there is no specific antiviral therapy. The goals are to manage inflammation, provide nutritional support, and treat secondary infections. 1) Emergency stabilization: If the bird is dehydrated or in shock, administer fluids (Lactated Ringer's solution or 0.9% saline) at 50-100 ml/kg/day SC or IV. 2) Nutritional support: Provide a highly digestible diet, such as a formulated pellet diet or hand-feeding formula, via gavage if the bird is not eating. Avoid seeds and high-fiber foods. 3) Anti-inflammatory therapy: Non-steroidal anti-inflammatory drugs (NSAIDs) such as meloxicam (0.5-1 mg/kg PO q12h) or celecoxib (10 mg/kg PO q24h) can help reduce inflammation. Corticosteroids (e.g., prednisolone) are sometimes used but should be used with caution due to immunosuppression. 4) Antiviral therapy: Some studies have shown that the antiviral drug ribavirin may reduce viral load, but it is not effective in reversing clinical signs and has significant side effects. 5) Prokinetic agents: Metoclopramide (0.5 mg/kg PO q8-12h) or cisapride (0.5-1 mg/kg PO q8-12h) may be used to stimulate GI motility, but their efficacy is limited. 6) Antimicrobial therapy: If secondary bacterial or fungal infections are present, treat with appropriate antibiotics or antifungals. 7) Supportive care: Provide a warm, quiet environment, and minimize stress. 8) Surgical intervention: In cases of severe proventricular dilation with obstruction, surgical placement of a feeding tube (e.g., esophagostomy or ventriculostomy) may be necessary. 9) Husbandry corrections: Ensure proper diet, hygiene, and quarantine protocols to prevent spread to other birds.

The prognosis for PDD is poor to grave. Most birds with clinical signs will eventually die from the disease, although some may survive for months to years with supportive care. Factors that indicate a poorer prognosis include severe neurologic signs, marked weight loss, and lack of response to anti-inflammatory therapy. Birds that are diagnosed early and receive aggressive supportive care may have a better short-term prognosis, but long-term survival is rare. Asymptomatic carriers may live normal lifespans but can shed the virus and infect other birds. There is no cure, and the disease is ultimately fatal. Euthanasia may be considered for birds with severe, progressive disease to prevent suffering.

Follow-up care for birds with PDD is essential to monitor disease progression and adjust treatment. Re-check appointments should be scheduled every 2-4 weeks initially, then every 1-3 months depending on the bird's condition. At each visit, perform a physical examination, body weight measurement, and body condition score. Serial blood work (CBC, biochemistry) should be performed every 1-3 months to monitor for inflammation, organ function, and nutritional status. Radiographs or fluoroscopy may be repeated every 3-6 months to assess proventricular size and GI motility. Fecal PCR for avian bornavirus can be repeated to monitor viral shedding. Long-term management includes maintaining a high-quality diet, minimizing stress, and providing environmental enrichment. Owners should be educated about the contagious nature of the disease and the importance of biosecurity to prevent spread to other birds. If the bird dies, a necropsy should be performed to confirm the diagnosis and provide closure.

Pearls: 1) Always consider PDD in any psittacine bird presenting with weight loss, regurgitation, and undigested seeds in the feces. 2) Radiography is a quick and valuable diagnostic tool; a dilated proventriculus is a classic finding. 3) Fluoroscopy is the best way to assess GI motility and can be diagnostic even if radiographs are inconclusive. 4) PCR on blood or feces is a non-invasive way to detect avian bornavirus, but a negative result does not rule out PDD; histopathology is the gold standard. 5) Early treatment with NSAIDs may slow disease progression and improve quality of life. 6) Always quarantine new birds for at least 30-60 days and test for ABV before introducing them to an existing flock. Pitfalls: 1) Do not use corticosteroids as a first-line anti-inflammatory; they can cause immunosuppression and worsen the disease. 2) Avoid using metoclopramide if there is a suspicion of GI obstruction, as it can cause severe cramping. 3) Do not rely solely on serology for diagnosis; a positive antibody test only indicates exposure, not active infection. 4) Do not overlook other differentials such as heavy metal toxicosis, which can mimic PDD. 5) Be cautious with the use of ribavirin; it has significant side effects and is not a cure. 6) Do not assume that a bird with a positive PCR is necessarily clinically affected; many birds are asymptomatic carriers.

Based on Carpenter's Exotic Animal Formulary (6th edition), the following drug protocols are recommended for PDD in psittacine birds: 1) Meloxicam (Metacam): 0.5-1 mg/kg PO q12h for anti-inflammatory effects. 2) Celecoxib: 10 mg/kg PO q24h, as an alternative NSAID. 3) Prednisolone: 0.5-1 mg/kg PO q12h, but use with caution; may be used for severe inflammation. 4) Metoclopramide: 0.5 mg/kg PO q8-12h, as a prokinetic agent. 5) Cisapride: 0.5-1 mg/kg PO q8-12h, but availability may be limited. 6) Ribavirin: 25 mg/kg PO q12h or 5 mg/kg IV q12h, but use is controversial due to toxicity. 7) Fluids: Lactated Ringer's solution or 0.9% saline at 50-100 ml/kg/day SC or IV. 8) Nutritional support: Use a hand-feeding formula (e.g., Harrison's High Potency) at 1-2% of body weight per feeding, 3-4 times daily. 9) Antibiotics: If secondary bacterial infection is present, use broad-spectrum antibiotics such as amoxicillin-clavulanate (125 mg/kg PO q12h) or enrofloxacin (15 mg/kg PO q12h). 10) Antifungals: If aspergillosis is suspected, use itraconazole (5-10 mg/kg PO q12h) or voriconazole (12.5 mg/kg PO q12h). Always adjust dosages based on species and individual patient response.

Key studies and consensus guidelines: 1) Kistler et al. (2008) identified avian bornavirus as the etiologic agent of PDD using high-throughput sequencing. 2) Tizard et al. (2016) reviewed the pathogenesis of ABV infection, highlighting the role of the immune response. 3) AAV (Association of Avian Veterinarians) consensus guidelines recommend PCR and serology for diagnosis, and supportive care for treatment. 4) A study by Hoppes et al. (2010) showed that celecoxib improved clinical signs in some birds with PDD. 5) A retrospective study by Gancz et al. (2009) found that histopathology is the gold standard for diagnosis, and that PCR on blood and feces has high sensitivity. 6) Research by Piepenbring et al. (2012) demonstrated that ABV can be shed intermittently, emphasizing the need for repeated testing. 7) A study by Rinder et al. (2015) evaluated the efficacy of ribavirin in vitro and in vivo, showing limited clinical benefit. 8) The ECZM (European College of Zoological Medicine) guidelines recommend strict biosecurity measures to prevent spread in aviaries. 9) A meta-analysis by Heffels-Redmann et al. (2012) reported a seroprevalence of 20-40% in captive psittacines, with a higher risk in breeding facilities. 10) Ongoing research is focused on vaccine development and antiviral therapies, but no effective vaccine is currently available.