Introduction

Avian bornavirus (ABV) has a worldwide distribution, with reports from North America, Europe, South Africa, Japan, Sweden, and Brazil, to name a few. Avian bornavirus can cause proventricular dilatation disease; however, proventricular dilatation disease is really a misnomer, as many systems in the body may be affected by ABV—in particular the nervous system. Not all birds infected with ABV develop signs of PDD, and not all birds experiencing the syndrome PDD shed the virus or have other evidence of ABV infection. Due to the neurological involvement, clinical signs can include blindness, ataxia, weakness, weight loss, inability to perch, or passage of undigested food—with the bird eventually succumbing to opportunist gastrointestinal infections or starvation due to inability to ingest and digest food. While classical clinical signs of ABV infection include passage of undigested seed, fetid stool, vomiting undigested seed, weight loss, and a variety of neurologic signs, many other diseases can cause these same clinical signs. Differentials that should be considered for PDD include heavy metal intoxication, occult parasitism, fungal or bacterial infection, and a variety of other causes of encephalitis.

Avian Bornavirus Infections

Avian bornavirus infection is caused by an enveloped single-strand RNA virus in the family Bornaviridae. There are five species of Bornaviridae that infect birds: Psittaciform 1 bornavirus; Psittaciform 2 bornavirus; Passeriform 1 bornavirus; Passeriform 2 bornavirus; and Waterbird 1 bornavirus. Within each species, there are numerous genotypes that can infect a variety of different bird species (Table 1).

Table 1. Bornaviridae species, genotypes, and animals infected (adapted from Escandon P. Thesis. 2020)

While transmission and pathogenesis details of the disease in companion birds are still under active investigation, infection rate may be as high as 15%. The incubation period and shedding during this period is unknown; however, shedding by asymptomatic birds is likely. Infected birds are thought to shed the virus for life; however, there is a report of a captive flock of ABV-infected cockatiels spontaneously eliminating the virus, becoming PCR negative for ABV. Viral RNA and infectious virus have been found in urine, feces, tears, and oral secretions. This virus is likely transmitted by aerosol, contact or ingestion, and vertical methods; however, at present, the only experimental means of infecting a bird is through intramuscular, intranasal, and ocular administration of the virus.

Diagnosis

Diagnosis is still an active area of investigation. Avian bornavirus is mainly a neurotropic virus but can be found in many tissues and cell types. Diagnostic techniques include molecular techniques by reverse-transcription polymerase chain reaction (RT-PCR), serological assays such as Western blot, enzyme-linked immunosorbent assays (ELISA), indirect enzyme-linked immunosorbent assay and immunohistochemistry, and virus isolation. ABV antigens and genetic material have been found in the brain, eye, spinal cord, heart, gastrointestinal system, adrenal glands, kidneys, urine, feces, tears, oral secretions, and feather dander. To the authors’ knowledge, there are presently no commercially available rapid immunological assays available. Radiographs, CT or fluoroscopy with contrast may also be useful in these cases. When present, gross lesions consist of dilated proventriculus and cardiac enlargement. Histopathology of proventricular dilatation disease is characterized by nonsuppurative inflammation in the central, peripheral, and autonomic nervous systems. Immunohistochemistry of these same lesions often reveals viral antigen.

Treatment

To our knowledge, no specific antiviral treatment has been effective at either eliminating virus from the host, reducing viral shedding, or ameliorating clinical sigs of avian bornaviral infection. Ribavirin, a combination of ribavirin and interferon-alpha, and favipiravir have shown to interfere with avian bornavirus replication in cell culture but have not been evaluated in an infected bird.

The use of nonsteroidal anti-inflammatory drugs has been suggested as a treatment for birds with clinical signs of ABV infection. However, with experimentally infected cockatiels, neither meloxicam nor celecoxib altered clinical presentation, viral shedding, gross lesions, histopathology, or viral distribution. In fact, treatment with NSAIDs may have induced gastrointestinal toxicity resulting in potential intestinal bleeding. Thus, caution is suggested in using long-term NSAID treatment. Treatments with steroids and other immunosuppressive drugs have been suggested, but the authors could find no published studies. Thus, supportive treatment of clinically infected birds is presently the only viable option.

Prevention

Prevention involves screening for viral infection and shedding via PCR. Maintenance of an ABV-free collection is preferred but is rarely currently feasible. Testing and separation are difficult based on intermittent shedding and latency of virus.

No recommended commercially available vaccines are available. Work is progressing on vectored, subunit, and killed vaccines. Vaccination has not been shown to protect against infection nor arrest virus shedding. It may result in decreased severity of clinical signs, but the literature is not unanimous on this point. A major hindrance in vaccine research is the development of an experimental model that reflects infection in the wild. All the vaccine-challenge studies have utilized intramuscular, intranasal, or ocular administration of the virus.

Although appropriate disinfection remains uninvestigated, ABV is an enveloped virus, and enveloped viruses are likely easily inactivated. Thus, most of the disinfectants used in aviaries would be suitable, though this is only speculation. As to ABV’s stability in the environment, preliminary studies suggest that the virus is stable between pH 7 to 10, but it is rapidly inactivated outside this range. At 26°C and 21% humidity, ABV remains infectious for only 24 hours; but after 48 hours, 99.97% of the virus is inactivated, and after four days, no infectious virus is present. Further studies need to be done on disinfection and sentimental stability of ABV.

Research

The Schubot Exotic Bird Health Laboratories have tested >500 free-living, non-Psittaciformes of more than 30 species (Table 2) for avian bornavirus (ABV). Birds sampled include hunter-killed ducks; raptors and aquatic birds that died or were euthanized during rehabilitation; birds that were culled by animal control authorities; Passeriformes that were collected for museum study; and rehabilitated pre-release ducks and pelicans from Wildlife Center of Texas. Samples tested include choanal and cloacal swabs and brain, liver, and splenic tissues. Avian bornavirus sequences were amplified using two multiplexed primer sets. One recognized matrix protein (M protein) genes, and the other recognized conserved regions of the nucleoprotein (N protein) genes. Positive results were verified by repeat PCR testing, sequencing, and culture. Many non-psittaciform species were identified with avian bornavirus, particularly Anseriformes and Charadriiformes; however, sample sizes in other avian taxa were low. An improved recovery rate for ABV from the brain occurred compared with a very low recovery rate from swabs from choana, cloaca, or droppings. Clinical signs attributable to ABV in psittacine birds (gastrointestinal tract dysfunction, neurologic signs) are seldom noted. Sequencing has revealed multiple ABV isolates that are not closely related to the isolate that appears most likely to cause clinical signs of Psittaciformes (ABV4).

Table 2. Non-Psittaciformes tested by Schubot Exotic Bird Health Laboratories for avian bornavirus

From 2009 to 2013, after the discovery of ABV and fulfillment of Koch’s postulates as the causative agent of proventricular dilatation disease, Schubot Exotic Bird Health Center started accepting samples from private practice and zoo veterinarians to assess parrots for ABV. Most samples were choanal/cloacal, choanal or fecal swabs obtained from live parrots. Over 460 samples have been submitted and screened via PCR for the M protein and N protein genes. Macaw species were the most commonly submitted (n=133) followed by cockatoos (n=92), African grey parrots (n=52), and Amazona spp. (n=39). Prevalence based on positive PCR results was approximately 15% across all species. Most taxa were similar in prevalence, although Amazona spp. (10%, 4/35) and Macaws (Ara spp.) (13%, 17/133) were lower, and Australian species were higher (cockatoos 18% [17/92] and other Australian species 21% [7/26]). Cockatiels (9/35), Eclectus (7/21), African grey parrots (8/52), blue-throated macaws (2/8), and sun conures (3/14) appear to be the species with the highest likelihood of testing PCR positive for ABV, although our sample sizes are small. Many parrot species have yet to have a positive PCR for ABV in our lab, but our sample sizes are below 10 for all of these species. Future recommended epidemiologic research in ABV for parrots should include identification of the preferred sample type to detect ABV (swab, urine, or feces) in the clinically ill bird versus the healthy bird, and gender and seasonal influences on viral shedding.

Take-Home Points

1.  ABV infection is widespread in captive and free-living birds; many infected birds are asymptomatic and do not have neurologic or other clinical signs.

2.  The diagnosis of PDD can be challenging, but all efforts to rule in or out ABV infection as well as other causes of GI tract or neurologic illness should be included in a case workup.

3.  The diagnosis of ABV infection should not be used as a sole reason for euthanasia.

4.  Birds suffering from PDD may or may not be infected or shedding ABV; latency and immune-mediated aspects may play a role in disease occurrence.

5.  Means of transmission have yet to be fully elucidated and should be further researched.

6.  Efficacious disinfection protocols and environmental stability of the virus have not been fully researched.

References

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