When confronted with an unwell parrot, veterinary practitioners are often well versed in differential diagnoses relating to bacterial or fungal diseases, nutritional deficiencies or husbandry concerns, but viral diseases are often overlooked. Viral diseases are often underlying in a number of clinical syndromes and should be considered in ill birds, especially juveniles and birds with immunosuppression. Viral diseases are often encountered when birds are young, usually when in a mixing environment such as during transportation or at a pet shop. Viral diseases can remain latent for months to years and should be on your differential diagnosis list when assessing unwell parrots.
Beak and feather disease virus
Psittacine beak and feather disease (PBFD) is caused by beak and feather disease virus (BFDV), a circovirus well known for causing birds to become completely bald. However, disease presentation can be far more diverse than this. Acute forms of the disease cause rapid depression, anorexia, regurgitation and death in juvenile parrots, while the more chronic form results in feather dystrophy. With each sequential moult, these dystrophic feathers replace healthy feathers, resulting in poor quality plumage (Zsivanovits, 2018). Cockatoos show particularly drastic clinical signs as they lose their powder down feathers first, creating areas of alopecia. The lack of powder down then results in a shiny pseudo-discoloration of the beak (Raidal, 2016). Beak deformity can also occur, with poor quality, crumbling keratin of the rhinothecal and gnathothecal tomium, progressive elongation (Figure 1) and necrosis of the rostral palate (Greenacre, 2005). In parrots with green feathers, the presence of feathers that are discoloured yellow but are otherwise normal can be an initial clinical sign suggestive of PBFD (Raidal, 2016).
Transmission is via faeces, crop secretions and feather dander (Raidal, 2016). Diagnosis can be made by polymerase chain reaction (PCR) testing of a blood sample or a feather with feather pulp present, such as in a newly growing feather (Zsivanovits, 2018). A study in peach-faced lovebirds (Agapornis roseicollis) has shown a higher proportion testing positive with PCR on blood compared to feather samples (Kalhesi, 2005). Histopathology of skin and dystrophic feathers can be useful for diagnosis and to differentiate from feather-destructive behaviour but can result in false negatives if the disease is not severe (Raidal, 2016). Unfortunately, there is no treatment for BFDV and affected birds succumb to the virus or subsequent secondary complications. Affected birds should be separated from non-affected individuals, as they will continue to shed the virus (Zsivanovits, 2018). BFDV is very stable in the environment, and so a thorough disinfection should occur if there is any suspicion of the virus.
Budgerigar fledgling disease is a disease of viral origin, caused by polyomavirus (Greenacre, 2005). Clinical signs include a high mortality rate in nestlings, abnormal feathers including stunted primary feathers, skin discoloration and haemorrhage, and abdominal distension (Zsivanovits, 2018). Acute disease can also occur in juvenile parrot species other than budgerigars, resulting in depression, anorexia, poor crop motility, diarrhoea and polyuria with a mortality rate of up to 41 percent (Greenacre, 2005). Older birds with more robust immune systems often develop subclinical disease following exposure and shed the virus intermittently following this. Chronic presentations of polyomavirus are seen in adult birds with poor feather quality, intermittent anorexia, immunosuppression and secondary bacterial or fungal infections (Greenacre, 2005).
Chronic presentations of polyomavirus are seen in adult birds with poor feather quality, intermittent anorexia, immunosuppression and secondary bacterial or fungal infections (Greenacre, 2005)
Transmission of the virus has been shown via secretions, especially urine, and vertical transmission has been proven in budgerigars (Greenacre, 2005). A PCR can be performed on abnormal feathers or feathers with pulp, faeces, a combined choanal and cloacal swab, or a blood sample (Zsivanovits, 2018). Serology can be performed on blood samples and used in conjunction with PCR (Molloy et al., 2021). Control of the virus is usually by destocking adult birds and halting breeding for six months, while also considering removing smaller psittacines, such as budgerigars and cockatiels, from a breeding collection (Zsivanovits, 2018).
Bornavirus has been shown to be the causative agent of proventricular dilation disease (PDD) (Honkavuori et al., 2008; Kistler et al., 2008). PDD was first encountered in the late 1970s and termed “macaw wasting disease”, and since then has been shown to affect more than 60 parrot species (Gregory et al., 1994). Clinical signs do not present until late in the disease process and are related to non-purulent inflammation of peripheral nerves, particularly those of the gastrointestinal tract (Lierz, 2016). This manifests as lethargy, weakness, regurgitation and weight loss, with characteristic undigested seed identified in droppings. Central nervous system signs can also be seen, including ataxia, opisthotonus (Figure 2), muscle tremors and seizures (Lierz, 2016). Most birds show clinical signs at a young age, with an average age of 3.8 years, and a reported range of 10 weeks to 17 years (Graham, 1991).
Diagnosis can be via PCR from urine, faeces or cloacal swabs (Hoppes and Shivaprasad, 2020); however, false negatives can occur. Serology can be performed but it is important to note that the presence of avian bornavirus antibodies does not necessarily indicate the presence of PDD. Radiographs, fluoroscopy or CT can show evidence of proventricular distension; however, this can also be caused by other clinical syndromes such as gastrointestinal foreign bodies, heavy metal toxicosis, endoparasites or Macrorhabdus ornithogaster (Hoppes and Shivaprasad, 2020). Use of contrast in the gastrointestinal tract can aid diagnosis. A proventricular diameter to keel height ratio has been developed to identify proventricular distension on a lateral radiographic projection (Figure 3; Dennison and Paul-Murphy, 2008). Ante-mortem diagnosis can be achieved by a combination of diagnostics; however, the only definitive diagnosis is via histopathology of the crop, proventriculus, ventriculus or adrenal gland, usually at post-mortem, showing evidence of myenteric ganglioneuritis (Hoppes and Shivaprasad, 2020). There is no current treatment for bornavirus and PDD, but COX-2 selective non-steroidal anti-inflammatory drugs have been shown to increase life expectancy in affected birds (Hoppes and Shivaprasad, 2020).
Psittacid herpesvirus 1
Psittacid herpesvirus 1 consists of four major genotypes and is the causative agent for Pacheco’s disease and mucosal papillomatosis. Pacheco’s disease is characterised by acute death within hours of showing clinical signs such as lethargy, anorexia, biliverdinuria, neurological signs, regurgitation and haematochezia (Zsivanovits, 2018). Mortality rates are high and patients rarely recover following disease; however, prophylactic use of acyclovir has been shown to reduce mortality in an outbreak situation (Phalen, 2006).
Mortality rates are high and patients rarely recover following disease; however, prophylactic use of acyclovir has been shown to reduce mortality in an outbreak situation (Phalen, 2006)
Mucosal papillomas are most frequently reported in Amazon parrots, macaws, hawk-headed parrots and conures, but have been reported in multiple other parrot species (Phalen, 2016). These often present as parrots that strain to defecate or have frank blood in their droppings (Figure 4), with cloacal papillomas identified on physical examination or cloacoscopy. Papillomas can occur anywhere along the mucosa of the gastrointestinal tract, and lesions higher in the gastrointestinal tract, such as in the proventriculus or ventriculus, can result in regurgitation (Zsivanovits, 2018). Oral papillomas have also been reported at the edges of the choana or at the base of the tongue (Phalen, 2016). Bile duct carcinoma and pancreatic duct carcinomas can arise secondary to mucosal papillomatosis, resulting in clinical signs of weight loss, poor feather quality and beak and nail overgrowth (Phalen, 2016). Diagnosis for psittacid herpesvirus 1 is made via PCR of blood or combined choanal and cloacal swabs. Histopathology of the liver and spleen is useful in cases of suspected Pacheco’s disease, as affected individuals will have intranuclear inclusion bodies (Phalen, 2016). Removal of papillomas has been attempted with cryosurgery, electrocautery, laser surgery and dissection; however, regrowth may occur following resection (Zsivanovits, 2018).
Knowledge of avian viral diseases is essential for any veterinary surgeon consulting with psittacine species. They should feature on the differential diagnosis list for any sick parrot, with widely available diagnostic testing in the UK. Unfortunately, the viral diseases in parrots discussed here are without cure, with the mainstay of treatment being controlling the spread of the viruses.