Pathogenesis of Avian Bornavirus in Experimentally Infected Cockatiels

Anne K. Piepenbring

, Dirk Enderlein, Sibylle Herzog, Erhard F. Kaleta, Ursula Heffels-Redmann, Saskia Ressmeyer, Christiane Herden, and Michael Lierz

Author affiliations: Justus Liebig University Giessen, Giessen, Germany

Abstract

Avian bornavirus (ABV) is the presumed causative agent of proventricular dilatation disease (PDD), a major fatal disease in psittacines. However, the influencing factors and pathogenesis of PDD are not known and natural ABV infection exhibits remarkable variability. We investigated the course of infection in 18 cockatiels that were intracerebrally and intravenously inoculated with ABV. A persistent ABV infection developed in all 18 cockatiels, but, as in natural infection, clinical disease patterns varied. Over 33 weeks, we simultaneously studied seroconversion, presence of viral RNA and antigens, infectious virus, histopathologic alterations, and clinical signs of infection in the ABV-infected birds. Our study results further confirm the etiologic role of ABV in the development of PDD, and they provide basis for further investigations of the pathogenetic mechanisms and disease-inducing factors for the development of PDD.

Proventricular dilatation disease (PDD) is a significant cause of disease-related fatalities among birds, primarily psittacines (1–3). PDD has been observed in >50 psittacine species. Large parrots, including many endangered species, are the most frequently and most severely affected birds (4). PDD constitutes a threat to all parrot flocks and aviaries worldwide and endangers the protection and conservation of captive and wild psittacine species.

PPD is caused by a nonpurulent inflammation of the autonomic nervous system of the upper gastrointestinal tract, the peripheral and central nervous tissue, and the cardiac conduction system (5,6). Gastrointestinal and neurologic signs can appear alone or in combination (4,7,8). The clinical signs are nonspecific, and PDD can be definitively diagnosed only by pathohistologic detection of lymphoplasmacytic infiltrates of ganglia in the upper gastrointestinal tract. However, a negative finding cannot exclude the presence of PDD (4,6,8,9).
In 2008, 2 independent groups of research scientists described a new virus, avian bornavirus (ABV), which was amplified from samples from PDD-affected birds. Since then, 6 different ABV genotypes have been detected in psittacines birds. Additional genotypes have been detected in a canary (Serinus canaria), wild Canada geese (Branta canadensis), and trumpeter swans (Cygnus buccinator) (3,10–12). Recent studies substantiate the crucial role of ABV as the etiologic agent for PDD (10,11). Several scientific groups found ABV in 60%–100% of PDD-affected birds studied (10,11,13–15). Surveillance studies in aviaries showed that not all birds were affected after exposure to PDD-diseased and ABV-positive birds, and clinical signs and infection status varied considerably in birds that were affected. In addition, some ABV-positive birds showed no clinical signs (16–20). These facts indicate that host factors as well as features of the infectious agent, ABV, play a key role for disease induction (18).

Initial studies of experimental infections in birds fulfilled Henle-Koch postulates by using small numbers of animals. Gancz et al. (21) inoculated 3 cockatiels by multiple routes with brain homogenate containing ABV-4. Sixty-six days postinoculation (dpi), PDD-associated signs, including characteristic histopathologic lesions, developed in 2 of the 3 birds, and test results were positive for ABV-4. The implications of these findings were obscured by the fact that the brain homogenate also included sequences with partial analogy to viruses of the family Astroviridae and family Retroviridae. After inoculating 4-day-old mallards (Anas platyrhynchos; n = 15) with ABV-4, Gray and colleagues (22) detected ABV RNA in the feces and antibodies against ABV in serum, but they did not detect any clinical signs or PDD-associated lesions. Later, Gray et al. (23) inoculated 2 adult Patagonian conures (Cyanoliseus patagonis) with ABV-4. The conures were known to be chronic carriers of psittacine herpesvirus, but they appeared to be healthy. Antibodies against ABV were detected 33 dpi, and shedding of viral RNA was detected 62 dpi. Clinical PDD signs developed in both birds, after which 1 bird died and the other was euthanized. Histopathologic analysis showed typical PDD lesions. It was not determined whether the herpesvirus infection was a potentiating factor (21–23).

Reliable studies that include sufficient numbers of animals to address the host variability over an adequate investigation period are needed to clarify the pathogenetic effects of ABV infection on the development of PDD. More precisely, this implies the need for investigating the course of clinical signs, seroconversion, histopathologic lesions, and virus shedding and distribution in the tissues of affected birds. To further the understanding of ABV and the associated disease, we infected 18 cockatiels with ABV by using 2 different inoculation routes and monitored them for 33 weeks. Our findings show that a persistent ABV infection was induced by in all 18 birds and that it was possible to reliably reproduce all of the known natural ABV disease patterns.