Clinical Signs of ABV Infections / Proventricular Dilatation Disease (PDD)
ABV is highly neurotropic; birds may experience the CNS or GI signs, or both.
The Gastrointestinal Signs of ABV Disease
The most frequently seen clinical GI signs include depression, anorexia and weight loss (due to poor motility), loss of body condition, regurgitation, passage of undigested food in the feces, poor absorption, inappetence and polyphagia (increased appetite). Crop biopsy and radiographs reveal dilation of the proventriculus, ventriculus and other organs.2 Histopathology slides show an infection of the nerves of the GI system.14
When this invasion of the nerve clusters by lymphocytes and plasmacytes reaches the GI system’s nerves, “symptomatic, persistent infections often result in intestinal colic and ganglioneuritis in the upper intestinal tract.”2, 22 The vagus nerve in the brain “supplies the first part of the intestinal tract—from the crop, proventriculus and ventriculus to the duodenum and the heart. When the vagus nerve malfunctions, there is a disruption of the normal GI motility and a thinning of the wall of the GI tract.” 14 This can result in the rupture of the proventriculus and immediate death. Finding the infected nerve ganglions confirms that the individual is positive for PDD.14 Other signs include varying degrees of abdominal enlargement, muscle atrophy, weakness, and polyuria. In addition, clinicians frequently observe secondary bacterial and fungal infections of the GI tract due to slow motility.20
Figure 4: Arrow indicates dilated crop four hours after feeding, with little or no mobility. (Courtesy S. Orosz)
The Neurological Signs of PDD
In some individuals, ABV assaults the Central Nervous System, leading to inflammation of the brain and spinal cord.22 ABV is a neurotropic virus, causing inflammation of the nerve ganglia.4 The immune response causes nerve dystrophy which leads to clinical disease. (Dahlhausen, personal communication)
CNS signs include “lack of coordination, ataxia, paresis (progressive, impaired movement), paralysis, head tremors, seizures, and motor and proprioceptive deficits.”2,5 Feather plucking and self-mutilation may also ensue due to a peripheral neuritis. The bird may experience difficulty balancing, moaning or crying due to digestive discomfort, hyperesthesia (oversensitivity to sensory stimulation), aggressive behavior, and intermittent head-shaking 4, 2Some practitioners believe that nervous system signs are more difficult to control and more rapidly fatal than GI signs. In tests, antibody titers were highest in birds with neurological signs.7
Figure 6: Signs of self-mutilation in an African Grey (Courtesy R. Dahlhausen)
Cardiac Impairment (Autonomic Nervous System)
In studies, “Viral antigen ABV has been identified in the cells which generate the electrical impulses that control the heart rate.” 2 Clinicians have discovered “lesions in tissues outside of the nerves, such as smooth or heart muscle fibers.” 22 Viral antigens have also been found in the “conduction pathways of the heart.” 22 These antigens cause acute death in otherwise normal-appearing individuals.” 22
The Avian Bornavirus may also be responsible for disorders of the eyes, leading to ocular lesions and blindness. Affected birds experience lesions within the fundus (retina) of the eye and decreased visual acuity.10 In most cases of cortical blindness, the eyes respond to correct treatment.2
Figures 6: Displacement of organs caused by dilated proventriculus and ventriculus. (Courtesy R. Dahlhausen)
Figure 7: A necropsy photo shows the dramatically reduced pectoral mass in a cockatoo. Wasting occurs when the proventriculus dilates as the disease progresses. The proventricular wall thins due to the altered mixing pattern of food; the food is not digested or absorbed properly. This, combined with reduced absorption of nutrients, causes the bird to use the muscle mass for its energy source. (Courtesy R. Dahlhausen)
Figure 5: Gross lesions in a blue and gold macaw. Note the enlarged size of the proventriculus and ventriculus. Normally, the proventriculus would not be larger than the liver, but here the organs are displaced due to enlarged proventriculus (in blue at top) and ventriculus (in grey, below the proventriculus. (Courtesy L. Bauck)
Auto-Immune System / Steps Involved
The nerve cell ingests the virus; the immune system fails to recognize and attack it. It is now an auto-immune disorder resulting in infection. Lymphocytes and plasmacytes respond.
The lymphocytes produce anti-ganglioside antibodies which triggers a cytokine storm.
The leakage of the proteins from the immune complexes initiates inflammation and symptoms.
The inflammation brings about more lymphocytes, but these are also infected. This becomes a recurring cycle.
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