Nervous and digestive symptoms

 

Nervous and digestive symptoms appear progressively. In the digestive form of the disease, the bird becomes progressively incapable of properly evacuating its digestive tube and digesting its food. This causes an accumulation of food in the crop and dilated stomachs (ventricle and proventriculus), resulting in regurgitations, vomiting, undigested seeds in droppings, and progressive weight loss in the animal.

 

The nervous form of the disease is characterised by loss of balance (sudden falls from its perch, particularly at night), poor co-ordination, abnormal head movements, seizures or onset of blindness.

 

The two clinical forms can possibly be connected. Death typically follows within a few months.

 

A newly identified virus, avian bornavirus

 

A viral aetiology was suspected for a long time without being able to specify the source. In 2008, two teams (Kistler et al. and Honkavuori et al.) independently identified in the US and Israel a new RNA virus, avian bornavirus, as the causative agent of this disease. Prior to this we only knew of mammalian bornavirus, the cause of encephalitis in horses, sheep and sometimes other pets.

 

Symptoms are mainly related to immune inflammatory reaction caused by the presence of the virus in nerve cells. The virus itself does not attack the body's cells.

 

Many healthy carriers

 

This disease is characterised by a large population of healthy carriers, that is, by a large number of birds that have been infected with the virus, which are contagious, but do not necessarily develop the disease. Among psittacidae, the African Grey Parrot, cockatoos, and Amazons more easily develop the disease while cockatiels are often healthy carriers.

 

Proventriculitis develops preferentially in dense populations of birds kept in a closed environment. Transmission occurs through ingestion of water or food contaminated by droppings, by inhaling dried droppings dust, and likely from parents directly to their chicks since the virus is present in the ovaries and testes.

 

Incubation varies from 11 days to 2 months. An infected bird can, however, remain a healthy carrier for months or years and excrete the virus intermittently before developing symptoms.

 

A difficult-to-reach diagnosis

 

To remove all doubt when suspecting the disease, a few additional tests are necessary:

• X-ray: results from an x-ray will show an increase in the size of the stomach as well as the presence of gas in the intestines. Although highly suggestive of a proventriculitis, this anomaly is not, however, specific to the disease.
• Screening tests: Now that we know the causative agent of this disease, the avian bornavirus, it is possible to develop screening tests.

There are two types of the latter:

 

1.     a serology, which searches for antibodies that develop in the blood during infection, and

2.     a PCR test, which can directly search the virus’ DNA in a sample taken from the bird.

 

These tests are currently under development and are not yet available in France. At any rate they cannot yet establish a firm diagnosis. In fact, a positive serology cannot differentiate a healthy carrier from a sick one and the reliability of PCR tests depends on the actual presence of the virus in the sample.

 

Stool or oral mucosa samples are therefore not recommended because the virus is only intermittently present there. Nor does a PCR test seem always capable of detecting the presence of the virus in blood. An interesting sampling spot for a PCR analysis appears to be the base of the feather, provided that it is torn off since this will expose a nerve ending where the virus is almost always present. Research is currently underway to try to develop a rapid and reliable test.

• Histological analysis

In the absence of reliable screening tests, the safest method still involves removing a small portion of digestive tissue and delivering it to a specialised laboratory to conduct a histological analysis, that is, the microscopic examination of the removed tissue to reveal the characteristic lesions caused by the virus. This procedure typically involves removing a small fragment of the crop; it is a low-risk, superficial operation that requires only a brief anaesthetic.

 

In conclusion, a serology is for now not a simple and reliable test for arriving at a PDD diagnosis on an individual. However, once it is available in France, this test will be easy to use and valuable in checking a population of birds such as a flock in which you want to eradicate PDD. The breeder must be well informed that a large proportion of his/her birds identified as positive will not develop the clinical disease.

 

The most reliable diagnostic protocol for an individual, therefore, is still for the moment a histological examination of a biopsy from the crop.

 

Treatment over the long term will yield a real improvement

 

Treatment aims essentially to reduce the symptoms since no medicine can currently eliminate the virus. It must be maintained for several months or several years. While a clear improvement is often observed, it is assumed unless otherwise noted that the birds remain chronically infected. Improvements typically occur within two weeks of administration. Evaluation criteria are weight gain, improvements in x-ray findings, and the progressive disappearance of lesions observed in repeated biopsies of the crop.

 

Treatment for inflammation with non-steroidal anti-inflammatory drugs (which are not cortisone-based) often brings real relief to the sick bird. It must also treat the reduction in gut motility and chronic regurgitation with gastrointestinal antispasmodics. To treat nerve damage, certain drugs used for Parkinson's disease in humans can sometimes be used.

 

Lastly, it is important to feed the bird with highly absorbable food of liquid consistency, such as a mash for feeding young birds, as that would bring it real comfort and promote weight gain. Infected birds often ingest foreign objects, such as pieces of wood, presumably to try to alleviate their digestive discomfort. Food supplements with plant fibres can meet that need.

 

Future prospects

 

The recent discovery of the avian bornavirus and the demonstration of its pathogenic role in proventriculitis of birds is therefore a fundamental step toward understanding this disease. All that remains is a widespread rollout of screening tests that are simple to use and available in France.

 

Dr J-F Quinton, February 2011