Polyomavirus in Budgerigars (French Molt)

The first generalized infection caused by any polyomavirus in any species of animal was described in young budgerigars and was called budgerigar fledgling disease (BFD). An avian polyomavirus related to the one recovered from budgerigars has been shown to infect several other species of birds. The clinical changes and progression of disease varies dramatically between budgerigars and non-budgerigar psittacine.

The polyomaviruses that infect birds are not known to infect humans or other mammals. The outcome of a polyomavirus infection in a budgerigar depends primarily on a bird's age when it is infected. Younger birds are most severely affected. In aggressive aviary outbreaks, most infected chicks may die within a several week period.

Chicks that survive the initial phase of an infection can have abnormally developed feathers frequently referred to as French Molt. Budgerigars with abnormal flight feathers are commonly referred to as "runners." Visibly similar feather lesions can be caused by any event that damages the blood supply to developing feathers.

In Europe a more chronic form of the disease is common, while in the United States and Canada an acute form of disease with high mortality is typical.

In some budgerigar flocks, 100 percent of the birds have been shown to have been previously infected. In other more intensively managed flocks, polyomavirus activity cannot be demonstrated in any of the tested birds.

Virus transmission is thought to primarily occur through direct or indirect contact with contaminated feather or fecal dust. Infected budgerigar hens can pass the virus through the egg.

The environmental stability of avian polyomavirus causes a considerable problem in the home or aviary.


There are several ways to detect polyomavirus in birds. Diagnostic tests your veterinarian may recommend include:


Home Care and Prevention

At home keep infected birds and those to which they have been exposed in isolation during recovery. Thoroughly clean and disinfect enclosures, food bowls and non-porous toys and perches. Discard porous (wood, natural fibers, rope, etc) objects that cannot be thoroughly cleaned and disinfected and do not replace them until birds are clinically normal and no longer shedding virus.

Finally, monitor fecal output to insure proper food consumption and digestion on a daily basis.

There are several things you can do to prevent pyomavirus infection. These include:

Avian polyomaviruses have a worldwide distribution and are known to infect Psittaciformes (parrots), Passeriformes (weaver finches, canaries), Galliformes (chickens and turkeys) and Falconiformes (falcons and hawks). The polyomaviruses that infect various birds are closely related but the symptoms and progression of disease in individuals and groups of birds vary dramatically.

The outcome of a polyomavirus infection in a budgerigar depends primarily on a bird's age when it is infected. Some chicks may develop normally for 10 to 15 days and then die without exhibiting any clinical signs. Other infected hatchlings may exhibit abdominal distention, hemorrhage under the skin and reduced formation of down and contour feathers. Clinical signs of disease or death are most common in 1- to 3-week-old budgerigars. Budgerigars older than 25 days of age are susceptible to infection but generally develop no signs of disease.

In aggressive aviary outbreaks, most exposed chicks can die within a several week period. The mortality rate associated with naturally acquired avian polyomavirus infections in young budgerigars may range from 25 percent to 100 percent. Older birds are considered relatively resistant to disease, while at the peak of viral activity, up to 100 percent of exposed budgerigars less than 15 days of age may die. The incubation period in chicks can range from several days to several weeks.

Some reports suggest that polyomavirus infections can cause reduced egg hatchability and early chick mortality. Other reports suggest that eggs from infected hens develop normally.

Chicks that survive the initial phase of an infection can have abnormally developed feathers, frequently referred to as French moult. This term is used to describe any slow, debilitating disease in budgerigars characterized by the progressive appearance of abnormal feathers. Any factor or infectious agent that causes damage to the blood supply of developing feathers can cause visible feather changes referred to as French moult. Avian polyomavirus, psittacine beak and feather disease (PBFD) virus, bacterial infections, fungal infections, exposure to toxins, malnutrition, drug reactions and metabolic diseases should be considered in any bird with abnormally developing feathers. In general, feather lesions caused by polyomavirus resolve after several months, while those induced by PBFD virus become progressively worse.

Infected budgerigars have been shown to shed virus in their feather dust and excrement. Virus may also be present in urates or secretions from the crop or respiratory tract. Crowding, poor air circulation, accumulation of excrement and stacking of enclosures increase the likelihood of polyomavirus transmission from infected to susceptible birds.

Polyomaviruses are considered resistant to severe environmental conditions and some disinfectants. Cool, moist conditions and freezing will preserve the infectivity of most viruses. The environmental stability of avian polyomavirus causes a considerable problem in the home or aviary. Infected hens can pass the virus through the egg.

Diagnosis In-depth

Polyomavirus should be considered in any budgerigar chick that drops dead with no premonitory signs. Polyomavirus should also be considered in chicks that die shortly after developing clinical signs that include depression, loss of appetite and bleeding under the skin (bruising) as well as in chicks with abnormally developing feathers.

Most cases of polyomavirus are suspected when viral factories called inclusion bodies are seen in the cells of affected feathers, liver, kidney or heart using a microscope. The disease is definitively diagnosed using viral specific DNA probes or by culturing the virus from infected tissues.

A virus neutralization assay is the most commonly used antibody test. In unvaccinated birds, detection of antibodies against the polyomavirus indicates that the bird has been previously infected.

Confirming an active case of polyomavirus is best achieved by combining tests that detect antibodies against the organism with tests that demonstrate that polyomavirus (culture), or a piece of polyomavirus genetic material (DNA probe test), is present in a sample. Tests like culture and DNA probe-based assays (PCR) are most valuable in birds with suggestive clinical signs, which are likely to be shedding polyomavirus from the respiratory and/or gastrointestinal tract. Culture remains the gold standard for documenting the presence of polyomavirus in a clinical sample. However, culture is problematic, time consuming and expensive in comparison to other tests. If a swab for DNA probe-based testing or culture is not slimy after sampling the choana and not coated with excrement after sampling the feces, then the sample is of poor quality.

Currently, there is no test that can be used to confirm that a bird does not have polyomavirus. However, a lack of detectable antibodies in serum coupled with an inability to detect the organism at a portal of exit provides supportive evidence.

Therapy In-depth

Currently, there is no effective therapy for polyomavirus-associated disease. In larger psittacine birds, several therapies, including various immunostimulants and antiviral drugs designed for other viruses, have been suggested. Included in the group of frequently mentioned therapies are interferon (a nonspecific immunostimulant), acyclovir (an antiviral drug with specific activity against some herpesviruses) and AZT (an antiviral drug with activity against some retroviruses). Anecdotal evidence suggests that some of these therapies may be effective in the treatment of large psittacine birds with clinical signs suggestive of avian polyomavirus. However, none of these treatments has been demonstrated to be effective in birds with documented (demonstration of virus in affected tissues) avian polyomavirus infection. Of these speculative therapies, interferon may be the most promising.

Other therapies that may be needed include fluids to correct dehydration and supportive nutrition if a bird has not eaten for several days or has lost considerable weight.


An inactivated vaccine can be used to reduce viral activity in multi-bird settings. Successfully vaccinated budgerigar hens have been shown to produce chicks that are resistant to disease during the most critical early weeks of life. To find an avian veterinarian in your area that can vaccinate for avian polyomavirus, contact Biomune (913-894-0230).