top of page


French Moult (Polyomavirus)


The following information outlines Dr Rob Marshall's views on the significance of Polyomavirus on the overall health and breeding success of exhibition budgerigars.


In my opinion, French Moult is the most serious disease of the modern day exhibition budgerigar. However, most experienced fanciers do not consider it so and continue to select birds with signs of French Moult for breeding. Feather abnormalities are usually the only sign of French Moult in Australian aviaries and the majority of youngsters regrow new healthy feathers very quickly. This is why most fanciers are not concerned about French Moult.

However, it is the permanent damage inflicted by the French Moult germ, Polyomavirus, on the immune system of the chicks that makes it such a serious disease. Infected youngsters are susceptible to many serious diseases, notably Chlamydiosis and Megabacteria that eventually lower the reproductive and show performance of the entire aviary.

There are no drugs to treat French Moult, but the fact that French Moult infects baby birds and not adult birds helps us to combat this complex disease. The disease is controlled and prevented by selecting birds for breeding which are free of the French Moult disease.


The Disease


French Moult is a peculiar virus disease infecting budgerigars under three weeks of age. It appears that the virus, Polyomavirus, by itself is relatively harmless and the apparent random nature of French Moult in the breeding cabinets is explained more by the immune response of the individual birds to the virus rather than the virus being particularly nasty. French Moult places the entire health of aviary at risk, because the virus inflicts permanent damage on the immune system of the young birds. They remain infected for life and perpetuate the French Moult to future generations of nestlings. The elimination of French Moult from the budgerigar aviary is a high priority and one of the most important steps for creating a healthy aviary.


The Symptoms


The symptoms of French Moult vary according to the age and condition of the budgerigar when it is exposed to the virus.


  • Addled eggs or dead in shell.


  • Swollen abdomen and retarded growth in 10-15 day old babies.

  • Deaths of babies.

  • Feather abnormalities.


There are more baby deaths when the infection occurs under 15 days of age. Some die at the time of hatching and others develop normally for 10-15 days then die suddenly without premonitory signs. The dead babies may have a distended abdomen, haemorrhage under the skin and reduced formation of down and contour feathers. Some show nervous signs of tremors, fits and staggers before dying.


Budgerigars infected when older than fifteen days of age are considered relatively resistant to the disease. They rarely die but show varying degrees of feather abnormalities. It is not known how much the virus damages the immune system of these older chicks. Many adult "carriers" show a loss of flight feathers or tail feathers. Frayed and bent tails are also seen in many "carrier" birds as a result of other health problems.


The Causes of an Outbreak of French Moult


As a rule, Polyomavirus infections reside in a latent state and infections become patent following periods of stress.

Each breeding season most aviaries experience one or two nests with French Moult. This is not an outbreak situation, but the babies from these nests are best culled before they reach the young bird aviary. One or both of the parents must be a carrier and the carrier bird(s) must also be culled. Refer to a later section for detecting the "carrier" bird. An outbreak is present when many nests experience French Moult in an aviary previously free of the disease.


Most importantly, an outbreak of French Moult indicates a damaged or tired immune system.


The cause of the weakened immune system must be identified and rectified before the outbreak can be controlled. The cause of a damaged immune system is usually one or more of the following:


  • French Moult itself. Using birds with French Moult or carrier birds as breeders is the starting point for an outbreak.


  • Poor food (moulding disease) damages the immune system, predisposing the flock to Polyomavirus.

  • Other diseases (especially Chlamydiosis and Megabacteria) damage the immune system.

  • Inadequate nutrition during breeding.

  • Over-breeding. Most modern day budgerigar families can only tolerate rearing two rounds of babies unless special energy rich food are provided daily when rearing young. The exertion of rearing a third round of chicks lowers the natural resistance and is usually the trigger for an outbreak.

  • Red mites. These bloodsucking mites weaken the immune system of the breeding birds.

  • Poor hygiene and excessive dust levels. The massive amounts of feather dust produced in the budgerigar aviary must be reduced. It creates an infectious atmosphere not only for French Moult but also for other airborne diseases such as Circovirus, Chlamydiosis, Mycoplasmosis and Staphylococcus.


The Cause of a First Round Outbreak


The most common causes of a first round outbreak are:

  • New breeding birds with French Moult - first year breeders are most contagious.


  • An underlying disease - most commonly Chlamydiosis or food related disease (Moulding Disease).

  • A nutritional problem.


The Cause of a Second or Third Round Outbreak


More often outbreaks occur in the third round with the earlier rounds experiencing one or two nests only having French Moult. The underlying causes of such outbreaks are:


  • Another disease (Chlamydiosis, Megabacteria, Red Mite, Moulding Disease).


  • Over-breeding and inadequate nutrition.

  • A build up of dust levels in a breeding room with one or two French Moult nests creates a Polyomavirus and Chlamydiosis rich environment, which may then quickly infect previously healthy nests. Chlamydiosis plays a significant role in Polyoma virus outbreaks in Australia.
























































The Treatment of an Outbreak of French Moult


The following treatment starts after the cause of the immune system damage is understood:


  • Identify the infected and 'carrier" birds by looking at each nest carefully for signs of French Moult. Record the results of previous rounds and the details of nests with French Moult. Write down the ring numbers and breeding details of those birds without French moult. The nests with no French Moult will be your foundation pairs for future breeding seasons.


  • Cull every chick with French Moult in the nest boxes and aviary.

  • Check every parent for signs of French Moult. Cull adults with signs of French Moult.

  • Remove and vacuum clean all dust, sand, food, grit from breeding cabinets.

  • Disinfect the breeding cabinets, breeding room, aviary, food and water containers. Repeat each week for four weeks.

  • Identify and treat the underlying cause of the immune system damage. For example, get new food or treat for Chlamydiosis and so on.

  • Fortify the nutrition of the birds with vitamins, protein and energy during and for four weeks after the treatment programme.

  • Re-pair selected pairs. Monitor results carefully.

  • Control re-entry of French Moult by careful new bird selection and quarantine.


The Control of French Moult


Keeping the aviary free of French Moult is achieved by the following rules.


  • New birds with French Moult are not to be used for breeding.


  • Good nutrition and the prevention of other disease allows the flock to naturally resist French Moult.

  • Regular cleaning and disinfection minimises the levels of virus in the breeding cabinets and aviary.

  • Every baby and adult bird with feather signs of French Moult is culled.


Andrew McFarlane's soft food recipe is eagerly consumed by breeding birds.

Information on the significance of Polyomavirus on the overall health and breeding success of exhibition budgerigars.

"The Budgerigar" is an internationally acclaimed definitive text on exhibition budgerigars.

  • Wix Facebook page
  • Wix Twitter page
bottom of page