By Dr   BSc, BVSc, MRCVS, MACVSc (Avian health)

‘French Moult’ is an old name for an old disease, which only in more recent times has become more fully understood. The disease in England was first noticed in birds from France, a hundred or so years ago, and so the English called the disease ‘French Moult’. Interestingly, the French noticed the disease in birds from England and so they called the disease ‘British Moult’. Anyway, it is likely that the agents that cause the disease traveled to Europe with Australian budgies and so, if anything, the problem should be called ‘Aussie Moult’.

When aviculturalists say that a bird has ‘French Moult’, they are usually referring to a parrot, often a budgie, that has what are called dystrophic feathers. These feathers have persistent feather sheathes, numerous fret marks, blood and inflammatory debris in the feather shaft, are generally stunted, and wear prematurely.

Although any condition that causes general ill-health or damages the feather follicle during the moult can compromise feather quality, birds affected by French Moult are infected with one of two viruses, namely Polyoma virus or Circo virus. Both viruses have the potential to inflame feather follicles, so that the feathers that grow from them are deformed. If this was all that the viruses did it would, perhaps, not be so bad but the changes seen in the feathers represent only part of the overall disease syndrome and for this reason it is important that aviculturalists have a good understanding of
these diseases so that appropriate control measures can be undertaken.

Let’s look at each of these viral diseases more fully.

Polyoma virus
Polyoma virus primarily is a budgerigar disease but can affect any parrot and also a range of passerines. The symptoms displayed by the birds and the final effect of the virus depend very much on the birds’ age, species, immune status and general health at the time of exposure.

The disease in budgies
If Polyoma virus infects nestling budgies, many chicks fail to thrive, become dehydrated, develop swollen abdomens and may bleed easily, resulting in bruising. Such chicks have been described as ‘little red marbles’. Many chicks less than 25 days old will die, with peak mortality occurring between 15 and 19 days of age. Some neonates die quite quickly just being found dead in the nest box. Older chicks are more likely to survive but are likely to develop feather abnormalities. Such youngsters are called ‘French moulters’ or ‘runners’. If mature birds are exposed to the virus, many will become infected but only a few will develop disease. These older birds will often mount an immune response and may well clear the virus from their systems within 4 – 6 months. While infected, however, they will be shedding the virus and are therefore a source of infection. Not all adult birds, however, will clear the virus. Some will remain persistently infected and intermittently shed the virus (particularly when under stress), therefore remaining a potential source of infection.

The disease in other parrots
Eclectus, Asians, Conures, Neophemas, Lorikeets and Lovebirds are all particularly susceptible, while Cockatoos are generally resistant. The feather changes seen in budgies, although they can occur, are less of a feature in other parrots. Some infected nestlings will die very quickly, with few or no warning signs, while others develop non-specific signs, such as vomiting, diarrhoea, breathlessness, decreased appetite, lethargy and bruising, and may be sick for days or weeks before dying. Older infected youngsters are more likely to survive, while birds affected as adults may show no signs but as with budgies some of these birds will become subclinical carriers of the disease. Some birds will develop a chronic form of disease where they generally just fail to thrive.

The disease in Passerines
Polyoma virus can cause deaths in nestlings, fledglings and adult finches and canaries. Birds that survive may develop feather and beak abnormalities.

What to do if you are suspicious of Polyoma infection?
The first thing to do is to consult your avian vet to get an accurate diagnosis. Often the history of disease within the aviary and the symptoms displayed by the birds are very suggestive but it makes no sense to start on an extensive control program without making 100% sure that this is in fact the problem. In Australia, the disease is usually confirmed by autopsying a recently dead bird and collecting tissue samples that are then forwarded to an avian pathologist for examination. With special stains, the pathologists are able to visualize the virus infecting the bird’s tissues. In some countries, a variety of ‘live bird’ tests are available. These include a PCR/DNA probe test, which can detect viral DNA in blood, tissue or cloacal samples and also blood tests to check for exposure (antibodies) to the virus. Frustratingly, these tests are not currently available in Australia.

So, what should one do if the disease is confirmed? A variety of protocols has been suggested, ranging from extensive culling to having a break in breeding through to doing nothing and allowing the birds to build up their own immunity.

It is impossible to eradicate the disease through culling because it is not possible in Australia to detect asymptomatic carriers. While totally depopulating and restocking is also not practical, because the disease is common, it is likely that the disease would simply be reintroduced with new birds.

It does seem that in many well managed budgie aviaries, the birds do develop a significant natural immunity to the virus and that although it is frustrating to lose any birds to disease, losses become low level and possibly tolerable.

In controlling the disease, it is important to remember two critical points: firstly, that the older a bird is at the time of exposure, the less likely it is to become unwell, and secondly, that the majority of adult birds that become infected remain asymptomatic and, after being infected and shedding the virus for 4 – 6 months, recover, clear the virus from their systems and are no longer infected. Continuous breeding provides an ongoing pool of young birds that are not only susceptible to the virus but will also be shedding the virus when infected. It also places breeders under significant stress, with the result that more are unlikely to clear the virus from their systems and become carriers and those that do become carriers are likely to shed the virus more frequently. Because of these and other factors, a significant break in breeding is therefore likely to lower the incidence of disease in an aviary and has been reported in some situations to even eradicate the disease. Predictably, outbreaks of Polyoma virus tend to be persistent in budgie aviaries where constant breeding occurs, while the disease appears self-limiting in aviaries breeding other parrots in distinct breeding seasons.

The virus in the environment is quite a tough one and can survive severe environmental conditions (56‹C for 2 hours) and many disinfectants. Bleach (sodium hypochlorite) and chloramines, however, are effective. Birds that are infected shed the virus in their droppings, saliva and feather debris and so contamination of the environment and the stability of the virus can be a problem in the aviary.

Aviculturalists whose aviaries are free of Polyoma virus must make every effort to prevent entry of the virus. This means introducing stock only from reliable sources and preferably keeping new introductions in quarantine for 4 – 6 months. In some countries, fanciers have the option of DNA probe testing of birds prior to entry.

Aviculturalists whose aviaries are affected by Polyoma virus can reduce (or possibly eradicate) the disease by three straightforward measures:
ongoing hygiene – this will decrease viral build up in the environment, meaning that fewer birds will be expose and those that are will receive a lower viral dose;
ongoing good care – the better is the birds’ general level of care, the more likely they are to not only clinically recover but also the less likely they are to fail to clear the virus and become carriers. Overcrowding, poor nutrition, and ineffective control of parasitic disease increase the vulnerability of the birds to disease generally.
breeding in discrete seasons – it is primarily juveniles and young adults that will be shedding the virus and so stopping breeding for 4 – 6 months will always result in a decreased incidence of the disease.

Adults that are asymptomatic carriers (that intermittently shed the virus) can be expected to infect some of their own nestlings and so retaining only proven breeders of healthy youngsters can be expected to further reduce the incidence of the disease.
These and other measures such as separating breeding birds into smaller units, separating birds of different species, and appropriate testing where available may be applicable in certain aviaries. Your avian vet can further advice on factors affecting a particular aviary’s unique situation.

Circo virus
The disease caused by Circo virus has many similarities with that caused by Polyoma virus and indeed the two cannot be distinguished clinically. Both cause similar symptoms in unwell birds with young birds being more severely affected. Birds that survive may develop dystrophic feathers and be described as ‘French moulters’. Some infected birds will mount an immune response and recover, while others will develop ongoing problems. The diagnosis and control of both diseases have many similarities.

There are, however, several critical differences. Polyoma virus infection is primarily a budgerigar disease; Circo virus primarily affects cockatoos, although it is being recorded in an increasing list of parrots. The main difference, however, is that birds infected with Polyoma virus that develop feather changes usually recover and feather changes regress. Also with Polyoma virus, most birds that are unable to clear the virus from their system and become carriers remain well in themselves. With Circo virus infection, many more infected birds are unlikely to clear the virus from their system. Most birds that develop abnormal feathers develop complications associated with the virus’s ability to compromise the functioning of the immune system and develop a progressive disease and die. Lorikeets, budgies and lovebirds are more likely to recover but a death rate of more than 95% can be expected in cockatoos within 12 months of feather changes being observed. Both viral infections can occur concurrently and indeed aviaries infected with Circo virus can expect a higher incidence of Polyoma virus infection because of the former’s immunosuppressive ability. Fortunately testing for both Circo virus antigen (ie the virus itself) and antibodies (ie indicative of exposure and immune response) is available in Australia.

Circo virus has a long incubation period (up to 20 months or longer). This coupled with the fact that, unlike Polyoma virus, many infected birds fail to mount an immune response and fail to recover, makes a break in breeding an ineffective control tool. As with Polyoma virus, however, ongoing hygiene and good care will help control the virus within the aviary. Birds that test consistently positive may be considered as pets only, being kept well away from other birds. Such birds have no place in a breeding facility. Because of the more severe nature of Circo virus infection and availability of testing for individual live birds, control focuses more on testing, quarantine and prevention of entry of the virus into the aviary in the first place. As always, your avian vet will be able to advise what is best in your situation.