For some time now, different strains of circoviruses have been known to cause disease in chickens and in psittacine birds (eg., parrots, budgerigars, cockatiels). Since 1986, a third strain of circovirus has been suspected to have caused illness in pigeons, first in Canada, then in Australia in 1989, and in the USA in 1990, where sporadic cases have occurred in pigeons since that time (Figure .) How are circoviral infections in pigeons recognised? The short answer is -- with difficulty. If case histories of affected birds are examined, it is seen that a wide variety of clinical signs may occur in different birds. For example, the first case in the USA involved a flock of 50 racing pigeons in which 3 to 5 young birds were affected each week. Signs included loss of appetite and dullness, with death occurring in 3-4 days.

In another flock, affected birds had diarrhoea, dullness, rapid loss of weight, inability to fly, and death.  In other cases, affected birds developed (variously) breathing difficulties, loss of weight, emaciation, nasal discharge, and pox-like changes on the beak. Affected young pigeons in western Canada developed olive green to chocolate brown, messy gelatinous droppings, weight loss and death in several days. Given all of these different findings, it is difficult to find common patterns of disease as a basis for suspecting a circoviral infection. However, the occurrence of of one or more diseases in young pigeons may well provide the first tell-tale clues that circovirus is active in your birds.

Like many other infections, circovirus infections seem to be most important in young pigeons. Information from the veterinary literature on this subject indicates that infected birds ranged in age from six weeks to one year. Presumably, older birds have an age-related resistance, or have developed solid immunity as the result of earlier infections. In one case that I investigated, only four of 42 young birds died -- yet other birds of similar age in that loft did not develop signs of disease and remained completely healthy, even though there was direct daily contact between sick and normal birds. Both first and second round pigeons were affected.

The main problem with circovirus infections in pigeons and other affected species, seems to be a primary result of their ability to cause massive destruction of cells that direct and co-ordinate body defences against a variety of other diseases. This is an important point. If body defences are severely weakened or even destroyed by these viruses, it is in the very nature of other disease-producing agents -- viruses, bacteria, moulds, and parasites such as the canker organism -- to take advantage of this weakened situation, and to begin to invade where defences are weakest.

To illustrate this important point, case studies of this disease indicated that pigeons infected by circoviruses had a variety of other infections as well -- for example, those caused by Chlamydia sp. (the cause of a condition called variously, chlamydiosis, or psittacosis when it occurs in psittacine birds, and ornithosis when it occurs in other classes of birds, etc.), canker organisms, adenovirus, paramyxovirus, poxvirus, herpesvirus, moulds, as well as infections caused by E. coli and Salmonella (paratyphoid) species of bacteria. This wide range of secondary and tertiary infections in pigeons known to be infected by circovirus points up the highly important role of this viral agent firstly in destroying defensive cells in the body, and secondly, in allowing a wide variety of other disease-producing agents to invade the tissues.

It is quite possible, and indeed likely, that as circoviruses spread among our birds, we will see more outbreaks of other diseases, and these happenings may be our first clue that circoviruses are operating in our flocks. Spread of this virus -- and many other types of viruses of course -- is virtually an iron-clad guarantee because of the nature of our sport. Contact in pigeon trailers and transporters, at shows, sales, the introduction of new birds to our lofts, etc., are all excellent means of spreading a variety of infectious disease-causing agents, including circoviruses.

Is there any way at present to prevent circoviral infections in our birds? The practical answer seems to be no, simply because of what we do in the sport -- train and race our birds together, and exchange or purchase birds from a variety of sources within and outside our own areas. Given these circumstances, we likely have to accept the inevitability of circoviral infections in our birds, much as we have had to with paramyxovirus, herpesvirus and adenovirus infections.

We can hope that early exposure of our birds to this agent will immunise them sufficiently to prevent serious problems with the secondary diseases that may well herald the existence of circoviral infections in our birds in the first place. We can also act vigourously and quickly to obtain veterinary diagnoses and information on treatment of these other conditions -- especially canker, and E. coli and Salmonella sp. infections when they occur. If you submit sick birds to your veterinarian or veterinary diagnostic laboratory, and you have had one or more recent outbreaks of the same or different diseases, especially in young birds, be sure to ask the staff to undertake a microscopic (histological) examination of a variety of tissues, including the very important spleen and bursa of Fabricius, as well as other lymphoid tissues, to search for "footprints" of circoviral infection.

It is in these tissues that this virus causes the most serious changes, and it is here that evidence of circoviral infection can best be demonstrated. The disease produces characteristic "inclusion bodies" (clusters of millions of viral particles) in infected cells of the organs just mentioned.

These inclusions, which appear to contain large aggregations of viral particles, have a blue-black grape-cluster appearance when tissues are stained with the commonly used H and E stain (Figure 1). Your veterinarian or laboratory personnel will understand the term "H and E".

Electron microscopic examination of these inclusion bodies will further reveal the characteristic appearance of the viral particles they contain. Circoviral infections are not likely to disappear in the near future, and as the virus spreads, there will likely be more cases of the secondary diseases mentioned earlier to indicate that circovirus is active in a number of lofts. Forewarned is forearmed. We can help our own situations by getting accurate veterinary and laboratory diagnoses of circoviral infections and the diseases that follow it. Vigourous and rapid treatment of these secondary diseases are likely to be our main defence against losses triggered by infection with circovirus.

Even now, this infection may be much more widespread among our birds than we realise, firstly, simply because of the widespread bird-to-bird and loft-to-loft contact among racing pigeons, and secondly, because of the number and variety of outbreaks of other infectious diseases that occur in many flocks. In other words, we may have many more outbreaks of diseases such as paratyphoid, canker, adenoviral disease etc. among pigeons because of the masked or hidden activity of circoviruses.

Most viral diseases in themselves are simply not treatable, and it certainly seems that circoviral infections will fall into the same category. Treating the secondary and tertiary bacterial and parasitic diseases unleashed by the circoviral damage to the immune system is about all that can be accomplished at the moment. Even so, treating the current disease condition, whether it is canker, paratyphoid or E. coli infections etc., is a highly important approach, to allow birds to survive long enough for the damaged immune system to repair or partially repair itself.