Gordon A Chalmers, DVM
Lethbridge, Alberta, Canada
Article Published 19 September 2007

Circoviral infection in pigeons was recognized for the first time in the USA in California in 1990, and as early as 1986 in Canada and 1989 in Australia. It is now a worldwide infection in pigeons, especially in young pigeons. Typically, this disease affects weaned pigeons between one and four months of age.

In affected pigeons there is a broad range of signs including depression, weight loss, respiratory distress, diarrhea and poor racing performance. One characteristic feature of this disease is an upsurge in a number of other diseases among young birds in a loft – canker, coccidiosis, herpesviral infection, E. coli, paratyphoid and other bacterial infections, etc.. These conditions occur because circovirus, much like the AIDS virus in humans, causes severe damage to the developing immune system. The result is that infected young pigeons become susceptible to a variety of diseases such as those just mentioned. In support of this observation, investigators from Germany and Spain found a broad range of bacterial, viral, fungal and parasitic diseases in 91% of young pigeons infected with circovirus – in fact, some affected birds had multiple infections.

A scientific paper published in 2006 on circoviral infection in pigeons highlighted significant new knowledge on this important infection (1). The Belgian and Irish authors of this work studied pigeons from a single loft in which so-called Young Bird Sickness (YBS) and circoviral infections had occurred previously. They collected cloacal (rectal) swabs from 10 pairs of adult pigeons during the week before these birds were paired. Later, they collected cloacal swabs from the 19 youngsters hatched and raised by these 10 pairs, at 15 and 28 days of age, and again at 37 and 51 days of age. These youngsters were weaned at 28 days of age and moved to a separate loft.

They also collected throat and cloacal swabs plus a blood sample from 20 other healthy adult birds ranging in age from two to nine years. Post mortem examinations were also performed on these 20 birds and many organs were sampled for examination.

As well, 22 eggs were obtained from 12 breeding pairs and incubated through to hatching. Numerous organs from these newly hatched youngsters were collected within 8 hours of hatching.

All samples were examined by use of a test known as PCR (Polymerase Chain Reaction) for evidence of circoviral DNA. Results showed that all 20 cloacal swabs taken from the 10 breeding pairs in the week prior to breeding were negative for circoviral DNA. However, cloacal swabs from four 15-day-old youngsters from these breeders were positive for circoviral DNA, but only one youngster was positive at 28 days of age. At 37 days of age - nine days after weaning – only three youngsters were positive, but at 51 days of age, samples from all 19 youngsters were positive. It is also interesting to note that all of these youngsters remained healthy during this period.

Further, three throat swabs and five cloacal swabs from the 20 healthy adults were positive for circovirus. Also, various organs from 13 of these 20 birds were positive for circovirus. The most common sites in which circovirus was evident were the trachea (windpipe) – 13 cases; pharynx (throat) – 12 cases; lung – 8 cases, with fewer positive findings in spleen – 7 cases; kidney – 5 cases; and liver – 3 cases. Very interesting was the finding of circoviral DNA in the organs of 8 of 22 newly hatched youngsters!

Although circovirus has now been found in young pigeons in many countries, it is a bit surprising that this study found it in 65% of apparently healthy adults - which obviously indicates a prime source of virus for youngsters. It is notable that a 2001study from France detailing an outbreak of circoviral infection in broiler pigeons detected the virus in young pigeons but not in older pigeons examined.

The detection of circovirus in recently hatched youngsters supports the idea that circovirus can be spread from parents to their offspring through the egg.
Previously, YBS has been associated primarily with Type I adenoviral infections that have been complicated with bacteria such as E. coli, etc.. However, in late 2005 new information surfaced from Germany in a published study that explored losses of youngsters in 18 lofts from different areas of the country (2). The authors of this study examined 51 youngsters, 45 of which were from lofts where birds ranging in age from three to 20 weeks were affected by YBS. As well six youngsters from lofts that were not affected by this disease were also examined in the same study.

Most of these youngsters (47 of 51) were submitted alive. Presenting signs in affected birds included reluctance to fly, green to black diarrhea, vomiting, lack of appetite, ruffled feathers, dullness, a fluid-filled crop, excess urination and sneezing. Post mortem examinations were done on all submitted birds, and many samples of organs plus blood were collected for examination.

Numerous species of bacteria, most commonly E. coli and Klebsiella species, plus yeasts and molds were isolated from the tissues of these birds. As well a microscopic parasite known as Spironucleus (Hexamita) species was found in the intestines of 27.5% of the affected birds. Spironucleus and richomonas (the organism causing canker) species were found together in about 16% of the affected birds. Coccidia were found in only one bird; roundworms and threadworms were found in five birds. Herpesvirus was isolated from three pigeons in the same loft. Although all of the birds with signs of illness and some apparently healthy birds were infected with circovirus, only YBS-affected birds had evidence of circoviral DNA circulating in their bloodstream.

On this basis, the authors suggested that examination of blood samples might be useful in detecting and confirming the diagnosis of circoviral infection in its early stages. It is very interesting and enlightening that adenovirus was NOT evident in any of the affected birds in this study; however circoviral DNA was detected by PCR in ALL of the affected birds!

Both of these important studies, one involving birds from only one loft and the other involving birds from 18 lofts, appear to signal the not surprising emerging dominance of circovirus in YBS, quite possibly replacing adenovirus as the chief viral agent involved in this disease. Even so, there is no treatment for the viral diseases themselves; only the involved secondary bacteria, yeasts/molds and parasites might be treated effectively. The somewhat disturbing finding of circovirus in newly hatched youngsters – likely indicating transmission through the egg - adds a new dimension to the means by which this virus is able to transmit itself to susceptible birds.

Although circovirus was found in the tissues of apparently normal youngsters as well as YBS-affected youngsters, it seems likely that the major effects of this virus occur after the virus invades and severely damages the immune system.

A recent article by Dr Conradie from South Africa reported that YBS seems to have changed from a typical adenoviral problem with vomiting and diarrhea of short duration in youngsters to a disease with a more prolonged course and affecting older birds to some extent as well. These older birds had a milder, more vague condition. A disease problem associated with adenovirus in earlier years now seemed to have changed to one associated with circo/herpesvirus. Dr Conradie felt that some of the poor and late returns in old bird races in 2005 were an example of the disease affecting older birds.
On the issue of vaccination against PMV and possible effects from circoviral infection, Tom Pennycott, a veterinary researcher in Scotland, has recommended that the vaccination of youngsters against this infection should begin when the birds are 3-4 weeks old.

His reasoning is that if fanciers wait until the birds are older, say four months or more, they may well encounter circovirus in the meantime. If this should occur and the immune system is badly damaged by the circovirus, birds are not likely to respond effectively to vaccination, which continues to leave them highly susceptible to infection by the paramyxovirus. Early vaccination is more likely to insure that immunity to PMV can develop before possible exposure to circovirus occurs. Ideally youngsters should be revaccinated in 4-6 weeks; all old birds in the loft should be vaccinated once a year.

References
1) Duchatel, JP, D Todd, JA Smyth, JC Bustin and H Vindevogel. 2006. Observations on detection, excretion and transmission of pigeon circovirus in adult, young and embryonic pigeons. Avian Pathol 35: 30-34.

2) Raue R, V Schmidt, M Freick, B Reinhardt, etc.. 2005. A disease complex associated with pigeon circovirus infection, young pigeon disease syndrome. Avian Pathol 34: 418-425.