|
The Emerging Problem
Of Pigeon Circovirus
Dave J. Rupiper DVM
Kenneth T. Briggs DVM, Ph.D.
A new disease concern for pigeon fanciers is being recognized with increasing frequency.
With reports of infected flocks in northern and southern California, Canada, and
Australia, it now appears that Pigeon Circovirus may become a major problem among
mixed-aged flocks. This is an example of a disease that has been around for at least
a decade, but that went unrecognized until recently. 1.
Circoviruses are a newly described group that includes Pigeon Circovirus, Psittacine
Beak and Feather Disease (PBFD) and Chicken Anemia Agent (CAA). Detailed laboratory
studies confirm that these viruses are similar to one another (they share some portions
of their DNA), but there is no known cross-over of disease agents from one group
of birds to another. PBFD is certainly the best known of the three, affecting more
than 35 species of "hookbills" (parrots, macaws, cockatoos, and their allies).
This disease results in characteristic abnormalities in feather growth, beak lesions
and long-term loss of body condition. It is known that both PBFD and CAA cause disruption
of the tissues comprising the lymphoid system. This system (including the spleen,
thymus, Bursa of Fabricius, and both gut- and bronchial-associated lymphoid tissues)
houses cells that are responsible for most immune functions of the body: making antibodies,
recognizing disease organisms, processing antigens, and so forth. Because CAA and
PBFD viruses harm the lymphoid organs, birds having these diseases are "immunosuppressed".
That is, these viruses cause a form of acquired immunodeficiency (like human AIDS).
One result is that secondary infections with other viruses, bacteria and fungi are
common among birds with PBFD and CAA, even when birds are vaccinated against the
secondary problems. In other words, Circoviruses compromise the birds' ability to
benefit from vaccination!
Disease symptoms seen in birds with Pigeon Circovirus somewhat resemble those seen
with PBFD and CAA. Pigeons initially show respiratory symptoms, weight loss, diarrhea,
and stop eating, often dying after a short (2 to 5 day) period of illness. Many symptoms
seen in Circovirus - infected birds are really attributable to disease caused by
secondary infections that take hold because Circovirus has suppressed the bird's
immune responses. Among these secondary infections, researchers have found Chlamydia
(Ornithosis/Psittacosis), Herpes virus, Pasteurella (Cholera), Paramyxovirus
- 1, Trichomonas (Canker), and Aspergillus (among others). Interestingly,
in some Circovirus - infected flocks, there will be illness, but no mortality, while
in other flocks, 100% of the squabs will die.
When these pigeons are presented for post-mortem exam, the most common findings are
destruction of cells in the spleen, thymus and Bursa. The latter two organs normally
shrink in size after hatching and are tiny or absent in the adult bird. In the youngster,
however, they are very important as sites for immune system tissues. With these primary
target organs, it is not surprising that all pigeons with confirmed Circovirus infections
thus far have been one year or less in age. Infections have occurred both in racing
lofts (in one case a bird returned two days late from a race and other individuals
were performing poorly.), and in squab operations; a variety of breeds are represented.
The relatively high frequency of infections among racing pigeons suggests that inadequate
attention has been paid to shipping conditions and quarantine. Dr. Leslie Woods,
of U.C. Davis feels that if Pigeon Circovirus holds true to the pattern set by PBFD
and CAA, then infection may be by a fecal-oral route. As yet, there are no known
wild bird reservoirs for the virus, so it is uncertain how racers are encountering
the disease.
Several factors conspire at present to hinder the prevention and diagnosis of Circovirus
in Pigeons. So far, researchers have not been able get the virus to replicate in
the lab. This step is required before rapid diagnostic tests can be developed or
vaccine trials can be done. Additionally, the presence of so many secondary infections
in Circovirus - positive flocks can lead to errors in treatment and diagnosis. Unfortunately,
the other Circoviruses are very resistant to treatment with heat, disinfectants and
detergents. 2. If this also holds for Pigeon Circovirus, it may be that the only
way to salvage an infected flock is to eliminate all young birds for at least a year
(ie, let the virus die - out naturally). Even this measure may fail, however, if
the virus is able to exist in a "carrier state" in some adult birds. These
and other details of the epidemiology (natural history and patterns of infection)
of Pigeon Circovirus are completely unknown.
Our advice is to pay careful attention to hygiene and quarantine procedures when
shipping birds, especially racers. Fanciers who import squabs, particularly into
flocks that already have many young birds, should observe strict quarantines for
2 to 4 weeks after arrival. Until flocks can routinely be vaccinated, this newly
recognized, immunosuppressive virus represents a real threat to 'open - entry' flocks
and racing lofts.
References
1. Woods LW. Circovirus Infections in Pigeons. Proc Annu Conf Assoc Avian Vet,
1993; 156-157.
2. Gerlach H. Viruses. In Ritchie B, Harrison GJ, Harrison LR, (eds.). Avian Medicine:
Principles and Application. Lake Worth, FL, Wingers Publishing, 1994; 862-948
|