VOLUME
54 (4) 1999
SEASONAL DEPENDENCE OF Chlamydia psittaci SHEDDING IN PIGEONS
A. Lublin, E. Leiderman. and Y. Weisman
Kimron Veterinary Institute. P.O.B. 12, Beit Dagan 50250
Summary
In previous reports we demonstrated an increased Chlamydia psittaci prevalence in
many species of birds in summertime. This study was designed to find whether
shedding of C. psittaci in pigeons is associated with seasonality and corticosteroid
hormones. Shedding of C. psittaci was examined by indirect immunofluorescence in a
pigeon flock during 12 consecutive months. Chlamydial antibodies were measured
using a commercial ELISA kit, and corticosterone blood levels were measured using a
RIA kit. The annual peak of chlamydial shedding was in July-August with a nonsignificant increase in chlamydial antibodies in these months. Corticosterone levels
did not show significant seasonal differences. Increased shedding of C. psittaci in
pigeons appears to be seasonal by dependent while the possible mediation of
glucocorticoids needs to be examined further.
Introduction
Pigeon chlamydiosis (C. psittaci) is very widespread, especially as a carrier state
without clinical symptoms (1,2,3). Feral pigeons are probably the most reported
commonly chlamydia-infected avian species (4), and shedding of chlamydia
from infected pigeons into the environment has to be considered. In a serosurvey, Bourke et al. (5) reported on high titers of C. psittaci antibodies in 39%
of 370 pigeon fanciers and farmers in the U.K. Pospisil et al. (6) reported on
86.6% positives by direct immunofluorescence method in organs of adult wild
town pigeons.
In previous reports we demonstrated an increased prevalence of C. psittaci in
Columbiformes in the warmer months of the year (7,8). The prevalence of
chlamydia-positive pigeons ranged from 27% during October-February (the cold
period of the year) to 42% in March-September (the hot period of the year), and
reached 46-47% in the hottest months, June-August, compared to 10-28%
during the coldest months (December-February). Since cloacal shedding of C.
psittaci is also increased following high dose-corticosteroid administration (9), it
can be questioned whether the weather-related alterations in shedding patterns
are associated with seasonal corticosteroid changes.
Materials and Methods
During 1995, a flock of 65 adult, semi-domesticated caged pigeons (Columba
livia domestica) was tested at monthly intervals for chlamydial antigen shedding
in cloacal swabs using a commercial immunofluorescence kit (Chlamydia-Cel
Vet IF Test, Cellabs Diagnostics, Australia) (10), and for chlamydial antibodies
in serum using a commercial ELISA kit (ImmunoComb Test Kit, Biogal Galed
Labs., Israel) (11). In 35 of the serum samples, corticosterone concentration was
determined using a commercial RIA test kit (Coat-A-Count Rat Corticosterone,
Diagnostic Products Corporation, USA).
The chlamydial antigen score was assessed by counting the fluorescent
elementary bodies per microscope field (12,13) and scored as 0 to 3 in +1
increments. Chlamydial antibodies were determined semi-quantitatively and
scored between 0 and 4 in +0.5 increments. Corticosterone concentrations were
determined using a calibration curve with known standards between 0 and 2000
ng/ml.
Results
Average percentages of chlamydia-shedding pigeons during the experiment (immunofluorescence of cloacal swabs).
Trending curve of the average percentages of chlamydia-shedding pigeons during the experiment.
The response of chlamydial antibodies, correlating the titer, which was monthly
averaged in the range of 1.0-1.5, showed a parallel increase (statistically nonsignificant) during July-August.
Average monthly percentages of chlamydial antibody levels in the pigeons during the year.
In addition, chlamydial antigen scores were averaged according to the four seasons (SeptemberNovember, December-February, March-May, June-August) and differences were compared by Duncan’s
multiple range test. As Shown..
Corticosterone levels, determined in December 1994 and January 1995 as representative of the cold
seasons (n=20 sera), and in August 1995 as representative of the hot season (n=15 sera), did not show
significant differences and were about 35 ng/ml in both periods.
Discussion
The significant increase in shedding of C. psittaci in the hottest period of the
year may indicate a kind of non-specific shedding response of carrier birds to
stressful stimulations, or some specific shedding mechanism that is activated
by heat. Dorrestein and Wiegman (9) showed that high-dose corticosteroids
cause C. psittaci shedding in birds, and this finding supports a possible
explanation to increased shedding in heat, as some glucocorticoids-dependent
mechanisms are associated with elevation of these at high environmental
temperatures. Increase of corticosterone levels as a response to heat exposure
of short duration was observed in turkeys (14), chickens (15) and mice (16),
even though levels of metabolic hormones like glucocorticoids are diminished
after long exposure to hot conditions (17,18). The increase in glucocorticoids
may be associated with adaptive changes leading to temperature acclimation
(14). For those reasons, possible involvement of corticosteroids in the
increased shedding of chlamydia was taken into consideration. Nevertheless,
corticosteroids changes were not observed in this annual experiment, which
may be due to opposed seasonal changes in both hot and cold weather (18).
Considering chlamydial antibodies, although the level of antibodies is less
dependent on external factors than shedding, a similar increase in the level of
antibodies was observed in the hot months of the year. The increased shedding
of C. psittaci in summertime may explain summer outbreaks of psittacosis in
wild bird populations, such as an outbreak including mortality of more than
400 gulls in North Dakota during summer, 1986 (19), as well as our findings
of significant higher prevalence of C. psittaci in summertime, especially in
small birds belonging to the passerine, columbine and psittacine groups (7,8).
As far as we know, there are almost no publications which associate outbreaks
of psittacosis in wild birds with seasonal effects and more observations need to
be performed.
To summarize, increased C. psittaci shedding was observed in pigeons in hot
weather, without a parallel increase in corticosteroid levels. It is possible that
corticosteroids have some interaction with high ambient temperatures as a
stress factor, in itself, but this needs further examination.
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