A Bovine Model of Respiratory Chlamydia psittaci Infection: Challenge Dose Titration

This study aimed to establish and evaluate a bovine respiratory model of experimentally induced acute C. psittaci infection. Calves are natural hosts and pathogenesis may resemble the situation in humans. Intrabronchial inoculation of C. psittaci strain DC15 was performed in calves aged 2–3 months via bronchoscope at four different challenge doses from 106 to 109 inclusion-forming units (ifu) per animal. Control groups received either UV-inactivated C. psittaci or cell culture medium. While 106 ifu/calf resulted in a mild respiratory infection only, the doses of 107 and 108 induced fever, tachypnea, dry cough, and tachycardia that became apparent 2–3 days post inoculation (dpi) and lasted for about one week. In calves exposed to 109 ifu C. psittaci, the respiratory disease was accompanied by severe systemic illness (apathy, tremor, markedly reduced appetite). At the time point of most pronounced clinical signs (3 dpi) the extent of lung lesions was below 10% of pulmonary tissue in calves inoculated with 106 and 107 ifu, about 15% in calves inoculated with 108 and more than 30% in calves inoculated with 109 ifu C. psittaci. Beside clinical signs and pathologic lesions, the bacterial load of lung tissue and markers of pulmonary inflammation (i.e., cell counts, concentration of proteins and eicosanoids in broncho-alveolar lavage fluid) were positively associated with ifu of viable C. psittaci. While any effect of endotoxin has been ruled out, all effects could be attributed to infection by the replicating bacteria. In conclusion, the calf represents a suitable model of respiratory chlamydial infection. Dose titration revealed that both clinically latent and clinically manifest infection can be reproduced experimentally by either 106 or 108 ifu/calf of C. psittaci DC15 while doses above 108 ifu C. psittaci cannot be recommended for further studies for ethical reasons. This defined model of different clinical expressions of chlamydial infection allows studying host-pathogen interactions

Circulating and broncho-alveolar interleukin-6 in relation to body temperature in an experimental model of bovine Chlamydia psittaci infection.

In rodent models of experimentally induced fever, the important role of interleukin-6 (IL-6) as a circulating endogenous pyrogen is well established. Studies employing larger animal species and real infections are scarce. Therefore, we assessed bioactive IL-6 in peripheral blood and in broncho-alveolar lavage fluid (BALF) of calves after intra-bronchial inoculation with vital Chlamydia psittaci (Cp), with inactivated Cp, or with BGM cells. Only calves inoculated with vital Cp developed fever (peak at 2-3 days after challenge) and significantly increased IL-6 activity. Controls inoculated with either inactivated Cp or BGM cells also expressed increased bioactive IL-6, but no fever developed. Activity of IL-6 in BALF was significantly higher compared to blood serum. This experimental model of Cp infection revealed no apparent relation between IL-6 in blood and body temperature, but did reveal a relation between IL-6 and other markers of inflammation in BALF. We conclude that a local inflammatory response in the lungs of infected calves caused fever, which developed by mechanisms including other mediators besides IL-6

Study design.

<p>d ai/h ai = days/hours ante inoculation; dpi = days post inoculation.</p

Activities of IL-6 (I.U./mL) in BALF and blood serum assessed at the same day in calves.

<p>Activities of IL-6 (I.U./mL) in BALF and blood serum assessed at the same day in calves.</p

Study design.

<p>d ai/h ai = days/hours ante inoculation; dpi = days post inoculation.</p

Results of multiple regression analysis relating IL-6 to other markers of inflammation in broncho-alveolar lavage fluid in all inoculated calves.

<p>Results of multiple regression analysis relating IL-6 to other markers of inflammation in broncho-alveolar lavage fluid in all inoculated calves.</p

Activity of IL-6 (I.U./mL) in blood serum of healthy calves aged 6–8 weeks at two different days within one week.

<p>Activity of IL-6 (I.U./mL) in blood serum of healthy calves aged 6–8 weeks at two different days within one week.</p

Relative changes of IL-6 in blood serum of calves and rectal body temperature in different treatment groups.

<p>Relative activity of IL-6 in blood serum (white) and rectal body temperature (RT) (grey) are presented box and whisker plots for <i>n</i> ≥ 6 and as plots for <i>n</i> = 3. <i>Cp</i>: <i>Chlamydia psittaci</i>. x-axis: relative changes of IL-6 in blood serum in % and rectal body temperature in °C. y-axis: time. -24 h: 24 hours before inoculation; 4 h: 4 hours after inoculation. Post-inoculation values of animals were compared to pre-inoculation values of the same animals with the Wilcoxon signed rank test, then <i>P</i>-values were adjusted according to Holm (# 0.05 < <i>P</i> ≤ 0.1; * 0.01 < <i>P</i> ≤ 0.05; ** 0.001 < <i>P</i> ≤ 0.01).</p

Results of multiple regression analysis relating IL-6 to other markers of inflammation in broncho-alveolar lavage fluid in calves inoculated with 10⁸ ifu of vital <i>Chlamydia psittaci</i>.

<p>Results of multiple regression analysis relating IL-6 to other markers of inflammation in broncho-alveolar lavage fluid in calves inoculated with 10⁸ ifu of vital <i>Chlamydia psittaci</i>.</p