Respiratory function and pulmonary lesions in pigs infected with porcine reproductive and respiratory syndrome virus

Abstract

Pulmonary dysfunction was evaluated in pigs infected with porcine reproductive and respiratory syndrome virus (PRRSV, isolate VR-2332) and compared to clinical and pathological findings. Infected pigs developed fever, reduced appetite, respiratory distress and dullness at 9 days post-inoculation (dpi). Non-invasive pulmonary function tests using impulse oscillometry and rebreathing of test gases (He, CO) revealed peripheral airway obstruction, reduced lung compliance and reduced lung CO-transfer factor. PRRSV-induced pulmonary dysfunction was most marked at 9-18 dpi and was accompanied by a significantly increased respiratory rate and decreased tidal volume. Expiration was affected more than inspiration. On histopathological examination, multifocal areas of interstitial pneumonia (more severe and extensive at 10 dpi than 21 dpi) were identified as a possible structural basis for reduced lung compliance and gas exchange disturbances.

Fig. 1

(A) Detection of PRRSV in serum by PCR. (B) Titres of antibodies against PRRSV in serum by ELISA. Data are presented as means ± SD; days −11 to 10: n = 12 per group; days 14–21: n = 8 per group. ELISA: 0, negative (s/p ratio = 0.0–0.39); 1, seropositive 1 (s/p ratio = 0.4–0.999); 2, seropositive 2 (s/p ratio = 1–1.49); 3, seropositive 3 (s/p ratio = 1.5–1.99).

Fig. 2

Body temperature (A) and respiratory rate (B) of pigs exposed to PRRSV and controls. Box-and-whisker plot represents median value, 25% and 75% percentiles (box), range, outlier values (o) and extreme values (*). Days −6 to 9: n = 12 per group; Days 10–21: n = 8 per group. # Indicates significant differences between PRRSV challenged pigs and controls (W-test, P ⩽ 0.05).

Fig. 3

Tidal volume per kg body weight (Vt/kg; A) and minute ventilation per kg body weight (MV/kg; B) of pigs exposed to PRRSV (n = 8) and controls (n = 8) as measured with the impulse oscillometry system. Box-and-whisker plot represents median value, 25% and 75% percentiles (box), range, outlier values (o) and extreme values (*). a.i. Represents data measured prior to inoculation (averaged per pig). # Indicates significant differences between PRRSV challenged pigs and controls (W-test, P ⩽ 0.05).

Fig. 4

Medians of spectral respiratory impedance within the frequency range of 3–15 Hz separated for respiratory impedance during inspiration (A) and respiratory impedance during expiration (B) of pigs exposed to PRRSV (n = 8) and controls (n = 8) 12 dpi. Rrs, respiratory resistance; Xrs, respiratory reactance, both during expiration (Rrs,ex, Xrs,ex, respectively) and inspiration (Rrs,in, Xrs,in, respectively). # Indicates significant differences between PRRSV challenged pigs and controls (W-test, P ⩽ 0.05).

Fig. 5

Transfer factor of carbon monoxide corrected for haemoglobin in relation to body weight (TL CO_Hb/kg; A) and metabolic body weight (TL CO_Hb/kg^0.75; B) in pigs exposed to PRRSV (n = 8) and controls (n = 8). Box-and-whisker plot represents median value, 25% and 75% percentiles (box), range, outlier values (o) and extreme values (*). a.i. Represents data measured prior to inoculation (averaged per pig). # Indicates significant differences between PRRSV challenged pigs and controls (W-test, P ⩽ 0.05).

Fig. 6

(a) Pulmonary lesions in the right cranial apical lung lobe of a pig 21 days after inoculation with PRRSV isolate VR-2332. Interstitial pneumonia with thickened interalveolar septa, septal infiltration of mononuclear cells (arrow, example), hypertrophy and hyperplasia of alveolar epithelial cells (arrowhead, example) and alveolar exudate (*, example). HE stain, bar = 200 μm. (b) Morphology of unaltered pulmonary tissue in the left cranial pulmonary lobe of a control pig 21 days after sham inoculation. Note open alveolar spaces and thin, even interalveolar septa due to intrapulmonary instillation of fixative. HE stain, bar = 200 μm. (c) Distribution of PRRSV antigen in the right cranial apical lung lobe of a pig 21 days post-inoculation with PRRSV isolate VR-2332. PRRSV antigen as shown by the presence of granular red reaction product is present in alveolar macrophages (arrowheads, examples) and in alveolar walls (arrows, examples). APAAP, bar = 100 μm.