Key Messages

• 100 pigs were inoculated with four different inoculum: Control, VR2431, VR2385 and LV.
• All PRRSv infected groups showed clinical signs: dyspnoea, tachypnoea, fever, ear drooping, reddened conjunctiva, chemosis, and skin cyanosis.
• The intensity of the disease was more severe in the VR2385 group with lethargy, anorexia, higher fever and more severe pneumonia.
• Also, lesions (macroscopic and microscopic) were detected in respiratory and lymphoid systems in all three groups.

Article Brief

The purpose of this study was to compare the pathogenicity of 3 PRRSV isolates (two US and 1 European) and confirm if strain differences are also found in an experimental infection.

100 caesarean-derived colostrum-deprived (CDCD) were inoculated at 4 to 5 weeks of age with four different inoculum: Control, VR2431, VR2385 and LV.

Pigs were clinically monitored (body temperature, coughing, diarrhoea, inappetence, lethargy) and necropsied at 1, 2, 3, 5, 7, 10, 15, 21 and 28 days post-inoculation (DPI).

After 2 days, a few pigs in LV and VR2431 had mild dyspnoea and tachypnoea due to handling stress. Other signs detected were, ear drooping, reddened conjunctiva, chemosis and skin cyanosis. Rectal temperatures reached 39.4°C and 40°C in LV and VR2431 inoculated pigs but 14 days after inoculation, all pigs in these two groups had recovered.

VR2385-infected pigs had moderate respiratory disease characterized by abdominal respiration, tachypnea, and coughing. Mean rectal temperatures peaked at 41°C at 2DPI. They showed lethargy, anorexia, skin cyanosis, rough hair and chemosis.

Pneumonia scores went from 0 to 31 per cent in LV-infected pigs and from 0 to 27 per cent in VR2431-infected pigs. Nine pigs were necropsied from each group at 10 DPI (Table below). In lungs, histopathology confirmed microscopic lesions: mononuclear cells infiltration, hypertrophy and hyperplasia by type 2 pneumocyte, and necrotic macrophages and debris were found in alveolar space. Lung lesions were similar in type and extent in all groups but more severe in VR2385 infected pigs, in this case the estimated pneumonia score ranged from 28 to 71 per cent.

Lymphadenopathy in mediastinal and middle iliac lymph nodes was also produced by the 3 PRRSv strains compares: change in colour and size enlargement. Microscopically, follicular hypertrophy, hyperplasia, and necrosis were produced in tonsils and spleens.

PRRSV was isolated in all pigs from different tissues (lungs, sera and tonsils).

The difference in severity of the 3 strains detected on the field was confirmed as well in the clinical picture and lesions found in the experimental infection. One of the strains (VR2385) was clearly more pathogenic than the LV and VR2431.

Reference

P.G. Halbur, P.S. Paul, M.L. Frey, J. Landgraf, K. Eernisse, X-J. Meng, M.A. Lum, J.J. Andrews and J.A. Rathje. 1995. Comparison of the pathogenicity of two US porcine reproductive and respiratory syndrome virus isolates with that of the Lelystad virus. Vet. Pathol. 1995. 32:648.

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