Figure 1: Histological picture of the lung tissue in uninfected (control) piglets
1A. In the left (piglet no. 5). The normal lung structure at low power view shows thin interalveolar septa devoid of infiltrate; in the peribronchial (and/or perivascular) connective tissue a few mononuclear cells (mainly lymphocytes) can be seen
1B. In the right (piglet no. 2). Unlike to 1A, this Figure shows areas of thickened interalveolar septa due to the accumulation of mononucle- ar cells (mainly of lymphocytes). Such focal mild non-specific interstitial infiltrate (MNSII) was found in the lungs of 5 out of 9 uninfected controls (Table 1)

Figure 2: Histological findings in the lungs of PRRSV infected piglets
2A. In the left above (piglet no. 16). At low power view some areas of the lung tissue even in the infected animal showed rather less ex- tensive thickening of interalveolar septa (infiltration by mononuclear cells referred to as mild non-specific interstitial infiltrate, MNSII); note the dilatation of small vessels (magn x100)
2B. In the right above (piglet no. 16). In contrast to the area shown above, another lung area of the same reveals typical UIP with more widespread thickening of interalveolar septa and their abundant mononuclear cell infiltration (along with hyperemia, i.e. dilatation of capillaries and small blood vessels).
2C. In the left below (piglet no. 25). The lung tissue of an animal who developed typical UIP shows widespread mononuclear infiltration of interalveolar septa and peribronchial connective tissue (a lymphatic follicle like structure can be seen, magn. x100)
2D. In the right below, the same piglet as above (no. 25). The mononuclear infiltrate in the peribronchial area consists mainly of lym- phocytes along causes thickening of interalveolar septa (magn. x240)

Figure 3: Extensive interstitial pneumonia in PRRSV infection
Lungs of the piglet no. 40 show severe interstial pneumonitis: the infiltration of interalveolar septa by mononuclear cells (mainly lymphocytes) is widespread so that the original lung structure can be hardly seen. In addition, the abundant hyper- emia along with extensive proliferation of connective tissue is clearly visible (magn. 220x)

Figure 4: Staining for N-antigen in the respiratory pathway and spleen
A. In the left above (piglet no. 34). The lungs of infected animals reveal overwhelming positive staining for N-protein, namely in the bronchial epithelium, in parabronchial mucinous glands and occasionally in the flat epithelium cells lining the aveoli (magn. 80x).
B. In the right above (piglet no. 12). The N-protein can be seen in the cytoplasm of ciliary epithelium cells lining the bronchi along with the negative goblet cells (magn. 120x).
C. In the left below (piglet no. 45). The N-protein can be seen in the cytoplasm of cells lining the alveolar wall and in mononuclear phagocytes which infiltrate the interalveolar septi (magn. 400x).
D. In the right below (animal no. 44): the spleen showing lymphatic follicles consisting mainly of lymphocytes positive for the N-protein (magn.x120)

Figure 5: N-protein in the pharyngeal area of PRRSV infected animals
A. In the left (piglet no. 26). In the tonsilar squamous epithelium, the N-protein is expressed mainly wthin cytoplasm of actively growing cells of the suprabasal and intermedial layers including a few basal epithelium cells (magn. 220x)
B. In the right (pig no. 13). N-protein can be seen in the acini of a submandibular salivary gland as well as in the marginal sinus of adjacent lymph node (magn. 220x).

Figure 6: Presence of vRNA in the serum of infected pigs (viremia has been detected on days 6 and 8 post-infection)






MNSII* in the lungs (4/9), no N-antigen seen neither in bronchi (0/9) nor
in tonsils (0/9)



In the lungs the prevalence of UIP**(23/28), less frequently MNSII (5/28); the N-antigen was seen in bronchial epithelium (21/28) and/or alveolar wall (5/28). Outside of lungs, the N-antigen was detected in tonsillar epi-
thelium (13/28), spleen (3/28) and salivary gland (1/28).

*mild non-specific interstitial infiltrate (in the peribronchial area and/or interalveolar septa)
** severeinterstitial infiltrate corresponding to the diagnosis of „Usual Interstitial Pneumonia” (for details see „The respiratory system”, pp. 556-617, in: E. Rubin, Farber J.L.[eds], Pathology 2nd ed., Lippincott Company, Philadelphia, 1994)
Table 1: Survey of histological lesions and the N-antigen presence in infected piglets


Animal group


ELISA (antibody)



on day 11

on day 18

Negative control





Positive infected

23/28 (82%)

5/28 (18%)

2/28 (7 %)

16/16 (100 %)

Table 2: The comparison of UIP with serological response