Pathogenesis and antigenic characterization of a new East European subtype 3 porcine reproductive and respiratory syndrome virus isolate

Abstract

Background: Porcine reproductive and respiratory syndrome virus (PRRSV) is divided into a European and North American genotype. East European PRRSV isolates have been found to be of the European genotype, but form different subtypes. In the present study, PRRSV was isolated from a Belarusian farm with reproductive and respiratory failure and designated "Lena". Analyses revealed that Lena is a new East European subtype 3 PRRSV isolate. The main purpose of this investigation was to study the pathogenesis and antigenic characteristics of PRRSV (Lena).

Results: Obvious clinical and virological differences were observed between the animals inoculated with a recent European subtype 1 PRRSV isolate (Belgium A) and animals inoculated with PRRSV (Lena). Three out of six pigs inoculated with PRRSV (Belgium A) had anorexia and low fever at 3, 4 and 5 days post-inoculation (dpi). High fever, anorexia and depression were prominent signs in most pigs inoculated with PRRSV (Lena) between 2 and 28 dpi. Four pigs out of ten died during the experiment. Arcanobacterium pyogenes was isolated from lungs of one animal that died, and Streptococcus suis was isolated from lungs of one animal that was euthanized. The difference in viral titres in sera from PRRSV (Belgium A) and PRRSV (Lena)-infected pigs was statistically significant (p < 0.05) at 7, 10, 14 and 21 dpi. The highest viral titres in sera ranged from 10(4.8) to 10(6.1) TCID50/ml for PRRSV (Lena) whereas they ranged from 10(3.1) to 10(4.8) TCID50/ml for PRRSV (Belgium A).The replication of PRRSV (Lena) was further studied in depth. Viral titres ranged from 10(2.5) TCID50/100 mg to 10(5.6) TCID50/100 mg in nasal secretions between 3 and 14 dpi and from 10(2.8) TCID50/100 mg to 10(4.6) TCID50/100 mg in tonsillar scrapings between 3 and 21 dpi. High viral titres were detected in lungs (10(2.3)-10(7.7) TCID50/g tissue), tonsils (10(2.0)-10(6.2) TCID50/g tissue) and inguinal lymph nodes (10(2.2)-10(6.6) TCID50/g tissue) until 35, 28 and 35 dpi, respectively.To examine the antigenic heterogeneity between the East European subtype 3 isolate Lena, the European subtype 1 strain Lelystad and the North American strain US5, sets of monospecific polyclonal antisera were tested in immunoperoxidase monolayer assays (IPMAs) with homologous and heterologous viral antigens. Heterologous antibody titres were significantly lower than homologous titres (p = 0.01-0.03) for antisera against PRRSV (Lena) at all sampling time points. For antisera against PRRSV (Lelystad) and PRRSV (US5), heterologous antibody titres were significantly lower than homologous titres at 14 and 21 dpi (p = 0.01-0.03) and at 10 and 14 dpi (p = 0.04), respectively.

Conclusions: Lena is a highly pathogenic East European subtype 3 PRRSV, which differs from European subtype 1 Lelystad and North American US5 strains at both the genetic and antigenic level.

Figure 1

Body temperature and respiratory disease scores in pigs inoculated with PRRSV (Lena). (A) Body temperature of pigs at different time points post-inoculation with PRRSV Lena. Temperature >40°C was considered as fever (dotted line). (B) The respiratory disease scores ranged from 0 to 6: 0 = normal; 1 = mild dyspnea and/or tachypnea when stressed; 2 = mild dyspnea and/or tachypnea at rest; 3 = moderate dyspnea and/or tachypnea when stressed; 4 = moderate dyspnea and/or tachypnea at rest; 5 = severe dyspnea and/or tachypnea when stressed; 6 = severe dyspnea and/or tachypnea at rest. Stress was induced by holding the pig for 45 sec. The numbers above the dots represent the number of animals.

Figure 2

Virus titres in sera of pigs at different days post-inoculation with PRRSV (Lena) and PRRSV (Belgium A). Symbols represent mean titres, whiskers above and below are standard deviations. Titres lower than 10^1.0TCID₅₀/ml were considered to be negative. *The difference is significant between virus titres.

Figure 3

The cross-reaction of antisera with homologous and heterologous IPMA antigens. Symbols represent mean titres, whiskers above and below are standard deviations. Titres lower than 1.7 log4 were considered to be negative. *The difference is significant between homologous and heterologous antibody titres.