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  • Author or Editor: Ann C. Vorwald x
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SUMMARY

Objective

To determine the effect of congenital and early postnatal infection of pigs with porcine reproductive and respiratory syndrome virus (PRRSV) on postnatal survival and growth.

Animals

20 pregnant gilts and their pigs and fetuses.

Procedure

16 pregnant gilts (principals) comprising 4 groups (4 gilts/group) were exposed oronasally to 4 strains of PRRSV (a vaccine strain, and 3 field strains) at or about day 90 of gestation. Four pregnant gilts (controls) were kept under similar conditions, except for exposure to PRRSV. Samples collected from pigs before ingestion of colostrum and samples and specimens collected from pigs at selected times thereafter were tested for PRRSV and homologous antibody. Pigs were observed for clinical signs and were weighed at birth and at weekly intervals until they were euthanatized and necropsied at about 3 weeks of age.

Results

At least some members of all litters of principal gilts were infected congenitally. Most noninfected, liveborn littermates became infected within the first week of life. Infection of pigs with field strains did, and infection of pigs with the vaccine strain did not, adversely affect postnatal survival and growth rate. All infected pigs had generalized lymph node enlargement.

Conclusion

Exposure of pregnant gilts to either attenuated (vaccine) or virulent (field) strains of PRRSV can result in congenital infection. Vaccine as well as field strains can be transmitted postnatally from infected to noninfected littermates. Pigs infected with field strains have a poorer rate of survival and growth than do noninfected pigs.

Clinical Relevance

Because attenuated (vaccine) PRRSV can cause congenital infection and be transmitted postnatally from congenitally infected to immune-naive pigs, the use of attenuated virus during gestation is, at best, questionable. (Am J Vet Res 1998;59:52–55)

Free access
in American Journal of Veterinary Research

Summary

Pregnant gilts were exposed to porcine reproductive and respiratory syndrome virus (prrsv ) by iv inoculation at or about gestation day 30 (3 gilts), 50 (3 gilts), 70 (3 gilts), or 90 (5 gilts) to investigate the likelihood of transplacental infection with prrsvat various stages of gestation. At or about 3, 6, and 9 weeks after exposure, gilts were either euthanatized while still pregnant or allowed to farrow. Gilts and pigs were observed for clinical signs of infection, and gilts, pigs, and fetuses were tested for prrsvand homologous antibody. All gilts were healthy throughout the study, except that farrowing was sometimes difficult and prolonged, and 2 gilts failed to farrow the entire litter. One gilt farrowed on day 111 of gestation; all others farrowed on day 114 or later. Porcine reproductive and respiratory virus was isolated from significantly (x2 test, P < 0.01) more fetuses and live and stillborn pigs of the 5 gilts that were infected at 90 or 92 days of gestation than from the fetuses and live and stillborn pigs of the 9 gilts that were infected at 72 or fewer days of gestation (ie, 33 of 44, 75% vs 3 of 78,4%). After initial infection, prrsvwas isolated from gilts and their pigs for a maximum of 3 weeks and 8 to 11 weeks, respectively. Findings of this study, with regard to the temporal aspects of transplacental infection, may help explain why natural epizootics of prrsv-induced maternal reproductive failure are often recognized principally as problems of late-term gestation and neonatal survival.

Free access
in American Journal of Veterinary Research

Abstract

Objective

To determine the safety and efficacy of vaccination of pregnant gilts with an attenuated strain of porcine reproductive and respiratory syndrome virus (PRRSV).

Animals

16 pregnant gilts.

Procedure

Pregnant gilts free of antibodies for PRRSV were assigned (4 gilts/group) to the following groups: group I, untreated controls; group II, vaccinated on day 60 of gestation; group III, vaccinated on day 60 of gestation and exposed to virulent PRRSV on day 90 of gestation; and group IV, exposed to virulent PRRSV on day 90 of gestation. Safety and efficacy of vaccination was evaluated by group comparisons of prenatal and postnatal survival of fetuses and pigs, respectively, and by the condition and rate of weight gain of liveborn pigs.

Results

Collective (prenatal and postnatal) death losses up to day 15 after farrowing (conclusion of study) were similar for groups I (7/47, 14.9%) and II (7/44, 16.9%) but were greater for group III (18/49, 36.7%) and were greater still for group IV (23/37, 62.2%). Mean body weight 15 days after farrowing was greatest for pigs in litters of group I (4.46 kg) and progressively less for the other groups (3.87, 3.76, and 2.18 kg for groups II, III, and IV, respectively).

Conclusions

Using these conditions, vaccination of gilts during midgestation appeared to be safe. However, it provided only partial protection against subsequent exposure to virulent virus.

Clinical Relevance

Attenuated-PRRSV vaccines may have to be administered to naive gilts > 30 days before conception to provide maximum protection throughout gestation. (Am J Vet Res 1999;60: 796–801)

Free access
in American Journal of Veterinary Research

Abstract

Objective

To determine clinical consequences of exposing pregnant gilts to strains of porcine reproductive and respiratory syndrome (PRRS) virus (PRRSV) isolated from field cases of “atypical” or “acute” PRRS in vaccinated herds.

Animals

20 pregnant gilts and their pigs and fetuses.

Procedure

8 pregnant gilts (principals: 4 groups [2 gilts/group]) were exposed oronasally at or about 45 days of gestation to 1 of 4 strains of PRRSV and necropsied 6 weeks later. Nonexposed controls (2 additional pregnant gilts) were kept under otherwise similar conditions. The experiment was repeated, except that principals were exposed at or approximately 90 days of gestation and allowed to farrow. Clinical observations were made at least twice daily, and samples and specimens from gilts and their fetuses and pigs were tested for PRRSV and homologous antibody.

Results

Exposure of pregnant gilts to PRRSV at or approximately 45 days' gestation resulted in low prevalence of transplacental infection and fetal death. Exposure of pregnant gilts to PRRSV at or approximately 90 days' gestation resulted in higher prevalence of transplacental infection and fetal death. Moreover, 1 gilt aborted and many liveborn pigs of other litters were weak and unthrifty. Clinical signs of disease and reproductive failure were especially severe for a field strain of PRRSV isolated from an epizootic that fit the strictest definition of atypical PRRS. Controls remained clinically normal and free of PRRSV.

Conclusion and Clinical Relevance

Some strains of PRRSV now circulating in US swine herds are more virulent than those encountered in the past. Clinical PRRS in vaccinated herds suggests need for a new generation of vaccines. (Am J Vet Res 1998;59:1540-1544)

Free access
in American Journal of Veterinary Research

Abstract

Objective

To compare the virulence of selected strains of porcine reproductive and respiratory syndrome virus (PRRSV) relative to reproductive performance of pregnant gilts.

Design

16 pregnant gilts (principals) were exposed oronasally to 4 strains (vaccine strain RespPRRS, field strains VR-2385, VR-2431, and NADC-8, 4 gilts/strain) of PRRSV on or about day 90 of gestation. 4 pregnant gilts (controls) were kept under similar conditions, except for exposure to PRRSV. Samples and specimens obtained from gilts, pigs (before ingestion of colostrum), and fetuses were tested for PRRSV and homologous antibody.

Animals

20 pregnant gilts.

Procedure

The virulence of each strain of PRRSV was evaluated mainly on the clinical status of the corresponding litters at farrowing.

Results

Most gilts remained clinically normal throughout the study and farrowed normally at or near the expected farrowing time. All virus strains crossed the placenta of principal gilts to infect fetuses in utero. The number of late-term dead fetuses (which appeared to be the best measure of relative virulence) ranged from 0 for litters of control gilts and gilts exposed to strain RespPRRS, to 38 for gilts exposed to strain NADC-8. All principal gilts became viremic and developed antibody against PRRSV. All strains persisted in alveolar macrophages of at least some principal gilts for at least 7 weeks after exposure.

Conclusion

Strains of PRRSV vary in virulence.

Clinical Relevance

The effects of PRRSV on reproductive performance are strain dependent and this should be considered in making a tentative diagnosis on the basis of clinical observations. (Am J Vet Res 1996;57:834–839)

Free access
in American Journal of Veterinary Research

Abstract

Objective

To determine stability of the restriction fragment length polymorphism (RFLP) pattern of a porcine reproductive and respiratory syndrome vaccine virus and patterns of other viral strains as they replicate in pigs.

Sample Population

Field samples of porcine reproductive and respiratory syndrome virus (PRRSV) and samples from 2 weaned pigs, 2 nursery-age pigs, and 5 gilts experimentally infected with PRRSV.

Procedure

PRRSV was isolated from field samples, experimentally infected pigs, or pigs that were in contact with experimentally infected pigs. For each virus, RNA was isolated from infected cells, and RFLP patterns were determined.

Results

61 % of field samples had 2-5-2 RFLP patterns characteristic of the vaccine virus, 32% had field virus RFLP patterns, and 7% had intermediate RFLP patterns that indicated a virus with a close relationship to the vaccine virus. Viruses isolated from experimentally infected pigs had no change in RFLP patterns after up to 13 weeks of in vivo replication and transmission to contact pigs.

Conclusions and Clinical Relevance

RFLP patterns distinguish the vaccine and field strains of PRRSV; however, as the vaccine virus spreads among a swine population, the RFLP pattern can change to a related intermediate pattern. A glycine at residue 151 of open reading frame 5 is another marker for the vaccine virus; this glycine is rapidly lost and eventually replaced with arginine as the vaccine virus replicates in pigs. (Am J Vet Res 1999;60:463-467)

Free access
in American Journal of Veterinary Research

Abstract

Objective

To determine the origin and clinical relevance of selected strains of porcine reproductive and respiratory syndrome (PARS) virus (PRRSV).

Animals

38 pigs without antibodies for PRRSV.

Procedure

A seemingly uncommon restriction endonuclease digestion site in a commercially available vaccine strain of attenuated PRRSV was tested for its stability and prevalence under defined conditions. Selected field strains of PRRSV, with or without the restriction-site marker, were subsequently tested in pigs for virulence and for their ability to replicate competitively in pigs simultaneously given the vaccine.

Results

Under experimental conditions, the restriction-site marker was stable during long-term infection of pigs. It was not detected in any of the 25 field strains of PRRSV that were isolated before use of the vaccine or 21 of 25 field strains that were isolated after use of the vaccine but that, on the basis of previous testing, were believed unrelated to the vaccine strain. Conversely, it was detected in 24 of 25 field strains that were isolated after use of the vaccine and that, on the basis of previous testing, were believed to be direct-line descendants of the vaccine strain. Putative vaccine-related strains caused more pronounced pathologic changes than did the vaccine strain alone, and they predominated during replication in pigs also given the vaccine strain.

Conclusions

In some swine herds, the vaccine strain may have persisted and mutated to a less attenuated form.

Clinical Relevance

The potential for persistence and mutation of specific strains of virus should be an important consideration when designing vaccination programs involving attenuated PRRSV. (Am J Vet Res 1999;60:334–340)

Free access
in American Journal of Veterinary Research

Abstract

Objective

To determine the predominant strain of progeny virus in samples obtained from cell cultures and pigs exposed simultaneously to attenuated and virulent strains of porcine reproductive and respiratory syndrome virus (PRRSV).

Sample population

Cell cultures and twenty 4-week-old pigs.

Procedure

Cell cultures and pigs were simultaneously exposed to various relative concentrations of an attenuated, cell-culture-adapted vaccine strain and a virulent field strain of PRRSV. Progeny virus obtained at selected intervals thereafter was tested to determine strain identity by use of restriction fragment length polymorphism (RFLP) analysis.

Results

Progeny virus from infected cell cultures comprised the attenuated strain, alone or in combination with the virulent strain, except when cultures had been exposed to a large excess (> 100,000-fold) of the virulent strain. Progeny virus from infected pigs comprised only the virulent strain regardless of the relative concentrations of the 2 strains to which the pigs had been exposed.

Conclusions

During concurrent replication in cell cultures, the attenuated strain quickly predominated. Conversely, during concurrent replication in pigs, the virulent strain quickly predominated.

Clinical Relevance

It is unlikely that only an attenuated strain of PRRSV would be identified by RFLP testing of samples obtained from pigs concurrently infected with a virulent strain of PRRSV. Nevertheless, the ability of a cell-culture-adapted attenuated strain of PRRSV to predominate during cell culture passage (the first step in the current RFLP testing procedure) indicated that, if possible, samples should be obtained from pigs that do not have a history of direct or indirect exposure to attenuated-virus vaccine. (Am J Vet Res 1999;60:119–122)

Free access
in American Journal of Veterinary Research