Diagnosing acute infections of porcine reproductive and respiratory syndrome virus in swine

The goal of this paper is to illustrate which specimens and diagnostic methods have been the most efficacious for the diagnosis of porcine reproductive and respiratory syndrome (PRRS) virus (PRRSV) infections by summarizing results from PRRS field investigations. In the fall of 1996 a number of swine farms in the midwest USA experienced acute reproductive losses that were characterized by a high incidence of sow mortality, abortions at all stages of gestation, stillborn and weak-born pigs, and respiratory disease in young pigs. Except for the severity of the disease, the clinical signs in these cases were compatible with PRRS although preliminary tests had not detected PRRSV. These cases were initially referred to as Sow Abortion and Mortality Syndrome (SAMS) and diagnostic investigations intensified. PRRSV was still the number one suspect etiology; however, serology was not useful since most of these affected herds were seropositive for PRRSV due to previous infections or vaccination. PRRSV was detected by immunohistochemistry in some of the affected pigs and sows suggesting it had a role in SAMS. The syndrome became known as Atypical or Acute PRRS and with the testing of adequate samples, PRRSV was isolated from these cases indicating it played a significant role in the reproductive failure. The best samples to demonstrate reproductive failure (based on success of virus isolation) have been presuckle blood samples from weak-born pigs followed by blood from stillborn pigs and fluids from aborted fetuses; however, virus was isolated only rarely or almost never from fetuses aborted after or before 100 days of gestation, respectively. The best sample to consistently demonstrate PRRSV infection in a group of swine older than 3-5 days of age has been lung lavage fluid. During the course of this investigation we visited 10 farms that had clinical signs compatible with Atypical PRRS. PRRSV was isolated from each case using MARC-145 cells and all virus isolates had the same 1-4-2 RFLP pattern; however, when sequenced there were genetic differences among the isolates (nucleotide difference for OFR5 was 1-9%). An important factor in the success of isolating virus from these cases was the collection of an adequate sample size since not all weak-born pigs are infected with PRRSV in utero and PRRSV-induced abortions do occur without transplacental infection of fetuses (especially abortions earlier than 100 days of gestation). The severe disease observed in the field has been reproduced in young pigs and pregnant gilts infected with Atypical PRRSV isolates propagated in cell culture. This is in contrast to previous studies where swine were not dramatically affected after experimental PRRSV infection. The application of diagnostic tests for PRRS epizootiology is demonstrated with the following field study that started about August 1, 1998 when a veterinarian notified our laboratory that a number of swine farms in his practice experienced epizootics of abortions and sick sows. We received or collected samples (sera from affected sows, aborted fetuses, weak-born pigs, and affected neonatal pigs) from 7 of these affected farms and PRRSV was isolated from at least one animal on each farm. All isolates from 6 of the 7 farms (Farms A, B, C, D, E, and G) had a 1-4-1 RFLP pattern and the 7th farm (Farm F) had a 1-4-1 viral isolate in one pig and a 1-7-1 viral isolate in a second pig. We were suspicious the epizootics on these farms may have been related due to the fact that 6 of these farms were located relatively close to each other (Farm D was located about 20 miles from this cluster of farms), the onset and severity of reproductive failure was similar among farms, and the 1-4-1 RFLP pattern was found on each farm. We continued the genetic analysis with nucleotide sequencing of ORF5 and comparison of the amino acid sequence of each isolate on each farm. Farms A, D, E, and G had 100% homology and when using the ORF5 sequence of these farms as a consensus sequence, farms B and C had 99.5% homology. The 1-4-1 isolate and the 1-7-1 isolate from farm F had 97.25% and 96.75% homology with the consensus sequence, respectively. Except for the fact that each farm had a group of pregnant sows in the same stage of pregnancy, no common denominator or relationship was detected among the farms that would account for the presence of a similar if not identical PRRSV isolate. The findings from this cluster of PRRS epizootics suggests area spread of PRRSV without any direct transmission by swine, humans, or fomites. We assume, as others have hypothesized, that this area spread represents air borne transmission although the possibility of a biologic or mechanical vector (birds, insects) cannot be ruled out