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Detection of PRRS Seroconversion and Persisting Antibody
Titers in Pigs
INTRODUCTION
The temporal pattern of porcine reproductive and respiratory syndrome (PRRS) viral infection yields serum antibody at 7–9 days post-infection (dpi), immediately followed by an acute viremic phase, a late infectious stage involving low, yet persisting viral tissue load, and potential clearance by 150 dpi.1 While the virus has the ability to persist in the pig even after clinical signs of infection subside, the role of this persistence in causing further outbreaks remains unclear.2
The studies reported here assessed the efficacy of the IDEXX HerdChek* PRRS-2XR Antibody Test Kit to detect early serum antibody production and the seropositivity in pigs maintaining a low viral load of persistent infection or long-term nonpersistent infection. The results from these studies will provide useful information for herd management based upon low levels of potentially infectious virus in pigs and encourage the use of IDEXX ELISA results on a herd basis, instead of analyzing just the individual results.
MATERIALS AND METHODS3,4
Thirty-five PRRS-negative six-week-old pigs were inoculated intramuscularly (IM) with a field strain of PRRSV 103 TCID50 (Carthage Veterinary Services; Carthage, IL), and were bled to obtain serum every two days for 14 days. A second study involved IM inoculation of 109 six-week-old naïve pigs with PRRS VR2332 104 TCID50 (Iowa State University PRRS-CAP; NC229). Serum was collected at days 0 and 7 post-infection, and every two weeks between 14 and 182 dpi. Ten animals were rechallenged with VR2332 at day 193. Pigs were identified as persistently infected by tissue necropsy. All serum samples were tested on the IDEXX HerdChek* PRRS-2XR Antibody Test Kit according to the package insert. The negative control group consisted of 50 uninfected pigs that were in the same facility with the exposed group for 202 days.
RESULTS
Seroconversion at early-stage infection assessed by the IDEXX ELISA yielded positive S/P values ranging from 0.4 to 3.9 by 14 dpi. By 8 dpi, 42.9% of animals were seropositive, while 91.4% of animals seroconverted by day 10 (Figure 1). All animals tested negative prior to PRRSV inoculation (data not shown). On the day of inoculation, one pig tested weakly positive. The majority of animals seroconverted by 14 dpi.
Figure 1. IDEXX PRRS-2XR Antibody Test Kit average S/P values during early PRRS virus infection
The mean S/P values ± standard deviation for long-term serodetection remained positive from 14 to 202 dpi (Figure 2). All animals seroconverted during the study. At 126 dpi, 95.7% of the remaining animals were seropositive (66/69) while 100% of animals were positive at day 193 (19/19). All negative control animals tested negative on all days. Most of the pigs identified as persistently infected by tissue necropsy remained positive for the duration of the study. Twelve of 15 nonpersistently infected pigs also remained seropositive throughout the study (Data not shown).
Figure 2. IDEXX PRRS-2XR Antibody Test Kit average S/P values of long-term infected animals post-infection
The red line represents the mean number of seropositive pigs on any given day. The light area around the red line shows where the majority of the positives fell.
The unexposed pigs remained negative through the entire 202 days of the experiment as shown in Figure 3. Most of the S/P levels were lower than 0.1.
Figure 3. IDEXX PRRS-2XR Antibody Test Kit average S/P values of long-term negative control unexposed pigs
DISCUSSION
The results from the studies presented demonstrate that the IDEXX HerdChek* PRRS- 2XR Antibody Test Kit detects seroconversion as early as eight days post-exposure and detects long-term serum antibody in persistent and nonpersistent PRRS viral infection as late as 193 days post-infection under these study specifications. Thus, the IDEXX HerdChek* PRRS-2XR Antibody Test Kit may be useful in identifying early PRRS virus infections as well as detecting extended antibody titers.
SUMMARY
The IDEXX HerdChek* PRRS- 2XR Antibody Test Kit has excellent detectability 10 days after exposure.
The IDEXX HerdChek* PRRS-2XR Antibody Test Kit has excellent detection of persisting antibodies more than 150 days after exposure in pigs identified as both persistent and non-persistent for PRRS virus.
Results from the IDEXX HerdChek* PRRS-2XR Antibody Test Kit should be interpreted at the herd level. Pig-to-pig variations in antibody response may mislead interpretation of results when analysis is performed on individual animals.
Technical Tip
Testing of Alternate Samples Types or Species
Customers sometimes ask if they can use an IDEXX test kit to test a sample type or species type that is not specified in the package insert. Unless the test kit insert states that testing is for multi-species or different sample types, IDEXX does not have that specific claim and cannot stand behind results obtained from off-insert testing. To have a specific claim, we need to have supporting data and approval by different governing bodies. Oftentimes during the development process, alternate sample types or species are not evaluated so we are unable to make those claims in our insert. When in doubt, check your test kit insert or call your local IDEXX representative.
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