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HerdChek* Porcine Reproductive and Respiratory Syndrome Test Solutions

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May 2009 Issue

Overview of Porcine Reproductive and Respiratory Syndrome (PRRS)

Porcine Reproductive and Respiratory Syndrome (PRRS) first appeared in the late 1980s as a “mystery disease” infecting swine in the United States. Confusing clinical signs made the disease difficult to differentiate from other swine diseases, including classical swine fever, parvovirus, pseudorabies, swine influenza, encephalomyocarditis, and Chlamydia and Mycoplasma infections. In 1990 the disease appeared in Europe.

Researchers now know that PRRS is a pulmonary alveolar macrophage infection caused by a virus similar to the equine arteritis virus. There are two known genotypes of PRRS virus: Type 1 (European) and Type 2 (USA). But both genotypes have been detected in both Europe and the United States. Each genotype has multiple strains; multiple strains of the virus can coexist on the same farm.

The disease affects swine worldwide, causing severe economic losses.

Economic Impacts

Economic losses stem primarily from reproductive failures, including premature births, late-term abortions, weak newborns, decreased farrowing rates and an increased number of stillbirths. The infection also causes respiratory disease that can occur in most stages of the production cycle. A virulent mutation of the virus is believed to have caused over 400,000 pig fatalities during a disease outbreak in China from 2006 to 2007.¹

Economic losses from PRRS in the United States are estimated at $580 million annually, significantly more than losses caused by other swine diseases.²

Infection Rates

PRRS infection rates worldwide are still unknown, but may be as high as 80% in some areas. Other areas may be free of infection. In the United States, it is estimated that 60% of swine herds may be affected.²

Transmission

PRRS can be transmitted by many routes, making it highly contagious. The virus is present in secretions, such as nasal discharges, saliva, semen and urine. Dams can transmit the virus to fetuses before birth. The virus can also be transmitted from infected objects, such as clothing and tools, or by intermediate carriers, such as mosquitoes. Animals are particularly susceptible to infection through skin breaks, including those caused by husbandry practices such as ear notching, or those caused by scrapes and bites from animal interactions.

The PRRS virus can cause persistent infections—infections maintained by an animal after its clinical signs have disappeared. These persistently infected animals look healthy, but continue to shed the virus and infect other animals.

Detection and Control

Fortunately, the PRRS virus is fragile and becomes inactive in the presence of heating and drying. Disinfectant techniques are effective at inactivating the virus.

Control requires early detection so that infected animals can be removed and so that sites can be disinfected to prevent spread of the disease and consequent economic losses.

PRRS can be diagnosed by serology methods and by an ELISA such as the HerdChek* PRRS 2XR Virus Antibody Test Kit. The HerdChek* PRRS 2XR Virus Antibody Test Kit is effective at detecting both Type 1 and Type 2 genotypes of the PRRS virus. For more information about the uses of the HerdChek test, see the next article, “Practical Uses for the HerdChek* PRRS 2XR Virus Antibody Test Kit.”

For more information about the PRRS virus, see the June 2007 issue of Animal Health Updates.

Practical Uses for the HerdChek* PRRS 2XR Virus Antibody Test Kit to Monitor and Manage PRRS in the Field

by Amber Stricker, MS, DVM
Suidae Health & Production, Algona, IA

Introduction

Porcine Reproductive and Respiratory Syndrome (PRRS) is a devastating virus that costs the swine industry millions of dollars each year. As veterinarians, it is our goal to monitor and manage this virus to the best of our ability in order to keep losses to the producer as minimal as possible.

To achieve this goal, it is important to monitor if and when seroconversion to the PRRS virus is occurring. By understanding this timeline we can make better decisions about vaccine placement and treatment protocols. Serology is an easy and inexpensive tool to help in this process.

Sample Size Calculation

Cross-sectional profiles can be used when there is a time constraint and a quick snapshot of herd health is desired. Longitudinal profiles are valuable to track individual pigs every 3–4 weeks and determine when and if they are seroconverting.

Determining the correct sample size is critical to achieving reliable and accurate data that can be extrapolated to the target population. With serological profiles the objective is most often to simply detect disease. Therefore, in infinite populations (>1,000 animals) the sample size can be determined based on the following formulation where α= the desired confidence level (typically 95% or 99%) and ρ= the expected minimum prevalence (typically 5% or 10%).¹

With the objective of simply detecting disease, a confidence level of 95% and a prevalence of 10% is most often used to determine sample size. In this case, for a population of 1000,
n = 29 animals. Of course there are several methods to determine the appropriate sample size, and therefore this formulation is to serve only as a reference. Adjustments to sample size should be made in consideration of time, money, frequency of sampling and objectives.

Clinical Applications for the HerdChek* PRRS 2XR Virus Antibody Test Kit

There are many ways that serological profiling can be used to monitor and manage PRRS in the field. The HerdChek* PRRS 2XR Virus Antibody Test Kit is considered the “gold standard” for detection of PRRS antibodies.² Below are three examples of cases where the HerdChek* PRRS 2XR Virus Antibody Test Kit has been used at Suidae Health and Production to aid in clinical decision making.

Case #1: PRRS negative herd
This sow farm was weaning a combined 2400 pigs per week from three different sites and commingling them in wean-finish barns. All three sites were PRRS negative.

A longitudinal profile was used to confirm the high health status of the pigs throughout the first 6 weeks in the barn. A total of 8 pigs from each of the three sow farms was tagged and bled at 0, 3 and 6 weeks postweaning.

As seen in Figure 1, ELISA results revealed that all 24 pigs stayed negative through 6 weeks postweaning and therefore confirmed the PRRS negative status of the sow farm.

Case #2: Evaluation of planned PRRS exposure in naïve gilts
PRRS naïve gilts in isolation were being acclimated to a PRRS-positive sow farm via natural exposure. The internal gilt isolation was made up of 4 individual rooms with 140 gilts in each room. Ten gilts were bled from each room and the samples submitted for PRRS ELISA titers.

The results below indicate that 100% of the gilts were positive for PRRS, based on a cutoff of 0.4 S/P, and therefore the natural exposure was a success.

Case #3: Cross-sectional study of PRRS positive finishing pigs
The owner of this site was purchasing 3000 weaned pigs per week from three different sow farm sources and commingling them in nurseries for 8 weeks. The pigs were bled upon exit from the nursery prior to movement into the finishers and were found to be PRRS negative. However, in mid to late finishing the pigs were experiencing high levels of respiratory disease on some of the sites. A cross-sectional serum profile was done in order to obtain quick results.

As shown below, the pigs were entering the finisher as PRRS negative but were seroconverting to PRRS at 12 weeks of age. This indicates that the PRRS was coming from an outside source, rather than from the nursery.

Interpreting Results

Once ELISA results are obtained, accurate interpretation of positive and negative results is key. No test is 100% sensitive and 100% specific, so there is always a potential for false negatives and false positives.

When interpreting negative and positive ELISA results, keep in mind all of the possible implications. Remember to consider the following possibilities when interpreting a negative result:

• It is less than 9 d.p.i. and pigs have not yet seroconverted.
• Pigs have already seroconverted, and serum antibody levels have declined below the S/P ratio of >= 0.4.
• Pigs have cleared the infection.
• It is a false result caused by low test sensitivity.
• Pigs are free from infection.

Positive ELISA results can also be confusing at times. Keep in mind that antibodies from previously infected or vaccinated herds cannot be differentiated, and therefore in these cases the ELISA cannot provide a definitive diagnosis of PRRS. Also, maternal antibodies may be present in nursing piglets and in nursery-age pigs up to 5 weeks of age.²

Acknowledgements
Case examples provided by Dr. Jason Kelly and Dr. Mitch Christensen, Suidae Health and Production, Algona, IA.

Tips for Using Wash Solutions

Many IDEXX ELISA test kits require the use of a diluted wash buffer between incubation steps. To maintain a high level of ELISA test kit performance, keep these guidelines in mind when using wash solutions:

• Use high-quality distilled or deionized water to mix with the specified volume of a wash buffer concentrate.
• Consult the test kit’s package insert to check the amount of wash buffer required to run each plate. The wash volume needed may vary depending upon the washer. Automated washers tend to use more wash buffer than manual washers because they need to be primed and then flushed after use.
• Unless otherwise specified, the diluted wash solution must be prepared fresh each day you are testing. Please read the insert carefully and prepare only what you need, because many inserts do not allow storage of extra wash for future use.

Your local IDEXX Technical Service team or your local IDEXX Area Manager will be happy to assist you if you have questions regarding volume, use or storage of wash solutions.

Events Around the World

• Verona, Italy—May 21–23, 2009
  XLI Congresso Nazionale Società Italiana di Buiatria
• Porte Alegre, Brazil—May 25–28, 2009
  21st Brazilian Poultry Congress/27th FACTA Conference
• Berlin, Germany—May 28–30, 2009
  5th International Symposium on Turkey Production
• Arbon, Switzerland—June 8–9, 2009
  Constanze Scientific Research Meeting
• Stavanger, Norway—June 12–16, 2009
  7th International Sheep Veterinary Conference
• Madrid, Spain—June 17–20, 2009
  14th International Symposium for the World Association of Veterinary Laboratory Diagnosticians (Booth #8)
• Seattle, WA, USA—July 11–14, 2009
  AVMA Annual Convention
• Vechta, Germany—July 19–23, 2009
  14th ISAH Congress 2009
• Minneapolis, MN, USA—August 9–14, 2009
  10th International Colloquium of Paratuberculosis

Enter the IDEXX PAS 2010 Calendar Photo Contest

Are you a PAS customer with a good camera and a ready eye for memorable images? Then we invite you to enter the 2010 IDEXX PAS Calendar Photo Contest.

The new IDEXX Pourquier* Bluetongue Blocking ELISA now provides increased sensitivity for the detection of antibodies against bluetongue virus (BTV). The IDEXX Pourquier* Bluetongue Blocking ELISA detects antibodies in both infected and vaccinated cattle, sheep and goats.

Specificity remains 100% for tested populations.

Improved Sensitivity
As shown below, performance reports from the Institute of Animal Health Pirbright (the OIE reference laboratory for BTV) and CIRAD (the French reference laboratory for BTV) indicate high sensitivity for the improved BTV assay, for both vaccinated and infected animals.

New Interpretation Cutoff
The new IDEXX Pourquier* Bluetongue Blocking ELISA has an interpretation cutoff of 70–80%, compared to 35–45% for the previous assay.

The rest of the assay protocol remains unchanged, allowing easy transition to the new kit. Sample and conjugate incubation times remain 45 minutes at room temperature to allow full automation. The table below compares protocol parameters for the new and the previous kits.

Summary
The new IDEXX Pourquier* Bluetongue Blocking ELISA provides:

• Greater sensitivity
• A new cutoff of 70–80% S/N
• 100% specificity in tested populations
• BTV antibody detection in both infected and vaccinated animals

Use the new IDEXX Pourquier* Bluetongue Blocking ELISA with either serum or plasma samples from cattle, sheep and goats.

• Available in 2-, 5-, and 10-plate kits
• Works with full automation
• Compatible with xChek* software

For more information about the new IDEXX Pourquier* Bluetongue Blocking ELISA, please contact your IDEXX Pourquier representative.

Sandra studied veterinary medicine in Hannover, Germany, and received her veterinary degree in 2002. From 2003 to 2005, while completing her thesis, she worked as a scientist for basic human research at the Medical Academy of Hannover. In 2006, she completed her studies and now has a doctorate degree.

Before joining IDEXX Laboratories, Sandra spent a year working for an ambulance practice as an equine veterinarian. She also worked for three years at a small laboratory called ZeckLab. In addition, she has experience managing scientific research projects in biological testing for industry, in routine sample processing, and as a consultant in customer service and technical support.

Sandra was born and raised in Braunschweig, a town in the middle of Germany. Since 2004 she has lived in the township of Wedemark, near Hannover. Her main interest is riding horses in classical dressage.

Sandra can be reached on her mobile phone at +49-160-907-974-88 or through e-mail at sandra-heine@idexx.com.

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