One of the challenges when conducting a disease elimination project is to ensure that the pathogen has indeed been eliminated from the herd. Although this sounds like a simple question, knowing for sure that a pathogen is no longer in the herd is often difficult to know with 100% confidence. In addition, the ability to answer such a question will depend on the characteristics of the pathogen, the availability of diagnostic tests and the clinical outcome of such infection.
In the case of PRRSV (porcine reproductive and respiratory syndrome virus), this question is crucial in order to determine when a PRRSV elimination program has been completed successfully and determine when negative gilts can be introduced into a herd. Furthermore knowing that PRRSV has been successfully eliminated will aid in future disease investigations should the herd become re-infected.
In a breeding herd undergoing PRRSV elimination through herd closure, the prevalence of sows and gilts harboring the virus will decrease over time as the virus is eliminated from the tissues of infected animals. In turn, the prevalence of positive pigs during lactation should also decrease. A caveat though is the risk that virus remains circulating in the breeding herd because of management practices such as cross-fostering and holding back older pigs that may maintain the virus circulating in the farrowing rooms.
What subpopulation of animals to target?
When considering establishing a monitoring program in a breeding herd we have 3 populations to consider: gilts, sows and piglets. Gilts may represent higher risk of infection if they got infected last. Sows may represent a lower risk if the herd was considered “stable” at the time of herd closure meaning that they had gone through the infection process when they were gilts. A difference here may be if the elimination project started after an acutely infected herd where all animals, sows and gilts became infected at the same time. Piglets should also be considered since some of them may be born viremic and represent a source of infection and the ones born negative may become infected during the lactation period. From a practical point of view, gilts in pens and piglets in the farrowing room are the target populations.
Once the closure period of a PRRSV elimination project concludes, sampling recently introduced replacement animals presumed negative at entry is a must. If the virus is still present in the population, this naïve population is the most susceptible and represents the best sentinel. Monitoring in this population should start after the gilts had enough time and opportunities to interact with previously positive sows (>30 days) and ideally takes place for several months. Confidence that a PRRSV elimination program has been successful builds over time if this population remains negative. Continued sampling of pre-weaned piglets is also advised, however sampling of this population should be done in conjunction with sampling sentinel gilts. Relying only on the piglet population may result in delayed finding positive piglets should the PRRSV elimination program have failed.
Frequency and sample size
It is important that consecutive samplings are taken over time and not just one sampling . Linhares et al (2012) reported the need to conduct PRRSV monitoring repeatedly over time in the piglet population when herds were trying to reach negative status. In this study, 17 farms out of 60 had at least 1 month of PCR negative results followed by PCR positive results and 4 farms had at least 2 consecutive months of PCR negative results followed by PCR-positive results. In addition, targeting the oldest piglets and poor doing piglets may increase the chances of detecting positive animals (Cano et al., 2008).
Gilts can be a good population to target. If gilts are in pens, it is best to consider collection of oral fluids.
As we mentioned above, as prevalence in a population decreases, the sample size required to detect at least one positive case can increase dramatically. Cano et al (2008) reported a prevalence of 7% of weaned pigs in one sow herd. To detect such a low prevalence with 95% confidence, we need to sample 41 pigs assuming a 100% accurate test. In practice many routine protocols settle for 30 samples which allows one to detect at least one positive sample given at least a 10% prevalence and 95% confidence interval (60 samples if at least a 5% prevalence detection is desired). The likelihood of missing positives decreases if sampling is conducted over time. Therefore, time is our friend when implementing a systematic routine monitoring program.
The test selected to diagnose PRRSV is crucial. Because there is not a differential ELISA test able to differentiate infected vs. vaccinated animals, seropositive results should be considered the results of an infection. Therefore serology is of limited use to monitor previously infected animals and piglets with passive immunity. Serology on the other hand should be the test of choice when monitoring is targeted to include naïve populations such as replacement animals originating from a presumed negative source.
In addition, positive serological results should be confirmed with an IFA test for confirmation given that the specificity of the ELISA is not 100%.
PCR (polymerase chain reaction) has become the test of choice to monitor piglets. In piglets the best sample is blood rather than oral fluids (oral fluids may be difficult to obtain at a young age). PCR of tonsil samples may also be used to identify adult carrier animals, however the difficulty of obtaining a sample discourages this procedure for implementation on a routine basis.
In summary, proving herd negativity during a PRRSV elimination project can be challenging and costly. However, sampling the most susceptible population(s) over time increases the chances to classify a herd according to its true status. For PRRSV this includes monitoring presumed negative gilts serologically after they have been introduced in the herd. Monitoring pre-weaned piglets over time by PCR is also recommended in order to determine when the sows have stopped shedding and determine when it is safer to introduce negative replacements.