Diagnosis – PRRSControl

What is the probability of finding a false positive in ELISA test?

Gyula Balka — Mon, 09 Oct 2023 06:36:42 +0000

If your herd tests positive for PRRS via ELISA but only 1 sample out of the 15 tested positive, what is the likelihood that it is a false positive? Can PRRS be transmitted to other pigs if you slaughter pigs and sell their raw meat in the open market? Must you cull all pigs if your country is free of PRRS?

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What type of samples or tissues can we use to check the PRRS status of my farm with RT-qPCR?

HIPRA Swine Unit — Tue, 09 May 2023 08:42:35 +0000

PRRS diagnosis provides valuable information for decision-making for the control and prevention of the disease. Consequently, new sampling methods (processing fluids and tongues) are proposed to facilitate sampling and increase PRRSV detection. The objective of this study was to analyze the evolution of RT-qPCR results and type of samples received in Diagnos (HIPRA diagnostic laboratory, Spain) from 2018 to 2021.

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How to use ELISA and PCR for PRRS detection and monitoring?

Enric Marco — Wed, 06 Apr 2022 06:34:40 +0000

Verify seroconversion of all animals on a farm is a method used today on multiplication and commercial farms in Spain/Europe? If it is, in what proportion of these evaluations are PCR and ELISA used? Is the gold standard always used or are other commercial options available? At what ages should samples be taken and how often should this be done to get a clear picture of what is happening on the farm? Using ELISA to detect seroconversion is not the best method to verify if a farm has a homogeneous immune status that would prevent the PRRS virus from recirculating internally. After an infection, not all animals develop an immune response so, given a positive ELISA result and depending on the cases, there can always be a percentage of animals that do not have a response and remain negative, without this implying that they have not been infected. Also, the duration of detectable antibodies with the ELISA test has a high individual variability, with no correlation with the duration of actual protection, which can also increase the percentage of negatives when serological monitoring of a population is done. In general, ELISA is used to check the health status […]

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What can we do when a “false positive” is suspected in a serology test?

Enric Marco — Tue, 08 Feb 2022 14:59:14 +0000

On a farrow to nursery farm with 200 sows, 20 blood samples were taken from sows, gilts, and piglets. All samples were negative for PRRS by ELISA except one, which was from a 5th parity sow. No pathology associated with PRRS was observed, and no vaccination against this disease was done. The farm has good performance, 90% fertility and 14 piglets weaned per sow. How should we interpret these analytical results? No ELISA test is 100% specific. Specificity is the test’s probability of correctly detecting negative animals. When referring specifically to the PRRS virus, the best ELISA tests on the market are capable of correctly detecting 94-99% of negative animals, but there is always a small percentage that can give a positive ELISA reaction without actually having been infected by the virus: these are known as false positives. The causes can be multiple and in many cases, it is related to the presence of proteins in the blood of the animal detected positive that can be identified as antibodies against PRRS. If the farm has not had clinical signs suggestive of the disease, the positive is most likely a “false positive“. The way to confirm this would be to retest […]

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“Test and Removal” technique

Enric Marco — Tue, 03 Aug 2021 09:52:15 +0000

On a PRRS negative GP farm, a PRRS PCR positive result was detected during regular sampling test. If we do test and slaughter for all herds immediately, will it be possible to return to a PRRS negative status after then? The elimination of PRRS by what is known as the “Test and Removal” technique was described in 2001 by Scott Dee, who carried it out successfully on chronically infected farms. In the case of a recent infection, the risk is that by the time you have the results, the virus will have already infected other animals, spreading the infection. The infection spreads much more rapidly among animals that have never had contact with the virus than among animals that already possess a certain immunity, hence the difficulty of using such a technique at the beginning of the infection. However, it could be an option in those cases in which the positives are relatively isolated from the rest of the animals and there is also the possibility of having urgent PCR results. But in any case, consideration should be given to re-testing the status of the pigs that remain, a few days after the removal of the positive’s ones (5-10 […]

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PRRSv infection in a boar stud in Mexico, how to manage it in terms of management and diagnostic?

Enric Marco — Fri, 23 Apr 2021 09:30:39 +0000

Our boar stud broke with PRRS one month ago (PCR positive and ELISA positive with very high titers). We gave 2 doses of Inmuno PRRS and then took new samples and grouped them in 3 pools of 5 samples each depending on the titer (high, medium, or low). We are waiting for the results, but is this the right way to make pools? And to introduce new boars, do we have to wait for the current animals to be negative and for the titers to drop? ELISA positive results; How should they be interpreted? First, it should be noted that ELISA readings (S:P ratio, commonly called titers) are not directly related to protective immunity. Interpretation of ELISA serological test readings should not go beyond saying whether or not there has been contact with the virus (Christopher-Hennings J, et al. 2002). Therefore, grouping the animals to be tested by high, medium, or low values should have no impact on the PCR values to be obtained. How many samples can a pool contain? As for how many samples a pool can contain, it will depend on the expected viral concentration. Under commercial conditions usually pools of 5 samples are taken, […]

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Can PRRSv infection cause 35% of mortality in the post-weaning period?

Enric Marco — Mon, 31 Aug 2020 12:16:51 +0000

In the post-weaning period we observe PRRS seroconversion with a mortality of up to 35%. The piglets present symptoms consistent with circovirosis, although the histological and serological results only indicate the presence of PRRSv. The sows are PRRS stable and the piglets are negative at birth. What could be happening? Post-weaning PRRS seroconversion is not always a consequence of producing viremic piglets but rather it is often an infection that occurs in the post-weaning phase as a consequence of the virus circulating in previous batches. Importance of vaccination of sows against PCV2 When these seroconversions are accompanied by a very high mortality, as seems to be the case here, it is very possible that other pathogens are involved ; perhaps the most frequent being PCV2. It is a common practice to vaccinate piglets and gilts on farms, but not to vaccinate sows. When this type of vaccination program is used there is a possibility of forcing the virus (PCV2) to circulate in gestation, which can cause circulation in late gestation or even in farrowing. In both cases, problems can arise post-weaning. When the virus circulates in late gestation, it can cause myocarditis in the fetuses, which will later increase […]

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How should the quarantine, acclimation, and monitoring of replacement gilts be carried out on PRRS-positive farms?

Enric Marco — Tue, 28 Jul 2020 12:53:26 +0000

Length of the quarantine: When we want to introduce negative replacements to a PRRS positive farm, the quarantine, or acclimation rather, must be long. “Long” meaning a period of not less than 2 months, because during this period our intention is to infect the new arrivals, giving them time to recover from the infection and lose their status as virus shedders before introducing them into the farm and starting to work with them. PRRS status of the gilts when entering the breeding herd: The aim is for the gilts to be introduced to acquire immunity to PRRS, and therefore become ELISA positive, but without shedding the virus; that is, they become PCR negative. How to perform the acclimatization? There are many techniques to achieve this status, although some work better than others, especially when it comes to the risk of producing disease. 1. Virus exposure with shedding animals: The most common method is to put the gilts in contact with animals on the farm that we know are shedding the virus. This contact should last about 15 days and should allow direct contact between the shedders and the gilts. 2. Live virus inoculation: Another more effective, but more […]

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Is it expected, for sows vaccinated with a MLV PRRS vaccine, to be all positive by ELISA test?

Enric Mateu — Thu, 17 Oct 2019 10:32:57 +0000

The answer to the question depends on whether it refers to naïve sows vaccinated against PRRS for the first time or to animals that have received multiple vaccinations with a MLV PRRS vaccine. Naïve sows vaccinated with MLV PRRS vaccine for the first time When naïve sows are vaccinated for the first time (modified live PRRS vaccine) about half of them seroconvert between 7-10 days after vaccination and by day 21 after vaccination all sows develop antibodies against PRRSV. ELISA values (optical densities or S/P ratios) may vary very much between animals. In contrast, when sows are multiple vaccinated with PRRS vaccines, a variable proportion of them may test negative by ELISA. Seroconversion of multivaccinated sows Moreover, in many animals already seropositive, re-vaccination with a PRRS vaccine does not produce a clear seroconversion in ELISA. It is worth to note, that by using different ELISA kits, the individual animals testing negative may be different. In other words, the ELISA used has influence in the result. In our experience, when animals are examined using several tests simultaneously, overall less than 5% of the vaccinated sows can be catalogued as real low or non-responders. Since ELISA results […]

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Externally purchased gilts were PCR PRRS virus positive at destination farm however previous test in the source herd were negative. Could the transport stress induce viremia in latent infection? What about the possibility of infection during transport (3 hours driving and the car was disinfected)?

Pedro Lopes — Thu, 02 May 2019 14:02:13 +0000

The PCR technique is usually very sensitive and it detects very small amounts of PRRS virus in blood or other samples, but there are some limitations to the interpretation of the results. PCR technique limitations Firstly, I would want to make sure that the gilts are confirmed PRRS virus negative in the source herd before being transported. To detect a 10% prevalence level with a 95% confidence you need to test 29 gilts. Depending on the PCR test you can pool 3 to 5 blood samples per test, but individual testing of 100% of the gilts will give you 100% assurance about the final result (take into consideration that negative sub-populations to PRRS virus are common in positive farms). If all gilts are confirmed PCR negative at the source herd, latent infection can be excluded. Stress / contamination during transport as a risk of PRRS virus infection Transport is a stressor and induces some degree of immunosuppression so it can induce viremia in a latent infection. Contamination during transport is also possible but it will take some days after contamination before you can detect PRRS virus in the blood by most PCR kits. I would like […]

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