A year ago, we were all up to our eyeballs dealing with issues surrounding a novel strain of influenza in humans that quickly became known as the “swine flu.” Time marches on, as does the popular press. The news cameras turned to chronicling an ongoing parade of other natural and unnatural disasters, and we can generally agree that it’s one of those issues that is good to see in the rearview mirror.

Flu marches on, too, so this anniversary is an opportunity to take stock of what happened with the virus after the spotlights disappeared.

Everyone is certainly aware that the pandemic 2009 H1N1 (p2009H1N1) influenza virus was first reported in the United States after it was diagnosed in pigs that were exhibited at the Minnesota State Fair last August (2009). Cases were later reported in commercial pigs in five states as part of the voluntary U.S. Department of Agriculture’s flu surveillance program www.usda.gov/wps/portal/usda/?navid=USDA_H1N1. Many other pig-producing countries have similarly reported finding the virus in pigs. At this point, detecting the virus in pigs is not a news story. It is, however, necessary to keep track of what types of influenza virus we’re finding in swine herds so we can keep up with them from a diagnostic and control standpoint.

Increased Testing for Respiratory Disease
There was some concern in several circles last summer over the perceived reluctance of producers and veterinarians to test pigs for influenza during the media frenzy immediately following the initial outbreak of p2009H1N1 in people. We, in fact, experienced a decline in case submissions for respiratory disease during June 2009 (see Figure 1). However, this decline was confounded by two things that historically drive a decline in diagnostic submissions: 1) a normal dip seen in the summer and 2) some profound economic challenges. The rest of the year, seasonal submission rates were normal in early fall and throughout the winter, although the total number of cases has exceeded past levels. This is a clear indication that people have not shied away from pursuing diagnostic testing for respiratory disease in pigs.

In addition to the high number of samples submitted for testing, the proportion of samples that tested positive for influenza virus has also been above recent levels (Figures 1 and 2). Some of this is the result of surveillance being done on farms that are specifically testing known positive populations to determine when the pigs are being infected with the virus. However, there is a general observation among producers and practitioners that there was plenty of influenza virus activity in pigs this past fall and winter.

We continue to characterize a portion of the influenza viruses that we detect. We can show from diagnostic submissions to the University of Minnesota Veterinary Diagnostic Laboratory (VDL) that there is a mix of influenza subtypes circulating in pigs. Figure 3 shows the distribution of influenza subtypes from cases for which the influenza virus has been sequenced at the Minnesota VDL year-to-date for 2010. Figure 3 shows that p2009H1N1 has accounted for a proportion of the cases sent to us. This information should not be extrapolated to the general swine population, since the number of cases with sequence information is very low.

New Tests for Flu
Some progress has been made in improving the diagnostic tests for influenza in pigs. Clinical signs typical of influenza in a group of pigs can be a good indicator of an outbreak. But the types and severity of signs can vary considerably, so the observations alone aren’t diagnostic evidence for flu. Confirming the presence of virus is possible by several methods at diagnostic laboratories, and further characterization of the virus can be helpful for developing control strategies.

PCR Tests Are Good
In June 2009, the USDA released an updated version of the flu PCR (polymerase chain reaction) detection test to all participating laboratories in the country. The test was able to detect all different types of flu in animals and was very sensitive in that it was able to find virus even at low levels. The test is excellent for screening samples for flu prior to testing them further to determine subtype, strain (pandemic or otherwise), or for virus isolation. Furthermore, the test performed better than virus isolation alone, histopathology alone, or previous versions of the PCR.

Once a sample is screened positive for flu by PCR, there are also several other PCR tests that accurately determine if the subtype of flu (H1N1, H1N2, or H3N2) or if it is pandemic H1N1. All these tests are done on the same sample and can be completed in as little as one day if necessary. So we can now detect flu with greater ease, which is good news for everyone as it saves time, money and effort.

New Flu ELISA
Serum samples can be taken from pigs that have recovered from flu to determine if antibody against the virus is present. The most robust test currently available to confirm prior infection is the IDEXX multispecies ELISA (enzyme-linked immunosorbent assay) that detects antibodies against the nucleoprotein (NP) protein, an internal viral protein, which is common to all strains of influenza. Therefore, this test can be used similar to the IDEXX PRRSV ELISA test, telling us that the pigs have been exposed to some form of the virus and is not limited to just detecting one strain or type of virus, vaccine or otherwise. Like the screening PCR test, this influenza multispecies ELISA is also excellent for monitoring populations of pigs for exposure to flu and can help determine timing of infection.

Pigs will develop detectable titers by a week after infection. A more thorough method of serologic testing is to take serum samples at the time of illness and again two to four weeks later from the same pigs and compare the titers. Pigs infected with the virus should exhibit a marked rise in antibody titer. Tests have been developed in most laboratories that will allow identification of the specific subtype with which the pigs have been infected.

Click to view graphs.

Marie Gramer, DVM
Jerry Torrison, DVM
torri001@umn.edu
University of Minnesota Veterinary Diagnostic Laboratory