Infectious disease continues to be a threat to swine production. In the United States, we are currently experiencing one of the most devastating epidemics in the last few decades with the introduction of porcine epidemic diarrhea virus (PEDV).
At the same time, swine farms have to deal with the increase in respiratory disease characteristic of the winter months. Even though the onset of porcine reproductive and respiratory syndrome virus (PRRSV) outbreaks in the fall appeared to be delayed by 2-3 weeks this year, by December there was a clear increase in the number of positive tests for PRRSV and swine influenza virus (SIV) at the University of Minnesota Veterinary Diagnostic Laboratory (UMVDL).
The presence of all of these current threats should not distract swine producers from the management practices in place to control other diseases. Porcine circovirus associated disease (PCVAD) appears to be under control in most of the farms. This disease is caused by porcine circovirus type 2 (PCV2), a small virus that is present in all farms. This disease caused devastating losses in U.S. swine herds during 2003-2006. The virus causes severe progressive wasting.
Because PCV2 affects the immune system of the pig, affected pigs are more susceptible to other viral and bacterial infections. Fortunately, the disease was controlled in most farms with the introduction of very effective vaccines. The rate of detection of PCV2 at the UMVDL has dropped since the introduction of the vaccine (Figure 1).
Figure 1. Proportion of Samples That Tested Positive for PCV2 by Polymerase Chain Reaction Over Time.
Furthermore, in most of the positive cases, only small amounts of virus are detected and pigs do not develop PCVAD. However, there are sporadic cases in which the full clinical picture of PCVAD is observed. These are usually instances in which the vaccine was not properly administered or a decision was made to stop vaccination. Therefore, it is important to remember that the threat of this viral disease is still there, and that we need to stay alert to possible outbreaks and perform adequate diagnostics if PCVAD is suspected.
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The best way to diagnose PCVAD is by having the veterinarian perform necropsies of affected pigs. Expert swine veterinarians will be able to perform a tentative diagnosis of PCVAD based on clinical signs and lesions. However, for a definitive diagnosis, samples need to be sent to the diagnostic laboratory for microscopic examination and detection of the virus. A complete set of tissues is preferred. But the most important tissues to collect are lymphoid tissues: lymph nodes, spleen, ileum and tonsil. At the diagnostic laboratory, a veterinary pathologist will be able to identify the characteristic microscopic lesions of PCVAD and also detect large amounts of PCV2 in lymphoid tissues (Figure 2).
Figure 2. Microscopic Image of Lymphoid Tissue Infected by PCV2. Infected Cells are Stained (brown) with Immunohistochemichal Techniques.
In summary, although PCVAD is much less frequent than a few years ago, it remains an important disease of swine in the United States. Proof of that is that most pigs are vaccinated against PCV2. This virus is still present in all farms and any problems in the vaccination program may result in an outbreak.
While dealing with all of the challenges of other viral threats such as PEDV, PRRSV and SIV, it is important to remain disciplined with the management practices that are keeping other diseases such as PCVAD under control.
Finally, PCVAD should be considered as a differential in the case of diseases including wasting, respiratory problems and increased mortality, even if the farm is currently vaccinating for PCV2.
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