Porcine circovirus type 2 vaccines have played a significant role in reducing the economic losses related to porcine circovirus associated diseases.
That’s just one consensus from a panel of swine veterinary experts during a roundtable discussion following this year’s American Association of Swine Veterinarians Annual Meeting in
Even though vaccine use has significantly improved the situation facing producers, PCV2 and PCVAD remain important challenges to the pork industry. The panelists acknowledged that important questions have surfaced as vaccine use continues to evolve.
“The situation obviously is better,” says Paul Yeske, swine veterinarian with
Very good control
“With the extensive use of vaccines, we’ve had very good control,” says Joe Connor, swine veterinarian with Carthage Veterinary Services,
He points out that veterinarians in his clinic vaccinate nearly all of their clients’ pig flows. “We think it’s controlling both clinical and sub-clinical infections, and we don’t think it’s shifting clinicals to sub-clinicals whatsoever,” Connor says. He emphasizes that vaccines have proven to be a good tool to control PCV2 and the resulting PCVAD.
“We’re still trying to impress on all producers that they have the capability to reduce the amount of virus in the environment based on how they manage pig movement within litters and within rooms and pig flow, as well as with sanitation,” Connor says.
Of course, there are important questions regarding vaccination timing, the impact of maternal antibody on vaccine effectiveness, and the influence of sow-herd vaccination that still need definitive answers. But studies are in progress today that will help answer those questions.
“There are a lot of things that we don’t yet know about PCV2,” agrees
Yeske emphasizes that producers need to continue to concentrate on keeping other diseases, such as porcine reproductive and respiratory syndrome, swine flu and Mycoplasma, under control. Another unanswered question is whether disease dynamics within systems and production flows have changed as a result of widespread PCV2 vaccination.
Although there has been progress, it remains a challenge to differentiate PCVAD from other common diseases such as PRRS, SIV and Mycoplasma. “We have to do good diagnostic work-ups to understand what’s going on,” Yeske says.
Connor looks at poor animal response in addition to diagnostics to help him determine whether PCV2 is involved in a herd.
From a diagnostic standpoint, Opriessnig says she can differentiate systemic PCV2, respiratory PCVAD and enteric PCVAD from subclinical PCV2 infection. “It depends on how the lesions are distributed and how much PCV2 antigen is associated with the lesions,” she notes. “If it’s systemic, then I would expect very severe lesions in lymph tissues and lung tissues, and maybe in the intestine associated with a moderate to high amount of PCV2 antigen.”
Vaccination itself is a good way to differentiate, Rodibaugh says. “If you vaccinate and don’t get a response from the circovirus vaccine, that may be an indirect indication that PRRS could be the significant player.”
“What a lot of us are doing today is using the vaccine as part of the diagnostic test,” Yeske agrees. Sound post-mortem diagnostic tools are now available to determine if PCV2 may be causing disease, but he doesn’t believe that the tools or the understanding of results exist for ante-mortem diagnosis. “That’s a challenge for us to better understand as an industry,” he adds.
Maternal antibody influence
One question currently being debated and researched is the maternal antibodies’ influence on PCV2 vaccine efficacy. Answers so far have been inconclusive, but experts expect more definitive insights to surface within a few months.
“We think we have had some issues with maternal antibodies in some herds,” Rodibaugh says. At the same time, he acknowledges that there are conflicting data in this regard.
Opriessnig has conducted a laboratory study testing the vaccine in the face of maternal antibodies. “The vaccine in our study was very effective; maternal antibodies did not inhibit it,” she says.
Still, the veterinarians are waiting for the researchers to sort this out. “We’ve been somewhat conservative in using vaccine in suckling pigs until we get the maternal antibody influence clarified,” Connor says. “Part of the discussion with our clients has been that we know it’s an efficacious vaccine. So right now we want to capture all the performance improvement. We can wait another six months to understand the maternal antibody interference.”
Standardized assays to measure maternal antibody also have been lacking. “Confusion with the testing standards is part of the problem,” Yeske says.
Measuring vaccine success
Connor uses several parameters to determine if PCV2 vaccines are working as expected. “One parameter is the percent of pigs placed that are marketable as Grade-A pigs,” he explains. “This is a combination of mortality plus morbidity, minus the number of pigs that are not Grade A. Another parameter is average daily gain, and a third one is feed conversion.”
He advises looking back for a clear reference. “If we look back two years ago, there were several close-outs where 89 percent to 91 percent of pigs placed would hit the Grade-A market. Today, that number will be 95 percent and more, with some wean-to-finish groups at 97 percent. So there has been a tremendous productivity shift.” He believes this can be directly attributed to PCV2 vaccinations.
“The No. 1 factor that is easiest to look at is mortality and the percent of pigs going to the primary market,” Yeske agrees. While feed efficiency and average daily gain improvements also are indicators, they can be “a little more challenging to obtain.”
Producer attitudes provide anecdotal evidence that the vaccines have been effective. “The most common comment that we get from producers is that they are going back and treating sick pigs again — and those pigs respond,” Yeske notes. “They are getting success from putting in the hard work to prevent and control diseases.”
Prior to the advent and widespread use of PCV2 vaccines, producers became discouraged because in many systems, disease problems remained overwhelming. At least today that has changed. PK
Editor’s note: The roundtable took place following the 2008 American Association of Swine Veterinarians Annual Meeting in San Diego, Calif., and was sponsored by Fort Dodge Animal Health.
Can PCV2 be eradicated?
Porcine circovirus type 2 vaccinations have helped control porcine circovirus associated diseases, but will it be possible to one day eradicate PCV2?
“It’s my understanding that it’s a very hardy virus, so our techniques to manage it through transport introduction are not here yet,” says Joe Connor, DVM, Carthage, Ill. “We need additional technology to eliminate the virus from common transfer methods from one site to another.”
Minnesota veterinarian Paul Yeske agrees, “We don’t know if we have the technology today to clean up the environment.” He believes, however, that eradication will be a long-term industry goal. “It’s been a very costly disease for producers and for the industry, and we need to explore the opportunities to eradicate it.”
Although it’s been possible to produce pigs that are PCV2-negative in a research setting, Tanja Opriessnig, Iowa State University veterinary diagnostician, doesn’t believe it’s possible to do this in a sustainable way industry-wide right now.
“First of all, once you eradicate PCV2 from a herd, you will need an ongoing source of negative replacement stock and that does not exist anywhere right now,” she says.
What about Transmission Via Boar Semen?
Iowa State University veterinary researcher Tanja Opriessnig’s studies show that porcine circovirus type 2 can be transmitted through boar semen.
“But the overall PCV2 load in the semen was low,” she explains.
Illinois swine veterinarian Joe Connor was involved in a study involving this very question. “We monitored boar semen and serum in vaccinates and non-vaccinates.” He points to the following three take-home messages from the study:
1) Boars do not consistently shed PCV2 in semen; it will be intermittently diagnosed (PCR) positive.
2) Vaccinating ready-to-use boars (6 months old) did not make any difference in terms of virus shedding or the percentage of positive animals.
3) There was porcine circovirus type 2A and type 2B, and there was a transmission between the boars within the stud.
As a result, Connor recommends that boars be PCV2 vaccinated at weaning and revaccinated when they enter the boar stud population. Opriessnig agrees with that assessment and embraces Connor’s recommendation.
A Defining Case
The American Association of Swine Veterinarians has developed an official case definition for porcine circovirus associated diseases. AASV used USDA’s Health and Human Services Centers for Disease Control model for describing the clinical expression of a new disease when a definitive diagnosis is not known.
Here are highlights of the case definition that AASV developed:
1. PCVAD can be subclinical or include one or more of the following clinical manifestations concurrently:
Multisystemic disease with weight loss (formerly known as porcine multisystemic wasting syndrome).
High mortality such as doubling of the herd’s historical mortality rate without introduction of a new known pathogen.
Respiratory symptoms, including pneumonia.
Porcine dermatitis and nephropathy syndrome.
Enteric signs, including diarrhea and weight loss.
Reproductive disorders, including abortions, stillbirths and fetal mummification. (Diagnosis requires the presence of fetal myocarditis associated with porcine circovirus 2 antigen in lesions.)
2. PCVAD is a broad categorization of multisystemic diseases that are confirmed by documenting the following histopathological findings in affected pigs:
Depletion of lymphoid cells in lymphoid tissues of growing pigs.
Disseminated granulomatous inflammation in one or more tissues (such as the spleen, thymus, intestines, lymph nodes — sternal, bronchial, inguinal and mesenteric — lung, kidney, liver and tonsil).
Detection of PCV2 within the lesions of growing pigs.
Joe Connor, DVM, Carthage, Ill.
Max Rodibaugh, DVM, Frankfort, Ind.
Paul Yeske, DVM, St. Peter, Minn.
Tanja Opriessnig, veterinary diagnostic pathologist, Iowa State University