Ten years ago, a pig-wasting disease associated with porcine circovirus type 2 (PCV2) almost simultaneously struck Canada, California and France.
Today, PCV2 is known as one of the most important viruses in the global pig industry, according to circovirus researcher Tanja Opriessnig, Iowa State University.
Different manifestations of PCV2 led to the American Association of Swine Veterinarians coining the phrase, porcine circovirus-associated disease (PCVAD), at their annual meeting in March 2006.
Those manifestations include: postweaning multi-systemic wasting syndrome, PCV2-associated pneumonia, enteritis, reproductive failure, and dermatitis and nephropathy syndrome, she says.
Methods of Control
Three methods have been used for the control of PCVAD:
Serotherapy, or the collection of serum from infected animals for injection to prevent and control PCVAD, was practiced widely in Europe prior to availability of commercial vaccines, Opriessnig said in a talk at the Leman Swine Conference in mid-September in St. Paul, MN.
While clinical disease and mortality rates were significantly reduced, results were not reproducible in other countries, she explains.
Autogenous vaccines prepared from lung or lymphoid tissue homogenates obtained from PCVAD pigs were used in a number of U.S. hog operations when there were limited supplies of commercial vaccines. Experimental tests of three different autogenous products revealed that PCV2 was not effectively inactivated by the formalin treatment, and use of the autogenous products was eliminated when PCV2 vaccines became widely available, Opriessnig says.
Four commercial, killed PCV2 vaccines are marketed, and all but Merial's sow vaccine is federally licensed for use in the United States (Table 1).
Today, PCV2 vaccines are among the three most commonly used vaccines in swine production in the United States, says Opriessnig. About 73 million pigs were vaccinated for PCV2 in 2007, compared with 70-80 million for Mycoplasmal pneumonia and 25 million pigs vaccinated for ileitis.
Issues of Concern
PCV2 vaccines have been shown experimentally to reduce PCV2 viremia (infection in blood) after challenge, and reduce lymphoid lesions and nasal and fecal shedding of PCV2 organisms. In the field, the vaccines have greatly reduced mortality, while improving performance parameters, cutting medication costs and boosting reproductive performance.
“Due to their remarkable efficacy in the field, PCV2 vaccines have been referred to as ‘miracle vaccines’ by some producers and practitioners.
“However, it must be kept in mind that a PCV2 vaccine will not protect against the effects of infections with other pathogens or lack of common-sense husbandry, management and disinfection practices,” Opriessnig points out. “It also needs to be considered that vaccine efficacy cannot be expected to be 100% in every instance.”
While PCV2 vaccines are very effective, care must be taken in relying on serology as a measure of vaccine efficacy. “It is generally believed that the success of PCV2 vaccines is due to induction of a strong cellular immune response, and experiments to further examine this are underway,” she comments.
The question of maternal antibodies interfering with PCV2 vaccine efficacy has sparked a lot of debate. According to Opriessnig, at least two controlled research trials have shown that these antibodies do not interfere with PCV2 vaccine efficacy.
As to vaccine timing, Opriessnig recommends PCV2 vaccines be administered as early as possible and at least 3-4 weeks ahead of expected exposure to the virus. “This may mean vaccinating pigs at 2-3 weeks of age in herds where PCVAD occurs in the nursery. Later administration may coincide with circulation of field PCV2 strains and is not always successful,” she explains.
Can PCV2 vaccines be effectively administered to infected pigs? While the answer is unknown, based on the widespread prevalence of PCV2 infection, the high percentage of subclinically infected pigs and the high success rate of PCV2 vaccination, it can be speculated that PCV2 viremia does not interfere with vaccine efficacy, Opriessnig assures.
Combination vaccines reduce labor and material cost by reducing the total number of injections per pig. The combination of PCV2 and Mycoplasmal pneumonia (Boehringer Ingelheim) holds promise because both vaccines are given at about the same age, Opriessnig states.
All PCV2 vaccines are killed products. PCV2 live vaccines would offer the benefit of not having to be administered intramuscularly, therefore lowering cost and labor requirements, she says.
Live vaccines might also prove more effective in overcoming high levels of maternal antibodies or concurrent infections with immune-suppressing viruses such as porcine reproductive and respiratory syndrome. A live PCV2 vaccine protected against PCV2 lesions and viremia following disease challenge in experimental studies, Opriessnig says.