News from the National Pork Board

Porcine Circovirus Associated Diseases Workshop stimulates research discussion

Over 50 scientific researchers, veterinarians, allied industry representatives, and producers met on August 17 to 18, 2006, to discuss and vote on porcine circovirus associated diseases (PCVAD) research priorities. The Pork Checkoff’s PCVAD Strategic Planning Workshop featured several speakers covering diverse areas of PCVAD topics. The speakers included Dr Phil Gauger, USDA/NADC; Dr Paul Dorr, North Carolina State University; Dr Kent Schwartz, Iowa State University Diagnostic Laboratory; Dr Kurt Rossow, Minnesota Diagnostic Laboratory; Dr Keith Erlandson, Carthage Veterinary Services; and Dr Marnie Mellencamp, PIC/Genus.

Phil Gauger reviewed PCVAD from an immunology perspective. The National Animal Disease Center (NADC) has investigated samples from several states focusing on a viral etiology. One common finding among affected herds is detection of porcine circovirus type 2 (PCV2), leading to the sequencing of field isolates. Gauger presented information on the different field isolates, PCV2a and PCV2b, and their relationship with isolates from Canada and Europe. His group has identified other viral pathogens as coinfections and occasionally isolated Mycoplasma hyopneumoniae.

Questions formulated as a result of this research include whether the strain found in these cases is related to high mortality, whether passively acquired PCV2a immunity protects against PCV2b infection, and whether PCV2b affects the immune system.

Paul Dorr addressed coinfections occurring with PCV2. Dorr reviewed a cross-sectional study examining 800 pigs necropsied from 41 farm flows. Key findings in this study included the fact that PCV-positive pigs are more likely to be co-infected with swine influenza, Mycoplasma species, and porcine reproductive and respiratory syndrome virus (PRRSV). In this study, 3-week-old PCV-positive pigs perpetuated swine influenza virus (SIV) circulation in the nursery. Classic porcine respiratory disease complex was more prominent in the three-site production systems and in the 9-week-old pigs. More systemic involvement was observed in the early finisher. No significant differences in 24-week-old pigs were noted, but this age group had the greatest prevalence of PCV2-positive pigs. This raises the question as to whether there is another triggering mechanism.

Questions raised include whether PCV2- infected pigs are more likely to be co-infected with other pathogens than non-PCV2-infected pigs; what are the most likely co-infectors; and how animals are affected by age and production system.

Kent Schwartz presented Iowa State Diagnostic Laboratory’s perspective. Numbers of PCVAD cases submitted to the diagnostic lab are on the rise. The diagnostic approach varies by case, depending on the practitioner’s questions. Polymerase chain reaction (PCR) is the most sensitive diagnostic tool to determine whether PCV2 is present. Immunohistochemistry (IHC) and possibly quantitative PCR help determine whether PCV2 is associated with the disease process. To investigate other infectious diseases that may be present, a complete diagnostic workup is needed, requiring samples for serological testing and tissues to be submitted. Sequencing and a dendogram can be utilized to determine if a different PCV2 strain is present. In a majority of cases, PCV2 is not a singular infection. Cofactors include PRRSV, SIV, Mycoplasma species, and bacterial agents causing septicemia or pneumonia, or a combination of these. In the basic diagnostic workup, Schwartz includes serological samples from 10 animals at 6, 11, and 16 weeks of age and tissue workup from two or three acutely affected pigs and two or three chronically infected pigs. Tissues should include lung, brain, lymph nodes, tonsil, kidney, ileum, liver, stomach, and spleen. Tests on tissues include histopathology, bacteriology, PCR, and IHC. On the farm, Schwartz recommends examining pigs at the age of peak mortality, 3 weeks pre-peak mortality, and 6 weeks pre-peak mortality.

Kurt Rossow reviewed PCV2 lesions from the Minnesota Diagnostic Laboratory perspective. Porcine circovirus type 2 appears to be involved with the increase in mortality seen after the spring of 2006. High mortality is most commonly associated with other pathogens and the 3-2-1 PCV2 genotype. Lymph node lesions are located in the germinal center and initially lack lymphoid depletion. However, lesions become more “typical” with time. Rossow presented lesions in the spleen and lung, as well as those associated with enteritis. Staining is always more intense and widespread when copathogens are involved. Herd PCV2 sequences are 4-2-2 or 3-2-1 or both. Co-infections may include SIV, PRRSV, Mycoplasma species, and enteric agents. The outcome of the infection varies depending on co-infections involved or systemic distribution.

Keith Erlandson addressed PCVAD from a practitioner’s and a producer’s perspective. Erlandson conveyed how quickly the disease appeared to affect hogs in his area. Initially, he observed PCVAD as an individual pig problem, then as isolated cases of high mortality. Producers who have had vaccine available report a reduction in mortality. Several questions that need to be addressed include determining which part of the genome is the best to sequence and how to interpret that sequence; researching sire and dam line differences; investigating possible interaction with PRRS serum exposure and sow herd PCV2 status; and investigating the possibility of other agents. Other on-farm applications would include determining if and how farm-to-farm transmission occurs, as well as the best methods for disinfection. Producers need a better understanding of potential triggers and methods to control the disease now and over time.

Marnie Mellencamp addressed the issue of potential unknown agents interacting with PCV2 to produce PCVAD. Mellencamp described the role of endogenous and exogenous retroviruses in vertebrate genomes. Porcine endogenous retroviruses (PERV) have been investigated for their significance in microbiological safety in xenotransplantation. No disease association has been reported in pigs. Three subtypes of PERV provirus were discussed. Currently, there is no conclusive proof for existence of exogenous porcine retroviruses. Mellencamp presented a study to investigate the presence of retroviremia in serum of commercial pigs and to seek variations in viremia within and across pig populations. Conclusions of this study revealed that further research is necessary to dissect the pathogenic potential for porcine retroviruses in endemic disease.

On August 18, participants were divided into five breakout groups. Each group was charged with listing a maximum of five priorities for each research topic. The topics covered included immunology, epidemiology, pathogenesis, diagnostics, and vaccinology-interventions-treatments. After each topic was discussed, the entire group scored all priorities on a scale of 1 to 10, with 1 being a low priority and 10 being the highest priority. A summary of this effort follows.

During the discussion, several considerations which may apply to many of the topics were identified. Each topic affects each stage of production, age of the animal, and the organ system affected. With many diseases, management practices vary with each stage of production and age of the animal. The organ system involved will affect the treatment options and may influence whether immunity is protective. Transmission may also vary with these factors.

Immunology

Understanding the immunological aspect of this disease is critical when developing tools to reduce its economic impact. Several unknowns exist in PCVAD immunology research. Determining the mechanism of protection was the highest ranking priority in this section (Table 1). However, much of the immunity discussion focused on establishing protective immunity and the roles of cell-mediated immunity (CMI) and humoral immunity (HI). Other specific research questions resulting from the exercise and their rankings are shown in Table 1.

Epidemiology

Little is known concerning how the relationship of PCV2 and potential cofactors has affected the distribution of this disease throughout the United States. It is critical to understand how PCVAD is transmitted in order to initiate control measures. The transmission of PCVAD received the highest research priority ranking (Table 2). Understanding the role of fomites, wildlife, transportation, geography, and the effect of serum injections is imperative to develop biosecurity protocols. The degree of shedding in boar studs-semen, feces, and oronasal secretions may also affect transmission of PCVAD. The role of horizontal and vertical transmission is yet to be fully understood. Molecular epidemiology ranked second, including sequencing versus restriction fragment length polymorphism (RFLP), the difference between isolates from diseased herds and herds without the disease, differences in clinical presentation of current outbreaks compared to historic outbreaks, and determining mutation rate and significance. Other specific research priorities and rankings are shown in Table 2.

Pathogenesis

What agents and conditions lead to the development of PCVAD? Unfortunately, there is no easy answer to that question. Therefore, the attendees formulated many priorities which, if answered, may lead to controlling this disease. The need for research to identify the initial and secondary PCV2 replication sites ranked as the highest priority in this area (Table 3). Investigating the causes of lymphoid depletion and what the consequences are, as well as understanding the effect of viral strain variation on virulence, was a close second. Other specific research questions the attendees want answered and their rankings are shown in Table 3.

Diagnostics

Diagnostics are the cornerstone of research in PCVAD. Without validated, standardized diagnostics, it is difficult for laboratories to communicate, and for producers and veterinarians to interpret, the diagnostics performed. The need for validated, standardized tools across the diagnostic laboratories in North America is critical, with the highest ranking from the attendees (Table 4). Those tools might include PCR (quantitative, multiplex, and pooling), ELISA, sequencing, and identifying antigen in feces, semen, saliva, colostrum, serum, and tissues. Developing standardized diagnostic protocols ranked second highest. These protocols might include clinical case definitions, sample collection, laboratory interpretation, cofactor evaluation, and environmental tests. Other specific research questions and their rankings are shown in Table 4.

Prevention and treatment

Considering the impact this disease is having on many production systems, finding a method to prevent a clinical outbreak or treat affected animals in an economical manner is imperative. Research determining vaccine efficacy in the face of cofactors, when pigs are exposed to multiple strains, to decrease viral shedding in boars (semen), sows, and pigs (oronasal and fecal), to provide fetal protection, and to induce CMI, if needed, ranked the highest priority in this area. Other research areas concerning vaccine efficacy include vaccine timing in pigs (considering passive antibody decay) and timing in sows (considering maternal colostral levels and uniform sow titers). As multiple vaccines become available, attendees were concerned whether the pig’s immune system will respond the same way to the different vaccines and whether their effect on viral load will be similar. The second highest ranking priority was the need to identify management and biosecurity practices to prevent transmission, both horizontal and vertical. Producers need to know effective disinfectants and potential biosecurity breakdown areas. Validated biosecurity practices to decontaminate trailers and buildings need to be provided to producers. Other specific research questions attendees identified are shown in Table 5.

Research priorities scoring the highest rankings overall are shown in Table 6.