Porcine circovirus type 2 (PCV2) is a small, non-enveloped DNA virus that has significantly impacted global swine production. Identified in the late 1990s, PCV2 belongs to the Circoviridae family and has a circular, single-stranded DNA genome. It has become one of the most widespread and economically impactful pathogens in the swine industry.
Porcine Circovirus Associated Disease (PCVAD)
Porcine Circovirus Associated Disease (PCVAD) is a collective term for the diverse clinical manifestations linked to PCV2 infection. The most recognized form is postweaning multisystemic wasting syndrome (PMWS), which commonly affects pigs in the nursery to early growing phases, often coinciding with declining maternal antibodies. Pigs with PMWS exhibit gradual wasting, leading to marked spines, and may also show respiratory distress. Other signs include pale skin, a rough haircoat, and enlarged lymph nodes. In herds without prior exposure, mortality rates from PMWS can reach 20%.
PCVAD also encompasses porcine dermatitis and nephropathy syndrome (PDNS), which typically affects pigs aged 11 to 14 weeks, although its prevalence is generally low, around 1%. Affected pigs often appear lethargic and anorexic, developing distinct red to purple, circular, crusty lesions on their skin, commonly starting on the hind legs and spreading towards the abdomen. These skin lesions may eventually fade or leave scars, and in severe cases, the condition can be fatal.
PCV2 can also contribute to reproductive failure in breeding sows. This can manifest as late-term abortions, stillbirths, and mummified fetuses. While less common in adult breeding herds due to natural immunity, reproductive issues are more frequently observed in newly established or naive herds with susceptible gilts.
Transmission and Spread in Herds
PCV2 spreads efficiently among pigs through various routes, making its control challenging within swine populations. Direct contact with infected animals is a primary mode of transmission, as pigs shed the virus through oronasal secretions, including saliva, nasal fluids, feces, and urine.
Indirect transmission also plays a substantial role, with fomites such as contaminated equipment, vehicles, clothing, and even farm staff’s boots serving as carriers. PCV2 is quite stable in the environment and can persist for several months, making thorough cleaning and disinfection protocols important for reducing environmental contamination. Vertical transmission, from an infected sow to her piglets, can occur both in utero and post-natally through colostrum and milk. While boars can shed the virus in semen, the practical significance of this route for reproductive failure is generally considered low unless semen has a very high viral load.
Several factors can increase the risk and spread of PCV2 within a herd. High stocking density, which increases pig-to-pig contact, can exacerbate the disease. Co-infection with other common swine pathogens, such as Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) or Mycoplasma hyopneumoniae, can enhance the severity and progression of PCVAD by impairing the pig’s immune response. Farm stressors like poor ventilation, weaning stress, and mixing different age groups also contribute to disease development and spread.
Diagnostic Procedures
Confirming a PCV2 infection requires a multi-component diagnostic approach, as clinical signs alone are not sufficient due to their similarity to other swine diseases. The accepted “gold standard” for diagnosing PCVAD involves three distinct criteria.
First, characteristic clinical signs, such as wasting, respiratory distress, or skin lesions, must be observed in affected pigs. Second, microscopic lesions must be identified in lymphoid tissues (e.g., lymph nodes, tonsils, spleen) upon histopathological examination. These lesions commonly include lymphoid depletion and multinucleated giant cells. Third, the PCV2 virus or its genetic material must be detected within these specific microscopic lesions.
Laboratory techniques are employed to fulfill the third diagnostic criterion. Immunohistochemistry (IHC) is a common method used on tissue samples to visualize the viral antigen within affected cells. Polymerase chain reaction (PCR), particularly real-time PCR, is another widely used test that detects PCV2 DNA in various samples like blood, oral fluids, feces, or urine. While PCR can detect the virus, the level of viral load often helps differentiate between subclinical infection and active disease, as higher viral loads are typically associated with clinical PCVAD.
Vaccination and Control Strategies
Commercial PCV2 vaccines represent the most impactful development in controlling PCVAD in swine. These vaccines are widely utilized in pig production, significantly reducing the severity of clinical signs and decreasing viral shedding. Vaccination protocols typically involve administering vaccines to piglets, often at a young age, to provide protection as maternal antibodies decline. Many producers also vaccinate sows to provide passive immunity to their offspring through colostrum, contributing to herd-level control.
Beyond vaccination, implementing robust biosecurity measures is a complementary strategy for managing PCV2. An “all-in/all-out” production system, where groups of pigs are moved through facilities and rooms are thoroughly cleaned and disinfected before the next group, helps break the chain of infection. Proper cleaning and disinfection protocols using effective disinfectants can reduce the environmental load of PCV2, known for its environmental stability.
Controlling pig movement, both within and between farms, helps prevent the introduction and spread of the virus to susceptible populations. This includes strict quarantine periods for newly introduced animals and careful management of visitors and vehicles entering the farm. Managing other concurrent diseases, such as Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) or Mycoplasma hyopneumoniae, is also important, as co-infections can worsen the impact of PCV2 and reduce the effectiveness of vaccination. Maintaining optimal environmental conditions, providing proper nutrition, and minimizing stress are additional management practices that support overall pig health and resilience against PCV2.