Circoviruses have been present in swine for several decades (without any apparent clinical disease), and a sufficient explanation for the current rise in the prevalence of circovirus-associated diseases is still lacking.

History

Porcine circovirus type 1 (PCV1) was first identified in 1974 and was recognized as a non-disease-causing agent that frequently occurred in laboratory tissue cultures. In 1991, swine veterinarians in western Canada began reporting cases of nursery pigs falling ill with clinical signs that included a progressive loss of body condition, visibly enlarged lymph nodes, difficulty breathing, and less commonly diarrhea, pale skin, and jaundice.

The name "postweaning multisystemic wasting syndrome" (PMWS) was applied to the disease. In 1997, a new circovirus (PCV2) was isolated from these outbreaks. Since that time, researchers have been able to successfully reproduce the disease by inoculating pigs with PCV2 in the laboratory.

PMWS has now been reported from most pig-producing countries of the world. Additional PCV2-associated diseases have also been described including reproductive failure, respiratory disease in finishing pigs, and an unusual skin and kidney disease referred to as porcine dermatitis and nephropathy syndrome (PDNS).

Agent and Pathogenesis:

Porcine circovirus type 2 (PCV2), is a small, non-enveloped, circular DNA virus that is only known to cause disease in pigs. Like parvovirus, PCV2 is stable in the environment and is resistant to many common disinfectants.

PCV2 is significantly different from PCV1 and can be easily distinguished by laboratories through testing blood samples, tissues, or the virus itself. PCV1 can readily infect pigs but has not been associated with any clinical disease.

Exactly how PCV2 causes disease is not well understood. Most researchers that have attempted to reproduce the disease in the laboratory have relied on intranasal or intravenous inoculation techniques. Monocytes and macrophages (types of white blood cells) found in the lung, tonsil, spleen, lymph nodes, and other tissues involved in the immune response are frequently found to contain virus after infection.

Over time, PCV2 infection results in the depletion of lymphocytes (white blood cells that are critical to the immune response) from tissues such as tonsils and lymph nodes that normally contain high numbers of these cells, leaving a very distinct microscopic lesion. The severity of the microscopic lesion, and the amount of PCV2 in the lesion, have been reported to correlate well with the severity of the clinical signs.

It has been widely speculated that the virus’ predilection for damaging the immune system leaves the pig immune-suppressed but this has not been substantiated experimentally. Interestingly, other research has shown that an activated immune system is required for PCV2 infection to result in clinical disease.

Epidemiology, Transmission, and Clinical Manifestation:

PCV2 infection in pigs can cause a wide variety of clinical signs and syndromes. Recently, swine blood samples collected from as far back as 1969 have been tested for antibody against PCV2 and found to be positive.

Additionally, a majority of swine farms today are seropositive for PCV2 while only a small percentage of them have actually shown signs of PCV2-associated disease. Researchers have learned that simply inocu-lating pigs with the virus will not reliably reproduce clinical disease. So, while PCV2 is strongly implicated in several disease syndromes, other appropriate environmental and management conditions must exist for the diseases to become apparent.

PMWS - PMWS is the classically described PCV2-associated disease and it is generally accepted that pigs must demonstrate all of the following criteria in order to establish the diagnosis:

• Wasting, weight loss, ill thrift, or failure to thrive;
• Microscopic lymphoid depletion or monocyte-macrophage associated inflammation in any organ; and
• Detection of PCV2 in the affected tissues.

Confusion resulting from observations that PCV2 infection does not necessarily mean a pig has PMWS; wasting or ill thrift can accompany many diseases, and the non-specific nature of some of the microscopic lesions has necessitated the development of the three disease criteria. Additional clinical signs that have been reported include jaundice, fever, stomach ulceration, diarrhea, and sudden death. Early reports of PMWS were primarily restricted to nursery age pigs.

However, some swine populations, especially in the U.S., seem to have a tendency to experience disease in early finishing pigs as well. Generally, less than 5% of the pigs in an affected group show symptoms of PMWS though some producers have reported a prevalence greater than 40%. Most affected pigs do not recover. On farms with a history of PMWS, not all groups of pigs will show clinical signs. These numbers can vary substantially between countries.

Co-infecting pigs with PCV2 and PRRS virus or parvovirus has been shown experimentally to result in a much more predictable and severe form of PMWS then PCV2 alone. Field experience and diagnostic laboratory cases have confirmed that PCV2 is seldom found alone.

PDNS - Another syndrome that appears to be related to PCV2 infection is porcine dermatitis and nephropathy syndrome (PDNS). Pigs affected by this condition generally remain alert and non-feverish but have numerous dime-sized, ulcerative, raised lesions on their skin, most frequently on the flank, rear legs, and belly. These lesions may coalesce and be covered by a crusty, black scab. PDNS may be fatal or pigs may spontaneously recover without treatment.

On post-mortem examination, the kidneys are enlarged, pale, and may have pinpoint hemorrhages on the surface. Microscopically, the kidney lesions have characteristics suggestive of immune-mediated hypersensitivity. The syndrome has not been reproduced experimentally but research points to a possible role for PCV2 and/or certain strains of Pasteurella multocida.

PRDC - In the U.S., the most common disease associated with PCV2 infection is pneumonia. PCV2 as a component of porcine respiratory disease complex (PRDC) has been well documented at diagnostic laboratories. Co-infections with PCV2 and Pasteurella multocida, swine influenza, PRRS virus, or Mycoplasma hyopneumoniae appear to create a synergistic environment that results in prolonged and unusually severe respiratory disease outbreaks.

These outbreaks are often reported to be unresponsive to antimicrobial therapy and can result in group mortality that is 2 – 10 times higher than normal. Other Diseases Associated with PCV2 - PCV2 infection has also been suggested as a cause of reproductive failure, enteritis, and CNS disease. These syndromes are less frequently reported than PMWS, PDNS, or PRDC and have not been thoroughly studied or reproduced in the laboratory. Research is continuing to further understand their importance.

Diagnosis:

Several techniques are available for diagnosing PCV2 infection. It is important to remember that PCV2 infection is widespread so pigs will frequently be positive on a blood test in the absence of any clinical signs in the pig or on the farm.

Good microscopic lesion evaluation and immunohistochemical staining techniques are available at most lab-oratories accustomed to diagnosing swine diseases and can be regarded as the gold standard for testing. Virus isolation is rarely requested or necessary. The microscopic lesions found are relatively unique but must be interpreted with regard to the clinical signs and history that accompany the case.

Treatment and Prevention:

Veterinarians and producers are aggressively pursuing means to prevent and control PCV2-associated diseases. No vaccine currently exists in the U.S. or internationally, though work is being done at private and university laboratories.

As PCV2 is a virus, antibiotic therapy is generally not very helpful in preventing or treating out-breaks. Some reports have suggested that when a bacterial infection is occurring simultaneously with PCV2 infection, some benefit may be derived from treatment with antibiotics.

Most success in managing the disease has come from attending to the details of biosecurity, sanitation, production strategies, environment, and control of other diseases. Biosecurity practices should include avoiding the introduction of live animals, minimizing visitor traffic on the farm, rodent, insect and bird control, and practicing shower-in and shower-out procedures. Complete cleaning and disinfection of facilities between groups of pigs appears to be helpful in preventing disease transmission as well as the severity of infections. The Security/Biosecurity Guides developed by the National Pork Board provides a thorough discussion of all these topics.

Production strategies believed to help in controlling PMWS include acclimatization of replacement animals prior to entering the herd, ensuring adequate intake of colostrum by all piglets, minimizing crossfostering activity, reducing group size, and strict adherence to all-in, all-out pig flow.

Control of the environment to assure good ventilation, removal of gases and humidity, and maintenance of appropriate temperature will also help to minimize the impact of PCV2 infection. Control of other farm pathogens is critical to avoiding the potential problems created by co-infection by PCV2 and other organisms.

In addition to the steps outlined above, strategic and appropriate vaccination, prompt treatment of ill pigs, and use of a hospital pen for removal of extremely sick pigs will help to reduce the risk of transmitting PCV2-associated diseases between pigs.

Summary:

PCV2 infects swine worldwide, and yet only a small proportion of pigs develop PMWS and other PCV2- associated diseases. The reason for this is unknown. Based on published research data, it is clear that PCV2 infection is necessary for the development of PMWS but it is generally not capable of producing the disease on its own. The most critical question for researchers studying PCV2-associated diseases is identifying which factors cause a PCV2 infection to progress to a PCV2-associated clinical disease.

Frequently Asked Questions:

What’s the difference between Circovirus, PMWS, PDNS, and PRDC?
Circoviruses are a family of viruses. Porcine circovirus type 2 (PCV2) is a specific strain of the virus known to infect and cause disease in pigs. PMWS (postweaning multisystemic wasting syndrome), PDNS (porcine dermatitis and nephropathy syndrome), and PRDC (porcine respiratory disease complex) are general terms used to describe the clinical disease syndromes caused by PCV2, usually in combination with one or more other agents.

Where did PCV2 come from?
Circoviruses have been known about for many years and have been determined to be in the pig population as far back as 1969. PCV2-associated diseases have been increasing in prevalence worldwide since the early 1990’s for reasons that are not known.

How is PCV2 diagnosed?
PCV2 infections can be readily diagnosed at most laboratories that are accustomed to working with pigs. It is important to remember that PCV2 infection is widespread in most pig populations but only causes disease in a low percentage of farms.

How do I treat pigs with PCV2-associated diseases?
Because the organism is a virus, antibiotic therapy is often not helpful. However, because PCV2 rarely acts alone, treatment strategies should be aimed at reducing the prevalence of other common diseases or infections such as PRRS virus, parvovirus, Mycoplasma hyopneumoniae, and Streptococcus suis. Good production practices such as all-in, all-out pigflow, proper environmental control, and sanitation and disinfection of facilities will also help to minimize the effects of the disease.

Is there a vaccine available for PCV2?
No vaccine is currently available though several research groups are working on their development.

How do I keep the PCV2 off my farm?
PCV2 has a worldwide presence and it is likely that your pigs have already been exposed to the virus. Researchers are continuing efforts to better understand why most farms are infected yet relatively few experience PCV2-associated losses. Enforcing strict biosecurity and farm security protocols will minimize the risk of introducing additional viruses or synergistic disease agents that may trigger PCV2 to become clinically significant.

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