Porcine circovirus associated disease is a challenging and costly disease for the U.S. pork industry today. Diagnosing PCVAD is based on pig weight loss and poor growth, as well as respiratory disease, diarrhea, reproductive failure, skin lesions (aka PDNS) and microscopic lesions, including depletion of lymphoid immune cells and presence of the virus in affected tissue. Pigs can be exposed to porcine circovirus type 2 by contaminated feces and urine, direct contact with other pigs, dam-to-fetus transmission or possibly through boar semen.
Following exposure and infection, PCV2 enters the blood stream. The presence of virus in the blood is called viremia, and it can be classified as primary or secondary.
Primary viremia is the virus’s first entry into the blood following viral multiplication in an initial mucosal site or entry portal.
Secondary viremia results from re-infection of the blood with new virus produced through multiplication in key systemic target tissues, such that virus concentration may reach substantially higher levels — titers.
The establishment of viremia in pigs exposed to PCV2 can have serious implications including:
- Viremia is the main vehicle for infection of important target tissues, which impacts the disease severity in an individual pig.
- Viremia has a dramatic impact on the virus’s capacity to be transmitted to other animals in the herd.
The magnitude of viremia is influenced when the amount of virus in the blood exceeds the rate at which the host can clear the virus. In addition, differences between PCV2 isolates, co-infections with other viruses or bacteria, pig susceptibility and immune factors could affect the viremia level and disease progression. Sustained viremia is an essential requirement for transmitting viral infections from animal to animal.
From clinical and epidemiological standpoints, it is important that viremia be prevented through the appropriate prophylactic interventions, such as vaccination.
However, vaccines are known to differ in their capacity to prevent viremia. There are several well-documented cases where vaccination has effectively prevented viremia, to a degree that would be compatible with the occurrence of sterilizing immunity. The enhanced clinical effectiveness of a vaccine that is capable of inducing sterilizing immunity is based on the premise that preventing infection at the start of the disease process will provide the most consistent protection under the various field situations that impact disease severity.
Viremia prevention or reduction is an important consideration for controlling PCVAD with vaccination. Even if it appears that PCVAD has been controlled based on reduced herd mortality, appearances can be deceiving. If viremia is still present, the potential for virus transmission and additional losses due to PCV2 are enhanced. Controlling viremia is critical to a successful PCV2 vaccination program. PE
Building a Glossary of Terms
Speaking the same language is critical to understanding the options for solutions. Here’s a refresher on some of the herd-health related terms that need to be crystal clear.
Clinical – Disease in animals of sufficient severity to be observed by the farm staff or measured by record analysis.
Epidemiological – The relationship of various factors that influence disease frequency and distribution in a population or herd.
Isolate – Bacteria or virus obtained in a pure culture from a field case or location.
Lesion – Pathological damage, injury or loss of tissue or organ function; this may be visible to the eye or by microscopic examination.
Lymphoid – Refers to the lymphatic system tissues (lymphocytes, lymph nodes and lymphatic vessels) as well as lymph fluid, which circulates through lymphatic vessels into the blood stream.
Mortality – Death as a statistic, a number or percentage.
Mucosal – Refers to mucous membranes such as the intestinal, respiratory and reproductive tract linings. The mouth, nose and larynx are mucosal surfaces where the initial infections often occur.
Porcine circovirus – A group of small DNA viruses, some of which can cause disease (Type 2) and some that are considered non-pathogenic (Type 1).
Sterilizing immunity – Immunity that completely destroys all microorganisms or neutralizes their pathogenic products.
Tissue – Specialized body cells that together perform certain special functions.
Titer – The quantity of an antibody or microorganism in blood or tissues.
Vaccination – The act of exposing an animal to a vaccine. Often done by injection but can be accomplished via exposure of mucosal surfaces such as oral (in the water) or intra-nasal instillation. Developing protective immunity after vaccination depends on the animal’s responsiveness to the vaccine, the vaccine’s inherent effectiveness, severity of the challenge and vaccine administration accuracy.
Viremia – Virus presence in the blood either as free-floating or cell-associated virus. Cell-associated virus is often found in lymphocytes.