Dissecting the details of PCVAD

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Since the discovery of porcine circovirus type 2 (PCV2) and it’s characterizations in Western Canada, the significance and dissemination of porcine multisystemic wasting syndrome — or PMWS — around the globe has grown and caused economic damage to swine industries. Veterinarians John Harding and Ted Clark, made the connection between PCV2 and PMWS in 1996.

A ubiquitous virus, PCV2 is among the smallest viruses in mammals and it’s the only circovirus that causes damage in swine. Antibody to PCV2 has been found in stored swine serum as far back as 1969. One research paper even shows that clinical signs in Spain and England may have occasionally surfaced since 1989. U.S. veterinarians have talked about finding PCV2 in tissue submissions for more than 10 years, but have not understood the significance of finding this virus as a co-infection. A mild and sporadic occurrence of growth retardation and weight loss, resulting in stalled pig growth has been the hallmark of this problem — at least until more recently.

An American Association of Swine Veterinarians task force took into consideration the clinical signs that Canadian and U.S. veterinarians were describing, and decided that the name porcine circovirus associated disease, or PCVAD, best captured the numerous clinical expressions. In both cases the name indicates that PMWS (specifically the wasting portion) is not the only clinical expression; although it is among the more economically damaging aspects.

A heighten awareness of PCVAD has swept across all major swine producing areas in the United States and Canada in recent years. The rapid appearance of PCVAD-related problems is only one interesting aspect of a very complex challenge.

At last March’s AASV annual meeting, a task force was charged with gathering information and addressing members’ concerns. The task started with a special session held during the meeting, where members could describe and discuss what they had observed related to PCVAD. The task force immediately went into action and developed a long list of issues that needed to be addressed. The task force also worked closely with the National Pork Board to educate the industry and disseminate information.

Currently, a number of variable clinical expressions are being described and associated with PCV2 infection. However, because PCV2 exposure in swineherds is commonplace, it’s frequently found in pigs submitted for diagnostic work-up. Thus, veterinarians needed a set of criteria to determine when a disease manifestation was likely associated with PCV2 infection. PMWS is recognized as a major clinical symptom of PCVAD but not the only one. Research has yet to confirm porcine dermatitis nephropathy syndrome, or PDNS, as one aspect of PCVAD. However, it is one of the clinical expressions included at this time.

It is important to know that while individual animals may exhibit clinical signs, the herd does not always experience PCVAD. This fact can contribute to a misdiagnosis.

To address this, the AASV task force adopted the Center for Disease Control’s approach to define “cases” in human medicine where a definitive etiology is not known. The criteria outlined in the sidebar were selected as the basis of a case definition so that all researchers and veterinarians will know what constitutes PCVAD. However, the case definition is considered to be a dynamic document which will be altered as additional information becomes available.

Numerous pork production newsletters and corporate news reports by the corresponding chief executive officers have mentioned PCVAD as a leading cause for economic concern. However, high mortality rates are only one aspect. Rising levels of under-weight market hogs and culled pigs are two more important aspects of lost opportunity. The increased requirements on animal health, labor and housing make up other major costs. One report showed that when increased mortality, decreased average daily gain and declining feed efficiency were calculated for a six-month period, the opportunity lost was $6.60 per pig.

Today, veterinarians are carrying out on-farm trials to find answers for their clients. Researchers are scrambling to test theories about how PVC2 virus can be associated with clinical expressions and mortality that can vary widely. Pharmaceutical and biological companies are striving to produce, obtain approval and supply quality vaccines.

Indeed, vaccines will be a tool to help lessen the devastating losses associated with this disease complex.

Tom Gillespie, DVM, Rensselaer Swine Service, Rensselaer, Ind., is past president of American Association of Swine Veterinarians.

Building a case definition

Defining what makes up porcine circovirus associated disease will be a work in progress. For now, PCVAD can be subclinical or include one or more of the following traits concurrently:

1. Multisystemic disease with weight loss (formerly known as PMWS).

2. High mortality: Doubling of the historical mortality rate without introducing a new known pathogen to the herd.

3. Respiratory signs, including pneumonia.

4. Porcine dermatitis and nephropathy syndrome.

5. Enteric signs, including diarrhea and weight loss.

6. Reproductive disorders, including abortions, stillbirths and fetal mummification. (Diagnosis requires the presence of fetal myocarditis associated with PCV2 antigen in lesions.)

PCVAD is a broad categorization of multisystemic diseases with the following histopathological findings in affected pigs:

1. Depletion of lymphoid cells in lymph tissue of growing pigs.

2. Detection of PCV2 antigen within the lesions.

3. Disseminated granulomatous inflammation in multiple tissues, such as the spleen, thymus, intestine, lymph nodes (sternal, bronchial, inguinal and mesenteric), lung, kidney, liver, tonsil and so forth.

4. Reproductive diagnosis requires the presence of PCV2 antigen in lesions associated with fetal myocarditis.

 

 

 

 



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