The Definition of Porcine Circovirus Diseases (PCVD)

The definition of Porcine Circovirus Diseases is still in progress. Scientists from a European consortium on PCVD research (European Project No.: 513928 from the VI Framework Programme, Priority 5. Food Quality and Safety) discussed and agreed on a PMWS herd case definition. The agreed definition, can be found at the web site of the project (www.pcvd.org). This case definition on herd level could change if new knowledge on the etiology, epidemiology and pathogenesis of PMWS becomes available.

PMWS herd case definition

Criterion 1. Clinical appearance on herd level

The occurrence of PMWS is characterized by an excessive increase in mortality and wasting post weaning compared with the historical level in the herd. There are two options for recognizing this increase, of which the first point (1a) should be used whenever possible:

1a: If the mortality has been recorded in the herd, then the increase in mortality may be recognized in either of two ways:

• Current mortality  mean of historical levels in previous periods + 1.66 x standard deviation (SD)
• Statistical testing of whether or not the mortality in the current period is higher than in the previous periods by the chi-square test

(In this context, mortality is defined as the prevalence of dead pigs within a specific period of time. The current time period is typically one or two months. The historical reference period should be at least three months).

1b: If there are no records of the mortality in the herd, the following approach may be used: an increase in mortality exceeding the national or regional level by 50% is considered indicative of PMWS.

Criterion 2. Pathological and histopathological diagnosis of PMWS

Necropsy should be performed on at least five pigs per herd. A herd is considered positive for PMWS when the pathological and histopathological findings, indicative for PMWS, are all present at the same time in at least one of the autopsied pigs. The pathological and histopathological findings are those described above.

Maybe one of the surprising points would be the necropsy of 5 pigs; this value is based on epidemiological features. If we consider the possibility of detecting at least one affected pig with the condition that is the cause of problems in 50% of the total diseased animals with 95% confidence, 5 pigs are needed (independently of the number of diseased pigs).

As it can be seen, the presented PMWS herd case definition tries to establish objective (statistical) criteria to delimit what can be considered or not as a disease concern into the affected farm.

The given herd case definition is especially applicable to outbreaks of disease, but much more difficult to apply on enzootic scenarios. Therefore, although this latter point has not been appropriately tackled, it could be inferred that PMWS can also be considered a herd problem when a clinical process characterized by a continuous significant incidence of postweaning death/wasted pigs complying the individual diagnosis of PMWS in a group of 5 pigs from the farm (3 individual criteria mentioned before). In this situation, point 1b could be applied.

A piglet affected by PCVD