Risk and/or triggering factors of porcine circovirus

Porcine circovirosis (PC) is probably one of the best examples of a multifactorial disease in the swine species. Although there is no longer any doubt that porcine circovirus type 2 (PCV2) is the vital factor for the disease to occur, we also know that this virus is ubiquitous and it is extremely difficult to find a farm that is completely free from its infection. Therefore, whether we have a very significant impact of the disease or simply a sub-clinical infection does not depend essentially on the viral agent (probably only in part, according to the PCV2 genotype that may be infected), but rather on those other factors such as risk, triggering or even worsening factors.

Initially, the most crucial factor cited was management. Nevertheless, this is a a very wide and unspecific concept. The origin of this is found in Brittany (France). Between 1995 and 1997, at the beginning of an epizootic called “maladie de l’amagrissement du porcelet” (MAP) (which we now call PC), it was observed that the farms that were most affected and that had the most losses, generally presented obvious deviations from what we would consider to be a suitable management of the animals and facilities. This is why Dr. François Madec (AFSSA, France) made a list of management practices with the aim of improving the anomalous situation observed. These practices were clearly orientated towards the French type of farm (mainly, closed cycles of 100 to 400 sows), and indeed they had a positive effect on MAP when they were suitably applied. In other words, good management reduces the possibilities of suffering PC or at least tends to minimize the effects of the disease. However, it is important to point out that in the majority of cases it is not possible to recuperate the production values from before the clinical process just with these practices. Therefore, despite being an absolutely fundamental point for the correct functioning of any pig farm, the improvement of management practices does not per se constitute the crucial element for the control of PCV. However, it is true to say that controlling PC in a situation of inadequate management can be virtually impossible.

Concomitant diseases constitute a second fundamental point when speaking about PC risk factors. Both from the epidemiological and experimental points of view, it has been shown that co-infections of PCV2 with porcine reproductive and respiratory syndrome virus (PRRSV), porcine parvovirus (PPV) and Mycoplasma hyopneumoniae can lead to serious losses associated to PC on the corresponding farm. This is why, with the lack of a vaccine against PCV2, it has been systematically recommended that the most important way to control PC is through the control of concurrent diseases. The same thing happens here as occurs with management: controlling the co-infection does not necessarily lead to the disappearance of PC, but it does reduce its impact. This is also a reflection of the pathological complexity that our farms suffer nowadays, which makes it difficult to decide “what causes what” in each case. Specifically, PRRSV has been regarded as the most important pathogen in the development of PC, and the automatic question when the disease occured was always whether PRRSV was also present. However, we should also ask ourselves the same question but the other way round: in the case of suffering from a clinical infection on account of PRRSV, to what point is the observed symptomatology not shared also by the co-existence of PC? It has been demonstrated retrospectively that some processes of respiratory disease in transition and early fattening which occurred at the beginning of the 1990s and were then attributed to PRRSV, were in fact concomitantly typical cases of PC. Therefore, it is clear that the clinical distinction between two diseases such as PRRS and PC is extremely difficult and it is easy to confuse one with the other if the adequate diagnostical tools are not applied.

Porcine genetics is also considered to be a key factor in the susceptibility and/or resistance to PC. Despite not being taken seriously before 2002, there was a growing amount of evidence that suggested that some genetic lines or concrete families were more susceptible to the disease. Currently, this is a clearly demonstrated fact, but the mechanism of resistance or susceptibility and the genes involved are as yet unknown. In fact, the change of genetic line of the male pig was widely used in Spain, mainly with success, with the aim of reducing the impact of the disease a few years ago.

Logically, this article could go into much more detail simply by commenting on the concrete risk factors associated to the appearance of PC (table 1). However, it should be emphasized that the determination of these factors is specifically based on the epidemiologic studies of the case-control type, which also implicates that the results obtained in one study may be different or even contradictory to those obtained in another. Therefore, although the scientific-technical world agrees on the three mentioned generic groups of triggering factors (management practices, concomitant diseases and genetics), the concretion of the three can be a difficult task. This is especially clear when we encounter a situation of PC on a specific farm and it is necessary to eliminate or attempt to eliminate the main triggering factor of the disease. How can this concrete risk factor be determined? What impact will its elimination have? How easy will it be to eliminate? (i.e. having the PRRSV infection as the “apparent” principal risk factor does not mean we can eliminate it easily – “apparent” because nobody can be sure that it is the principal or only risk factor). This is why the arrival of vaccinal products of contrasting success against PCV2 has for the most part caused PC to be forgotten as a multifactorial disease.

Table 1. Summary of some of the most significant concrete risk/protection factors in relation to PC. It is important to point out that this table represents a compilation of data from different studies which does not mean that each and every one of those studies obtained the same results; in some cases, these results were contradictory and some of them could be considered as having spurious effects or simply statistically significative relations but with an apparent absence of biological significance.