The answer is quite simple: a priori, there is no limit.



In order to further explore this subject, some of the available data are discussed below.

In the past, it was thought that recombination between different PRRS virus strains was an uncommon phenomenon. However, examination of full-length genome sequences has demonstrated that:

1. Recombination is very frequent.
2. It occurs all along the genome, in both type 1 and type 2 strains.

In fact, recent studies have demonstrated that several strains in both genotypes are the result of the recombination; in some cases that recombination implies at least three different strains, or even more. Obviously, for recombination to happen, it is necessary that at least two different strains infect the same pig close in time. There are evidences that two or more PRRS virus strains can infect the same individual pig. Therefore, if recombination is much more common than we thought in the past, we cannot rule out the simultaneous coexistence of more than one strain on the same herd/farm, especially in the biggest ones. In fact, several studies have identified from two to eight strains in a same herd over a period from three months to four years. 

Quasispecies. RNA viruses replicate with extremely high mutation rates, causing significant diversity. When this occurs, groups of diverse variants can simultaneously exist; this phenomenon is known as quasispecies. Thus, a quasispecies is a cloud, group or distribution of diverse variant viruses related by a similar mutation or mutations that collectively contribute to the characteristics of the population. In naturally PRRS virus-infected pigs, the virus exists as a quasispecies distribution of related but distinct viral variants. This phenomenon occurs independent of the strain and the host. Immunomodulation and quasispecies generation have been speculated to play a crucial role in PRRS virus infection and its chronic/persistent course.

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