PRRS is “porcine reproductive and respiratory syndrome”, which pretty much sums up the disease. It’s caused by — you’ll never guess — Porcine reproductive and respiratory syndrome virus (PRRSV), an arterivirus that emerged in 1987. That was the year I left large animal veterinary practice, so I never had a chance to deal with PRRS clinically.
Twenty-three years may not seem like all that long a time, but if you’re an RNA virus that’s a lot of generation times and a whole lot of time for mutations and evolution, and PRRS viruses are an evolutionarily mess. 1 There are North American type PRRSV viruses and European type viruses, there are mysterious clusters of related viruses, there are clusters of related diseases, there are thousands of sequences, and it’s just kind of baffling what’s gone on with the whole schtick.
A new paper2 has tried to sort out part of the mess by analyzing some 8624 North American-type PRRSV sequences, from nearly a dozen countries, and working out evolutionary relationships between them all. 3 (The focus on the North American series — the Type II PRRSV — is because this group seems to be a more common source of disease; although the European strains are far from rare themselves.)
There were a couple of interesting points that parallel some other viruses:
1. Feral vaccines. It’s already known, or at least strongly suspected, that some of the modified-live PRRSV vaccines have started to go feral on a small scale (not nearly as dramatically as the vaccinia virus I mentioned a while ago), and that’s supported by this genetic analysis:
In the vaccine-associated sublineage phylogenies (data not shown), there were a number of well-supported small clusters that might reflect the small-scale transmission of the vaccine viruses in the field … 2
As well as vaccinia, there are other live vaccines that are known to spread into the population. The sort of limited transmission that seems to be showing up here is more typical of this sort of thing than are the vaccinia instances I talked about before.
2. The amazing flying pigs. Even though this is just one of the two major sub-groups of PRRSV and it’s less than 25 years since it emerged, they came up with nine fairly distinct lineages of the virus (see the figure to the right). As you’d expect the lineages speak to the history of the virus — which is to a large extent the history of the pigs that carried the virus.4
This version of the virus probably started out in North America (though how it got there … ?) and then got introduced into other countries on several independent occasions. Two of these introductions were in the late 1980s, shortly after the North American emergence. Aside from that there’s evidence of a bunch of smaller introductions:
… lineage 1 had several Thai sequences clustered with early Canadian sequences … ; lineage 8 contained highly pathogenic Chinese strains and their relatives … ; and lineages 8 and 9 had several Italian isolates which were distributed separately along the phylogeny …, indicating independent introductions of PRRSV from the United States to Italy. 2
They were even able to identify smaller-scale travel patterns, between individual states in the USA:
Iowa plays a central role because its viruses were introduced recurrently to all nine other states (Fig. 5B). The remaining states were not just receiving sites. Their local strains also were transmitted to other states repeatedly, but within a narrower range. … Our phylogeographic analyses reveal, for the first time, an interstate PRRSV traffic network in the United States. … The result also indicates that long-distance spread is a frequent process for PRRSV … 2
This is a reminiscent of the history of the pandemic H1N1 influenza virus, when it was still in swine. (Remember that pandemic H1N1 is genetically a mixture of a North American swine influenza strain and a Eurasian strain.) There’s a large national and global traffic in pigs, and even though most countries are reasonably careful in the way they handle incoming pigs it’s not a guarantee against virus introduction. I’m not singling out pigs, either — other kinds of livestock also are global travellers, and obviously so are humans. But it’s a reminder that it isn’t just humans and their viruses that can quickly travel and spread around the globe.
- More correctly, our understanding of their evolution is a mess. The viruses are doing just fine.[↩]
- Shi, M., Lam, T., Hon, C., Murtaugh, M., Davies, P., Hui, R., Li, J., Wong, L., Yip, C., Jiang, J., & Leung, F. (2010). Phylogeny-Based Evolutionary, Demographical, and Geographical Dissection of North American Type 2 Porcine Reproductive and Respiratory Syndrome Viruses Journal of Virology, 84 (17), 8700-8711 DOI: 10.1128/JVI.02551-09[↩][↩][↩][↩]
- That’s a lot of viruses, but the sampling is heavily biased to a limited number of places, especially the USA [and especially a few regions within the USA] so it’s probably an underestimate, and maybe a severe underestimate, of the global diversity.
- Or of the pig’s fluids. I think that especially in the early days of the emergence, the virus was spread by the boar semen used for artificial insemination.[↩]