To identify hitherto-unknown rodent-associated herpesviruses, we captured M. musculus, R. norvegicus, and 14 other rodent species in several locations in Germany, the United Kingdom, and Thailand. … we detected 17 novel betaherpesviruses and 21 novel gammaherpesviruses but no alphaherpesvirus.  … it is also possible that rodent alphaherpesviruses either never developed or became extinct earlier during herpesvirus evolution. 

–Ehlers B, Kuchler J, Yasmum N, Dural G, Voigt S, Schmidt-Chanasit J, Jakel T, Matuschka FR, Richter D, Essbauer S et al. (2007) Identification of novel rodent herpesviruses, including the first gammaherpesvirus of Mus musculus. J Virol 81:8091–8100.  doi:10.1128/JVI.00255-07

(My emphasis)

RodentiaComment: Herpesviruses are an ancient, ubiquitous family of viruses that as a group infect essentially every species of mammal, and probably virtually every species of bird and reptile as well.  

In mammals, herpesviruses are divided into three families (alpha, beta, and gamma-herpesviruses) that are distinct in their biological activities.  But the ancestral herpesviruses seem to have been alpha-herpesviruses; herpesviruses of birds and reptiles are all alpha. Presumably the beta- and gamma families arose from the ancestral alphas, some time after mammals diverged from their own, reptilian, ancestors.  

It’s puzzling, then, that mice and rats — which are, as this quote shows, infected with dozens of different herpesviruses — don’t appear to have any natural alpha-herpesvirus infections.  Why?  I have no idea.  Could there be some intrinsic incompatibility between the alpha-herpesvirus lifestyle and rodents?  Or could there have been some strange bottleneck that caused rodent herpesviruses to go extinct tens of millions of years ago?  

Making things even more puzzling is the fact that rabbits apparently do have at least one alpha-herpesvirus.  One was just isolated from domestic rabbits in Alaska (of all places).1  We know almost nothing about this virus yet, so it’s possible that rabbits aren’t the natural host; but if they are, then the rodent lineage presumably lost their complement of  alpha-herpesviruses somewhere after the rabbit lineage branched off.2

  1. Jin L, Lohr CV, Vanarsdall AL, Baker RJ, Moerdyk-Schauwecker M, Levine C, Gerlach RF, Cohen SA, Alvarado DE, Rohrmann GF (2008) Characterization of a novel alphaherpesvirus associated with fatal infections of domestic rabbits. Virology 378:13–20.[]
  2. Horner DS, Lefkimmiatis K, Reyes A, Gissi C, Saccone C, Pesole G (2007) Phylogenetic analyses of complete mitochondrial genome sequences suggest a basal divergence of the enigmatic rodent Anomalurus. BMC Evol Biol 7:16.[]