Host specificity II: host switching by viruses

Morbilliviruses

Biology

In general morbilliviruses have a surprisingly low mutation rate (surprising because RNA copying is messy, and because single-stranded viruses don't provide a "reference copy" for fixing mistakes); they don't tend to escape by genetic change in some of the ways that (e.g.) influenza and AIDS do.

Since they typically lead quickly to either recovery and death, morbilliviruses often need large populations to sustain themselves through a continuous flow of new susceptible hosts. Otherwise they can "burn out" local populations (measles needs a host population of about 250,000 to persist over long periods of time). Morbilliviruses do not tend to have biochemical mechanisms for persisting over a long period in a particular host individual, although there are possible exceptions.

Since morbilliviruses "live fast and die young" both within hosts and (of necessity) within host populations, they are always perched on the brink of extinction. Interestingly, morbilliviruses as a whole have a wide host range, and some viruses within the family have wide host ranges. Is this a "solution" to the ecological problem that morbilliviruses face?

Examples

• Measles virus (MV) is a fairly host-specific virus, which infects humans and closely related primates (although population structure within wild primates doesn't allow persistence: critical community size is 300K for humans). Inexperienced human populations experience high mortality, which could suggest some population specificity, but this probably has more to do with host resistance than with parasite variation. (Also, host mortality is not necessarily correlated with parasite fitness: we'll talk about this at much greater length when we get to discussing virulence.)
• Rinderpest (RPV): infects bovids, both domestic and wild. Native to central Asia (which is a center of endemism for many different taxa, including diseases), it did sometimes spread into Europe but didn't cross the Sahara until the 19th century. Carried with livestock of an Italian expeditionary force, it spread through sub-Saharan Africa in 10 years, with huge ecological, social, and historical consequences (which we may discuss more later, when we get to "community and ecosystem consequences of parasites"). For our purposes, the interesting thing is that rinderpest affects both domestic bovids and a wide spectrum of wild species.
• Peste des petits ruminants (PPRV/"goat plague"): original source in West Africa, went the other way (now endemic in mideast and southern Asia). similar to rinderpest, affects goats and sheep and a range of wild hosts
• Phocine & cetacean distemper viruses: there have been recent discovery of a whole batch of these viruses in causing marine mammal die-offs of various kinds: these epidemics are really hard to observe, but bodies do wash up (North Sea phocine distemper virus epidemic in 1988 is probably the most carefully recorded example), and reverse transcriptase PCR (RT-PCR) techniques are helping a lot because one can extract virus from decomposing/ed samples (and sequence the virus, if there's enough left, to find out phylogenetic relationships)
• Canine distemper virus: domestic dogs (obviously), also affects wild dogs, hyaenas, and almost every other carnivore you can think of (polar bears, raccoons, pandas, seals, ferrets, lions, etc. etc. etc.). Responsible for many outbreaks in small and endangered wildlife populations: e.g. blackfooted ferrets, lions of the Ngorongo crater (30% (?) mortality in 1994). Often spread from domestic dogs (which often have high, stable population densities) but also (?) in some cases apparently circulating/persistent within wild populations (polar bears, crabeater seals).

Host shifting

• the most "macro" level, the distinction between different large branches of the phylogeny (measles, rinderpest/PPRV, CDV/PDV)
  
• at an intermediate level, the (apparently) more recent origin of a variety of different strains of marine mammal viruses, probably from CDV (these viruses may also be traded between different marine mammal populations, or persistent within some particularly large populations). Molecular data
  
  (RPV=rinderpest; PDV=phocine distemper virus; DMV=dolphin morbillivirus; CDV=canine distemper virus; PMV=porpoise morbillivirus; PPRV=peste des petits ruminants; MSMV=monk seal morbillivirus (WA, West Africa, and G, greece); MV-EDM=measles virus, Edmonton (vaccine) strain)
• canine distemper virus seems to have very few limits, although there are recognizable differences between strains; however, these strains are mostly geographical rather than grouped by species, e.g.:
  

Evolutionary ideas