Dog and cat (William Cheselden, 1733)Emerging diseases don’t arise from nothing; they are established diseases that move away from their previous niche. Usually when a pathogen tries to move out of its comfort zone, it drives down a dead end; it might infect a few individuals, but it doesn’t spread effectively, and it fizzles out.

Of course, we don’t usually hear about the fizzles; we hear about the successes (HIV) and the partial successes (SARS, avian influenza, Sin Nombre virus, and many more). Even so, many people don’t seem to know much about one of the most spectacularly successful emerging diseases of the 20th century.  

In 1978, dogs around the world suddenly began to die, developing a bloody diarrhea and rapidly (often overnight) progressing to fatal dehydration. This turned out to be due to canine parvovirus 2 (CPV2); dogs were already known to have a parvovirus (CPV1) but this one was new, and lethal. It exploded around the world within a few months, infecting millions and killing thousands of dogs.

The good news was that this virus turns out to be effectively controlled by the immune system. Its initial explosion was through a naïve ecosystem, dogs that had never been exposed to anything like this virus and that had no immunity to it. A single exposure to the virus gave long-lasting immunity, and this immunity could be passed to pups by nursing. Along with the rapid development of a new vaccine, the worldwide dog population rapidly become, by and large, resistant to CPV2. The virus is still with us, but because of herd immunity usually isn’t a cause of epidemics any more. 1

Canine parvovirusCanine parvoviruses is one of the very few, if not the only, example of a pandemic species-switching virus in the modern age of molecular biology. We’ve been able to watch the virus mutating, spreading, and responding to the environment almost from the outset. (We don’t know exactly when the virus entered the dog population; it must have been before 1978, perhaps well before, and circulated in a small pool of animals for a while before its sudden eruption. These initial stages would be very interesting to analyze as well, to see what, if anything, allowed the virus to switch from cycling through a small population to infecting everything.)

The actual origin of CPV2 is a little controversial. The original suggestion was that it arose from a feline parvovirus, feline panleukopaenia virus (FPV). In fact, it was believed for a while that it might have arisen from contamination of canine vaccines, by the feline virus — which would help explain the almost simultaneous appearance worldwide. But now it’s agreed that CPV2 could have arisen from a parvovirus of some other carnivore — mink, or perhaps foxes or raccoons — instead of from FPV, and almost certainly wasn’t from a vaccine contaminant. (FPV still looks like the most likely source, though.)

The actual mutations in CPV2 that allowed it to spread into dogs have been pinpointed — there are changes that allow the virus to infect canine cells, which FPV can’t do. Not surprisingly, though, CPV2 has continued to mutate rapidly since 1978. 2 It’s moved into a new ecosystem, and it’s adapting to that ecosystem in a hurry. New variants of CPV2 are now showing the ability to infect cats (again?); something to watch for.

Viruses that successfully switch hosts are rare, but potentially catastrophic, events. The lessons of CPV might help us understand how, say, influenza viruses might shift from chickens to humans (and this hasn’t gone unnoticed;3 I’m not the first to make the connection by any means). CPV is a unique resource, because we have examples of so many of the steps in the evolution of a virus that’s adapting to a new host.

  1. An excellent review of the disease itself is in: Carmichael LE (2005) An annotated historical account of canine parvovirus. J Vet Med B Infect Dis Vet Public Health 52:303-311. However, I think he’s overconfident, or outdated by now, in his assertion that the virus definitely didn’t arise from feline panleukopaenia virus.[]
  2. Shackelton LA, Parrish CR, Truyen U, Holmes EC (2005) High rate of viral evolution associated with the emergence of carnivore parvovirus. Proc Natl Acad Sci U S A 102:379-384.
    J Gen Virol 89 (2008), 2280-2289; DOI 10.1099/vir.0.2008/002055-0. Phylogenetic analysis reveals the emergence, evolution and dispersal of carnivore parvoviruses. Karin Hoelzer, Laura A. Shackelton, Colin R. Parrish and Edward C. Holmes[]
  3. Parrish CR, Kawaoka Y (2005) The origins of new pandemic viruses: the acquisition of new host ranges by canine parvovirus and influenza A viruses. Annu Rev Microbiol 59:553-586.[]