Parvovirus Primer

Dr. Susan Wilkinson, DVM

Generally, there are 2 recognized strains of parvovirus: CPV-1 & CPV-2. CPV-1 was first isolated in dogs in 1970. Then in 1978, a new strain CPV-2 swept through many parts of the world, causing extensive losses. CPV-2 is antigenically related to feline panleukopenia virus (FPV) and there has been some conjecture as to whether the CPV-2 strain is a variant of FPV that mutated from the use of modified live FPV vaccines.

Infected dogs shed the virus in their feces, and this is the usual vector for infection of a susceptible dog -- sniffing around infected feces. Parvovirus is a very hardy organism with the ability to survive a long time in the environment. So even though the feces that shed the virus are long gone, the virus survives and is able to infect susceptible dogs. The virions (infectious virus particles) are inhaled (or ingested), travel to the tonsils & local lymph nodes, then to the lymphoid organs (such as the spleen) where replication takes place, then on to the intestine. Once here is when you see the clinical signs -- the virions attach to the crypts of the intestinal villi (wavy finger-like projections that increase the absorptive surface area of the intestine). It takes approximately 3-5 days to reach this stage. As the absorptive surfaces of the small intestine are damaged, clinical signs include sudden onset apathy, vomiting, and the characteristic bloody diarrhea (with the FOUL smell), often with excessive mucus. The virus is shed in the feces from days 3-12 post infection, with peak shedding occurring at the onset of clinical signs -- a carrier state has not been demonstrated.

Effective vaccines are available (and ineffective ones too! but that's a whole 'nother story), but parvovirus has a nasty little paradox that makes it tough to ensure that puppies are effectively covered. As most people know, puppies receive the vast majority of their initial immunity via the colostrum (mom's first milk). There is only a short period of time (6-40 hrs.) when these vital immunoglobulins (Ig's) can be absorbed from the puppies' intestinal tract, so early ingestion of high quality colostrum is vital. The Ig's absorbed give the puppies a protective immunity to most of the diseases that the mom has been exposed to. Over time, however, this immunity wanes, and the pup must develop its own immune response. We help this process along by a series of vaccinations.

Here's where the parvo paradox rears its ugly head -- for as the maternal parvo immunity wanes, there is still enough maternal immunity present that it interferes with the vaccination response, yet at the same time, the maternal antibodies left do not provide a protective titre against the disease (this paradox is found to a lesser extent with other virues’). Thus the puppies are susceptible to infection. We try to overcome this by administering a series of vaccines. During this series the goal is to get 2 doses into the pup so that they mount first an initial (priming) immune response, and then with a second dose of vaccine, they mount what is know as an anamnestic response (they actually form protective antibodies). The problem is, you don't know when the maternal antibodies are low enough to allow the initial response to take place, thus the series of 3 to 4 doses of vaccine. There have been some breeders trying to overcome this by administering multi booster shots to their bitches. Yes, this does give the pups a longer protective maternal immunity, but then it just makes them susceptible later, often after they've completed their set of vaccinations (and their owners are happily confident that their pup is protected, when in fact it's not at all).

Vaccine manufacturers are well aware of this paradox and have been working to produce a vaccine that is more effective in overcoming maternal antibodies -- some are available on the market now, and initial trials have been quite favourable.

Now then, why do a lot of puppies get their first vaccines at 6 weeks you ask -- isn't this just a waste of money? Well yes & no. For those pups with a nice strong maternal immunity, then yes that initial dose of vaccine is likely ineffective. But for those pups that maybe didn't get enough colostrum, or maybe the colostrum was not as high in Ig's, or maybe they just didn't absorb it well -- then that initial vaccination may well be just in time for the falling maternal immunity and provide them with the priming dose they need.

Dr. Susan Wilkinson, DVM

November, 2003