How Infection Happens
PARVOVIRUS: HOW DOGS GET INFECTED AND WHAT HAPPENS TO THEM
Whether or not infection happens depends on the interaction of three things: host vitality (including immune experience/vaccination status), virulence of the virus (including virus strain and how many viral particles the host is exposed to), and environmental factors (other stresses and/or concurrent conditions). Obviously these three aspects interplay somewhat (a stressful environment will reduce host vitality, a dry environment will reduce the number of viral particles etc.)
WHY ONLY PUPPIES?
The most important factors in whether parvovirus infection occurs seem to be the experience the dog’s immune system has had with the virus plus the number of viral particles the host is exposed to. In the 1970’s and early 1980’s when the virus was new, all dogs young and old were susceptible but now, because the virus is present everywhere, all dogs, even the unvaccinated ones, have at least some immunological experience with this virus. Any exposure no matter how small is likely to generate some antibodies. Also, vaccination is a widespread process nowadays and it is likely that a dog has had at least one vaccine at some point. Will these antibodies be enough for protection? In general, the answer seems to be yes, depending on the dog's age at vaccination and nature of the exposure; infection in dogs over age one year is somewhat unusual. It is important to realize, however, that this observation should not be taken to mean that adult dogs should not continue their vaccinations. Even though infection is somewhat unusual in adult dogs, adult dogs should still continue their vaccinations as this is a life threatening disease for which treatment is expensive and no chances should be taken.
The younger the dog, the less immunologic experience and the more susceptibility to infection.
When puppies are born, they are completely unable to make antibodies against any infectious invader. They would be totally unprotected except that nature has created a system to protect them. Their mother secretes a special milk for the first day or two after giving birth. This milk is called "colostrum." It contains all the antibodies that the mother dog has circulating in her own body and in this way, she gives her own immune experience to her offspring. These antibodies are protective until they wear off sometime in the first 4 months of the puppy’s life.
How much colostrum an individual puppy gets depends on its birth order and how strong a nurser it is; not all puppies get the same amount of antibodies. Every nine days the antibody levels possessed by the puppies drops by half. When the antibody level drops to a certain level, they no longer have enough antibody to protect them and if they are exposed to a large enough number of viral particles, they will get infected. The age at which maternal antibodies disappear is different for each individual puppy and is not predictable.
We recommend that puppies be restricted from public outdoor areas until their vaccination series is completed at age 16 weeks.
There is a period of a week or so during which the puppy has no antibody protection left over from its mother but still is not yet competent to respond to vaccination. This window is where even the most well cared for puppies get infected.
The virus enters the body through the mouth as the puppy cleans itself or eats food off the ground or floor. A minuscule amount of infected stool is all it takes.
There is a 3-7 day incubation period before the puppy seems obviously ill.
Within the bone marrow, the virus is responsible for destruction of young cells of the immune system. By killing these cells, it knocks out the body's best defense and ensures itself a reign of terror in the GI tract where its most devastating effects occur. All parvoviral infections are characterized by a drop in white blood cell count due to the bone marrow infection. Seeing this on a blood test may help "clinch" the diagnosis of parvoviral infection. Also, a veterinarian may choose to monitor white blood cell count or even attempt to artificially raise the white blood cell count in an infected puppy through treatment.
The virus kills one of two ways:
HOW IS SURVIVAL POSSIBLE?
Even parvovirus cannot disrupt the entire immune system. Plus, every day that goes by allows more antibody to be produced. This antibody can bind and inactivate the virus. Whether survival is possible amounts to a race between the damaged immune system trying to recover and respond versus the fluid loss and bacterial invasion.
Next in the series: Treatment
Page last updated: 10/20/2017