How parvovirus infection happens

Whether or not infection happens depends on the interaction of three factors: host vitality, including immune experience/vaccination status; virulence of the virus, including how many viral particles the host is exposed to; and environmental factors. Obviously these three aspects interplay somewhat, as a stressful environment will reduce host vitality, a dry environment will reduce the number of viral particles, etc.{{more}}

Where does the virus come from?

Remember that this virus has been around since the 1970s, is hard to disinfect, and is shed in extremely large numbers by infected dogs. This means that there is virus everywhere: on every carpet, on every floor, in every yard and park. Virus is shed in the stool for the first two weeks or less after the initial infection, but only a tiny portion of infected stool – which could be months old depending on the environmental temperature and humidity – is needed to infect a non-immune dog. Some dogs become what is called subclinically infected, which means they do not appear particularly sick. These animals tend not to be confined, since no one knows they are infected; thus they can spread virus around a large area, depending on where they leave their droppings.

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 1970s and early 1980s when the virus was new, all dogs, young and old, were susceptible; but now that the virus is 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 (protection). 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, as infection in dogs older than 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 the vaccine generally comes combined to protect against other illnesses like leptospirosis, canine distemper etc. No chances should be taken. The younger the dog, the less immunologic experience and the more susceptibility to infection there is.

Natural protection?

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 specific type of milk called colostrum for the first day or two after giving birth. 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 four months of the puppy’s life.

How much colostrum an individual puppy gets depends on its birth order and how strong it nurses; not all puppies get the same amount of antibodies. Every nine days the antibody level possessed by the puppies drops by half. When the antibody level drops to a certain level, they no longer have enough antibodies to protect them and if they are exposed to a large enough number of viral particles, they will get infected.

I recommend that puppies be restricted from public outdoor areas until their vaccination series is completed, at about 14 weeks of age.

There is a period lasting a good 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.

To be continued next week…

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