Chu On This!
Where does the Parvo Virus come from? (Part 2 of the series)
By Cindy Yi-Ming Chu, DVM
Remember that this virus has been around since the 1970s, is hard to disinfect, and is shed in extremely large numbers by infected dogs. 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. 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 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. No chances should be taken.
The younger the dog, the less immunologic experience and the more susceptibility to infection there is.
We recommend that puppies be restricted from public outdoor areas
until their vaccination series is completed at age 16 weeks.
There is a period lasting a good week or so during which the puppy has no antibody protection leftover 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 to 7 day incubation period before the puppy seems obviously ill.
The virus kills one of two ways:
- Diarrhea and vomiting lead to extreme fluid loss and dehydration until shock and death result.
- Loss of the intestinal barrier allows bacterial invasion of potentially the entire body. Septic toxins from these bacteria result in death.
Parvo Vaccination Options/Prevention
The biggest problem in protecting a puppy against this infection ironically stems from the natural mechanism of protection that has evolved. As mentioned previously, puppies obtain their immunity from their mother’s first milk, the colostrum, on the first day of life. This milk contains the mother’s antibodies against parvovirus, and until these antibodies wane to ineffective levels they will protect the puppy.
The problem is that they will also inactivate vaccine.
Vaccine is a solution of inactivated virus, either live and weakened (attenuated or modified) or killed. This virus is injected into the puppy. If there is still adequate maternal antibody present, this vaccine virus will be destroyed just as if it were a real infection. There will be a period of about a week when there is not enough maternal antibody to protect the puppy but too much to allow a vaccine to work. (This period is called the window of vulnerability.) After this period, vaccine can be effective.
The next problem is that the age at which vaccine can be effective is different for each individual puppy.
To get around this, we vaccinate puppies in a series, giving a vaccine every 2 to 4 weeks until age 16 weeks. By age 16 weeks, we can be certain that maternal antibodies have waned and vaccine should be able to take. It should be recognized that some individuals, especially those of well-vaccinated mothers, must be vaccinated out to 20 weeks unless a high titer vaccine is used.
Vaccinating Adult Dogs
Classically, parvovirus vaccine has been administered annually to all dogs. Vaccine against canine parvovirus has been included in the distemper combination vaccine (the DHLPPC, “6 in one”)
(Please call us, 951-493-6771 or email us with any questions.)