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How Parvo Infection Happens
Whether or not infection happens depends on the interaction of three factors: host immune status; strength/strain of the virus, and environmental factors.
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 sub clinically 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.
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 off-spring (only of she has been properly vaccinated). These antibodies are protective until they wear off sometime in the first 2- 4 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 levels 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.
We recommend that puppies be restricted from public outdoor areas
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.
Upon entering the body, the virus seeks out the nearest rapidly dividing group of cells. The lymph nodes in the throat fit the bill and the virus sets up here first and replicates to large numbers. After a couple of days, so much virus has been produced that significant amounts of it have been released into the bloodstream. Over the next 3 to 4 days, the virus seeks new organs containing the rapidly dividing cells it needs: the bone marrow and the delicate intestinal cells.
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 GI tract is where the heaviest damage occurs. The normal intestine possesses little finger-like protrusions called villi. Having these tiny fingers greatly increases the surface area available for the absorption of fluid and nutrients. To make the surface area available for absorption greater still, the villi possess microvilli, which are microscopic protrusions. The cells of the villi are relatively short-lived and are readily replaced by new cells. The source of the new cells is the rapidly dividing area at the foot of the villi called the crypts of Lieberkuhn. Parvovirus strikes right at the crypt. Without new cells coming from the crypt, the villus becomes blunted and unable to absorb nutrients. Diarrhea in large quantities results, not to mention nausea. The barrier separating the digestive bacteria from the blood stream breaks down. The diarrhea becomes bloody and bacteria can enter the body, causing widespread infection (remember that that virus has also simultaneously destroyed the bone marrow's ability to respond immunologically).
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 antibodies 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.
Disinfecting After Parvo
Canine parvovirus is especially hardy in the environment. It is readily carried on shoes or clothing to new areas (which accounts for its rapid worldwide spread shortly after its original appearance). It is able to survive freezing temperatures in the ground during winter, plus many household disinfectants are not capable of killing it indoors.Given that this is such a tough virus to destroy, many people want to know exactly what they must do to disinfect an area that has contained an infected dog or how long they must wait before safely introducing a new dog to a previously contaminated area.
Here is what we know about how contaminated an environment is likely to be:
A parvoviral infection can be picked up ANYWHERE although it is easier to pick up an infection in an area where an infected dog has been simply because of the larger amounts of virus in a contaminated area.
A typical/average infectious dose for an unvaccinated dog is 1000 viral particles. For some dogs far less is needed. For other dogs, far more is needed. An infected dog sheds 35 million viral particles (35,000 TIMES the typical infectious dose) per OUNCE of stool.
Here's what we know about disinfection: