The disease is caused by a highly contagious virus that is transmitted mostly by dogs orally contacting infected feces. Being a virus, they contain only DNA or RNA, and are not capable of reproducing unless they invade a cell. Once inside the cell they take over and force the cell to produce so many new virus particles that the cell eventually bursts, releasing these new virus particles into the bloodstream and tissues so they can invade other cells. The only thing that can stop this is the immune system. Viruses are the smallest of known living organisms, and can only be seen with special microscopes called scanning electron microscopes, that cost millions of dollars. The parvovirus is extremely small (the Latin word for small is parvo) - just 1 thimble full of stool can contain millions of virus particles. It is easy to see why contamination occurs so readily. Incubation period varies from 5-10 days. As in many viral diseases of the intestinal tract, some dogs can pick up the disease and shed the virus without significant symptoms in themselves.
The majority of dogs presented with parvovirus show signs of fever, lethargy, vomiting, diarrhea, and lack of appetite. In severe cases the diarrhea is very watery and frequently bloody, with a telltale odor. They are very ill, with significant abdominal pain. The virus is so strong that it literally causes the lining of the intestines to slough. It is painful to eat, and with the severe diarrhea and vomiting that is present, they rapidly become dehydrated. They also have a disruption in their electrolytes (sodium, potassium, chlorine) that adds to the weakness. There is a complication that can occur from all the intestinal activity regarding vomiting and diarrhea. It is called intussusceptions, which is literally a telescoping of the intestine into itself. This will cause the intestine to die, resulting in death of the pup. Treatment is surgical; unfortunately, these pups are in no shape for surgery. Luckily we do not encounter this very often, if at all. In the peracute form of this disease the virus attacks the heart and causes rapid death. Fortunately, it is rare to encounter this nowadays.
As with all infectious diseases, minimizing exposure from infected animals is the most effective means of prevention. Since infected dogs shed large amounts of virus in their stool, contamination is always a possibility. The virus is quite resistant in the environment, especially in public areas that are not disinfected. This is a good reason to keep your pup away from these areas until it is older, worm free, and had its full series of dog vaccines.
Since this disease occurs mostly in puppies, worms (internal parasites) and poor nutrition add susceptibility. Puppies should be wormed frequently until they are 3 months old.
Any dog you already have in the household before you exposed it to a parvo dog you recently brought in (whether it died or recovered from the parvo) should be current on its vaccines and should have minimal exposure, if possible, to the contaminated areas. It is rare for an adult dog that is current on its parvo vaccine (yearly boosters) to get parvo.
If you had a dog die of parvo we recommend thorough cleaning with diluted bleach (1:30 with water, or 4 ounces of Clorox in a gallon of water) and waiting 1-2 months before introducing a new dog to the area. Spray the yard as best as possible with a hose and keep new dogs away from the area for 1-2 months. Never put bleach on your dog. Vaccines are highly effective. Ideally, we should vaccinate pups every 2 weeks starting at 6 weeks of age and lasting until 5 months of age. This is not realistic for most people though. Fortunately, parvo vaccines given at 8, 12, and 16 weeks of age are highly effective. Puppies should not be exposed to other dogs or the feces of other dogs until the vaccine series is complete.