Health Topics

Parvo Virus

(This article was originally published in Pets Magazine - Dog Care Guide and is reproduced here with the permission of the publisher.)

What is Parvo Virus?

Canine Parvovirus is an infection caused by a virus related to the feline panleukopenia (distemper) virus. It is not the same as cat distemper, and a cat with distemper will not infect a dog. The viruses are merely close relatives.

When it first showed up in the late 1980's, parvovirus was a completely new disease, not merely a previously unrecognized one, that very quickly became a worldwide problem. Virologists have speculated on the origins of the virus and believe that it is a mutant of the cat distemper virus. Whatever the origin, the virus rapidly adapted itself to dogs and caused a worldwide epidemic.


The clinical signs accompanying this disease vary form undetectable to severe and rapidly fatal. Dogs of all ages can be infected, but in general the younger the animal the more severe the disease. The dog may be lethargic and inappetent the evening before obvious signs appear. The dog then runs a temperature, is depressed, has vomiting and diarrhea and is dehydrated. The diarrhea may be very watery, be tinged with blood, or very bloody. The smell is very characteristic due to the bleeding from the bowel.

The virus can also attack the heart muscle, causing myocarditis, particularly in young puppies. This usually results in sudden death, often just at a time when we think we have the diarrhea under control and the pup seems to be responding well.

Current research suggests this fatal myocarditis results only when pups are infected within a very few days of birth. Since maternally derived antibody protects pups from parvovirus infection and since most adults have been vaccinated or previously infected, almost all puppies nowadays have at least some immunity during the very early critical stage.


The terrible diarrhea which occurs in this disease is due to the virus attacking the lining of the intestines. This causes cells to die, slough off and bleed. The condition is called viral enteritis. Unfortunately, treatment for viral enteritis is supportive only; there are no effective antiviral agents. The primary goal of therapy is to replace the fluid and electrolyte losses since dehydration and electrolyte imbalances themselves may be fatal. This is achieved with IV fluids. Broad spectrum antibiotics are used to combat secondary bacterial infection. Good nursing care is absolutely essential.


Parvovirus is shed in the stools, is extremely hardy and can survive heat and sub-zero temperatures for long periods of time. This means the virus may remain in the environment long after the feces is removed. Your shoes can carry it home to your dog.

What else is happening?

A 10 year study at the Western College of Veterinary Medicine in Saskatoon uncovered several interesting factors:

  • Some breeds are definitely more susceptible and at a higher risk than others. These are the Rottweiler, Doberman, German Shepherd and Pit Bull Terrier. Breeds at least risk were the Toy Poodle and Cocker Spaniel.
  • There is a seasonal component. High risk months are August and September. Low risk months are November, December and January.
  • Parvo continues to be a problem primarily of young dogs six weeks to six months of age. At this age the turnover rate of the cells lining the gut is very high. Since we know that parvovirus multiplies in active cells, infection during this time of increased gut cell turnover may result in a more extensive break in the gut mucosal barrier. This leads to more severe dehydration, protein loss, and systems absorption of toxin and bacteria into the victims body. Any concurrent injections, parasites or other viruses, will clearly exacerbate the condition.

How can vaccinated dogs get the disease?

We know there are dogs that have been vaccinated but still come down with parvovirus infection. This can occur either due to inadequate vaccination, i.e., not enough boosters at the right time or due to exposure during the "window of susceptibility". This window is inevitable in pups and can be as wide as 30 days, during which the pup may be susceptible in spite of the vaccination. This occurs because of the following facts:

  • Puppies receive maternal antibodies from the mother against parvovirus.
  • The better vaccinated the mother - the more antibodies she confers. The antibody concentration in the pup is inversely proportional to the litter size.
  • Vaccination of the pup while maternal antibody is present will not result in an immune response - so will not protect the pup from exposure.
  • Unfortunately, although the maternal antibodies are sufficient in number to interfere with response to vaccination - they are too low to protect from infection. Therein lies the window of susceptibility - there will always be a period from one to four weeks when during which the pup is at risk but is not yet immunizable. This is why we recommend that young pups should be kept away from public parks until vaccination is complete.

Vaccination Protocol

Because parvovirus is so stable in the environment it can survive up to seven months, epidemiologists estimate that 90 percent or more of the dog population would have to be successfully immunized to break the chain of infection. This is unlikely to happen so we can anticipate more and more parvo cases.

Because of the persistence of maternal antibody, we now recommend the following protocol:

  • Start at 6 weeks - boost at 8, 12, & 16 weeks and then annually.
  • If in a high risk area or dealing with high risk breeds that travel, vaccinate every six months.
  • Booster vaccination is a good idea prior to boarding your pet.