Everything You Need To Know About Parvo

Parvoviruses are a species-specific single-stranded, non-enveloped DNA virus.

The canine virus appeared by a mutation in the early 1970s. Subsequent mutations brought about the epidemics of canine parvovirus disease in 1978 – 1979. Since this was a new virus, the pet population did not have immunity to it. Most exposed dogs became very ill and many died.

The virus invades and kills rapidly dividing cells and this biological behavior explains the clinical signs that we see. 

Everything You Need To Know About Parvo

Susceptible cells

Especially susceptible are the cells that line the intestines and the bone marrow.

Destruction of the lining of the gastrointestinal tract causes severe vomiting and bloody diarrhea that is the hallmark of the disease. Bone marrow infections cause low white blood cell counts and compromise the immunity of the infected dog.

It is a double whammy because the intestinal damage allows bacteria to enter the bloodstream and the lack of immunity causes septicemia, which is often fatal.

In the early years, before we had vaccines for this virus, very young puppies had no immunity from their mothers and their developing heart cells were also damaged. This caused parvoviral cardiomyopathy, which was also often fatal.

Everything You Need To Know About Parvo
Canine Parvovirus. Photo Virus World

The infection

There is a 3 – 7 day incubation period from infection to clinical signs in susceptible dogs.

Once the virus establishes itself, it is shed in large numbers in the stool. Dogs with partial immunity may not become as seriously ill but will still shed virus in their stool.

Due to its ability to survive in the environment, and a large number of viruses shed by infected animals, the virus is ubiquitous, found almost everywhere that dogs go.

What does it look like

Many people think that bloody diarrhea, or diarrhea with the distinctive smell of blood (as in that dog smells like it has parvo), is always caused by parvovirus (CPV) infection.  

It is important to know that there are many reasons for this severe gastroenteritis in young dogs. We see a similar clinical picture with parasites, other viral infections, dietary indiscretion, and toxin ingestion.

How is it diagnosed

Many times I hear about a dog being diagnosed with parvovirus infection and it is an incorrect diagnosis. It is tempting to diagnose any dog with bloody diarrhea (very distinctive smell), with parvo. But indeed, parvo is hemorrhagic diarrhea caused by an infection with canine parvovirus. Therefore, the only way to diagnose it accurately is to get a positive fecal test showing virus being shed in the stool … There will be false positives if the dog was recently vaccinated. Most, I mean almost all, vaccines for parvovirus are modified live vaccines in the US. Can’t speak to the rest of the world.

The vaccine is very safe and very effective. In fact, once vaccinated correctly, dogs may have lifetime, or close to it, immunity.

However, any unvaccinated dog with vomiting and diarrhea, fever, anorexia, and a low white blood cell count is treated as a puppy with parvovirus infection until proven otherwise.  

Diagnosis can be confirmed with a positive fecal test for virus antigen. 

The treatment

The truth is, these dogs are very sick and whether they actually are infected with canine parvovirus or not, they will need to receive the same aggressive therapy if they are to survive.

Dogs that are suspected of being infected with CPV are isolated, placed on IV fluids and antibiotics, as well as anti-emetics (to stop vomiting) and GI protectants. Seriously ill dogs will often benefit from IV plasma transfusions. Until the infection is ruled out, strict quarantine needs to be observed and all contact equipment disinfected with a bleach solution.

By the time a diagnosis is made, the disease is often fatal.   


The best bet is prevention. All breeding bitches should be vaccinated so that they can pass some maternal immunity to their puppies. Puppies need to be vaccinated every 3 – 4 weeks, starting at 6-8 weeks of age (if they have maternal immunity) until they are at least 16 weeks old.

Fortunately, immunity is developed rapidly and is relatively long-lived.  

A booster at 1 year and then every 3 years should be adequate to maintain adequate protection. Most vaccine manufacturers will stand behind this vaccine schedule for dogs.

It is rare to see a vaccinated adult dog become ill from CPV infection. 

Vaccine failures

Reasons for vaccine failures includes, improper vaccine handling, vaccinating dogs that are not well enough to respond to the vaccine, poor maternal immunity, or stopping the initial series of vaccines before maternal immunity has waned.

There are some dogs that do not respond well to vaccines and cannot mount an immune response due to a defective immune system, but this is rare. The vaccine must have been stored safely. Since it is a modified live vaccine, they are not adjuvanted. While the virus could theoretically revert to the wild state and cause disease, I’ve never seen it.

There are also some dogs that are prone to adverse reactions to vaccines, but in the case of CPV, every effort should be made to get the initial vaccine series completed.

Titer testing

The goal of vaccination is immunity.

Owners that want to minimize the administration of vaccines while assuring that their dog is protected may opt for a blood test to measure antibody titers. 

This test will measure the level of antibodies to the virus in the blood and help determine if the pet is immune or not. We recommend titers annually. The blood can be sent to most reference labs and a new in house test has been developed for use in the veterinary clinic.

Related articles:
Canine Parvovirus
Canine Viral Infections—Parvo: Cosmo’s Battle with Parvovirus
Parvovirus in Dogs: Tidus’ Recovery From Parvo

Further reading:
Parvovirus: This Can Kill Your Dog in Less Than 72 Hours

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