What is Parvovirus?

By Dr. Zachary Glantz

An estimated 900 dogs are diagnosed with parvovirus every day in the United States, of which the vast majority are unvaccinated puppies. Despite such prevalence, a recent survey found only 44% of dog owners knew about parvovirus.

Canine Parvovirus (CPV) is a small, non-enveloped DNA virus that attacks rapidly dividing cells to replicate (gut cells, lymphatic tissues, and bone marrow are the most common). There are two types with a few subtypes. In the United States, CPV-2 is most common, with CPV-2b and CPV-2c as the most common subtypes in North America. Both subtypes have similar clinical symptoms and management.

CPV is shed from sick dogs, but many dogs will shed for up to a few weeks after recovering from the virus. It's extremely stable in the environment, making avoidance and disinfection challenging, since CPV can persist for months or even years under the right environmental conditions. Only a few chemicals can inactivate CPV, with bleach and accelerated hydrogen peroxide being most consistently effective. 

Dogs most often encounter parvovirus directly from the environment, or through fomites (like owners carrying it inside on shoes). Incubation is usually a few days to 2 weeks, and many dogs will rapidly decline, sometimes becoming unstable within 24-48 hours of the onset of the first clinical signs.

What Clinical Signs Does Parvovirus Cause?

Common clinical signs in dogs include diarrhea, poor appetite, vomiting, lethargy, and dehydration. Unfortunately, some puppies can become sick so quickly that the only symptom they have is poor appetite and lethargy. It’s not uncommon to diagnose parvovirus prior to diarrhea starting!

Since treatment early on improves the prognosis, any puppy with poor appetite and lethargy should be evaluated urgently, especially if they have not been thoroughly vaccinated, or have not completed their puppy vaccine series.

How Do Vaccines Work for Parvovirus?

Thankfully, CPV vaccines are highly effective. The challenge is getting good immunity prior to exposure to the virus. Puppies should be vaccinated on a schedule until fully protected.

While a single vaccine can provide full immunity, there are a few reasons why the vaccine would not work, such as persisting maternal antibodies and an immature immune system.

Puppies get antibodies from their mother, which are protective, but will also block immune response to the CPV vaccination. Maternal antibodies are typically present until about 8-13 weeks. Immune function in puppies can be mature enough for good vaccine response starting around 6-12 weeks.

Both of these factors mean there will be a “window of susceptibility” at some point, so even well-vaccinated puppies will have a few days/weeks where they could become sick if exposed to CPV. This is why puppies should not visit high-risk areas until fully vaccinated.

The AAHA vaccine recommendation for CPV for puppies is every 3-4 weeks starting at 6-8 weeks until 16-18 weeks. Many breeders and owners will focus on the number of vaccines, but the timing is much more important. A puppy that gets 3 vaccines (6wks, 9wks, 12wks) may still be at risk for CPV, while a puppy that got only 1 vaccine at 16 weeks is likely protected. 

Bottom line: Be thoughtful about where your puppy goes and what it’s exposed to, and follow a good vaccine protocol!

How is Parvovirus Treated?

Like most viral infections, the primary focus of treatment is supportive care to give the body time to mount an immunologic response against the virus. Rapidly replicating viruses, like CPV, may outpace the immune system. 

Common treatments include: fluid therapy, antibiotics, nausea support, pain management, electrolyte/glucose supplementation, nutrition.

Depending on the stability of the patient, some dogs may require hospitalization for days to recover, while some may respond well to outpatient care. 

Recently, a breakthrough treatment is now available. The treatment is a Canine Parvovirus Monoclonal Antibody (CPMA) which is administered as a single intravenous injection. CPMA is a synthetic dog antibody that binds to and neutralizes CPV, preventing it from entering and destroying cells. CPMA can dramatically reduce the damage caused by CPV, often improving prognosis and shortening the duration of treatment needed for recovery.

At Art City Vets, CPMA has allowed us to provide care for many dogs who otherwise would have required referral for hospitalization. We’ve also been able to treat dogs for much lower cost than days of hospitalization. If CPV is caught early, many dogs may be successfully treated with outpatient care, avoiding hospitalization altogether.

Amazingly, CPMA may also prevent exposed puppies from becoming sick. It’s common for one or two dogs in a litter to become sick from CPV, and instead of waiting to see who becomes sick, we can administer CPMA and in many instances prevent illness from developing, buying time for effective vaccination.

Important note: CPMA interferes with vaccines for parvovirus, so dogs who receive CPMA should get a DHPP vaccine 17-20 weeks later.

What is the Prognosis for Parvovirus?

Without treatment, 60-90% of dogs sick with parvovirus will die. Dogs that are very ill (dehydrated, cold, hypoglycemic), will nearly all die without intensive treatment.

Prior to CPMA, survival rates for parvovirus are typically as high as 90%, but nearly always require days of hospitalization. In a study from 2021, 322 dogs were hospitalized for parvovirus treatment, and 91% survived, however the median hospitalization time was 79 hours. Spending 3-4 days in the hospital costs thousands of dollars, and is not an option for many families, especially if a few puppies in a litter are sick.

Since CPMA is so new, there are limited published studies. In one study, 28 eight-week-old puppies were exposed to parvovirus, and 21 were treated with CPMA 4 days later. No other treatments were given to any dogs. All 21 of the CPMA-treated dogs survived, while only 43% of the untreated dogs did. Since many dogs aren’t diagnosed only 4 days after exposure, this study may not be a good representation of real life.

Looking at naturally infected client-owned dogs, a study in 2023 evaluated a shelter's treatment prior to CPMA (49 dogs) and including CPMA (63 dogs). They found survival rates with and without CPMA were similar (82% vs 78%), however the CPMA-treated dogs were hospitalized for a shorter time (median of 2 vs 4 days), and stopped shedding the virus earlier. Dogs with minimal clinical symptoms had the most profound difference, recovering 3.5 days faster when treatment included CPMA. Since dogs are unlikely to receive CPMA only 4 days after exposure, this study is more “real life” example of the effect of CPMA. 

Dogs who recover from CPV often will have normal, healthy lives, but are more likely to develop chronic gastrointestinal problems later in life.

Why is Hospitalization Important?

Many dogs presenting to the vet sick from parvovirus are already unstable, even if clinical symptoms have only been around for a day or so.

A study evaluating outpatient vs inpatient CPV treatment found survival rates were similar, however more metabolic disturbances developed in the outpatient group (hypoglycemia, hypokalemia). Of note, the study provided hospitalization for all dogs until deemed stable, and the average outpatient dog was still in-hospital for 3.8 days on average! This study occurred prior to CPMA.

A study on 95 dogs treated by a Philadelphia shelter with outpatient care found a survival rate of 83%, and found that smaller dogs and dogs who had been sick longer had a lower chance of survival. This study also occurred prior to CPMA.

With CPMA available, the decision to hospitalize often depends on the overall stability of the dog, as well as the financial means of the family. While CPMA can be a tremendous benefit, it’s not a miracle drug, and severe cellular damage to the gut and immune system takes time and supportive care to recover.

What Does This All Mean For My Dog?

• Vaccinate puppies based on well-established guidelines.

• Until puppies are fully protected, take caution to avoid exposure to parvovirus.

• Any sick puppy should be seen urgently by a veterinarian, especially if they are not fully vaccinated and have symptoms like lethargy, poor appetite, vomiting, and diarrhea.