Canine parvovirus (CPV) is a highly contagious viral disease of dogs that commonly causes acute gastrointestinal illness in puppies. The disease most often strikes in pups between six and 20 weeks old, but older animals are sometimes also affected. A rare variant of the disease may be seen in very young (neonatal) puppies is myocarditis (an inflammation of the heart muscle).

Cause
Symptoms and complications
Tests and diagnosis
Treatment
Prevention
Additional resources
Baker Institute and canine parvovirus

Cause

What causes parvovirus infection?

The virus that causes the disease known as “parvo”, canine parvovirus type 2 (CPV), first emerged among dogs in Europe around 1976. By 1978 the virus had spread unchecked, causing a worldwide epidemic of myocarditis and inflammation in the intestines (gastroenteritis). We now know the virus is not limited to dogs, but is capable of causing infections in wild canines such as coyotes and wolves, and other wild animals, including foxes, raccoons and skunks. CPV is closely related to feline panleukopenia virus (FPV), a virus that has been know since the 1920s to infect cats and mink and other animals. CPV probably arose as the result of 2 or 3 genetic mutations in FPV that allowed it to expand its host range to infect dogs.

Three decades after its first appearance, CPV strikes puppies with deadly disease much less frequently due to the development of effective vaccines in the late 1970s, but outbreaks still occur frequently, and vaccinating your dog is of the utmost importance. Puppies and adolescent dogs are especially susceptible to parvovirus, and you should avoid bringing your puppy to public places where there is likely to be lots of virus (animal shelters and kennels) until after their vaccinations are complete.

Why and how might my dog become infected?

Canine parvovirus can be found in almost any environment, but not every dog who comes into contact with the virus becomes infected. Several factors come into play in infection, including the immune status of the dog and the number of viruses the dog is exposed to. If the combination of factors is just right and a dog does become infected, a specific sequence of events is begun as the virus attacks the body.

What happens during infection?

Once a dog or puppy is infected, there is an incubation period of three to seven days before the onset of first symptoms. Inside the dog, CPV needs the help of rapidly dividing cells in order to successfully cause disease, and the virus usually begins by attacking the tonsils or lymph nodes of the throat. Once inside the lymph nodes, the virus typically invades lymphocytes (a type of white blood cell) for one or two days, creating many copies of itself. These viruses hitch a ride inside the lymphocytes, where they are sheltered from the host defenses, and enter the bloodstream. Many of these CPV-infected lymphocytes are ultimately killed, causing a reduction in the number of circulating lymphocytes, a condition called lymphopenia.

Once in the bloodstream, the virus again targets rapidly dividing cells, hitting hardest in the bone marrow and in the cells that line the walls of the small intestine. In very young dogs, CPV can also infect the heart, leading to inflammation of heart muscle, poor function, and arrhythmias.

In the bone marrow, the virus weakens the body’s ability to protect itself by destroying young immune cells and causing a drop in the protective white blood cell count. This probably makes it significantly easier for the virus to invade the gastrointestinal tract, where the virus does its worst damage.

The virus causes this destruction by targeting the epithelium of the small intestine, the lining that helps to absorb nutrients and provides a crucial barrier against fluid loss and bacterial invasion from the gut into the body. The cells that make up the epithelial surface are short-lived and are replaced continually by new cells born in the rapidly-dividing areas known as the crypts of Lieberkühn. The virus invades these crypts where new epithelial cells are born and disables the body’s ability to replenish the intestinal surface.

By preventing the replacement of old and dying cells with fresh new cells, the virus leaves the intestinal surface unable to adequately absorb nutrients, prevent fluid loss into the stool, or prevent bacteria from moving from the gut into the body. Severe diarrhea and nausea are the initial result, but eventually the intestinal surface can become so damaged that it begins to break down, and the bacteria that are normally confined to the gut penetrate the intestine walls and enter the bloodstream. This causes both significant fluid loss from diarrhea and widespread infection inside the body. To make matters worse, the body’s immune system is already weakened, as its ability to produce new white blood cells to combat infection has been hampered by the invasion of CPV into the bone marrow. CPV is not always fatal, but when it does kill, death is as a result of either dehydration and shock, along with the effects of septic toxins produced by the intestinal bacteria roaming throughout the bloodstream.

Symptoms and complications

Symptoms often associated with CPV include lethargy, depression, and loss or lack of appetite, followed by a sudden onset of high fever, vomiting, and diarrhea. If your dog is experiencing bouts of bloody diarrhea and/or vomiting, CPV is only one of several potential culprits. Your veterinarian can run several tests to help determine whether your dog is infected with CPV.

Tests and diagnosis

How will my vet diagnose CPV?

By far the most common and most convenient method of testing for the presence of CPV is the fecal ELISA test. ELISA is an acronym for enzyme-linked immunosorbent assay, a technology is similar to that used in home pregnancy tests. In an ELISA test, antibodies to parvovirus are immobilized on the surface of a testing chamber. A fecal sample is added to the chamber, and antibodies attach to parvovirus proteins that may be present in the stool. A color-changing chemical is then added to the chamber, and if parvoviruses have attached to the antibodies, the chemical will change color and indicate a “positive” result.  CPV fecal ELISA tests can usually be completed by your veterinarian in less than 15 minutes. Though the ELISA test is fairly accurate, it is can occasionally produce false positive or false negative results, so further testing may be necessary to confirm a diagnosis.

Veterinarians may also rely on a test that uses a techniques called polymerase chain reaction (PCR) to diagnose CPV from fecal samples. The CPV fecal PCR test detects small pieces of viral DNA that are specific to CPV in the stool of an infected dog. This test is very accurate (more so than CPV fecal ELISA), but requires that a fecal sample be sent to a laboratory that specializes in performing PCR-based testing, so it generally requires more time than a CPV fecal ELISA.

A simple measure of white blood cell count is often the clincher for a CPV diagnosis. Because one of the first things the parvovirus infects is the bone marrow, a low white blood cell count can be suggestive of CPV infection. If a dog has both a positive ELISA reading and a low white blood cell count, a fairly confident diagnosis of CPV may be made.

Treatment

What are the treatment options for dogs with CPV?

Treatment options for dogs suffering from CPV involve supportive care and management of symptoms. Treatment options will vary, depending on how sick the dog is, but certain aspects are considered vital for all patients.

A hospital stay is often necessary so that the dog can receive intravenous fluids and nutrients to replace the vast quantities lost via vomiting and diarrhea. An intravenous drip is preferred because the digestive tract of stricken dogs is usually in distress and can’t tolerate or absorb what the dog needs. Blood transfusions may also be helpful to boost low blood cell counts that may result from CPV infecting the bone marrow.

Antibiotics may be appropriate therapy for a dog suffering from CPV, administered either intravenously or as injections, to help fight the infection if intestinal bacteria have entered the bloodstream. In addition, medications to control nausea and diarrhea are sometimes useful. Many dogs will respond to medical therapy if it is initiated in a timely fashion, and those dogs that recover from CPV infection retain lifelong protective immunity against the strain that infected them.

Prevention

How do I vaccinate my pet against CPV?

Since the advent of a number of effective canine vaccinations for CPV, this infectious disease has become much less of a threat to dogs. This does not mean, however, that CPV does not remain a serious problem, and vaccination of your dog should not be considered an option – it is a must.

Veterinarians usually administer the CPV vaccine as part of a combination shot which includes, among others, the distemper, canine adenovirus, and parainfluenza vaccines. These shots are given every 3 to 4 weeks from the time a puppy is 6 weeks old until he is at least 16 weeks of age. A booster vaccination is recommended one year later, and then at one at three year intervals thereafter.

How else can I help prevent the disease?

The tiny parvovirus is extraordinarily hardy. They are capable of surviving for months outside an animal, even through the winter, and are resistant to most household cleaning products. Infected dogs can shed vast numbers of viruses, making it difficult to disinfect an area once it has been exposed to an infected dog. These facts highlight the importance of isolating any dog that is infected with CPV from other dogs. Given the fact that most environments (including dog parks, lawns, and even homes) are not cleaned with disinfecting products regularly, a puppy can be exposed to CPV without any warning, making the vaccine protection all the more important.

If your home and yard have been contaminated by an infected dog, there are steps you can take to disinfect them before introducing a new dog or puppy. Despite its relative resistance to cleaning agents, we do know that CPV can be inactivated by bleach. Cleaning with a solution of one part bleach mixed with approximately 30 parts water is an acceptable method for disinfecting any indoor area (including bedding, food/water bowls, and all surfaces) that once housed an infected dog. There is evidence suggesting that CPV loses some of its ability to infect an animal after one month in an indoor environment. Outside, you cannot (and should not) bleach your lawn, but rain or watering can dilute the concentration of the virus over time. This dilution, combined with the sanitizing effects of sunlight can bring the numbers of viruses down to an acceptable level in a few weeks.

Additional Resources

The AVMA brochure on parvovirus provides a brief overview of what pet owners can expect in canine parvovirus infections.

A more detailed resource for owners can be found at VeterinaryPartner.com.

For veterinarians, the Merck Veterinary Manual provides a comprehensive chapter on parvoviral infection.

Baker Institute and canine parvovirus

The Baker Institute for Animal Health has a long history of working to prevent and treat canine parvovirus infection. The virus first emerged in the United States, Europe, Asia and Australia in 1978, when a virus similar to feline panleukopenia virus crossed over from cats to cause a new type of disease among domestic dogs. Within two years the virus had spread worldwide, killing thousands of dogs and possibly infecting millions more. Baker Institute scientists, including Drs. Leland Carmichael and Max Appel, first isolated the virus later that same year, and by 1979 had developed the first vaccine for parvo. By 1981, Baker Institute scientists had created an improved attenuated vaccine for the disease. Today, Dr. Colin Parrish continues to study the virus and its evolution in order to determine whether existing vaccines provide adequate protection from modern strains of CPV.

For more on Parrish's recent research, such as the following paper recently published, visit our PUBLICATIONS.

Voorhees Ian EH, Lee Hyunwook, Allison Andrew B, Lopez-Astacio Robert, Goodman Laura B, Oyesola Oyebola O, Omobowale Omobowale, Fagbohun Olusegun, Dubovi Edward J, Hafenstein Susan L, Holmes Edward C, Parrish Colin R. Limited Intrahost Diversity and Background Evolution Accompany 40 Years of Canine Parvovirus Host Adaptation and Spread. J Virol. 2019 Dec 12;94(1).