Parvovirus in Dogs: What It Is and How to Prevent It
Canine parvovirus is highly infectious and dangerous to dogs. Preventing parvo in dogs is far easier than treating a dog infected with parvo.
The year was 1980, and I was working as a veterinary assistant for my local veterinarian. All of a sudden we started seeing a large number of puppies coming into the hospital frighteningly ill, with symptoms of vomiting, severe lethargy, and bloody diarrhea. Canine Parvovirus (CPV-2) had reached our small rural town.
It was incredibly scary. Dozens of our furry friends were dying before our eyes, and there seemed little we could do. Most of the puppies under five months old and about two to three percent of older dogs died from CPV-2 when it first emerged. There is a distinct odor to the feces of dogs who are infected with the disease and that memory is cemented in my olfactory system. I also recall that bleach seemed to have become my personal scent, and I kept a wardrobe in the garage so I could change my clothes before I went into my own home after a day at the veterinary hospital.
What is Parvovirus?
Canine parvovirus – CPV-2, more commonly known as “parvo” – is an extremely virulent pathogen of domestic and wild canids, first emerging in Europe around 1976. It is a tiny (parvo means “small” in Latin), round, non-enveloped single-stranded DNA virus that has been determined by phylogenetic analysis to have descended from a single ancestor sometime in the mid-1970s. It is theorized that it evolved as a mutated variant of the long-known feline panleucopenia virus (FPV) that infects cats, minks, and raccoons (but not canines or cultured dog cells), and it adapted to dogs via non-domestic carnivores such as minks and foxes. CPV-2 and all its known variants (CPV-2a, CPV-2b, and CPV-2c) are highly infectious and cause acute hemorrhagic gastroenteritis (bloody diarrhea). They can also cause myocarditis (inflammation of the myocardium, the middle layer of the heart wall).
Parvo spread worldwide rapidly; the virus is extremely hardy and easily transmitted, and dogs had no immunity to the new disease, causing a pandemic during the late 1970s and early 1980s. Within a few years, though, intense collaborative research resulted in the development of effective vaccines (allowing for prevention) and treatment procedures that resulted in survival, culminating in disease control. Today, parvo is nearly unheard of in areas where most dogs are vaccinated, yet it remains a scourge in areas where vaccination compliance is low.
Dog Breeds Susceptible to Parvovirus
CPV-2 infection occurs worldwide in domestic dogs and other members of the dog family (foxes, wolves, coyotes), but there is no evidence that it is zoonotic (transmissible to humans).
All dogs are susceptible, but it has been described that some breeds, specifically Rottweilers, Labrador Retrievers, English Springer Spaniels, American Staffordshire Terriers, Doberman Pinschers, Alaskan Sled Dogs, and German Shepherd Dogs are at increased risk for contracting the virus. The Koret Shelter Medicine Program at the University of California, Davis, suggests that “these breed tendencies likely evolve and change over time as a result of natural selection against susceptible lineages, and no one breed should be assumed to be more or less susceptible than another.”
However, according to one of the world’s leading canine vaccine researchers, Ronald D. Schultz, PhD, professor and Chair of the Department of Pathobiological Sciences at the University of Wisconsin-Madison, there are breeds (most notably the Rottweiler and Doberman) that respond less reliably to vaccinations and the CPV-2 vaccine in particular. This underscores the importance and value of confirming that your dog has been successfully immunized by performing a vaccine titer test after the last of his puppy vaccines have been administered. (More on that below.)
The virus can affect dogs of any age, but it is most prevalent and severe in puppies from six weeks to six months of age. If a puppy receives CPV-2 antibodies from its mother via colostrum ingestion, he should be protected from infection for the first few weeks of life; susceptibility increases as these maternal antibodies wane. Any unvaccinated dogs and incompletely immunized dogs are also very susceptible. Adult dogs who contract the disease generally respond better to treatment than puppies, but if a dog is on steroids, immune-suppressive medication, or is immunocompromised in any way, he may be at higher risk of death from the disease.
The incidence of CPV-2 infection (as well as other infectious diseases) is higher in areas where large numbers of dogs congregate, such as animal shelters, dog shows, pet stores, breeding kennels, and boarding/training facilities.
How is Parvovirus Spread?
Canine parvovirus is highly infectious. The virus is present in a concentrated form in the dog’s stool. Infected dogs may start shedding the virus in their stool within four to five days of exposure (which can be before clinical signs of the illness emerge); their stool will also be loaded with the virus throughout the period of their illness, and for about 10 to 14 days after clinical recovery. Once exposed, unprotected dogs will generally start showing signs of the infection within three to 14 days, with an average of five to seven days.
The parvovirus infection causes severe diarrhea – and so anything that comes into contact with that stool can spread the virus. An unvaccinated dog who sniffs infected stool, an infected dog’s rear end (the virus will be all over the hair on the dog’s rear end), or even a shoe that stepped in a bit of the infected stool can become infected. The virus can “catch a ride” on anything that might be exposed to feces from an infected dog, including clothing, shoes or boots, dog toys, and kennel equipment (such as bedding, resting platforms, hoses, bowls, buckets, mops, wheelbarrows, etc.), can spread the disease.
Unprotected dogs can contract parvo year-round, but infection appears to naturally increase during spring and summer months. It’s more prevalent in areas where vaccination of animals is not widely practiced.
Parvovirus Symptoms and Pathogenesis
CPV-2 preferentially infects and destroys the rapidly dividing cells of the gastrointestinal system, lymph tissue, and bone marrow. The virus enters a dog’s body through the mouth and begins attacking the tonsils or lymph nodes of the throat. In the lymph nodes, the virus replicates in significant amounts, releasing into the bloodstream after a couple of days. As CPV-2 requires rapidly dividing cells to successfully cause disease, it travels to locations containing these cells, primarily those in the bone marrow and intestinal system. When the virus infects the bone marrow, it destroys the new cells of the immune system (white blood cells) effectively knocking out the body’s best defensive mechanism and making it difficult to combat infection.
It also appears to make it easier for the virus to invade the gastrointestinal system, where the virus does severe damage. In the small intestines, the virus attacks the glands in the epithelial lining where new cells are formed for the intestinal villi that provide for absorption of fluid and nutrients. New cells are prevented from forming and the system becomes unable to absorb nutrients, resulting in diarrhea and nausea. The intestinal lining can become so damaged it breaks down, allowing the digestive bacteria normally confined to the gut to pass through the intestinal walls and enter the bloodstream, causing fluid loss and widespread infection.
At this point in an infection, because the virus has already damaged the immune system, it is difficult for the body to fight the infection. Canine parvovirus is not always fatal, but death can occur from either the septic conditions created by the intestinal bacteria in the bloodstream, or dehydration and shock.
Clinical signs of CPV-2 infection generally include a quick onset of inappetance, vomiting, extreme lethargy, depression, diarrhea (often severe and containing blood), and dehydration. Additionally, the abdomen may be tender or painful and appear bloated, the heart may beat rapidly, gums and whites of the eyes may become noticeably red. When CPV-2 first presents, there is a slight rise in temperature, but it gradually falls to below normal with the advancement of the disease, and the affected dog often develops hypothermia.
Dogs become weak and dehydrated quickly due to the damage being done to the intestinal tract and the inability to absorb nutrients and fluids. Severely affected dogs may be in a collapsed state, with prolonged capillary refill time, poor pulse quality, and/or tachycardia. Laboratory analysis may show evidence of leukopenia and neutropenia (low levels of white blood cells). Most deaths from CPV-2 occur within 48 to 72 hours following the onset of clinical signs; early treatment is critical.
Parvo Diagnosis in Dogs
In most cases, diagnosis of CPV-2 is fairly straightforward; a presumptive diagnosis can often be made based on the dog’s history, clinical symptoms, physical examination, and laboratory tests (such as white blood cell count). Sudden onset of vomiting and bloody diarrhea is highly suspicious in susceptible dogs.
Fecal testing confirms the diagnosis. In-hospital viral antigen detection tests, such as the enzyme-linked immunosorbent assay (ELISA), are most commonly used. These tests use the same type of technology as home pregnancy tests and are reported to be fairly accurate and sensitive for recently emerged strains of the virus and can be completed with results in less than 15 minutes. There are, however, some limitations to the test. Results can be skewed by recent vaccination with a live vaccine, showing a positive reading from detecting the live virus from the vaccine, not from a CPV-2 infection.
The polymerase chain reaction (PCR) test is another fecal-based test that can be used to detect the virus; it is highly accurate, but it requires a sample be sent to a laboratory, which can take valuable time. Sometimes more than one type of test may be necessary to achieve optimal diagnosis.
Treatment options will vary depending on how sick the patient is, and also on the owner’s ability to pay. Optimal care can be expensive and is most effectively administered in a veterinary hospital that provides 24-hour care. Aggressive treatment protocols involve supportive care and management of symptoms until the dog’s immune system recovers, usually for two to seven days. There are no drugs available that can kill the CPV-2 virus, but immediate supportive treatment is crucial to saving the life of the dog.
The basic treatment protocol employs broad-spectrum prophylactic antibiotics, steroids, and antiemetic medication (to stop vomiting), and restoration of fluid, electrolyte, and metabolic abnormalities primarily via intravenous therapy. The patients must also be kept warm, clean, and comfortable.
The patients should also be monitored for development of hypokalemia (dangerously low potassium levels) and hypoglycemia (low blood sugar). In severe cases, blood transfusions may be warranted to boost low blood-cell counts resulting from bone marrow infection. Recent evidence suggests that providing enteral nutrition (tube feeding) earlier in the patient’s illness is associated with faster clinical improvement, weight gain, and improved gut barrier function. When vomiting has ceased for 12 to 24 hours, it is recommended that water and a bland, easily digestible low-fat diet be gradually reintroduced.
CPV-2 can be fatal even with aggressive treatment. The disease is most severe in younger puppies, especially those younger than 12 weeks, and most puppies without veterinary intervention usually die. Successful outcomes require early diagnosis and treatment. If a dog survives the first three to four days, he will usually recover rapidly (usually within one week), and he will become immune to the virus for life.
Read "Canine Parvovirus Vaccine, Symptoms, and Treatment," for a closer look at how parvo is treated.
Home Treatment for Parvo
The gold standard of treatment for dogs or puppies who are infected with parvovirus can be expensive, and too often, owners who are given the worst-case estimate of the cost of this treatment will opt to have their canine companion euthanized, since they lack funds to pay for those estimated costs. In theory, though, the treatment that parvo patients need to recover is not in and of itself expensive; the greatest contributor to the cost is the amount of time and attention required to nurse the animal through the illness.
In order to try to save lives by preventing “economic euthanasia,” researchers at Colorado State University’s College of Veterinary Medicine and Biomedical Sciences Veterinary Teaching Hospital developed a protocol that could be made available to highly motivated owners who wanted a second option: a chance to care for the patient at home.
Their protocol is meant to be administered under a veterinarian’s supervision, and requires constant communication with and oversight from a willing veterinarian. The owner will have to administer subcutaneous fluids and other injections, monitor the patient’s body temperature and keep him especially warm, provide nutrition as needed by syringe-feeding a high-calorie canned food, and be alert to changes in the patient’s condition that may require a return to the veterinarian for lab tests and additional medications.
In a study comparing the efficacy of the protocol to conventional gold-standard treatment, CSU researchers determined that 80 percent of the patients treated with the home-care protocol recovered, compared to 90 percent of the conventionally treated patients.
See “New Home Treatment for Parvo May Prevent 'Economic Euthanasia',” WDJ November 2013, for more information. Here's a link to the Colorado State home treatment protocol.
Controlling a Parvo-Free Environment
The CPV-2 virus is extraordinarily hardy; it can remain viable for months, perhaps even years, outside its canine host. The virus survives through winter, through freezing temperatures, and is resistant to most household cleaning products. An infected dog can shed 35 million viral particles in an ounce of feces; a typical infectious dose to an unvaccinated dog is only 1,000 viral particles. That makes it very difficult to disinfect an area once it has been exposed.
Dogs with confirmed or suspected canine parvovirus should be isolated to prevent spreading the disease to other susceptible dogs and to limit environmental exposure. Only fully vaccinated older puppies and adult dogs should be exposed to the home environment of a dog recently diagnosed with CPV-2. Puppies especially should not be allowed to come into contact with the fecal waste of other dogs as this a prime vector for transmission of CPV-2.
Contaminated environments need to be sanitized through repeated cleaning to remove all organic debris and dirt, along with the virus, followed by use of disinfectants with proven efficacy. Effective choices include household bleach (5 percent sodium hypochlorite) at ½ cup per gallon of water (1:32 dilution). Like all disinfectants, bleach must be stored and used correctly in order to be effective. Bleach is an acceptable method for cleaning any indoor areas, surfaces, and equipment (including bowls and beds). Avoid using mops to clean.
Launder clothing, bedding, towels, etc., in hot water with detergent and bleach. Do not overload the washing machine, and use a clothes dryer rather than hanging items up to dry. Make sure that all objects that have come into contact with an infected dog have been discarded or disinfected.
Because CPV-2 can be found anywhere infected dogs have visited, the cornerstone to protecting your dog from the disease is vaccination. Since the development of the vaccine, the incidence of infection has been greatly reduced. A single modified live vaccine can confer protection within three to five days. Although parvoviruses continue to evolve, vaccination appears to confer reliable protection against all known strains.
As with other infectious diseases of dogs, puppies from mothers with immunity are protected by maternal antibodies, acquired via colostrum. This natural protection can, however, create a problem in prevention, as pups can become susceptible between the waning of the maternal antibodies and the administration of the vaccine. There is a high rate of infection reported in pups older than six weeks of age, about the time when maternal protection wanes in most puppies.
Most cases of CPV-2 infection tend to be in unvaccinated or partially vaccinated puppies. For this reason, until they are fully immunized, puppies should be provided with very limited exposure to public places, and should not be taken to areas where lots of dogs congregate or are walked. Until their immunization is complete, puppies should never be taken to dog parks or pet supply stores.
The age at which puppies receive their first vaccination (and subsequent vaccinations) varies widely, depending on their environment. Shelters usually vaccinate all puppies over four weeks of age upon admission to the shelter, and at least every three weeks after that until the puppies are older than 16 to 18 weeks. An experienced breeder with superior control over her dogs’ environment and visitors might wait until a puppy is eight weeks or older, and vaccinate as few as two times, with the last vaccination being administered after the puppy is 18 weeks old. The goal is to administer vaccine as quickly as possible after the pup’s maternal antibodies have faded, and since it’s impossible to predict exactly when this might happen, the vaccinations are repeated until the last possible time that the maternal antibodies could possibly interfere with immunization.
Adult dogs, over the age of one year, generally need only one vaccination. A vaccine titer test, administered at least three weeks after the dog or puppy’s final vaccination, can confirm whether the dog has developed protective antibodies.
Dogs who have been infected by CPV-2 and survived tend to be antigenically stable (immune to further infection).
See “Puppy Vaccines: Why Your Puppy Needs So Many Shots,” in the October 2016 issue for more about the number and timing of the vaccines needed to protect puppies from parvo and other diseases.
Today, survival rates in quickly diagnosed and appropriately treated dogs from CPV-2 infection are reported as being in the 75 to 90 percent range. In untreated dogs, the mortality rate often exceeds 90 percent. Note that these are averages and individual morbidity and mortality rates vary according to the age of the dog, the severity of infection, the presence of concurrent health problems, and the speed with which the illness was diagnosed and treated.
Local shelters and veterinarians can give you information about parvovirus outbreaks and the prevalence of the disease in your community. Often, shelter workers are aware of specific neighborhoods where parvo is particularly endemic.
Pixel: A Tiny Parvo Success Story
Two years ago I helped my sister foster and nurse a young pup recovering from parvo that came from the shelter where she worked as a registered veterinary technician. This pup arrived at the shelter with a female dog and two other puppies. All of the puppies appeared to be littermates, about five weeks old. Based on clinical signs, they were suspected of being infected with parvovirus; a SNAP test confirmed that two of the pups were positive.
The pups with parvo were isolated, treated, and survived, but the tiniest female, all 2.2 pounds of her (down from 3.5 pounds at intake) suffered from complications of maldigestion, malabsorption, anemia, and hypoglycemia. The disease had ravaged her tiny digestive tract and it was touch and go for days. The tiny pup could barely stand and when she did, it was in a hunched posture. Also, her body couldn’t maintain a normal temperature, so when she wasn’t curled up with my Border Collie, she had a special heating disc to keep her warm.
When she finally developed an interest in eating, it was a challenge to find something she would eat more than once – and when she did eat, the food often came out looking like it did when it went in. My sister provided her with constant care and love, and, happily she bounced back.
It was at this point this speck of a pup was given her name – Pixel – and her permanent home (with my sister of course). Today she eats ravenously as if to make up for lost time and she has no side effects from her severe illness.
Barbara Dobbins, a former dog trainer, writes about dogs and studies canine ethology. She lives in the San Francisco Bay area with her new puppy, Tico.