Canine Parvo Virus

Information Sheet
Canine Parvovirus bar

Table of Contents

Read Shelter Medicine Director Dr. Kate Hurley's article The Virus that Never Dies?and Controlling Parvo: Real Life Scenarios published in Animal Sheltering Magazine.


Introduction

Parvovirus is highly contagious, incredibly durable in the environment, and capable of producing severe or life-threatening disease in dogs. It is critical to prevent transmission of the disease and rapidly identify infected animals in order to provide medical care as deemed appropriate and protection to other dogs in the shelter. Inevitably, Parvovirus will be introduced into shelters from the surrounding community from time to time. If this occurs where preventive medicine is not practiced, in a crowded, busy shelter where staff is already stretched to the maximum, the response to the epidemic is a crisis mode, which is inefficient, very expensive, and much less effective at protecting animals from this deadly disease.
Parvovirus facts
  1. Parvovirus is very durable in the environment and can persist  for months or years.
  2. Bleach or potassium peroxymonosulfate (Trifectant or Virkon-S) have been proven to kill parvovirus if used correctly. For more information on parvocidal disinfectants, click here.
  3. There is no way to completely disinfect contaminated dirt and grass, although sunlight and drying has some effect. Mechanical decontamination through irrigation may also be helpful, but the area must be allowed to dry thoroughly between applications. Potassium peroxymonosulfate has relatively good activity in the face of organic matter, and can be sprayed on contaminated areas using a pesticide sprayer or other applicator. To be on the safe side, contaminated areas should be off limits to puppies for 1-6 months (choose the longer end of the waiting interval for moist, dark areas, if area can not be heavily irrigated or if it can not be sprayed with potassium peroxymonosulfate).
  4. Parvovirus can be spread on hands, feet, clothing, tools, rodents and flies traveling from kennel to kennel! Dogs may carry the virus on their fur and feet even if they themselves do not get ill. The virus enters the dog through the nose or mouth and has an incubation period of 3 days to 2 weeks (usually 5-7 days).
  5. Puppies under 6 months old are most likely to get severe disease. Rottweilers, Dobermans, pit bulls and mixes of these breeds are especially vulnerable. Adult dogs may get mild disease that is indistinguishable from diarrhea of any other cause. Affected dogs have mild to severe diarrhea, may be dehydrated and lethargic, have vomiting, or can develop severe to fatal secondary bacterial infections.
  6. Vaccination usually prevents disease in adult dogs that have received a vaccine at least 1-2 weeks before exposure, but does not prevent them from carrying virus on fur if exposed. Puppies up to 16 weeks of age may not be protected fully by vaccination.

Diagnosing Parvovirus

There are three ways to "test" for parvo: clinical, by complete blood count (CBC), and by fecal antigen tests. Read the " parvo cheat sheet" for a quick summary of diagnosis.
Clinically: Dogs with parvoviral enteritis typically have vomiting and diarrhea which typically is severe but may be mild. Lymph nodes may be enlarged and the dog may have a fever. Other diseases may mimic parvo, so a clinical diagnosis made without the aid of diagnostic testing is not precise.
CBC: Dogs with parvo typically have very few white blood cells. If the normal lower limits for white blood cell counts on the CBC is 6000 cells/microliter, a dog with parvo may have less than 1000. Although this also does not confirm parvo, it is strong evidence and a dog with some other disease and such a low white cell count requires aggressive treatment or euthanasia regardless.
Fecal antigen test: The fecal tests used in clinics and shelters are assaying for antigens of the virus itself. If the dog has the virus in its feces, the test should be positive. Always keep in mind that:
  • Tests must be performed exactly as the literature indicates or the test is invalid.
  • Tests should be performed on any dog with diarrhea that is also exhibiting signs of systemic disease: vomiting, lethargy, fever, loss of appetite, dehydration, or dogs with unusually copious, smelly/bloody diarrhea, or any dog with known exposure to parvo within the preceding 14 days who then develops diarrhea.
Interpreting test results
  • Some vaccinated dogs may have a false-positive test, i.e. the virus in their feces is vaccine strain. The vaccine-positive result can happen within about 3-14 days of vaccination. A study in kittens suggests this is uncommon with some tests - only 1/64 recently vaccinated kittens tested weakly positive on the Idexx brand test. However, 13/64 kittens tested positive, some strongly positive, on the Synbiotics brand test. (Levy, J. K. (2006). Impact of Vaccination on Parvovirus Testing in Kittens. International Veterinary Vaccines and Diagnostic Conference, Oslo, Norway.) Equivalent data is not available for puppies, however all puppies testing positive should be carefully isolated regardless of recent vaccine history.
  • Some dogs with parvo have negative tests, either because it is very early in the course of the disease or late in the course (most puppies will test negative by the time they've had clinical signs for 2-3 days). Tests should be repeated on dogs that are strong suspects or used in conjunction with a CBC or blood smear.
  • Parvovirus is very uncommon in dogs over four months of age vaccinated at least 1-2 weeks prior to exposure. Positive test results in these dogs should be verified by another test, such as a CBC (or necropsy if the dog dies or is euthanized). If multiple cases of apparent parvovirus are observed in vaccinated dogs over four months old, please contact the UC Davis Shelter Medicine Program.
  • Each shelter should have a decision tree about which tests to use and what to do with a dog once tests are performed. One example: if sufficient facilities exist, have a transitional area for dogs that have suspect parvovirus but things do not add up (a dog who has a positive test result but who also appears well, a dog with bloody diarrhea that has negative test results). Attempt to clarify the status of such dogs with a CBC. Carefully isolate, retest and monitor "transitional" dogs daily until they are clearly not infected with parvo and then place them up for adoption or they clearly have parvo and then euthanize or treat them.

Limiting the Spread of Parvovirus

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  • Vaccination against parvovirus is very effective and is the cornerstone of prevention. Vaccinate all dogs and puppies over 4-6 weeks of age immediately on intake, and repeat vaccinations in puppies every 2-4 weeks. Go with the low end of the age range and short end of the vaccination interval in shelters where parvo risk is high.
  • Use a known parvocidal disinfectant, correctly applied at the correct concentration. This is particularly important following a known parvo exposure. For more information on disinfection, see our Cleaning and Disinfection information sheet. For more information on parvo disinfection FAQS, click here.
  • Train staff in recognition of parvovirus, including correct use of the fecal antigen test. Create a written plan for what to do if a parvo case is identified.
  • Ensure availability of protective gear such as gloves, protective jumpsuits (long sleeves are a must!), and boots or shoe covers.
  • Puppies and at-risk dogs exposed to parvovirus should be isolated and quarantined for two weeks. For more information on quarantine and risk assessment, see notes on panleukopenia control. The considerations for quarantine in the face of parvo are the same, as these two viruses are very closely related.
Perform routine shelter maintenance to minimize the spread of infection in the event that infection does occur
  • Cleaning should be performed routinely (typically daily for tasks like cages and weekly for floors, walls, vehicles etc.) with a know parvocidal disinfectant.
  • All areas of the shelter are to be cleaned and disinfected routinely (except carpeted areas). This includes vehicles, front lobby, all walkways and common areas, after hours receiving, etc.
  • Areas of animal transport vehicles where dogs are kept should be cleaned and sprayed with a parvocidal disinfectant between each use.
  • Scrapers and scrub brushes should be designated for each kennel and used only for that kennel to minimize cross contamination.
  • Remove all gross organic matter from runs prior to disinfection. Scrape or scrub as needed to remove dried on debris.
  • If necessary, use degreasing solution to clean the kennel.
  • Rinse with water.
  • Spray with parvocidal disinfectant. Avoid splashing as much as possible (splashing can spread the virus). Do not use power washers or high pressure hoses during a parvo outbreak unless all animals have been removed from the area.
  • Allow to air dry, or squeegee if necessary.
  • Ideally wait ten minutes before letting dogs back into that area.
  • Spray walkways with parvocidal disinfectant after cleaning runs.
  • Do not allow puppies from unknown backgrounds into play yards, carpeted offices, or other hard to clean areas until they have been in the shelter at least two weeks.
Perform routine surveillance and keep track of trends.
  • Maintain a logbook for staff to record parvo test results in a convenient location and ensure that staff uses it! It should include the Parvo test date, animal number, and initials of the person running the test (which also allows supervisors to figure out who does not runs tests).
  • Monitor for severe or bloody diarrhea, vomiting, depression or other signs of illness. Also observe for dogs that are found dead in run or any incoming puppy that has an unthrifty appearance. If there is any doubt, that animal should be tested.
Balancing parvovirus risk and puppy socialization
It's easy to see the consequences when a puppy is exposed to parvovirus before vaccination has a chance to protect. This motivates us to carefully protect puppies from possible exposure to this deadly virus. However, the consequences of poor socialization are less immediately apparent, but may be equally severe. A puppy that has been isolated from other animals, people and a wide variety of experiences may grow up fearful or even aggressive. As we know in shelters, this may lead to fatal consequences if the dog is later surrendered due to these behaviors. We need to balance the risk of parvo with the very real risk of poor socialization, and do our best to provide puppies with the best of both worlds: a variety of experiences in a relatively safe and clean environment. For more on providing puppies with socialization opportunities while protecting them from parvo, please see our information sheet on "parvovirus and puppy socialization".


Client Information

  • Parvovirus is highly infectious and causes diarrhea (sometimes bloody), vomiting, lethargy, weakness, and fever. The virus also attacks white blood cells, leaving the infected dog much more susceptible to other bacterial infections.
  • Diarrhea can be mild or severe fatal. Diarrhea in an otherwise bright, alert, eating, drinking dog is more likely due to diet change, stress, parasites, or diet change, than to parvovirus.
  • The virus is very contagious and is spread by exposure to feces. Unfortunately, dogs from shelters often have been exposed to parvovirus and should be observed for 14 days after adoption to be sure they are not incubating the virus.
  • There is some risk that a dog incubating parvovirus will infect other dogs. The new dog and its feces should be kept away from puppies and unvaccinated dogs for 2 weeks. Dogs that have had at least two vaccines, with the last one at least 2 weeks prior to exposure to an infected dog are fairly protected.
  • If you suspect your dog has parvo, call your vet as soon as possible. The earlier the infection is diagnosed and treated with fluids, antibiotics, and nursing care, the more likely it is to do well.
Communicating with the general public when parvoviral infections occur in your shelter
  • The presence of cases of parvovirus in the shelter should be communicated to local veterinarians and to the public.
  • Adopters should be informed of the exposure status of their new dogs with a copy of any relevant medical records, and should sign an adoption with medical waiver form.
  • All adopters should be given a parvo information handout which should include a description of the disease and the associated clinical signs of an infected animal.
  • As always people who have recently adopted an animal should be strongly advised to have the animal examined by a veterinarian. In the event that the adopted animal had been kept in an area where there was even a remote chance that it could have been exposed to parvo the adopter should be informed and it should be made clear to them that they should notify their veterinarian so that they may examine the dog, perform any necessary diagnostic tests, and properly advise the owner on the care of their newly adopted animal.
Myths and facts about canine parvovirus “2c”:No evidence for substantially different properties.
From:  Kate Hurley, UC Davis Koret Shelter Medicine Program, and Colin R. Parrish, Cornell University. 8-29-2008
In recent publications and email communications a lot of attention has been given to the “new” canine parvovirus (CPV) strain, which has been designated CPV strain 2c. It is important to put this mutation into a context.  This virus has a single nucleotide substitution that changes one amino acid in the capsid protein.  That amino acid (of viral protein 2 (VP2) residue 426) had previously undergone a change from an asparagine to an aspartic acid to give the so-called CPV-2b strain, and in the CPV-2c variant that aspartic acid changed to a glutamic acid, a similar amino acid. 
Over the 30 years since canine parvovirus emerged in dogs it has undergone some evolution, with a small handful of changes that have been selected in the virus.  Some of those mutations created small changes in the capsid surface that influence the binding to the host antibodies, and the changes of VP2 residue 426 have a small influence on the binding of some antibodies.  However, dogs make a large variety of antibodies against the virus and these mutations only influence the binding of a small proportion of those, so that the immunity generated against any of the canine parvovirus strains still has a great deal of reactivity with all other strains of parvovirus.
The research to date shows that all currently available vaccines protect against all known strains of CPV, including the newer CPV-2c strain 1,2,3.  Some of the evolution of the CPV appears to give new strains small selective advantages in nature, and in some cases those can eventually replace the old variants.  For example the original CPV-2 strain from 1978 is not found in nature today as it was replaced in 1980 by the CPV-2a variant.  The CPV-2a is still the common virus circulating in the USA and around the world, and the CPV-2b and CPV-2c variants essentially differ from that 1980 virus at a only one or two amino acid positions 4,5.
There is no evidence that CPV-2c is a more serious threat to either shelter or owned dogs than the other CPV strains. It is not possible to distinguish CPV-2c from CPV-2b or -2a isolates based on clinical signs or parvo snap tests. CPV-2c causes similar clinical signs as the previously known strains, including mucoid or hemorrhagic diarrhea, leukopenia, and lymphopenia 4,5. Although a few reports suggest that CPV-2c may cause more severe clinical signs than -2a and -2b, others describe less-severe disease and lower mortality rates in CPV-2c infected dogs 6. There is no evidence, nor reason to believe, that the susceptibility of CPV-2c to disinfectants is different than other strains of parvovirus.  All parvoviruses are environmentally persistent and resist inactivation by some common disinfectants, including quaternary ammonium compounds and alcohol7,8.  However, all strains can be reliably inactivated by correctly applied disinfectants documented to inactivate parvoviruses, including sodium hypochlorite (household bleach diluted at ½ cup per gallon) and potassium peroxymonosulfate (e.g. Trifectant ®) 7,8,9.
CPV-2c is not known to be a particular diagnostic challenge, and is expected to cross react with commonly used ELISA tests just as other strains do10. To distinguish the CPV-2c from CPV-2a/2b requires DNA sequence analysis or PCR 3; however there seems to be little or no clinical use for these tests, as vaccine or management principles are not different.
Canine parvovirus continues to evolve, and it is possible that future variants may be altered in detection properties or vaccine susceptibility, but this has not been demonstrated to date.  If unusual cases of canine parvovirus infection are suspected please contact the UC Davis shelter medicine program. In the meantime, the appropriate precautions when CPV is diagnosed or suspected in a shelter are the same regardless of which strain of CPV is involved.
For more information on management of parvovirus in shelters, or to contact the UC Davis shelter medicine program, please visit www.sheltermedicine.com.
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