Canine Parvovirus (parvo) is a highly contagious viral disease that is one of the most common causes of diarrhea in dogs under 6 months of age. It first appeared in the late 1970s, and is now one of the most common serious dog disease problems encountered in animal shelters, replacing distemper for that distinction. 

It is reported in coyotes, foxes, and wolves and probably affects most, if not all, members of the canine family. Puppies are the most susceptible, and their clinical signs are worsened by concurrent infections with roundworms, other internal intestinal parasites, protozoa (such as Coccidia), viruses or bacteria.

In general, if aggressive therapy is initiated early in the course of the disease, the prognosis for puppies to recover can be excellent, although fatalities do occur. However, the mortality rate for puppies in shelters can be much higher because most shelters cannot diagnose, isolate or treat the cases. As for adult dogs, many of them become infected but never actually show clinical signs of disease. Rottweilers, Dobermans, Pit Bulls, German shepherds and Labrador retrievers seem to be at higher risk for the disease.

Cause
Transmission
Clinical Signs
Diagnosis
Treatment
Prevention and Disease Outbreak Management

Cause

The parvovirus is very stable in the environment and very resistant to most disinfectants. It can persist in organic material in the environment for over one year. Another member of this virus family is responsible for causing panleukopenia, which is more commonly known as distemper, in cats. (This feline parvovirus was present before the strain that affects dogs appeared. In fact, the first vaccination efforts to control canine parvo were made using feline panleukopenia vaccines.)

Different strains of parvovirus have evolved over the years since it was first discovered in dogs in 1978. The current strain infecting dogs in the US is CPV-2b, which has also been isolated in cats suffering from clinical signs of panleukopenia. In the shelter it is essential to separate dogs from cats, as cats can act as a reservoir causing further disease in dogs.

Transmission

Parvo disease is spread from dog to dog mainly through exposure to contaminated feces. It is also spread through contact with fomites (contaminated objects). Common fomites include hands, instruments, clothing, food and water dishes, toys and bedding. Insects and rodents can also provide a means for disease spread. The virus can remain on a dog's hair coat and serve as a means of transmission long after recovery from clinical disease.

The incubation period, or period between contact with the virus and the appearance of symptoms, is usually 4-6 days. Because the disease may be difficult for the shelter to detect during the incubation period, apparently healthy animals with parvo may be adopted out only to become ill a few days later in their new home, causing much heartache for the shelter staff and the new owners.

It is very important to know the shedding pattern of parvovirus in order to design an effective management, diagnostic and prevention strategy. Parvovirus can be shed in the feces 3-4 days after infection with the virus, which is generally before clinical signs of illness appear. The virus will also be shed in the feces for 14 days post infection.

Incoming puppies with a history of contact or exposure to the disease should be quarantined for 14 days if possible to watch for the onset of signs, and recovered animals should be isolated for 1-2 weeks before rejoining the general population. A recovered puppy should be bathed prior to rejoining the general population in order to remove parvovirus from the skin and hair coat.

Clinical Signs

Parvovirus affects the digestive system and the heart. The signs can vary widely:

• There can be sub clinical infection with no signs or mild signs of lethargy and appetite loss that only last for 1 or 2 days.
• The most common clinical symptoms that shelters see are varying degrees of vomiting, foul smelling diarrhea that can be very bloody, loss of appetite, fever, weakness, depression and dehydration.
• Affected puppies are also very leukopenic, meaning they have too few white blood cells.
• The heart symptoms are rarely seen today and usually occur in puppies infected in utero or during the neonatal period, but they can cause sudden death without other signs, sudden death weeks to months after apparent recovery from other parvo signs, or sudden onset of symptoms of congestive heart failure in puppies under 6 months of age.

View Our Interactive Parvo Timeline

Diagnosis

The disease is commonly diagnosed by an evaluation of the clinical signs accompanied by clinical laboratory tests. It must be remembered that not all cases of bloody diarrhea and vomiting are caused by parvovirus. In addition to clinical laboratory tests and blood counts, there are Elisa fecal test kits available that can be used in the shelter to test for parvovirus. Routine general surveillance testing for the disease is not recommended. These antigen tests look for the presence of virus in the feces.

Parvo testing can present problems if the limitations of the tests are not understood. False negative results can be obtained because parvovirus is shed for a few days only or intermittently. Test results are most accurate if the test is administered the first few days after infection, or within 5 days after clinical signs appear. Weak positive results are often a result of recent vaccination with a modified live vaccine (within 5-12 days of vaccination).

Immunologists disagree about the impact of vaccination on the tests. Some believe the vaccine antigen should not generate a positive result at all while others believe the weak positive is a true positive. However, a positive test accompanied by clinical signs, a low white blood cell count or history of exposure to parvovirus should be interpreted as positive regardless of vaccination. False negatives can also result from blood in the feces.

Treatment

Parvovirus can have a higher mortality rate in shelter puppies despite early or aggressive therapy because of concurrent debilitation, parasitism and stress. However, if appropriate treatment is provided, the prognosis for these puppies to recover can be excellent, comparable to that for any other puppy. There is no specific cure, so treatment consists of providing supportive care, which consists of antibiotics to fight off secondary bacterial infections, fluids to correct the dehydration, broad spectrum deworming and control of the vomiting and diarrhea.

It was previously believed that all food should be withheld until vomiting and diarrhea had stopped, but new treatment modalities suggest that early enteral nutrition via a feeding tube may actually hasten recovery in some cases. Puppies that survive for 3-4 days generally have a good chance of making a full recovery within a week, but it must be remembered that they will be shedding virus for up to 14 days post infection.

Decisions to treat parvo in the shelter should be thought out carefully. Consideration should be given to the fact that

1. Shelters are not hospitals and seldom have the resources to provide proper isolation and treatment.
2. It takes several days of intensive care therapy to treat.
3. Sufficient recovery to reach adoptability may take 1-2 weeks.
4. The ability of the virus to persist in the environment long term endangers the lives of both the current and future residents.
5. Recovered puppies should still be isolated for at least 1 week post recovery because they may still continue to shed virus (remember that they can shed for 2 weeks post infection).

If a strict isolation area managed by staff skilled in maintaining the integrity of an isolation protocol is not available, animals with parvovirus should be removed from the facility for treatment or euthanized to curtail their suffering and minimize disease spread.

Prevention and Disease Outbreak Management

• Quarantine exposed and in contact animals for 2 weeks if possible.
• Isolate sick animals immediately.
• Clean and disinfect the entire facility, not just the kennel cages.
  
  Fortunately for shelters, regular household bleach (sodium hypochlorite) is one of the most inexpensive means of neutralizing parvovirus. It should be diluted 1 part bleach to 32 parts water (or 4 ounces of bleach to a gallon of water) to make it safe to use around animals yet still be effective.

  Unfortunately, many quaternary ammonium products commonly used in shelters such as Parvosol, Kennel Sol or A33 are labeled as parvocidal, but multiple studies over the past several years have proven they are not very effective.
  
  Another parvocidal disinfectant is potassium peroxymonosulfate or Trifectant^®. It is readily available to shelters and has been proven to be parvocidal. Three advantages of Trifectant over bleach are that 1) it has better activity in the presence of organic material and 2) it does not have to be prepared fresh daily like bleach and 3) it is less corrosive to metal surfaces or mucus membranes than bleach.
  
  Whichever disinfectant is used, the surface must be clean before the disinfectant is applied, and it should remain in contact for 10 minutes before rinsing.
  
  

• Instruct staff, volunteers and visitors about the dangers of spreading disease via fomites, particularly on hands and clothing.
  
  • Vaccinate puppies in the shelter starting at 5-6 weeks of age using a high-titer, modified live parvo vaccine, with boosters every 2 weeks, until 14-16 weeks of age. In the more susceptible breeds of dog, it may be advisable to vaccinate until 16-18 weeks of age.
    
  • Adult dogs may be vaccinated once with a modified live vaccine. In high risk situations it may be advisable to repeat the vaccination in 2 weeks, although this is normally not necessary.
    
• Deworm routinely with a broad spectrum anthelminitic.
  
• Feed the best diet affordable for the life stage and condition of the animals.
  
• Segregate puppies by litter and age groups, and always from adults.
  
• Use disposable toys and food dishes.
  
• Install hand sanitizers that use a product that is 70% alcohol. Although alcohol does not kill parvo, it will kill other pathogens.
  
• Utilize shoe and boot covers in isolation areas. Foot baths are only effective if they are properly maintained, meaning they are cleaned and refilled regularly. The best disinfectant to use in a foot bath is Trifectant because of its superior activity in the presence of organic material. The disinfectant should cover shoe treads.
  
• Restrict the use of cleaning materials to individual rooms or wards.
  
• Avoid using mops to clean.
  
• Launder clothing, bedding, towels, etc, in hot water, a good quality detergent and bleach. Do not overload the washing machine, and use a clothes dryer rather than hanging items up to dry.
  
• Avoid using common drain troughs unless they are properly maintained and dogs cannot touch the water and debris as it passes in front of their cages. High pressure hosing systems should be used with care in parvo outbreaks, as this tends to spread disease particles.
  
• Reduce stress!
  
  • Establish routines for cleaning, feeding, playing.
    
  • Provide bedding and toys (disposable or disinfectable) for enrichment.
    
  • Turn lights off at night so animals can sleep.
    
  • Reduce noise levels, play soft music.
• Vaccinate incoming animals at intake. The modified live parvo vaccine is an excellent vaccine that provides fairly rapid protection against viral challenge.

For more detailed information about the management of parvovirus in shelters, please go to the shelter medicine website managed by the University of California Koret Shelter Medicine Program.

Lila Miller, DVM, is Vice President of ASPCA Veterinary Outreach.