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.
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.
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.
Parvovirus affects the digestive system and the heart. The signs can vary widely:
View Our Interactive Parvo Timeline
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.
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
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.
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.Photo Credit: Dr. Stephanie Janeczko