Canine Parvovirus (CPV)

In dogs, Parvovirus should be suspected whenever puppies are presented with lethargy, vomiting, &/or loose stools especially if there is blood in the stool (a hemorrhagic enteritis). Puppies 6 to 16 weeks of age are the most susceptible. This is due to the fact that puppies younger than this are usually protected by maternal antibodies they received in the mother's milk (colostrum). Older dogs have acquired immunity from vaccination or previous exposure and may not show significant clinical illness if they do become infected. Rottweilers, Doberman Pinschers, and English Springer Spaniels seem to be at greater risk for severe disease, but we see this devastating disease an all species of dogs.

CPV infects rapidly dividing cells of the intestine, bone marrow, and lymphatic tissues. The rapidly dividing cells of the intestine reside in valleys of the intestinal mucosal lining known as crypts. These cells then migrate to cover the hills of the mucosa known as villi. The virus is capable of severe enough damage to result in necrosis (cell death) of the intestinal crypt cells and this leaves no cells to cover the villi. The villi essentially are ulcerated, collapse, and digestive function is lost. Tremendous amounts of blood components and fluids are lost across this ulcerated mucosa and toxins and bacteria may be free to pass from the intestinal lumen to the bloodstream.

Rapidly dividing cells are also found in the bone marrow and the virus attacks these resulting in a drop in the dog's white blood cell count. Both lymphocytes and neutrophils are affected (these are types of white blood cells). Neutrophils are the predominant white blood cell found in circulation and function as part of the rapid first line of defense against infection. CPV infection causes low neutrophil counts by cell injury in the bone marrow and due to depletion of circulating cells responding to the tremendous amount of damage to the intestinal wall. The lower the white blood cell count, the worse the illness and counts less than 3000 mean the dog is not able to defend against bacteria found in the digestive tract and the environment.

Dogs become infected by coming in contact with stool or contaminated soil. Once the virus enters the dog's body, it first localizes in lymphoid tissues (day 2), then in the bloodstream (days 3-4), and then is found in intestinal crypts and the bone marrow (days 4-7) resulting in the onset of GI symptoms 5 days after infection. Symptoms include lethargy, anorexia, fever, vomiting (with or without diarrhea), bloody watery diarrhea, and blood in the vomitus. Due to the massive fluid losses across the intestine and the lack of intake, dogs rapidly become severely dehydrated, rapidly loose weight, and experience abdominal discomfort.

The severity of the illness is worse with stress; overcrowding; poor hygiene; other concurrent bacterial, viral or parasitic infections; and when secondary bacterial infections occur.

Puppies will die either from inadequate or late treatment, severe dehydration, electrolyte imbalances, hypoglycemia, anemia, hypoalbuminemia, hypothermia, and septic shock or overwhelming secondary bacterial infection due to low numbers of protective white blood cells. Hypoalbuminemia results from the loss of albumin from the bloodstream into the intestine and results in the inability to hold fluid within the blood vessels.

Diagnosis is by clinical signs, presentation (puppies 6-16 wks of age), white blood cell counts, and we use an in house ELISA test to identify virus in the stool. Virus can be detected in the stool only during the first few days of clinical illness. Virus is shed in the stool for no more than 2 weeks following infection.

Rehydration is the first cornerstone of treatment. We recommend the administration of intravenous electrolyte solutions (lactated ringers or plasmalyte) in large volumes with a continuous 24 hour IV drip. Healing and defense require that we promote a proper blood flow to all tissues. The rate of fluid replacement is tailored to the amount of fluid losses from vomiting and diarrhea. Dextrose may be added to the fluids both to provide energy to a body that cannot get it otherwise, but also to encourage increased vascular volume. After fluids have been initiated, potassium chloride is frequently added to the fluids to make up for total body potassium depletion. Antibiotics are routinely administered by injection to help defend against secondary bacterial infections, but in some cases the white blood cell count is dangerously low and aggressive combination antibiotic therapy is needed (aminoglycosides or fluroquinolones and penicillins, or single late generation cephalosporins such as ceftazidime). In cases where the blood count is falling, the dog's condition is expected to worsen, but when the blood cell count stops dropping and starts to increase, the dog is expected to improve. We also use drugs to help control vomiting (Reglan or metoclopramide, chlorpromazine, Anzemet, Zofran, or Cerenia), and in some cases these have a sedative effect to help the dog rest. In cases of severe bloody diarrhea, whole blood transfusions may mean the difference between success and failure. Again, when blood components are lost across the intestine, they may need replacement. Plasma transfusions accomplish this and are an especially good source of albumin. When plasma is not readily available, synthetic colloids such as hetastarch can be used to help hold fluid within the vascular system and to build vascular volume. Timely administration of these products can also mean the difference between success and failure.

Treatment costs for 24 hour intensive care can be quite costly. Anyone who has to pay the bill will wish they had pet insurance. Severe cases requiring more aggressive measures such as combination intravenous antibiotics, plasma transfusions, and Reglan continuous IV drips may result in charges in excess of twice the estimated amount in one 24 hour period. The usual course of infection results in 3-5 days of severe symptoms with most dogs recovering within a week. It may be an upward trend in the blood count, a willingness to take fluids by mouth, or just a general improvement in attitude that signals their recovery.

Some dogs experience only mild illness, but there are those that scrape the bottom for days, and there are dogs we loose in spite of our best efforts. Many clients elect to start with intensive care, but treatment must be tailored to fit their budget and the dog's condition. Once vomiting stops, small amounts of water and then bland food (ID) are offered and gradually the dog is returned to its original diet over the following 1-2 weeks. If concurrent or secondary infections are present, treatment for these needs to be finished. Once the virus runs its course, dogs usually return to normal with no long term ill effects and quickly make up for lost time. Dogs recovering from infection have a better acquired immunity than we can confer with vaccination.

Since its emergence in the late 70's, canine parvovirus has changed and the current viral agents may be even more deadly than those before. Infected dogs shed the virus for usually less than 2 weeks post infection. Once the virus is present in the environment it can persist and remain ineffective for long periods. Thus it is the ground that can serve as the largest reservoir and presents the greatest risk for exposure. The virus can be inactivated by sunlight, formalin, and sodium hypochlorite (bleach) in a 1:30 dilution. Prevention and control can be accomplished by strict attention to hygiene, isolation of puppies from environments that may contain the virus (the park, other dogs), immunizations, and acquisition of puppies from individuals or groups that adhere to these practices. Modified live vaccines provide protection from disease and prevent viral shedding but cannot be used in pregnant dogs. Killed vaccines can be given to pregnant dogs, but do not prevent viral shedding. In spite of early and continued vaccination against CPV, puppies can still become infected, in some cases due to interference from maternal antibodies that prevent them from responding to vaccination but are not protective against infection.

The best protection is vaccination. If you think your dog may have symptoms compatible with Canine Parvovirus infection, seek immediate veterinary care.