Canine Parvovirus (CPV)
In dogs, parvovirus should be suspected whenever puppies are lethargic, vomiting, and / 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 younger puppies are often protected by the maternal antibodies they received in their mother’s milk (colostrum). Older dogs have acquired immunity from vaccination or previous exposure and may not show significant clinical disease if infected. Rottweilers, Doberman Pinschers, and English Springer Spaniels appear to be at increased risk of severe disease, but we see this devastating disease in 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 in the lining of the intestinal mucosa known as crypts. These cells then migrate to cover the hills of the mucosa known as villi. The virus is capable of causing serious enough damage to cause necrosis (cell death) of the cells in the intestinal crypt, leaving no cells to cover the villi. The villi essentially ulcerate, collapse, and digestive function is lost. Huge amounts of blood components and fluids are lost through this ulcerated mucosa, and toxins and bacteria can pass freely from the intestinal lumen into the bloodstream.
The rapidly dividing cells are also found in the bone marrow and are attacked by the virus, causing a decrease 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 cells found in the circulation and function as part of the first line of rapid defense against infection. CPV infection causes low neutrophil counts due to cell injury in the bone marrow and due to the depletion of circulating cells that respond to the enormous amount of damage to the intestinal wall. The lower the white blood cell count, the worse the disease, and counts below 3000 mean that the dog cannot defend itself against bacteria found in the digestive tract and environment.
Dogs become infected by coming into contact with contaminated soil or feces. Once the virus enters the dog’s body, it is first localized in the lymphoid tissues (day 2), then in the bloodstream (days 3-4), and then found in the intestinal crypts and bone marrow (days 4- 7) resulting in the onset of gastrointestinal symptoms 5 days after infection. Symptoms include lethargy, anorexia, fever, vomiting (with or without diarrhea), bloody watery diarrhea, and blood in the vomit. Due to massive losses of fluids in the intestine and lack of intake, dogs quickly become dehydrated, lose weight quickly, and experience abdominal discomfort.
The severity of the disease worsens with stress; overpopulation; Poor hygiene; other competing bacterial, viral or parasitic infections; and when secondary bacterial infections occur.
Puppies will die due to improper or delayed treatment, severe dehydration, electrolyte imbalances, hypoglycemia, anemia, hypoalbuminemia, hypothermia, and septic shock or an overwhelming secondary bacterial infection due to low numbers of protective white blood cells. Hypoalbuminemia is the result of the loss of albumin from the bloodstream to the intestine and results in the inability to retain fluid within the blood vessels.
Diagnosis is based on clinical signs, presentation (puppies 6 to 16 weeks old), white blood cell count, and we use an internal ELISA test to identify the virus in feces. The virus can be detected in stool only during the first days of clinical illness. The virus is shed in the stool for no more than 2 weeks after infection.
Rehydration is the first cornerstone of treatment. We recommend the administration of electrolyte solutions intravenously (lactated Ringer or plasmalyte) in large volumes with a continuous 24-hour intravenous drip. Healing and defense require that we promote adequate blood flow to all tissues. The fluid replacement rate is tailored to the amount of fluid loss from vomiting and diarrhea. Dextrose can be added to fluids both to provide energy to a body that cannot otherwise obtain it, and to stimulate increased vascular volume. Once fluids have been started, potassium chloride is often added to fluids to compensate for total body potassium depletion. Antibiotics are usually given by injection to help ward off secondary bacterial infections, but in some cases the white blood cell count is dangerously low and a combination of aggressive antibiotics is needed (aminoglycosides or fluroquinolones and penicillins, or next-generation cephalosporins such as ceftazidime). In cases where the blood count is decreasing, the dog’s condition is expected to worsen, but when the blood cell count stops decreasing and begins to rise, 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 can mean the difference between success and failure. Again, when blood components are lost in the intestine, they may need to be replaced. 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 retain fluid within the vascular system and increase vascular volume. Timely stewardship of these products can also mean the difference between success and failure.
Treatment costs for 24-hour intensive care can be quite expensive. Anyone who has to pay the bill will wish they had pet insurance. Severe cases that require more aggressive measures, such as the combination of intravenous antibiotics, plasma transfusions, and Reglan continuous intravenous drip, can generate burdens in excess of twice the estimated amount in a 24-hour period. The usual course of infection results in 3-5 days of severe symptoms and most dogs recover within a week. It may be an upward trend in your blood count, a willingness to drink fluids, or just a general improvement in attitude that signals your recovery.
Some dogs only experience mild illnesses, but there are some that scratch their bottoms for days and there are dogs that we lose despite our best efforts. Many clients choose to start with intensive care, but the treatment must be tailored to their budget and the condition of the dog. Once the vomiting stops, small amounts of water and then soft food (ID) are offered and the dog gradually returns to its original diet over the next 1-2 weeks. If there are concurrent or secondary infections, it is necessary to end the treatment. Once the virus runs its course, dogs generally return to normal with no long-term ill effects and quickly make up for lost time. Dogs that recover from an infection have better acquired immunity than we can confer with vaccination.
Since its appearance in the late 1970s, canine parvovirus has changed, and current viral agents can be even more deadly than previous ones. Infected dogs shed the virus for less than 2 weeks after infection. Once the virus is present in the environment, it can persist and remain ineffective for long periods. Therefore, it is the soil that can serve as the largest reservoir and presents the greatest risk of exposure. The virus can be inactivated with sunlight, formalin, and sodium hypochlorite (bleach) at a dilution of 1:30. Prevention and control can be achieved with strict attention to hygiene, isolating puppies from environments that may contain the virus (the park, other dogs), vaccinations, and acquiring puppies from individuals or groups that adhere to these practices. Modified live vaccines provide protection against disease and prevent viral shedding, but cannot be used in pregnant dogs. Killed vaccines can be given to pregnant dogs, but they do not prevent viral shedding. Despite early and continuous 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 do not protect against infection.
The best protection is vaccination. If you think your dog may have symptoms consistent with canine parvovirus infection, seek immediate veterinary attention.
