What is Parvovirus? Causes, Treatment and Prevention

January 9, 2018 /

There are numerous threats to our pets by infectious organisms, but few as capable of causing severe illness or death than parvovirus.

The likelihood is high that once infected with parvovirus your pet will require significant medical intervention and, if left untreated, the disease typically progresses to death. Fortunately, parvovirus can be prevented through vaccinations and a series of lifestyle practices focused on avoidance of the virus or the locations it is known to lurk.

What is Parvovirus?

Parvo is known in medical terminology as canine parvovirus type 2 (CPV) and is commonly referred to as “parvo.” The virus is one of the littlest, measuring less than 30 nanometers in diameter, and has a name derived from the Latin term parvus, meaning small. Besides dogs, parvo is capable of infecting multiple species, including wildlife like foxes, coyotes, raccoons, skunks, and wolves. Cats also can be infected by parvo’s relative, feline panleukopenia virus (FPV). Although many species are at risk, in this article I am going to focus primarily on parvovirus as it affects our canine companions. Unvaccinated dogs, dogs less than four months of age, and those without fully functioning immune systems are most at risk. Certain breeds are more susceptible to infection with parvo, including American Pit Bull Terriers, Doberman Pinschers, English Springer Spaniels, German Shepherds and Rottweilers.

How Does Parvovirus Infect Its Host and Cause Illness?

Parvo is very virulent in that only a small number of viral particles are needed to cause infection. These particles can remain infectious outside of the body for approximately five months, depending on environmental factors that support or deter viral maintenance, such as humidity, sunlight, and temperature. Infection relies upon viral particles having direct contact with mucus membranes/body tissues. Parvo enters the body through the mouth and potentially through the nose, as the back of the nasal cavity connects to the throat in a common area called the pharynx. Infection occurs when the mouth contacts anything containing or having been contaminated by parvo, including:

Surfaces - cages/crates, floors, blankets, towels, etc.
Substances - feces, diarrhea, soil, grass, etc.
Body parts - patient’s own paws, fur, caretaker’s hands, etc.
Clothing - shoes, uniform, etc.
Medical equipment - stethoscope, rubber gloves, gowns, etc.

The lymph nodes at the back of the throat are the primary sites that are recipient to parvo, which then replicates inside white blood cells. White blood cells are immune system cells responsible for recognizing and fighting infection, managing inflammation, cleaning up tissue damage, and other tasks crucial to day-to-day organ system maintenance. As parvo replicates in the lymph nodes, the lymphatic vessels permit it to reach the bone marrow and the digestive tract. Inside the bone marrow, parvo negatively impacts the production of new white blood cells. When white blood cell production decreases, it increases the potential for infection with bacteria, fungi, parasites, and viruses. In the digestive tract, parvo damages the inner-most parts of the intestines. The villi are finger-like projections coating the inner intestine that are further lined by microvilli, which are microscopic protrusions into the lumen of the intestine. The lumen is the hollow, central portion of a tubular loop of intestine. The microvilli enhance the surface area for fluid and nutrient absorption and are covered by cells called the crypts of Lieberkuhn. These crypts are parvo’s prime intestinal targets and severe damage prevents formation of new cells to line the microvilli and reduces absorption of water and nutrients. When sufficient damage to crypt cells occurs, severe digestive tract upset called parvoviral enteritis ensues. Without the protection from the crypt cells, bacteria that normally thrive in the intestine can move into the blood stream and cause a blood-borne infection called sepsis. Severe dehydration, multi-system organ failure (kidneys, liver, etc.), low blood pressure, abnormal blood clotting (coagulopathy), and other severe secondary effects of parvo ultimately lead to death.

How is Parvovirus Diagnosed?

Parvo is diagnosed multiple ways, including the observation of clinical signs of illness and a veterinarian’s physical examination and diagnostic testing. Post-exposure, a pre-patent period of three to seven days is needed for parvo to incubate and produce clinical signs. Parvo’s clinical signs are similar to those caused by infections with microorganisms (bacteria, parasites, etc.), toxic exposures, and other causes and can include: Lethargy and behavioral changes - Dogs infected by parvo initially seem normal, but as the pre-patent period progresses they will appear lethargic and less-able to exercise. Lethargy worsens as parvo prospers inside the body and secondary changes occur. Decreased appetite and water consumption - Hyporexia (decreased appetite) and anorexia (no appetite) are common in pets suffering from parvo. Additionally, reduced water consumption leads to dehydration. Vomiting - Because of parvo’s severe damage to the digestive tract, and the secondary effects of the virus in the body affecting the immune system, vomiting is commonly seen. Diarrhea - Parvoviral diarrhea is typically characterized by being soft to watery, explosive, urgent, malodorous, and containing blood and/or mucus. The characteristic aroma is caused by parvo’s severe toxic effect in the intestines and the presence of blood. Having smelled this horrible malodor many times before I can personally attest to the distinctive smell of parvovirus-associated diarrhea. Elevated or decreased body temperature - Normal body temperature is 100-102.5F (+/- 0.5F). As the immune system mounts a response to parvovirus infection there’s an associated elevation in body temperature called hyperthermia. As dehydration occurs, or the body’s thermoregulatory abilities are overwhelmed, temperature plummets and hypothermia occurs. Abnormal mucus membrane appearance and prolonged capillary refill time - Parvo-infected dogs suffering from fluid or blood loss can have abnormally-colored gums, appearing pale pink, white, blue, or grey, and feel dry or tacky. Dehydrated patients show prolonged capillary referral time (CRT), which is the duration needed for the color to return to the mucus membranes upon being pressed to push out the blood. Normal CRT is one to two seconds. Since many of the clinical signs of parvo are also seen with a variety of other ailments, diagnostic testing is crucial in achieving an official diagnosis and directing appropriate treatment, including: Fecal testing - Infections from parasites, bacteria, and other viruses can cause symptoms similar to parvo. Therefore, fecal testing is crucial in ruling out some of these other potential causes of digestive tract illness. Parvo ELISA (enzyme-linked immunosorbent assay) is a type of test for antigen in feces which is commonly used as a first line of detection due to its advantages of being run in-facility and producing results within 15 minutes. An antigen is an infectious organism (like a virus, bacteria, parasite, etc.), toxin, or other substance that stimulates the immune system to produce antibodies. The ELISA test contains parvo antibodies, which bind to parvo antigen and produce a color change that is evaluated as a weak or strong positive result. Lack of color change can be interpreted as a negative result, yet false negatives can occur when there is minimal parvo being shed in the digestive tract, such as during the pre-patent period. False positives can also occur within a few days after vaccination, as the vaccine contains a modified-live virus. A positive ELISA test result combined with clinical signs of digestive tract upset merits cause for concern, further testing, and treatment. Parvo ELISA tests are also capable of detecting feline panleukopenia virus (FPV). Polymerase chain reaction (PCR) testing is a more-accurate fecal test than ELISA as it assesses antigens specific to a parvo-infected patient and can detect very small volumes of antigen. A disadvantage to PCR testing is that it must be sent to an external laboratory and takes days to get results. Blood testing - Numerous blood tests exist that help assess a patient's baseline whole body health and potential infection with parvovirus. When speculation exists as to a patient’s parvo positive status based on fecal testing, then a blood test called a complete blood count (CBC) can strengthen or reduce the likelihood of diagnosis. CBC assesses the levels of red and white blood cells and platelets existing in the circulatory system. Since parvo rapidly attacks the bone marrow, the CBC can reveal a low white blood cell count (leukopenia) that is characteristic of the illness. More specifically, a low lymphocyte count (lymphopenia) can be seen. Chemistry testing yields information about the kidney, liver, pancreas, electrolytes, calcium, blood proteins (albumin, globulin, total protein), muscles, thyroid, and more. Blood pressure - Parvo leads to dehydration which is associated with decreased blood pressure. When blood pressure decreases, there is a commensurate reduction in delivery of fluids, oxygen, nutrients, removal of toxins, metabolic wastes, other harmful substances from all body tissues. Radiographs (x-rays) - Since parvo negatively impacts the digestive tract, abdominal radiographs can yield helpful information about the stomach, small and large intestine, kidneys, liver, spleen, and other organs. Thoracic radiographs may also be needed to assess the chest cavity for concurrent illness, like aspiration pneumonia occurring because of a patient inhaling vomit. Radiographs are minimally invasive and require short-term mild restraint for proper positioning to attain the best-possible images. Ultrasound - More-advanced imaging besides radiographs may be needed when diagnosing and treating parvo. Ultrasound permits a more-thorough evaluation of abdominal organs than radiographs, as it creates a real-time, moving image (while radiographs allow for only a static image). As with radiographs, abdominal ultrasound is minimally invasive and requires only mild restraint for proper positioning.

How is Parvovirus Treated?

If left untreated parvo has about a 90% mortality rate. Yet, with appropriate treatment survival rates can be as high as 90%. Parvo is treated by supporting the body in its process of eliminating the virus. As so many systems are negatively impacted we veterinarians are always striving to get ahead of the virus’ damaging effects. Since parvo causes such severe illness, hospitalization is crucial to increase the likelihood of survival and permit a thorough approach to treatment, including: Isolation - Due to parvo’s high virulence, infected patients need to be confined to an area of a hospital where the virus won’t be carried to susceptible patients. Many hospitals are equipped with isolation rooms having negative pressure air flow to keep the virus from escaping and affecting other environments in the immediate area. Face masks, gowns, gloves, and other materials involved in the care-provision process should be single-use and disposed of or remain in the isolation room should such resources be limited. Since parvo is shed for up to 14 days post-infection, it’s crucial that all affected patients continue to be isolated and have their feces properly disposed of even if clinical signs of illness have resolved. Fluid therapy - Fluid therapy can be delivered many ways, including by mouth (per os, or PO), intravenously (IV, or into the vein), or subcutaneously (SC, or under the skin). Since parvo-infected patients show mild to severe digestive tract upset like vomiting, trying to get sufficient fluids in by mouth isn’t usually a viable treatment option. Intravenous fluids are the best option, as when fluids are delivered into the vein, tissues in-need of hydration are most-effectively hydrated through the rapid expansion of blood volume. Injectable medications (antibiotics, antacids and anti-nausea, B-Complex vitamins, etc.) can be given directly into the IV fluid line to have an enhanced effect as compared to PO or SC administration. Hospitalization is required for IV fluids, which increases the cost of treatment as compared than PO or SC options. Subcutaneous fluids are the next best option after intravenous, but delivering the volume of fluids needed to combat the severe dehydration caused by parvo-associated large bowel diarrhea can be logistically improbable. Thermoregulation - Low body temperature (hypothermia) is commonly associated with parvo infection and dehydration, so providing external sources of warmth can normalize body temperature. Warm towels and IV fluids, or forced-air warming systems, can be used provided sufficient care is used in cleaning the products post-treatment. Antibiotics - There are a variety of bacteria that live inside and on the surface of the body that can potentially sicken a patient should the bacteria proliferate either in the normal location or translocate to another body part/system. The digestive tract is one body system having millions of bacteria that serve their purpose in digesting food and maintaining normal function. Parvo’s damage to intestinal crypt cells stops the protective barrier and permits bacteria to access the blood stream and negatively impact organs like the liver, kidneys and heart. Anti-nausea and antacid drugs - Parvo’s severe effect on the digestive tract causes nausea and increases potential for vomiting. If vomiting occurs frequently then stomach acid can irrupt into the esophagus, which overtime can cause enough inflammation that tissue scarring occurs. Antacid medication (Famotidine, Omeprazole, etc.) reduces production of gastric acid and can both reduce the negative effects on the esophagus and nausea. Anti-nausea medication (Ondansetron, Maropitant, etc.) acts directly on the brain’s vomiting center to reduce nausea and potentially improve appetite. Appetite stimulants (Mirtazapine, cannabidiol , etc.) can promote a pet’s desire to eat. Probiotics - The digestive tract is the foundation of a pet’s immune system, as the small and large intestine are rich in a variety of beneficial bacteria serving to facilitate digestion and waste elimination called probiotics. The small intestines contain Lactobacillus, which are the most plentiful type and aid in digestion. The large intestine (colon) has plentiful Bifidobacteria and Enterococcus responsible for fecal formation and storage, toxin elimination, and water absorption. Probiotic supplements (such as Honest Kitchen's Instant Goat's Milk) should be selected with the emphasis in supporting both the small and large intestine. Dogs afflicted with proviral enteritis benefit from probiotic therapy as a means of promoting intestinal health in the face of the severe damage caused by parvo and the removal of the body’s own beneficial bacteria associated with antibiotic therapy. Fecal transplants are a novel treatment where a donor’s feces are ingested by the recipient in a capsule or in some food format to replenish lost bacteria. Blood product transfusion - Besides intravenous fluids, blood products like plasma and whole blood can be intravenously infused and can improve hydration, blood pressure, and the overall condition of the patient. Plasma is the component of blood lacking red blood cells and platelets (cells responsible for blood clotting) that is rich in important blood proteins like albumin, white blood cells, antibodies, blood-clotting factors, and other health-yielding substances. Whole blood has the same components as plasma but also contains red blood cells and platelets to enhance oxygen delivery and blood clotting and would be appropriate for a parvo patient suffering from low red blood cell count (anemia), low platelet count (thrombocytopenia), or when plasma isn’t available. Moist, human-grade, whole-food diets - Once vomiting is under control and the parvo patient is eating, moist food format provides much-needed hydration and enhances the removal of toxins and metabolic waste. I recommend all pets eat a moist diet regardless of their state of illness or wellness. Human-grade, whole-food diets are generally less-likely to contain mycotoxins as compared to feed-grade diets like kibble and some canned foods. Such is why I’m such an advocate of pets eating diets like The Honest Kitchen.

Can Parvovirus be Prevented?

Fortunately, parvo can be prevented through the combined efforts of vaccination and proper sanitary conditions. Although parvo may lead to some degree of immunity, attaining a protective level of immunity is unreliable and associated with a high degree of sickness. Therefore, vaccination is tantamount to achieving optimal immunity and minimizing morbidity and mortality. Parvo is considered a Core vaccination, which means immunization produces a reliably protective level of antibodies and the disease we are striving to prevent is one that is potentially fatal. Non-Core vaccinations (Bordetella, etc.) don’t always produce a protective level of immunity and the diseases against-which we are vaccinating generally don’t cause significant morbidity or mortality. Puppies are born with residual immunity from their mother that wanes over the days to weeks after they are weaned (no longer nursing) off their mother’s milk. Dogs start their puppy series of parvo vaccinations around six weeks of age, as vaccines given earlier in life when nursing may be less effective due to maternal antibody interference. Parvo vaccinations are given every three to four weeks until three or four have been given. Parvo can be given as a single vaccination agent, but it’s commonly combined with Distemper virus, Adenovirus, and Parainfluenza virus. Two to three weeks are necessary for a puppy to mount a sufficient immune response to vaccination. As a result, it’s crucial to prevent exposure to dogs of unknown vaccination status, those recently in the shelter system, and any dog showing clinical signs of an illness, along with locations where other canines congregate (daycare, dog parks, etc.) or defecate (common elimination areas on city streets, etc.). These guidelines still apply after the vaccination series is complete and performing a blood test called an antibody titer two to three weeks later can determine if a protective parvovirus antibody level has been produced or if a booster vaccination is needed. Prevention is truly the best medicine when it comes to parvo. Work with your veterinarian to create an immunization strategy that works best for your pup and create a lifestyle plan that minimizes exposure to the virus. Has your pet been infected with parvo? Feel free to share your perspective in the comments section.

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