Parvovirus Infection In Your Dog

Parvovirus Infection In Your Dog

Parvo Virus Enteritis – CPV

Ron Hines DVM PhD

 Which vaccines for your dog and when

Corona Virus 

Parvovirus is the most common and dangerous viral diseases affecting dogs around the world. Until ~1978, that title was held by the canine distemper  virus. Parvovirus is extremely contagious. While the distemper virus dies rapidly in the environment, the parvovirus is very stable – capable of retaining its infectivity for many months (some says up to 2 years, given the right conditions).

The parvovirus is a tougher cookie in other ways – unlike the distemper virus, parvo is resistant to many disinfectants, drying, hot days (up to 80 C/176 F for 30 minutes won’t reliably kill it). It is resistant to cold temperatures and acidic environments as well. Parvovirus also directly attacks cells of the dog’s immune system – the very cells responsible for the dog’s protection against virus. A third troubling ability of the parvovirus is its capacity to “reinvent” itself from time to time through its high rate of natural mutation, along with its ability to merge with other strains of parvovirus when it encounters them in the body. 

Cats have always been plagued by their own parvovirus, the one causing panleukopenia. Some time in the 1960s or 70s, the cat parvovirus managed to mutate just enough to jump to dogs. Parvovirus is a particularly devastating disease in puppies and adolescent dogs – the younger the dog, the more likely that symptoms will be severe.  The vaccines veterinarians have are very effective and give long-lasting immunity to parvo – if they are given to puppies at the proper time.

If My Dog Catches Parvo, What Signs Might I See?

A key location for the parvovirus is within the immune cell patches scattered throughout the walls of your dog’s upper intestines. (read here) There, the parvovirus causes severe damage, both directly and by killing the lymphocyte cells crucial in preventing pathogenic bacteria from invading the pet’s intestinal walls (located in the Peyer’s patches). Not surprisingly, starting 3-5 days after infection, great numbers of the virus pass out in the dog’s stool. Parvovirus can continue to be shed in the dog’s stool for at least a month. (read here)

Very young puppies are a special case. The parvo virus has a preference for cells within the dog that are rapidly dividing. In very young puppies, those cells are not just the rapidly dividing lymphocytes but also the cells of the developing heart. So young pups often die rapidly from heart failure. Others are left with their heart function permanently impaired. 

But the signs you are more likely to see in an older puppy, adolescent or mature dog relate to what is going on in its intestines. Those signs generally begin 3-5 days after exposure to the parvo virus (some say as early as 2 days and as long as 14 days after exposure but I have never encountered those extremes).

The earliest signs are depression and a disinterest in food (anorexia). They may approach their food and water dishes halfheartedly to sniff now and then, but the progressing inflammation of their intestines is taking away their appetite.

That is soon followed by vomiting and diarrhea. The diarrhea produced has a distinctive foul odor and the chocolate color of digested blood. Some associate the newer CPV2c strain of virus with less of the traditional parvo stool odor and color.

Both the vomiting and diarrhea cause dehydration that adds to the dog’s depressed, dull state. Occasionally, fever plays a part in dehydration as well. Dehydration leads to the loss of ability of plucked up skin to bounce back. It assumes a doughy consistency that your veterinarian will notice. That, and the sunken eyes of illness (cachexia), often lead to a build up of material at the inner corners (medial canthus) of the eyes. But any form of dehydration will do that.

Fever is common during the first few days of illness. That, and occasional anemia resulting from intestinal blood loss, can cause rapid respiration and heart rate (tachypnea/tachycardia).

In the early and mid stages of serious parvovirus cases, affected dogs abdomens are often tight and tucked in. If you poke or knead these tummies, it is often quite obvious that it is painful to the dog. 

For unknown reasons, certain breeds appear more susceptible to severe parvovirus disease. Those breeds include rottweilers, pit bulls, pinschers, spaniels and German shepherds. The tendency of some breeds to have blood clotting issues might be a factor. 

Later in the disease, a weak pulse, subnormal temperature and other signs of shock often accompany severe cases. Many associate that with the toxins, produced by invading secondary bacteria, circulating throughout the animal’s body. (read here)

With the dog’s immune system disabled and the intestinal lining partially destroyed (“leaking”/lost integrity) by parvovirus, bacteria normally confined to the intestinal contents now have free passage into the body.

Parvovirus disease can be a downward roller coaster ride with a bottom that no one can accurately predict. The depth of that bottom depends on your dog’s age and general health, the number and strain of the parvovirus involved and any partial immunity it acquired from its dame that still persists in its bloodstream. In puppies that are still nursing, some vets theorize that high levels of anti-parvo antibodies in their mother’s milk protect locally in the pup’s intestine against parvo virus penetration – even after the window for antibody absorption has closed. 

It is also highly dependent on the veterinary care that your dog receives. That care gives their bodies’ immune system time to bounce back and produce its own antibodies that neutralize the parvovirus.  No medicine vets have can disable or kill the virus. I have noticed that dogs that look no worse two days in a row almost always survive when supportive medications, fluids and good nursing care are provided.

There are dogs, particularly small puppies, which do not survive this initial phase. Their body reserves are just too limited to battle the virus. Some actually die before diarrhea begins. Other things being equal, severity and outcome of parvovirus infections has a lot to do with the age of the dog involved. Healthy adult dogs that become infected can show few if any symptoms at all. Infected puppies that are fortunate, still retain some of their mother’s antibodies that lessen the chances of the infection being fatal.

Not all cases of severe diarrhea with parvo-like symptoms are caused by the parvovirus. Coronavirus of dogs, although rarely fatal, can cause similar initial signs. So can hookworms, bacteria and dietary indiscretions. Read about that here

If My Vet Suspects Parvo, What Test Might My Veterinarian Run?

With experience, veterinarians learn to recognize likely cases of parvo. At one time, that was done through a physical examination, history (= “signalment”, the age, breed, vaccination status, home environment, of your dog). Parvovirus crosses the mind of all veterinarians when a young dog is brought in with vomiting, diarrhea, depression and a spotty vaccination history.

Today, the linchpins for a positive diagnosis are the in-office tests that confirm the presence of the parvovirus itself ; but your  veterinarians will often notice other things abnormal on its blood work too:

Early in parvo disease, all or potions of your dog’s white blood cell count are often low (=  leukopenia, neutropenia, lymphopenia). (read here) Throughout the disease, liver enzymes are often elevated. Blood ions: sodium, potassium, chloride and bicarbonate, are often out of balance as well. When substantial leakage damage has already been caused to the pet’s intestinal lining, blood albumin, and globulin are often low as well. Parvovirus damage can even extend to your dog’s pancreas. When it does, blood levels of pancreatic enzymes might be high as well. (read here)

Do Today’s Veterinarians Have More Specific Ways Of Identifying Parvo Cases?


They use specially designed ELISA tests that search for parvovirus in your dog’s stool. When those tests fail to catch a parvovirus case, it is usually because it is still too early or too late in the infection for enough of the virus to be present. Your veterinarian can run this test in about  in 8-10 minutes. These tests are never foolproof. As I mentioned, they can be falsely-negative early in infections when parvovirus numbers are still low or late in infections when most virus have already been eliminated. Some say that watery stools are more likely to give false-negatives but I do not know if that is true. There are mixed opinions as to whether recently administered parvovirus-containing vaccinations produce false-positive test results. I tend to believe that they sometimes do.

Veterinarians also use more sophisticated PCR tests on samples of your dog’s stool that amplify and detect the presence of parvovirus’ DNA. These test are considerably more sensitive.  But they require that a fecal sample from your dog be sent to a laboratory that specializes in performing the test. So it takes much longer to get the test results back (too long to begin appropriate treatment) and PCR tests cost more. These tests can also have the same problem of deciding if the virus present is from a recent vaccination or due to a “wild” pathogenic strain of parvo. (read here) We know that dogs can harbor and shed vaccine origin parvovirus for at least 28 days after being vaccinated. (read here)

When either test returns unexpected results, many veterinarians glace back at your dog’s neutrophil count. Wild strains of parvovirus usually depress a dog’s neutrophil count early in the disease – firming up your veterinarian’s diagnosis decision.

Veterinarians can also look for a rise in your dog’s blood anti-parvo antibodies. That stead rise in anti-parvo antibody titre occurs in dogs that are successfully recovering from a parvovirus infection.  Since those tests require early and later blood samples, they are of little help in making an immediate diagnosis for a dog in trouble. However, a high protective parvo antibody level (titre) in a sick dog at admission would make your vet think twice about a diagnosis of parvo. 

What Treatments Might Help My Dog?

Veterinarians do not yet have a medication that will kill parvovirus inside your dog’s body. What your veterinarian can offer your dog is supportive care and hopefully an effective antibiotic barrier to defend it against secondary bacterial invaders while we wait for your dog’s own immune system to activate (“kick in”).

Dehydration is a very common finding in all severe cases of parvovirus disease. And significant (5-10% or more) dehydration affects your dog’s ability to keep crucial blood electrolytes in a normal balance. (read here). Dehydration also lowers the volume of blood available to transport nutrients to and eliminate waste from all body organs. So replacement fluids (“IV fluids” such as LRS) that contain replacement electrolytes are one of the key element of successful parvovirus treatment.

Additional ingredients are often added the these fluids before they are administered. (things like bicarbonate to bring the pet’s acid:base balance back into normal range, glucose for dogs  whose blood sugar has dropped dangerously low, potassium for those with hypokalemia .  (read here

In moderate to severe cases of fluid loss due to diarrhea, vomiting and a lack of interest in drinking, those fluids are best given intravenously. In milder parvo cases, they can be given subcutaneously in moderate amounts throughout the day. 

The second most important treatment is providing your dog with antibiotics to protect it from bacterial invasion and the effects of bacterial toxins. Antibiotics such as cefovecin/Convenia® or cefoxitin are frequently used. Ampicillin and second and third generation cephalosporins are still safe alternative antibiotics for dogs fighting parvo – particularly when combined with drugs like gentamycin, once the dog is adequately rehydrated. Aminoglycoside antibiotics like gentamycin can cause kidney and other damage in dehydrated patients. (read here)

Antibiotics are a very important element of parvovirus treatment because dogs with parvovirus disease have a diminished ability to fight bacterial infections of all kinds. That is because the parvovirus disables their immune system. When dogs will accept them and keep them down, oral antibiotics (such as amoxicillin and metronidazole) are helpful as well.

Fever, when it occurs at all, is only present early in parvo disease. It is rarely ever present in young puppies suffering from the disease. Body temperature support if your dog’s temperature drops to subnormal (hypothermia) is also quite important. No system or organ in a dog’s body, including its immune system, functions well when it is chilled.

Your vet will do as much as possible to decrease the inclination for these dogs to vomit. Medications to do so include drugs that work directly to suppress vomit-inducing centers in the brain (antiemetics such as maropitant/Cerenia®, metoclopramide and perhaps ondansetron/Zofran® ) (read here) and medications that coat the lining of your dog’s stomach and intestines (gastroprotectants such as sucralfate/Carafate®).

If parasite eggs (particularly blood-sucking hookworms) are found in your dog’s stool, gentle wormers (vermifuges) such as pyrantel/StrongidT®  might be indicated as well. It is not uncommon for dogs suffering from the nausea of a parvovirus infection to experience a situation where their intestine propels its contents in the wrong  direction (reverse peristalsis), so I often find mature roundworms in the vomit contents of parvo-infected dogs. 

Dogs with milder cases of parvo infection can often be coaxed to accept small amounts of liquid or liquefied diets and gels (such as  NutriCal®, Lixotinic®, Virbac Rebound®  or Purina CN®, given at frequent intervals. Years ago, many veterinarians were instructed that dogs with parvovirus infections should not receive any food PO (by mouth) because their intestines needed a “rest period”. But many now feel that small, frequent feedings might improve a dog’s chances of survival. We really do not know.

The most common drug used today to counteract the abdominal pain of parvovirus is probably buprenorphineFlunixin/Banamine®, used to reduce post-surgical pain in dogs, can actually make parvo-related intestinal and stomach problems worse. (read here)

Using these medications and procedures, veterinarians can usually save ~70-85% of the dogs that arrive infected with parvovirus. Those that are the toughest are the fragile young puppies that are so susceptible to shock and sudden death. But if they can make it through the first three or four days, many do recover.  Some are left with long-term disabilities such as heart damage and intestinal maldigestion/malabsorption issues that make it difficult for them to maintain normal body weight. (read here & here) The antibiotics given to these dogs to save their lives kill the good as well as the bad bacteria. That results in other opportunistic bacteria eventually taking their place (dysbiosis). One study found that a fecal bacterial transplants (FMTs) from a healthy dog seemed to improve that problem. (read here)

Once your dog has been stabilized by your veterinarian, it is possible to treat many cases of parvovirus in your own home under your veterinarian’s staff supervision  –  if  you have the temperament to do so. Doing so is a very labor-intensive and emotionally draining process. But when done well, it can mean the difference between life and death when economics or circumstance do not permit longer in-hospital stays. Once your dog is stabilized, obliging veterinarian technicians can teach you how to give fluids subcutaneously and how to monitor your pet’s vital signs. Not all veterinary clinics have rules that allow that. When done right, success rates for at home treatment are high – but probably never quite as high as good in-hospital care. 

There was also a time when veterinarians though that a transfusion of blood serum obtained from a dog that had recovered from parvovirus might help save the life of a parvo dog. I had many successes doing that when young puppies were severely ill with parvovirus disease. I kept a blood donor Labrador retriever and pet at my animal hospital where repeated contact with the parvovirus and hyperimmunity was likely. But with time veterinary college professors disparaged that treatment. To the best of my knowledge, they had no evidence that blood serum taken from recovered dogs was not beneficial. Recently more studies confirm that such hyperimmune serum might be quite beneficial. (read herehere , & here)

There are also drugs that will decrease intestinal movement (motility) and therefore diarrhea (eg diphenoxylate/Lomotil®). However, the material in a parvo-infected dog’s intestine is usually filled with bacterial toxins. We want those toxins out of your dog – expelled in its diarrhea – not trapped in its body. So it is considerably safer to replace lost fluids by injection or orally but try to keep intestinal flow active.

There are dog owners who are flabbergasted at the current cost of parvovirus intensive care. They mistakenly assume that the hospital staff pockets that money. That is not the case. The current cost of a 10 ml bottle of Zoetis’ Convenia® antibiotic in the United States as I write this article is $324.35. Once reconstituted, I have to discarded the refrigerated remainder after 56 day. 

Does The Strain Of Parvovirus That My Dog Caught Matter And Is The Vaccine Always Effective?

There do appear to be stronger (more pathogenic/more virulent) and less pathogenic strains of parvovirus circulating throughout the world. But the reports are all veterinarian’s in-the-field impressions – not controlled studies. Without controlled studies, one can never be sure if one dog’s success in fighting off a parvovirus attack was due to characteristics of the virus or characteristics of that particular dog. We are just not sure of the facts yet. (read here)

There are also a number of reports that certain newer strains of parvovirus might also have the ability to override vaccine protection. (read here).  Others disagree. (read here) There is no confirmed evidence that I know of that any of the major pharmaceutical company’s parvovirus vaccines do not protect equally well against all strains of parvovirus your dog is likely to encounter. Discussions and chatter occur from time to time regarding a newer emerging parvovirus strain causing more severe disease or a strain with enhanced ability to override maternal antibody protection when parvo vaccines were given at too young an age. But none of that has every been documented. 

I believe that the most important factor governing how effective your dog’s “puppy shots” are going to be is at what age they were given and if the manufacturer’s instructions on injection technique were followed – not which brand was given or which strains of wild parvovirus your dog chances to meet. As with canine distemper (which is generally included in the same puppy shots) the vaccine’s effectiveness is cancelled when the residual immunity the puppy received from its mother (maternal antibodies passed to the pup through early nursing) is still present in your puppy’s body. The manufacturers of those vaccines claim their products can override maternal immunity. But how much and when varies from puppy to puppy and situation to situation. That is why that last vaccination, at ~18 weeks of age is so crucial to your pup’s wellbeing. Most receive full protection at 16 weeks of age. But you cannot be 100% sure – short of having its antibody titre measured. If that last vaccination fails, it is generally because the parvovirus was already incubating in its body. Vaccines take time to work. They do not protect your dog against viruses that are already present in its body.  Read about that here . Vaccinations are also more likely to fail when puppies are severely parasitized, nutritionally deprived or under other forms of stress. 

Can My Dog Catch Parvo More Than Once?

I have never seen nor heard of that happening. A dog that has recovered from a confirmed parvovirus infection is believed to be immune for the rest of its life. That is why frequent parvo revaccination of adult dogs is a waste or time – or worse.

If I Decide To Get Another Dog How Long Should I Wait? 

It is considerably easier to make your new dog immune to parvo than it is to clean up a parvo-contaminated environment. Wait a few months before considering purchasing or adopting another dog. The whole family needs to come to terms with the loss of a pet. If you long for immediate companionship, consider fostering an adult (vaccinated) dog from a rescue group. They are always hot to make those fostering situations permanent. Resist that until everyone has come to terms with their loss and is comfortable with your decision. When you do find your dog, see to it that it is immune to parvo before it joins your family. The blood antibody titre test for parvo that I mentioned earlier will confirm that.

What About The Parvo Cures I See For Sale On The Internet?

Some internet sites suggest vitamins, probiotics, prebiotics, glucosamine, various homeopathic remedies and other snake oil treatments will cure your dog of canine parvovirus disease. Others suggest a diet of mashed potatoes and cottage cheese to get your dog through the infection. None of that has ever been proven to be the least helpful. When you purchase those items online or in a pet store, at best, they will cause your dog no harm. 

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