Parvovirus Infection In Your Dog

Parvo Virus Enteritis – CPV

Ron Hines DVM PhD

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 80C/176F for 30min 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. (ref1, ref2, ref3, ref4)  

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. (ref) 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. (ref) 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. (ref)

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. (ref)

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 half-heartedly 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. (ref)

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. (ref1, ref2)

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. (ref)

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. (ref)   So can hookworms, bacteria and dietary indiscretions. (ref)

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

With experience, veterinarians learn to recognize likely cases of parvo. At one time, that was done through a physical examination, history (= signalment: e.g. age, breed, vaccination status, home environment, etc.). 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 the pet’s white blood cell count are often low (neutropenia, leukopenia, lymphopenia). Throughout the disease, liver enzymes are often elevated. Blood ions: sodium, potassium, chloride and bicarbonate, are often out of balance. When substantial damage has been caused to the intestinal lining, blood albumin and glucose levels are often low as well. Even pancreatic enzymes can be elevated. (ref)

Today, veterinarians have more specific ways of identifying parvo cases:

They use ELISA tests that search for parvovirus in your dog’s stool using custom-created tagged antibodies. (ref) When those tests fail to catch a case, it is usually because it is too early or late in the infection for enough of the virus to be present. The test can be run by your veterinarian in 8-10 minutes. (ref1, ref2) These tests are never foolproof. As I mentioned, they can be false-negative early in infection when virus numbers are low or late in infection when most virus have already been eliminated. (ref) 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 recent parvo-containing vaccinations produce false-positive test results. I tend to believe they sometimes do.

Veterinarians also use more sophisticated PCR tests on samples of your dog’s stool that amplify and detect the parvovirus’ DNA. These test are newer and 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 longer to get back the results and it costs more. These tests can also have problems deciding if the virus present is from a recent vaccination or due to a “wild” pathogenic strain of parvo. (ref) We know that dogs can harbor and shed vaccine origin parvovirus for at least 28 days after being vaccinated. (ref)

When either test returns unexpected results, many veterinarians go back to considering the degree the wild virus usually depresses a dog’s neutrophil count early in the disease in firming up their final diagnosis decision.

One can also look for a rise in anti-parvo antibodies in the blood of 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 (titer) on a sick dog at admission would make your vet think twice about a diagnosis of parvo. (ref)

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 pet is supportive care and an antibiotic wall against secondary bacterial attack while we wait for your dog’s own immune system to “kick in” (activate).

Dehydration is a standard finding in all severe cases of parvo. And significant (5-10% or more) dehydration affects your dog’s ability to keep crucial blood electrolytes at normal levels. (ref). It also lowers the volume of blood available to transport nutrients and waste to and from all body organs. So replacement fluids (such as LRS) that contain replacement electrolytes are probably the key element in successful parvovirus treatment.

Often additional ingredients are added the these fluids before they are given. (things like bicarbonate to bring the pet’s acid:base balance back into normal range (ref) , glucose for dogs  whose blood sugar has dropped, potassium for those with hypokalemia and starch compounds (eg hetastarch or pentastarch/Pentaspan®) to prevent leakage of blood into tissues (due to decreased osmotic (oncotic) pressure). (ref)

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 often be given subcutaneously.

The second most important treatment is providing the dog with antibiotics to protect it from bacterial invasion and the effects of bacterial toxins. Antibiotics such as cefovecin/Convenia® or cefoxitin are frequently given. 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.

These antibiotics are very important 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, is only present early in parvo disease – and rarely ever present in young puppies. As time goes by, body temperature support if the dog’s temperature drops to subnormal (hypothermic) is also quite important. (ref) No system or organ in the dog’s body functions well when 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® [ref] ) and medications that coat the stomach and intestines (gastroprotectants such as sucralfate/Carafate® ).

If parasite eggs (particularly hookworms) are found in your dog’s stool, wormers (such as pyrantel/ StrongidT®) might be given as well.

Dogs with milder cases of parvo 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. Some years ago, many vets thought that dogs with parvo should not receive food because their intestines needed a period of “rest”. But most now feel that small, frequent feedings improve their chances of survival.

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

Read more about standard parvo treatment protocols in detail here  and here

Using these medications and procedures, veterinarians can often save ~70-85% of the dogs that arrive infected with parvo. Those that are the toughest are the fragile young puppies that are so prone to shock and sudden death. If they can make it through the first three or four days, many do recover.  However, some are left with long-term disabilities. (ref)

Once your dog has been stabilized by your veterinarian, it is possible to treat many cases of parvovirus in your home under your veterinarian’s staff supervision   – if you have the temperament to do so. It is a very labor-intensive and emotionally draining process. But done well, it can mean the difference between life and death when economics or circumstances do not permit longer in-hospital care. Once your pet is stabilized, your veterinarian’s technicians will teach you how to give fluids subcutaneously and how to monitor vital signs. When done right, success rates for at home treatment are high – but never as high as for in-hospital care. (rptref)

There are dog owners 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 10ml bottle of Zoetis’ Convenia® antibiotic in the USA is $288.70. Once reconstituted, it must be discarded in less than 2 months.

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. Most infectious disease specialists no longer believe that is true.

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 keep intestinal flow intact.

Dogs that recover from a severe case of parvo can be left with difficulties digesting food and maintaining body weight. We do not know if that is because of scarring that occurred in their intestines, organ damage (ref), or changes in the bacteria that inhabit their intestines (their microflora). Those bacterial changes (dysbiosis) might have been due to the antibiotics given to save their lives. (ref)   Recent studies found that fecal bacterial transplants (FMTs) from a healthy dog seemed to improve that problem. (ref)

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 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. (ref)

There are also a number of reports that certain newer strains of parvovirus might also have the ability to override vaccine protection. (ref1, ref2, ref3).  Others disagree. (ref)

So through 2018, there is no confirmed evidence that the major maker parvovirus vaccines to not protect equally well against all strains of parvovirus your dog is likely to encounter. (ref1, ref2

Others associate some of the newer emerging strains with more severe disease (ref); still others, with enhanced ability to override maternal antibody protection when parvo vaccines were given at too young an age. (ref)

But I believe that the most important factor governing how effective your dog’s “puppy shots” are going to be is when they were given, not which brand was given or which strains of wild parvovirus your dog chances to meet. (ref) 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 the 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 for your pup’s wellbeing. Most receive full protection at 16 weeks of age. But you cannot be 100% sure – short of having it titer 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 against virus that are already in the body. (ref

Vaccinations are also more likely to fail when pups are severely parasitized, nutritionally deprived or suffering other forms of stress. (ref)

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 titer test for parvo that I mentioned earlier will confirm that.

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

Some websites suggest vitamins, probiotics, prebiotics, glucosamine, various homeopathic remedies, other snake oil treatments and mashed potatoes with cottage cheese to cure parvo. None of that has ever been proven to be helpful.  When you purchase those items online or in a pet store, at best, they will cause no harm.

I did for a time incorporate yunnan paiyao tablets and their safety pills in my treatment plan. I purchased them and their safety pills from a Chinese pharmacist in Manhattan. They were standard issue in the Chinese army to stem hemorrhage. No science confirms their benefit.

You are on the Vetspace animal health website