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|THE PET HEALTH LIBRARY
By Wendy C. Brooks, DVM, DipABVP
Educational Director, VeterinaryPartner.com
What is Parvo?
Parvoviruses are a large group; almost every mammal species (including humans) seems to have its own parvovirus. Fortunately, each virus is specific for which animal species it can infect (i.e. the pig parvovirus will not infect people, the canine parvovirus will not infect cats, etc.) The canine parvovirus will affect most members of the dog family (wolves, coyotes, foxes etc.).
Parvoviruses are smaller than most viruses and consist of a protein coat (a capsid) and a single strand of DNA inside. It is hard to believe that such a simply constructed organism could be so deadly; however, this virus has proved especially effective at infecting rapidly dividing host cells such as intestinal cells, bone marrow cells, cells of the lymph system, and fetal cells. Parvoviruses are not enveloped in fat the way many other viruses are. This makes parvoviruses especially hardy in the environment and difficult to disinfect away.
While the parvoviruses of other species have been well known for decades, the canine parvovirus is a relative newcomer. The original canine parvovirus, discovered in 1967 and called CPV-1 did not represent much of a medical threat except to newborn puppies but by 1978, a new variant, CPV-2 appeared in the U.S. This newer version seems to represent a mutation from the feline parvovirus (which is more commonly known as the feline distemper virus). Because this virus was (and is) shed in gigantic numbers by infected animals, and because this virus is especially hardy in the environment, worldwide distribution of the virus rapidly occurred. At this time, the virus is considered to be ubiquitous, meaning that it is present in EVERY ENVIRONMENT unless regular disinfection is applied.
Attempting to shield a puppy from exposure is completely futile.
In 1978, no dog had any sort of immunity against this virus. There was no resistance and the epidemic that resulted was disastrous. To make matters worse, a second mutation creating CPV-2a had occurred by 1979, and it seemed to be even more aggressive. Vaccine was at a premium and many veterinarians had to make do with feline distemper vaccine as it was the closest related vaccine available while the manufacturers struggled to supply the nation with true parvo vaccines.
Over thirty years have passed since then. The most common form of the virus is called CPV-2b. Virtually all dogs can be considered to have been exposed to it at least to some extent, which means that most adult dogs, even those inadequately vaccinated, can be considered to have at least some immunity. It is also worth mentioning the new particularly virulent strain of parvovirus: CPV-2c, which is rapidly becoming the second most common form of canine parvovirus. CPV-2c was discovered in the year 2000 and is able to infect cats. Cats vaccinated against feline distemper can be considered protected. Currently available vaccines cover all variants of canine parvo including CPV-2c as do all the commercially available diagnostic test kits.
Parvoviral infection has become a disease almost exclusively of puppies and adolescent dogs.
Parvoviral infection must be considered as a possible diagnosis in any young dog with vomiting and/or diarrhea. With proper hospitalization, survival rates approach 80%. Still, there are many myths and misunderstandings about this virus, how it is spread, and how to prevent it. The purpose of this article is to clear up these misconceptions and provide the public with an accurate information source.
The Virus in the Environment/Disinfection
Given that this is such a tough virus to destroy, many people want to know exactly what they must do to disinfect an area that has contained an infected dog or how long they must wait before safely introducing a new dog to a previously contaminated area.Because the canine parvovirus is not enveloped in fat the way the distemper virus is, canine parvovirus is especially hardy in the environment. It is readily carried on shoes or clothing to new areas (which accounts for its rapid worldwide spread shortly after its original appearance). It is able to survive freezing temperatures in the ground during winter, plus many household disinfectants are not capable of killing it indoors.
Here is what we know about how contaminated an environment is likely to be:
A parvoviral infection can be picked up ANYWHERE although it is easier to pick up an infection in an area where an infected dog has been simply because of the larger amounts of virus in a contaminated area.
A typical/average infectious dose for an unvaccinated dog is 1000 viral particles. For some dogs far less is needed. For other dogs, far more is needed. An infected dog sheds 35 million viral particles (35,000 TIMES the typical infectious dose) per OUNCE of stool.
Here's what we know about disinfection:
How Parvo Infection Happens
Whether or not infection happens depends on the interaction of three factors: Host vitality, including immune experience/vaccination status; virulence of the virus, including how many viral particles the host is exposed to; and environmental factors. Obviously these three aspects interplay somewhat as a stressful environment will reduce host vitality, a dry environment will reduce the number of viral particles, etc.
Where does the Virus come from?
Remember that this virus has been around since the 1970s, is hard to disinfect, and is shed in extremely large numbers by infected dogs. This means that there is virus everywhere: on every carpet, on every floor, in every yard and park. Virus is shed in the stool for the first two weeks or less after the initial infection but only a tiny portion of infected stool - which could be months old depending on the environmental temperature and humidity - is needed to infect a non-immune dog. Some dogs become what is called subclinically infected, which means they do not appear particularly sick. These animals tend not to be confined since no one knows they are infected, thus they can spread virus around a large area depending on where they leave their droppings.
Why only Puppies?
The most important factors in whether parvovirus infection occurs seem to be the experience the dog’s immune system has had with the virus plus the number of viral particles the host is exposed to. In the 1970s and early 1980s when the virus was new, all dogs young and old were susceptible, but now that the virus is everywhere, all dogs, even the unvaccinated ones, have at least some immunological experience with this virus. Any exposure no matter how small is likely to generate some antibodies. Also, vaccination is a widespread process nowadays and it is likely that a dog has had at least one vaccine at some point. Will these antibodies be enough for protection? In general, the answer seems to be yes as infection in dogs older than one year is somewhat unusual. It is important to realize, however, that this observation should not be taken to mean that adult dogs should not continue their vaccinations. Even though infection is somewhat unusual in adult dogs, adult dogs should still continue their vaccinations as this is a life-threatening disease for which treatment is expensive. No chances should be taken.
The younger the dog, the less immunologic experience and the more susceptibility to infection there is.
When puppies are born, they are completely unable to make antibodies against any infectious invader. They would be totally unprotected except that nature has created a system to protect them. Their mother secretes a specific type of milk called colostrum for the first day or two after giving birth. It contains all the antibodies that the mother dog has circulating in her own body and in this way, she gives her own immune experience to her off-spring. These antibodies are protective until they wear off sometime in the first 4 months of the puppy’s life.
How much colostrum an individual puppy gets depends on its birth order and how strong it nurses; not all puppies get the same amount of antibodies. Every nine days the antibody levels possessed by the puppies drops by half. When the antibody level drops to a certain level, they no longer have enough antibodies to protect them and if they are exposed to a large enough number of viral particles, they will get infected.
We recommend that puppies be restricted from public outdoor areas
until their vaccination series is completed at age 16 weeks.
There is a period lasting a good week or so during which the puppy has no antibody protection leftover from its mother but still is not yet competent to respond to vaccination. This window is where even the most well cared for puppies get infected.
The virus enters the body through the mouth as the puppy cleans itself or eats food off the ground or floor. A minuscule amount of infected stool is all it takes.
There is a 3 to 7 day incubation period before the puppy seems obviously ill.
Upon entering the body, the virus seeks out the nearest rapidly dividing group of cells. The lymph nodes in the throat fit the bill and the virus sets up here first and replicates to large numbers. After a couple of days, so much virus has been produced that significant amounts of it have been released into the bloodstream. Over the next 3 to 4 days, the virus seeks new organs containing the rapidly dividing cells it needs: the bone marrow and the delicate intestinal cells.
Within the bone marrow, the virus is responsible for destruction of young cells of the immune system. By killing these cells, it knocks out the body's best defense and ensures itself a reign of terror in the GI tract where its most devastating effects occur. All parvoviral infections are characterized by a drop in white blood cell count due to the bone marrow infection. Seeing this on a blood test may help clinch the diagnosis of parvoviral infection. Also, a veterinarian may choose to monitor white blood cell count or even attempt to artificially raise the white blood cell count in an infected puppy through treatment.
The GI tract is where the heaviest damage occurs. The normal intestine possesses little finger-like protrusions called villi. Having these tiny fingers greatly increases the surface area available for the absorption of fluid and nutrients. To make the surface area available for absorption greater still, the villi possess microvilli, which are microscopic protrusions. The cells of the villi are relatively short-lived and are readily replaced by new cells. The source of the new cells is the rapidly dividing area at the foot of the villi called the crypts of Lieberkuhn. Parvovirus strikes right at the crypt.
Without new cells coming from the crypt, the villus becomes blunted and unable to absorb nutrients. Diarrhea in large quantities results, not to mention nausea. The barrier separating the digestive bacteria from the blood stream breaks down. The diarrhea becomes bloody and bacteria can enter the body, causing widespread infection (remember that that virus has also simultaneously destroyed the bone marrow's ability to respond immunologically).
The virus kills one of two ways:
How is Survival Possible?
Even parvovirus cannot disrupt the entire immune system. Plus, every day that goes by allows more antibodies to be produced. This antibody can bind and inactivate the virus. Whether survival is possible amounts to a race between the damaged immune system trying to recover and respond versus the fluid loss and bacterial invasion.
Parvo: the Physical Illness and its Treatment
BE PREPARED FOR A 5 to 7 DAY HOSPITAL STAY AND SUBSTANTIAL EXPENSE. Treatment for parvoviral infection centers on supportive care. This means that the clinical problems that come up in the course of the infection are addressed individually with the goal of keeping the patient alive long enough for an immune response to generate. We do not have effective antiviral drugs and must rely on the patient’s immune system for cure.
INTENSIVE CARE IS NEEDED TO TREAT THIS INFECTION.
There are certain basic treatment principles that can be viewed as “must haves” in addressing the parvo puppy.
One of the ways parvo can kill is via the metabolic derangements that occur with extreme dehydration. It is crucial to replace the vast fluid losses (from vomiting and diarrhea) with intravenous fluids. Fluids are given as a steady drip rather than simply under the skin so that absorption into the circulation is direct. Potassium is usually added to the fluids in order to maintain electrolyte balance. Dextrose (sugar) is also frequently added as the stress of the disease may lower blood sugar, especially in a small puppy.
The second way parvo kills is through bacterial invasion of the circulatory system (sepsis). The intestine is normally full of bacteria and when the parvovirus ulcerates the intestine there is little to prevent the bacteria from marching easily into the bloodstream. With the GI tract damaged, antibiotics cannot be given orally. They are given either as shots or are added into the IV fluid bag.
Control of Nausea
Patient comfort is an important part of treatment for any disease but is especially important for parvo treatment as these puppies feel extremely nauseated. Again, the GI tract is too damaged for oral medication so medications are given as injections.
The vomiting typical of parvo infection is not only uncomfortable but can ulcerate the esophagus. The disease itself ulcerates the stomach and small intestine. Medications called gastroprotectants help heal ulcers and help minimize their formation. These medications include the injectable antacids (cimetidine, ranitidine, or famotidine) as well as sucralfate, which forms webbing over ulcers to facilitate healing.
Home Treatment for Parvo
Home treatment for parvo infection is a bad idea when compared to hospitalization and intensive care. Mortality rises substantially and the heavy diarrhea and vomiting lead to heavy viral contamination in the home. Still, if financial concerns preclude hospitalization, home care may be the puppy’s only chance. Fluids will have to be given under the skin at home as will injectable medicines.
Diagnosis of Canine Parvovirus
The ELISA test has become the most common test for parvovirus in puppies. ELISA stands for Enzyme Linked ImmunoSorbent Assay. This sounds complicated and high tech but is actually the same type of technology used in home pregnancy test kits. The parvo ELISA test is also a kit and is performed in the vet’s office in about 15 minutes or less. There are many different brands and testing is sensitive in its ability to detect virus in stool.A puppy with a bloody diarrhea could have a parasite problem, a virus other than parvovirus, stress colitis, an intestinal foreign body, or may simply have eaten something that disagreed with him. It is important to confirm the diagnosis of parvovirus before embarking on what could be the wrong treatment.
Parvovirus lesions in the GI tract are of a classical appearance. There is no mistaking them under the microscope. Unfortunately, tissue samples of the GI tract are not readily available and most infected puppies are not good surgical candidates. Still, if a puppy has died and the cause is unclear, submitting samples of the GI tract can generally confirm or rule out a parvovirus diagnosis provided the tissue has not degenerated.
It is also important to realize that puppies with parvovirus infection may have concurrent intestinal parasites or may have swallowed foreign bodies.
There will be complicating conditions that must be monitored. This means that other tests will be required during the management of the parvo patient. The above tests are simply those that can be used to confirm the parvovirus diagnosis.
Caring for the Recovered Dog; The Parvo Puppy Goes Home - What you Need to Know
Your puppy will be finishing up a course of antibiotics and may also be on some medication for nausea or diarrhea. It is important that you give your puppy the medication prescribed for the full amount of time it has been prescribed.
Your puppy should be considered contagious to other puppies for a good month so it is important to play it safe by restricting trips to the park, obedience school or other neighborhood areas. If your puppy is less than 16 weeks of age, she should not be allowed in public areas until the vaccination series is fully completed.
Humans are not susceptible to canine parvovirus infection though some strains can be contagious to cats. Adult dogs that have been vaccinated are not susceptible, either. Puppies, however, are at risk. If your sick puppy was indoors only, wait at least one month before any new puppies come to your home. If your sick puppy was outdoors, remember that it can take 7 months before the virus is eliminated from soil. (Freezing will preserve the virus so any time the ground is frozen does not count in this 7-month period.)
Your puppy may be bathed any time as long as you do not allow her to get cold or chilled after the bath. Bathing will reduce the amount of virus left on her fur and will help reduce contagion.
Follow your veterinarian's recommendations. Your puppy cannot be re-infected with this virus for at least 3 years (and probably is protected for life simply by virtue of this infection) but there are other viruses that your puppy should be protected against. Your veterinarian will give you a vaccination schedule for the future.
There should be no permanent ramifications due to this infection. The recovered puppy should lead a normal life once the recovery period is completed (1-2 weeks).
If you have any questions about your puppy's care after discharge, contact your veterinarian for assistance.
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