The genome is about 5000 nucleotides long. CPV2 continues to evolve, and the success of new strains seems to depend on extending the range of
hosts affected and improved binding to its receptor, the canine transferrin receptor. CPV2 has a high rate of evolution, possibly due to a rate of nucleotide
substitution that is more like RNA viruses such as Influenzavirus A. In contrast, FPV seems to evolve only through random genetic drift.
CPV2 affects
dogs, wolves, foxes, and other canids. CPV2a and CPV2b have been isolated from a small percentage of symptomatic cats and is more common than feline panleukopenia
in big cats.
Previously it has been thought that the virus does not undergo cross species infection. However studies in Vietnam have shown that CPV2 can
undergo minor antigenic shift and natural mutation to infect felids. Analyses of feline parvovirus (FPV) isolates in Vietnam and Taiwan revealed that more than 80%
of the isolates were of the canine parvovirus type, rather than feline panleukopenia virus (FPLV).[13] CPV2 may spread to cats easier than dogs and undergo faster
rates of mutation within that species. The virus does not transmit to birds, or humans , but it can be spread by them when a bird comes in contact with feces and
then the dog's environment, or when a cat goes to the groomers and returns with an exposed pet carrier.
Variants
There are variants of CPV type 2 called CPV-2a, CPV-2b and CPV-2c. The antigenic patterns of 2a and 2b are quite similar to the original CPV type 2. Variant 2c
however has a unique pattern of antigenicity. This has led to claims of ineffective vaccination of dogs, but studies have shown that the existing CPV vaccines
based on CPV type 2b, provide adequate levels of protection against CPV type 2c. However, there are reports that outdated vaccines based on the old CPV-type 2
may not afford sufficient cross-protection against the type 2c variant.
Pathophysiology
There are two forms of CPV2: intestinal and cardiac. Puppies are most susceptible, but more than 80 percent of adult dogs show no symptoms. With severe disease,
dogs can die within 48 to 72 hours without treatment by fluids and antibiotics. In the more common, less severe form, mortality is about 10 percent. Certain breeds,
such as Rottweilers, Doberman Pinschers, and Pit bull terriers as well as other black and tan colored dogs may be more susceptible to CPV2. Along with age and breed,
factors such as a stressful environment, concurrent infections with bacteria, parasites, and canine coronavirus increase a dog's risk of severe infection. Dogs who
catch Parvovirus usually die from the dehydration it causes or secondary infection rather than the virus itself.
Intestinal form
Dogs become infected through oral contact with CPV2 in feces, infected soil, or fomites that carry the virus. Following ingestion, the virus replicates in the
lymphoid tissue in the throat, and then spreads to the bloodstream. From there, the virus attacks rapidly dividing cells, notably those in the lymph nodes,
intestinal crypts, and the bone marrow. There is depletion of lymphocytes in lymph nodes and necrosis and destruction of the intestinal crypts. Anaerobic bacteria
that normally reside in the intestines can then cross into the bloodstream, a process known as translocation, and cause sepsis. The most common bacteria involved
in severe cases are Clostridia, Campylobacter and salmonellaspecies. This can lead to a syndrome known as Systemic inflammatory response syndrome(SIRS). SIRS leads
to a range of complications such as hypercoagulability of the blood, endotoxaemia and acute respiratory distress syndrome(ARDS). Bacterial Myocarditis has also been
reported secondarily to sepsis. Dogs with CPV are at risk of intussusception, a condition where part of the intestine prolapses into another part. Three to four days
following infection, the virus is shed in the feces for up to three weeks, and the dog may remain an asymptomatic carrier and shed the virus periodically. The virus
is usually more deadly when infested with worms or other intestinal parasites.
Cardiac form
This form is less common and affects puppies infected in the uterus or shortly after birth until about 8 weeks of age. The virus attacks the heart muscle and the
puppy often dies suddenly or after a brief period of breathing difficulty. On the microscopic level, there are many points of necrosis of the heart muscle that
are associated with mononuclear cellular infiltration. The formation of excess fibrous tissue (fibrosis) is often evident in surviving dogs. Myofibers are the site
of viral replication within cells. The disease may or may not be accompanied with the signs and symptoms of the intestinal form. However, this form is now rarely
seen due to widespread vaccination of breeding dogs.
Even less frequently, the disease may also lead to a generalized infection in neonates and cause
lesions and viral replication and attack in other tissues other than the gastrointestinal tissues and heart, but also brain, liver, lungs, kidneys, and adrenal
cortex. The lining of the blood vessels are also severely affected, which lead the lesions in this region to hemorrhage.
Infection in the uterus
This type of infection can occur when a pregnant female dog is infected with CPV2. The adult may develop immunity with little or no clinical signs of disease. The
virus may have already crossed the placenta to infect the foetus. This can lead to several abnormalities. In severe cases the pups can be still born or born
mummified, in mild to moderate cases the pups can be born with neurological abnormalities such as cerebellar hypoplasia.
Signs and symptoms
Dogs that develop the disease show symptoms of the illness within 5 to 10 days. The symptoms include lethargy, vomiting, fever, and diarrhea (usually bloody).
Diarrhea and vomiting result in dehydration and secondary infections can set in. Due to dehydration, the dog's electrolyte balance can become critically affected.
Because the normal intestinal lining is also compromised, blood and protein leak into the intestines leading to anemia and loss of protein, and endotoxins escaping
into the bloodstream, causing endotoxemia. Dogs have a distinctive odor in the later stages of the infection. The white blood cell level falls, further weakening the
dog. Any or all of these factors can lead to shock and death.
Diagnosis
Diagnosis is made through detection of CPV2 in the feces by either an EIA or a hemagglutination test, or by electron microscopy. PCR has become available to diagnose
CPV2, and can be used later in the disease when potentially less virus is being shed in the feces that may not be detectable by EIA. Clinically, the intestinal form
of the infection can sometimes be confused with coronavirus or other forms of enteritis. Parvovirus, however, is more serious and the presence of bloody diarrhea, a
low white blood cell count, and necrosis of the intestinal lining also point more towards parvovirus, especially in an unvaccinated dog. The cardiac form is
typically easier to diagnose because the symptoms are distinct.
Prevention and decontamination
Prevention is the only way to ensure that a puppy or dog remains healthy since the disease is extremely virulent and contagious. The virus is extremely hardy and has
been found to survive in feces and other organic material such as soil for over a year. It survives extremely cold and hot temperatures. The only household
disinfectant that kills the virus is bleach.
Weaning puppies can be vaccinated with a modified live virus low passage high titer vaccine at 6 weeks of
age, then every 3 to 4 weeks until 15 or 16 weeks. Puppies are initially protected through passive immunity derived from the mother. These maternal antibodies wear
off before the puppy's immune system is mature enough to fight off CPV2 infection. Maternal antibodies also interfere with vaccination for CPV2 and can cause
vaccine failure. Thus puppies are generally vaccinated in a series of shots, extending from the earliest time that the immunity derived from the mother wears off
until after that passive immunity is definitely gone. Older puppies (16 weeks or older) are given 3 vaccinations 3 to 4 weeks apart. The duration of immunity of
vaccines for CPV2 has been tested for all major vaccine manufacturers in the United States and has been found to be at least three years after the initial puppy
series and a booster 1 year later.
A dog that successfully recovers from CPV2 sheds virus for a few days. Ongoing infection risk is primarily from fecal
contamination of the environment due to the virus's ability to survive many months in the environment. Neighbors and family members with dogs should be notified of
infected animals so that they can ensure that their dogs are vaccinated or tested for immunity. Vaccine will take up to 2 weeks to reach effective levels of
immunity, the contagious individual should remain in quarantine until other animals are protected.
Treatment
Survival rate depends on how quickly CPV is diagnosed, the age of the animal and how aggressive the treatment is. Treatment for severe cases that are not caught
early usually involves extensive hospitalization, due to the severe dehydration and damage to the intestines and bone marrow. A CPV test should be given as early as
possible if CPV is suspected in order to begin early treatment and increase survival rate if the disease is found.
Treatment ideally consists of
crystalloid IV fluids and/or colloids, antinausea injections (antiemetics) such as metoclopramide, dolasetron, ondansetron and prochlorperazine, and antibiotic
injections such as cefoxitin, metronidazole, timentin, or enrofloxacin. IV fluids are administered and antinausea and antibiotic injections are given
subcutaneously, intramuscularly, or intravenously. The fluids are typically a mix of a sterile, balanced electrolyte solution, with an appropriate amount of
B-complex vitamins, dextrose and potassium chloride. Analgesic medications such as buprenorphine are also used to counteract the intestinal discomfort caused by
frequent bouts of diarrhea.
In addition to fluids given to achieve adequate rehydration, each time the puppy vomits or has diarrhea in a significant
quantity, an equal amount of fluid is administered intravenously. The fluid requirements of a patient are determined by their body weight, weight changes over time,
degree of dehydration at presentation and surface area. The hydration status is originally determined by assessment of clinical factors like tacky mucous membranes,
concentration of the urine, sunken eyes, poor skin elasticity and bloodtests.
A blood plasma transfusion from a donor dog that has already survived CPV
is sometimes used to provide passive immunity to the sick dog. Some veterinarians keep these dogs on site, or have frozen serum available. There have been no
controlled studies regarding this treatment. Additionally, fresh frozen plasma and human albumin transfusions can help replace the extreme protein losses seen in
severe cases and help assure adequate tissue healing.
Once the dog can keep fluids down, the IV fluids are gradually discontinued, and very bland food
slowly introduced. Oral antibiotics are administered for a number of days depending on the white blood cell count and the patient's ability to fight off secondary
infection. A puppy with minimal symptoms can recover in 2 or 3 days if the IV fluids are begun as soon as symptoms are noticed and the CPV test confirms the
diagnosis. However, even with hospitalization, there is no guarantee that the dog will survive.
Unconventional treatments
There is no specific antiviral treatment for CPV. However, there have been anecdotal reports of oseltamivir (Tamiflu) reducing disease severity and hospitalization
time in canine parvovirus infection. The drug may limit the ability of the virus to invade the crypt cells of the small intestine and decrease gastrointestinal
bacteria colonization and toxin production.[29] There is also anecdotal evidence suggesting that colloidal silver is effective at treating CPV although currently
regulatory authorities are discouraging its use due to potential toxicity issues and lack of demonstrated efficacy. Lastly, recombinant feline interferon omega
(rFeIFN-w), produced in silkworm larvae using a baculovirus vector, has been demonstrated by multiple studies to be an effective treatment.
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