Canine parvovirus

Canine parvovirus (CPV) is a contagious virus affecting dogs. The disease is highly infectious and is spread from dog to dog by physical contact and contact with feces. It can be especially severe in puppies.

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History

CPV is a relatively new disease that appeared in the late 1970s. It was first recognized in 1978 and spread worldwide in one to two years.^[1] The virus is very similar to feline distemper (also a parvovirus); in fact, they are almost identical, except for two amino acids in the capsid protein VP2.^[2] The early belief was that the feline distemper mutated into CPV; however, this has never been proven. The current prevalent theory is that CPV mutated from an unidentified parvovirus (similar to feline parvovirus (FPV)) of some wild carnivore.^[3] Interestingly, a strain of CPV2b (strain FP84) has been shown to cause disease in domestic cats, although vaccination for FPV seems to protect against it.^[4]

Two additional strains of canine parvovirus CPV2a and CPV2b were identified in 1979 and 1984 respectively.^[3] Most cases of canine parvovirus are believed to be of these two strains, which have replaced the original strain;^[5] however, they are indistinguishable by most routine tests. A third type, CPV2c (a Glu-426 mutant), has been discovered in Italy, Vietnam, and Spain.^[6]

Viral Structure and Pathology

CPV is a non-enveloped single-stranded DNA virus. The name comes from the Latin parvus, meaning small, as the virus is only 20 to 26 nm in diameter. It has an icosahedral symmetry. The genome is about 5000 nucleotides long.^[7] CPV 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.^[8] CPV has a high rate of evolution, possibly due to a rate of nucleotide substitution that is more like RNA viruses such as Influenzavirus A.^[9] In contrast, FPV seems to evolve only through random genetic drift.^[10]

Canine parvovirus 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.^[11] It does not transmit to birds, or humans; although each species has its own parvovirus. Canine parvovirus cannot be spread to species outside of the canid family and other carnivores, but it can be spread by them (for example, a bird comes in contact with feces and then the dog's environment, or a cat goes to the groomers and returns with an exposed pet carrier).

Infection

There are two forms of CPV: intestinal and cardiac. Cardiac form is less common and affects puppies infected in utero or shortly after birth. Symptoms of heart failure usually emerge in puppies less than eight weeks old.^[12] The virus attacks the heart muscle and the dog dies suddenly of heart failure. However, this form is now rarely seen due to widespread vaccination of breeding dogs.^[13]

Certain breeds, such as Rottweilers, Doberman Pinschers, Labrador Retrievers, and Pit bull terriers as well as other black and tan colored dogs may be more susceptible to CPV ^[14]

Dogs become infected through oral contact with CPV in feces, infected soil, or fomites carrying the virus. Puppies are most susceptible. They are initially protected by maternal antibodies received in colostrum while nursing. These antibodies wear off before the puppy's immune system is mature enough to fight off CPV infection. Maternal antibodies also interfere with vaccination for CPV and can cause vaccine failure. This is why 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.^[15]

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.^[16] In the fetus and newborn puppy, the virus attacks the heart muscle. Bacteria that normally live in the intestines then cross into the bloodstream and cause sepsis. 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.^[13]

Signs and symptoms

Most dogs (more than 80 percent) that are infected will show no symptoms.^[12] Dogs that develop the disease show symptoms of the illness within 3 to 10 days. The symptoms include lethargy, vomiting, fever, and diarrhea (usually bloody). Dogs with CPV are also at risk for intussusception, a condition where part of the intestine prolapses into another part. After a dog is infected, there is no cure, but dogs usually recover from the viral infection and associated symptoms within five days with aggressive treatment.^[12] However, diarrhea and vomiting result in dehydration and secondary infections can set in, causing death even in treated dogs. Risk factors for severe disease include young age, a stressful environment, and concurrent infections with bacteria, parasites, and canine coronavirus.^[12]

Due to dehydration, the dog's electrolyte balance is destroyed. Because of destruction of the normal intestinal lining, blood and protein leak into the intestines leading to anemia and loss of protein, and endotoxins escape into the bloodstream, causing endotoxemia. The white blood cell level drops, further weakening the dog. Any or all of these factors can lead to shock and death.

Diagnosis

Diagnosis is made through detection of CPV in the feces by either an ELISA or hemagglutination test, or through electron microscopy. However, the presence of bloody diarrhea and a low white blood cell count in an unvaccinated dog are strong indications of infection.

Treatment

Survival rate depends on how quickly CPV is diagnosed 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. If a puppy or dog suddenly stops eating, seems listless, and has vomiting or diarrhea, an immediate CPV test can pinpoint the problem, and help the puppy recover by getting it the care it needs more quickly, before the puppy becomes seriously ill.

Home treatment using IV fluids is an effective option, but hospitalization may be required. Treatment consists of IV fluids and colloids, antinausea injections (antiemetics) such as metoclopramide, dolasteron, ondansetron and prochlorpromazine, and antibiotic injections such as cefoxitin, metronidazole, timentin, or enrofloxacin.^[17] A veterinary technician inserts an IV catheter in a vein, and IV fluids are administered by inserting the needle in the rubber tip that is part of the catheter. The antinausea and antibiotic injections are given subcutaneously, intramuscularly, or intravenously. The fluids are typically a mix of a sterile, balanced electrolyte solution, with 3 cc of B-complex vitamins and 50 cc of 50 percent dextrose and an appropriate amount of potassium chloride added per 1000 cc bag. Analgesic medications such as buprenorphine are also used to counteract the intestinal discomfort caused by frequent bouts of diarrhea. The IV fluids and other medications can be given at home by the pet owner if the veterinarian feels the owner can handle such nursing requirements.

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, usually 50-100 cc. The fluid requirements of a patient are determined by their body weight, weight changes over time, degree of dehydration at presentation and surface area.

A patient's hydration status is originally determined by assessment of clinical factors like tacky mucous membranes, concentration of the urine, sunken eyes, poor skin elasticity and information gathered in bloodwork.

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. ^[17] Additionally, fresh frozen plasma and human albumin transfusions can help replace the extreme protein losses seen in severe cases.

An albumin level of 2.0 mg/dl or greater is needed for adequate tissue healing. Swelling of the feet (peripheral edema)and crackles on auscultation of lung fields (pulmonary edema) are indications that protein levels are dropping below acceptable levels.

Admitting the severely affected patient into the hospital is considered by many to be the standard of care. Emergency hospitals in urban areas often offer 24 hour critical care as do specialty and university hospitals. Since 24-hour supervision can improve the outcome, some veterinarians who do not have 24-hour staff allow the patient to go home at night with the catheter in place and fluids as needed. The owner then returns the patient to the veterinary hospital the next morning for daytime monitoring by the veterinary staff. For pet owners with limited funds, combining daytime veterinary care with nighttime at-home IV fluids can be effective and a cost savings.

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 sepending on the white blood cell count and the patients 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.

There is no specific antiviral treatment for CPV. Even with hospitalization, there is no guarantee that the dog will survive.

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.^[18]

There is also anecdotal evidence suggesting that colloidal silver is effective at treating CPV. Colloidal silver is known to have broad-spectrum antiviral properties and has been demonstrated in-vitro to effectively neutralize a number of viral pathogens, including the AIDS virus.^{[citation needed]} Colloidal silver was routinely prescribed for human use before penicillin's discovery, and was manufactured as a drug for human use until 1978. Currently regulatory authorities are discouraging use of colloidal silver due to potential toxicity issues and lack of demonstrated efficacy ref (http://www.tga.gov.au/docs/html/csilver.htm)

Prevention and decontamination

Direct contact with infected feces is not necessary for the disease to spread: viral particles on shoes, clothing, hair, and so on are all that is needed for the transmission. The disease is extremely hardy and has been found to be present in feces or other organic material (eg. soil) even after a year including extremely cold and hot temperatures. The only household disinfectant that kills the virus is a mixture of bleach and water, 1 part bleach and 30 parts of water.^[12]

Prevention is the only way to ensure that a puppy or dog remains healthy. This disease is extremely virulent and contagious. With severe disease, dogs can die in as little as 6 to 8 hours despite treatment. In the more common, less severe form, mortality is about 10 percent.^[2]

It is extremely important to vaccinate puppies and adult dogs against CPV. Weaning puppies should receive initial vaccination, by a licensed veterinarian, at 8 weeks of age, then every 3 to 4 weeks until 15 or 16 weeks of age with a modified live virus low passage high titer vaccine. Older puppies (16 weeks or older) should receive 3 vaccinations 3 to 4 weeks apart. ^[14] The duration of immunity of vaccines for CPV has been tested for all major vaccine manufacturers in the United States and has been found to be at least three years after initial puppy series and a booster 1 year later.^[19]

A dog that successfully recovers from CPV is still contagious for up to 2 months, so the dog must be kept away from other dogs and puppies. Neighbours and family members with dogs should be notified of infected animals so that they can ensure that their dogs are vaccinated and tested.

See also

• Parvovirus

References