Chapter 14 Intestinal Viruses
Parvoviruses, coronaviruses, and rotaviruses are established causes of viral enteritis and diarrhea in dogs and cats and are discussed in this chapter. In addition, numerous other viruses of uncertain significance and enteropathogenicity have been found in the feces or intestines of dogs and cats.
Canine parvovirus type 2 (CPV-2), a non-enveloped, single-stranded DNA virus, causes an acute, highly contagious enteritis of dogs that has been prevalent worldwide since the late 1970s. CPV is believed to have evolved from the feline panleukopenia virus or a closely related virus. Since 1980, variants designated CPV-2a and CPV-2b have evolved, the latter now being the predominant strain in North America. Both of these variants and a newer CPV-2c variant can also infect and replicate in cats.
CPV infection occurs by the fecal-oral route. During acute illness, and for about 1 to 2 weeks after recovery, massive amounts of parvovirus (over 1 billion virions per gram of feces) are shed in the feces of infected dogs. Because the virus can survive and remain infectious for 5 to 7 months in the environment, fomites and environmental contamination play a major role in transmission. A peak seasonal incidence in the months of July, August, and September was found in a Canadian study, but this may vary by climatologic region.
Dogs of any age can be infected, but the incidence of clinical disease is highest in puppies between weaning and 6 months of age. Puppies younger than 6 weeks of age generally are protected by passive maternal antibody, whereas most mature animals have been immunized or have seroconverted from natural exposure.
All dogs are susceptible to infection; however, certain breeds have a higher risk for parvovirus infection and appear to be more susceptible to a severe form of the disease. These include rottweilers, Doberman pinschers, American pit bull terriers, German shepherds, and possibly Labrador retrievers. The biologic basis for these breed susceptibilities is unclear.
Suspect parvovirus infection in young dogs that have an abrupt onset of vomiting and foul-smelling bloody diarrhea, especially if associated with severe depression, fever, or leukopenia or if these signs follow potential exposure to infected dogs or fomites.
Because of the difficulty in breaking through maternal antibody interference with vaccination in young puppies, prior vaccination does not necessarily exclude parvoviral infection, especially in puppies 6 to 20 weeks of age.
Serum chemistry abnormalities are variable and non-specific. Findings may include electrolyte abnormalities (most frequently hypokalemia), prerenal azotemia, hypoglycemia, hypoalbuminemia, increased bilirubin, and increased liver enzymes (alanine transaminase and alkaline phosphatase).
Gas and fluid distention of the gastrointestinal (GI) tract due to generalized ileus is frequent in parvoviral enteritis and must be differentiated from small intestinal obstruction (e.g., foreign body or intussusception). Carefully palpate the abdomen to help rule out mechanical obstruction, including complicating intussusception. Barium contrast radiography often reveals mucosal irregularity (corrugation or scalloping) and prolonged transit time.
Necropsy diagnosis of parvovirus is based on identification of the characteristic intestinal lesions of necrosis of the intestinal crypt cells with secondary villous collapse and dilatation of the crypts with necrotic debris. Myeloid degeneration and widespread lymphoid depletion also are seen. Parvovirus can be demonstrated in frozen tissue samples by immunofluorescence and in fixed specimens by PCR.
The treatment of parvovirus is mainly supportive and similar in most ways to the treatments used for other types of severe gastroenteritis. The intensity of the treatment depends on the severity of the signs. In dogs with fully developed clinical signs, withhold food and water for 12 to 24 hours to rest the GI tract, administer IV crystalloids to replace fluid and electrolytes, and give parenteral antibiotics to control bacterial complications. Initiate therapy whether or not definitive tests are done or while awaiting the return of results.