Viral infections

By | February 24, 2015

Canine and feline viral enteritis are usually diagnosed in younger unvaccinated animals. The animal’s age, history, clinical signs and haematological findings are important in ranking a viral aetiology as a likely cause of the animal’s diarrhoea.

Canine parvovirus

Dogs are susceptible to infection by two types of parvovirus. Canine parvovirus-1 (CPV-1) is a relatively non-pathogenic virus that is occasionally associated with myocarditis, pneumonitis and gastroenteritis in very young puppies. Canine parvovirus-2 (CPV-2) causes classic parvovirus enteritis 5-12 days after infection via the faecal-oral route. CPV-2b is a more recently recognized mutated form of CPV-2, which may be more pathogenic in some dogs. Dobermanns, Rottweilers, Pit Bull Terriers and Labrador Retrievers appear more susceptible than other breeds.

The virus replicates in the intestinal crypts and causes severe villous blunting, diarrhoea, vomiting and subsequent bacterial translocation. Presenting complaints can vary from lethargy and anorexia, to vomiting with or without blood. Diarrhoea can” be absent In the early stages of infection and usually occurs 24-48 hours after onset of vomiting. The diarrhoea can often be profuse and haemorrhagic. Protein-losing enteropathy characterized by panhypoproteinaemia can be seen in severe cases. Dogs are predisposed to sepsis, secondary to neutropenia associated with viral damage to bone marrow progenitors, and bacterial transtocation due to intestinal damage. Clinical signs can be exacerbated with concurrent infection with distemper virus, coronavirus, Salmonella, Campylobacter, Giardia or other intestinal parasites.

Clinical signs

Anorexia, lethargy, fever, vomiting, diarrhoea and dehydration are common. Hypothermia, icterus and disseminated intravascular coagulation (DIC) are typically seen in severe cases with bacterial sepsis or endotoxaemia.

Diagnosis

The diagnosis is often made tentatively on the basis of clinical details, history and physical examination findings. A history of sudden onset of vomiting and diarrhoea in a young dog, particularly if the puppy is unvaccinated or only partially vaccinated, warrants consideration. The presence of neutropenia on the haemogram in association with signs of enteritis is suggestive of CPV; however, salmonellosis or severe infections can also be associated with neutropenia. There are no pathognomonic findings on a serum biochemistry profile, although hypoglycaemia, hypokalaemia, prerenal azotaemia and increased bilirubin or liver enzymes are commonly found. Abdominal radiography is indicated to help rule out an intestinal foreign body, and may reveal intestinal gas, intestinal fluid and ileus. A faecal ELISA test for CPV-2 should always be performed on faeces, even if diarrhoea is not present. The test is considered sensitive and specific; however, the ELISA test result may be negative if the assay is performed too early in the clinical course of the disease, or after 10-14 days following infection. Other tests.that can be undertaken to confirm the diagnosis of CPV include electron microscopic evaluation of the faeces for the presence of the virus (CPV-1 is indistinguishable from CPV-2), haemagglutination inhibition for serum IgM, histopathology of intestinal lesions and immunohistochemistry of intestinal biopsy samples.

Treatment

Treatment is supportive and similar to other severe, acute infectious enteropathies. Intravenous fluid and electrolyte therapy is indicated, with particular attention given to potassium repletion. The intramedullary route can be utilized in very small puppies, although the subcutaneous route is likely to be inadequate. Dextrose solution (2.5-5%) is added to the intravenous fluids if the dog is hypoglycaemic. Plasma or colloids (dextran 70 or hetastarch) are indicated if the serum albumin concentration drops below 20 g/l. Antibiotics are administered to febrile or severely neutropenic dogs. If the animal is neutropenic, but afebrile, the administration of a first-generation cephalosporin is reasonable. Dogs in septic shock should be treated with a broad spectrum aerobic and anaerobic antibiotic (e.g. ampiciilin plus amikacin). Human granulocyte colony-stimulating factor (G-CSF) at 5 u.g/kg s.c. or i.v. q24h has been used to increase neutrophil numbers, but may not influence patient outcome, whereas some benefit has been shown for co-interferon. Anti-emetics, such as prochlorperazine, metoclopramide or ondansetron, are indicated if the vomiting is intractable. Metoclopramide is most effective when administered as a constant rate infusion at a dose of 1 mg/kg q24h. Gastric protectants, including H2 receptor antagonists and sucralfate, are indicated if there is evidence of secondary oesophagitis. Broad spectrum anthelmintics to treat concurrent intestinal parasites should be administered when the dog is no longer vomiting. Most dogs can be gradually weaned on to a bland diet of cottage cheese and rice, or a commercially available highly digestible diet; however, the presence of intractable vomiting might warrant the administration of partial or total parenteral nutrition.

Vaccination

Vaccination with a modified live parvovirus is effective at preventing the disease. Vaccines should not be given before 6 weeks of age because of interference by maternally derived antibody. Depending on the vaccine used, an initial course of vaccination is completed at 10-12 weeks.

The frequency of booster vaccinations is controversial, and serum antibody concentration, although a relatively crude measure of immunity, shows relatively good correlation between a ‘positive’ antibody titre and protection against canine parvovirus. However, the absence of antibody (an obvious decision point in establishing the ‘need’ to administer booster vaccine) does not necessarily correlate with susceptibility. Furthermore, there are no established test standards for measuring antibody titres in dogs and cats. One should expect, forthe same serum sample sent to two different laboratories, two different reference ranges and two quite different antibody titres reported. For example, one laboratory will report a ‘positive’ titre against canine parvovirus as >1:80, while another laboratory reports a value of >1:5 as ‘positive’. Interpreting test results can be confusing forthe clinician andfrustrating for the pet owner.

Feline parvoviral enteritis

Feline parvoviral enteritis (feline panleucopenia) is caused by a parvovirus distinct from CVP-2b, although CVP-2b can also infect cats and cause disease. Feline parvovirus has become an uncommon disease because of routine vaccination; however, outbreaks are occasionally seen in unvaccinated animals, particularly feral populations and catteries. The clinical signs are similar to those described for dogs with parvoviral enteritis.

Diagnosis

Diagnosis is based on the History, physical examination findings, results of a haemogram (neutropenia) and faecal ELISA. The ELISA test used to detect canine parvovirus has been reported to cross-react with feline parvovirus.

Treatment

The principles of treatment are virtually identical to those described for the dog with parvovirus enteritis.

Vaccination

Vaccination with a modified live parvovirus is very effective and is given to kittens at 10-12 weeks of age. A killed vaccine should be administered to pregnant queens, as transplacental infection of kittens with the vaccine strain can cause cerebellar hypopiasia.

The frequency of booster vaccinations is controversial and serum antibody concentration, although a relatively crude measure of immunity, shows good correlation between a ‘positive’ antibody titre and protection against feline parvovirus.

Canine coronavirus enteritis

Canine coronavirus is closely related to feline enteric coronavirus/feline infectious peritonitis (FIP) and transmissible gastroenteritis in pigs. Infection with canine coronavirus is associated with milder clinical signs than parvovirus, as the virus destroys mature enterocytes at the viilus tips while sparing the intestinal crypts. The bone marrow is typically spared, affording these dogs a relatively good prognosis with supportive therapy.

Although acanine coronavirus vaccine is available, there is no scientific evidence that this disease is a significant problem in household dogs. It is mainly a problem when large numbers of dogs are brought together under heavy stress (i.e. in dog shows or kennel situations).

Feline enteric coronavirus

Feline enteric coronavirus is related to FlP-producing strains of coronavirus, and invades the enterocytes at the tips of the villi. Infected cats may be asymptomatic, or develop mild, transient diarrhoea and fever. Infected cats can seroconvert and test positive on serological testing. In addition, feline enteric coronavirus may mutate to FIP virus.