Miscellaneous Antiviral Agents Used in Animals

Several drug classes continue to be investigated mainly because of their in vitro antiviral activities. Their potential clinical usefulness remains obscure in most instances. Included among these agents are thiosemicarbazones, guanidine, benzimidazoles, arildone, phosphonoacetic acid, rifamycins and other antibiotics, and several natural products.

Oseltamivir is a prodrug that, when hydrolyzed, yields the carboxylated metabolite that inhibits viral neuraminidases of human influenza viruses. Mature influenza viruses bud off from the cell in a sphere of host phospholipid membrane. The virus will adhere to the cell until neuraminidase has been cleaved from the sialic acid residues of the host cell membrane. Neuraminidases enable separation and subsequent release of viral progeny. Hydrolysis (conversion to the active form) occurs in the GI tract and liver. The use of oseltamivir for treatment of viral diseases in dogs (parvovirus and parainfluenza) has also been reported. Because canine parvovirus does not contain neuraminidase, any observed effect may be due to oseltamivir's action on bacterial neuraminidase from enteric bacteria that cause infections secondary to canine parvovirus. Therefore, because of the lack of evidence for efficacy in these viruses, the recommendation is to reserve oseltamivir for human populations to minimize the potential for formation of resistant influenza strains.

In adult horses with experimentally induced equine influenza, a dosage of 2 mg/kg, PO, every 12 hours for 5 days decreases the length of pyrexia and viral shedding. Pharmacokinetic studies suggest that more frequent dosing may be required to maintain therapeutic concentrations1; however, no safety or efficacy studies have been performed to validate that suggestion. In 2006, the FDA prohibited the extralabel use of the adamantane and neuraminidase inhibitor classes in chickens, turkeys, and ducks. Although resistance of influenza viruses is relatively low at this point, there is a serious concern for the potential emergence of resistant strains of influenza A, including H5N1 avian influenza virus in the human population. Extralabel drug use (ELDU) is not expressly prohibited in other species, but these antivirals should be considered to be reserved classes of drugs to preserve their efficacy in the human population, should not be used in food animals, and should be used only in instances of documented infection and only when other agents or measures are considered inadequate for patient survival.

Interferon alfa-2 is an immunomodulating cytokine. It has been used in cats for treatment of feline leukemia virus-associated disease as well as for treatment of feline infectious peritonitis and feline immunodeficiency virus infection.