Antiviral drugs

 

Although antiviral compounds are sometimes used for systemic infections such as feline immunodeficiency virus (FIV) infection, their main application in veterinary medicine is in ophthalmology, particularly for herpesvirus infections in cats and horses. Only limited data are available on the efficacy of antiviral compounds against veterinary viruses. Antiviral compounds are highly toxic, have a relatively narrow therapeutic index and should be used with great care.

Aciclovir has been used to treat both ocular and respiratory disease caused by feline herpesvirus 1 in cats. However published information on efficacy is equivocal and aciclovir has no activity against felid herpesvirus 1 in cell culture. Aciclovir appears to be more effective for conjunctivitis and keratitis caused by equine herpesvirus in horses; clinical improvement is usually seen within a few days. Aciclovir has also been used experimentally in equid herpesvirus 1 abortion outbreaks, but further studies are required before this regimen can be recommended.

Zidovudine has been used to treat cats with FIV infection. The drug produces at least temporary alleviation of clinical signs in a proportion of cats, and may increase both survival time and quality of life. However, zidovudine has no obvious effect on viraemia. Clinical improvement is generally observed 10 to 14 days after commencement of treatment. Zidovudine is less effective against feline leukaemia virus and provides no clinical improvement. The drug can cause severe anaemia in cats, and possibly hepatotoxicity at high doses. The side-effects of long-term treatment with lower doses have not been ascertained.

In addition to specific antiviral compounds, drugs which may enhance the immune response are sometimes used either alone or in conjunction with antivirals to treat virus infections. These include interferons and various other immune-stimulatory preparations.

Interferons are cytokines produced by the host cells under particular circumstances, including viral infection. Once produced these interferons are then able to produce a variety of effects including prevention of viral infection of other cells. There are several different types of interferon identified, and a recombinant interferon omega of feline origin is authorised in the UK. Although currently authorised for the treatment of dogs with canine parvovirus infection, this product is also effective in cats and many cat viruses have been shown to be susceptible in vitro. The exact mechanism of antiviral action of omega interferon is not fully understood, however, there is no direct effect against the virus, rather an inhibition of synthesis within infected cells by destruction of mRNA and inactivation of translation proteins.

Interferon treatment has been used for cats infected with feline calicivirus and has been shown to reduce acute stomatitis associated with this infection. Cases of feline chronic gingivostomatitis have also been treated with interferon and although the results of clinical trials are still awaited, there are anecdotal reports of treatment improving clinical disease. Interferon has also been used in cats infected with FeLV, FIV, or both and has shown some effect in these cases.

Immunoglobulins directed against specific viral infections are available for use in animals exposed to infection. Vaccines are rarely effective at controlling disease once infection has occurred (and certainly once clinical signs have developed) in an individual animal, although they will provide group protection. Treatment of viral infections in animals usually consists of nursing, control of secondary bacterial infections, possibly analgesics, NSAIDs, and other symptomatic therapies.

ACICLOVIR

Indications.

Dose: See Prescribing for reptiles

Prescription-only medicine: Aciclovir (Non-proprietary) UK

Tablets, aciclovir 200 mg, 400 mg, 800 mg

AMANTADINE HYDROCHLORIDE

Indications.

Dose: See Prescribing for ferrets

Prescription-only medicine:® Symmetrel (Alliance) UK

Capsules, amantadine hydrochloride 100 mg Syrup, amantadine hydrochloride 10 mg/mL

OMEGA INTERFERON

Indications. Parvovirus infection (enteric form); viral infections in cats

Contra-indications. Vaccination during and after treatment until full recovery

Side-effects. Transient hyperthermia, vomiting, slight leucopenia, erythropenia, thrombocytopenia

Warnings. Fluid therapy and other supportive treatment necessary; safety in pregnant or lactating animals has not been established; studies on long-term effects have not been established

Dose:

Dogs: by intravenous injection, 2.5 million units/kg once daily for 3 days

Cats ♦: feline calicivirus, by intravenous injection, 2.5 million units/kg. Repeat twice at intervals of 2 days Chronic gingivostomatitis, by subcutaneous injection, 1 million units/kg daily for 5 days, then by mouth, 50 000 units daily

FeLV, FIV infection, by subcutaneous injection, 1 million units/kg daily for 5 days

Prescription-only medicine: Virbagen Omega (Virbac) UK

Injection, recombinant omega interferon of feline origin 5 million units/vial, 10 million units/vial, for dogs

ZIDOVUDINE

(Azidothymidine, AZT)

Indications. Feline immunodeficiency virus infection in cats, particularly when clinical signs of immunodeficiency related disease are evident

Contra-indications. Renal impairment, hepatic impairment

Side-effects. Hepatotoxicity and anaemia in cats

Dose:

Cats: by mouth or by subcutaneous injection, 5-10 mg/kg daily in 2-4 divided doses

Prescription-only medicine:® Retrovir (GSK) UK

Oral solution, zidovudine 10 mg/mL

Injection, zidovudine 10 mg/mL. For dilution and use as an intravenous infusion

Combination preparations

Indications.

Dose: See Prescribing for reptiles

Prescription-only medicine:® Trizivir (GSK) UK

Tablets, abacavir (as sulfate) 300 mg, lamivudine 150 mg, zidovudine 300 mg