Antivirals are a group of drugs that are used to treat several viral infections in cats. Viruses and bacteria are both common causes of disease in cats; however, they differ in their treatment.
Viruses are intracellular parasites (parasites that hijack the cells of their host) that are made up of RNA or DNA and wrapped in a protein envelope (capsid). They use machinery within the host cells to replicate, which makes it challenging to develop drugs that target the virus without harming the host cells in the process. Most antiviral drugs available for cats were developed for use in people.
Bacteria are single-celled organisms that are capable of replicating on their own. Antibiotics are effective against bacteria; they can prevent bacterial reproduction or kill the bacteria by preventing it from building cell walls.
Antivirals interfere with steps in the viral replication cycle and are grouped into different classes depending on the step in which they work.
- Nucleotide analogue reverse transcriptase inhibitors (NtARTIs)
- Nucleotide synthesis inhibitors (NSI)
- Receptor homologues/antagonists
- Protease inhibitors
- Integrase inhibitors
- Interferons
What viral infections can antivirals treat in cats?
Unfortunately, there is not as broad a spectrum of antivirals as there are antibiotics. Antivirals are available to treat the following infections:
- FVR-1 (feline herpesvirus)
- FeLV (feline leukemia)
- FIV (feline immunodeficiency virus)
- FIP (feline infectious peritonitis)
- FCV (feline calicivirus)
Herpesvirus, FIV and FeLV are with the cat for life. After the initial infection, feline herpesvirus lies dormant in the nerve cells where it remains hidden but can reactivate at times of stress. Antivirals inhibit replication of the virus while the cat’s immune system mounts a response.
The goal of anti-virals for FIV and FeLV is to keep viral replication to a minimum.
FIP is a mostly fatal viral infection, but treatment can prolong survival time.
Antiviral drugs
Trifluridine
Trifluridine inhibits DNA polymerase and thymidine synthetase and is the most active agent against feline herpesvirus. It is a topical medication that is applied to the eye 5-6 times a day.
- Brand names: Viroptic
- Treats: Feline herpesvirus
- How is the drug supplied? Ophthalmic (eye) ointment
Ganciclovir
Ganciclovir triphosphate is a competitive inhibitor of deoxyguanosine triphosphate (dGTP) incorporation into DNA and preferentially inhibits viral DNA polymerases more than cellular DNA polymerases.
- Brand names: Cytovene
- Treats: Feline herpesvirus
- How is the drug supplied? Topical
Famciclovir
Famciclovir is converted to penciclovir, which then converts to penciclovir triphosphate. Penciclovir triphosphate inhibits DNA polymerase and selectively inhibits viral DNA replication. Famciclovir is used to treat conjunctivitis associated with the feline herpesvirus.
- Brand names: Famvir
- Treats: Feline herpesvirus
- How is the drug supplied? Oral tablets
Idoxuridine
Idoxuridine, which closely resembles thymidine, inhibits thymidylate phosphorylase and specific DNA polymerases, which are necessary for the incorporation of thymidine into viral DNA. It is used to treat corneal ulcers and conjunctivitis associated with feline herpesvirus.
- Brand names: Dendrid, Herplex
- Treats: Feline herpesvirus
- How is the drug supplied? Ophthalmic (eye) ointment
Zidovudine (AZT)
A thymidine analogue, zidovudine selectively inhibits FIV’s reverse transcriptase, which is the enzyme that the virus uses to make a DNA copy of its RNA. By inhibiting reproduction, viral load is reduced.
- Brand names: Retrovir
- Treats: Feline immunodeficiency virus and feline leukemia virus
- How is the drug supplied? Tablets and injection
Vidarabine
Vidarabine inhibits DNA polymerase and viral replication by incorporation into viral DNA. It is used to treat conjunctivitis and corneal disease associated with feline herpesvirus infection.
- Brand names: Vira-A
- Treats: Feline herpesvirus
- How is the drug supplied? Ophthalmic (eye) ointment
Interferon
Interferon is a naturally occurring protein made by cells of the immune system in response to certain viral infections. They do not kill the virus but bind to infected cells as well as neighbouring uninfected cells, which stimulate them to produce proteins that prevent the virus from replicating.
- Brand names: Intercat, Roferon, Intron A, Virbagen Omega
- Treats: Feline herpesvirus, feline calicivirus, feline immunodeficiency virus, feline leukemia virus and feline infectious peritonitis.
- How is the drug supplied? Injection and oral liquid
Prevention
While there are less effective anti-viral medications compared to antibiotics, which treat bacterial infections, there are several vaccinations available to prevent viral infections.
- F3: Feline herpesvirus, feline calicivirus and feline panleukopenia
- Feline leukemia
- Feline immunodeficiency virus
- Feline infectious peritonitis
- Rabies
F3 is a core vaccination that ALL cats should receive. Your veterinarian can recommend additional vaccinations which will vary depending on local regulations as well as risk factors.
What other treatments are available for viral infections?
In most cases, supportive care is offered while the cat’s immune system mounts a response to the infection. This will include fluid therapy to prevent or treat dehydration, nutritional support and medications to relieve symptoms.
Antibiotics are ineffective against viral infections but are sometimes prescribed to treat or prevent secondary bacterial infections which can develop.
As FIV and FeLV affect the cat’s immune system, pet owners must take steps to prevent opportunistic infections with regular health checks, diligent parasite control, avoid stress and keep cats indoors. Your veterinarian may tailor an immunocompromised cat’s vaccination schedule.
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