There’s been a lot of interest in our blog about the optimal famciclovir (Famvir) dose for feline herpesvirus. It’s lovely to have a dose that’s backed up by research – after all, anyone can claim success for any dosage regimen at all in a disease that’s usually self-limiting! BUT, the pharmacokinetically optimal dose (90mg/kg BID) is much higher than most of us have used in the past, making cost and patient tolerance even more problematic. With the help of Dr. Elena Contreras DVM, I’ve delved into this a little further to assess the options.
This is a long post that doesn’t have all the answers by a long shot, so here’s the executive summary for those who don’t want to read the whole thing:
- For now, we may be stuck with 90mg/kg BID as the optimal dose. For ocular infections, 40 mg/kg TID – or BID? – might be effective. (The drug is present in tear secretions and studies have measured tear concentrations).
- Consider having famciclovir compounded into a tastier and more easily dosed form, like a palatable powder that can be mixed with food and syringe-fed to inappetant cats.
- Money and time can be saved by (1) focusing on prevention; (2) only treating cats that really need it; (3) not using topical antiviral treatment as well as oral famciclovir, and (4) not using antibiotics when not indicated. That should leave a budget over for famciclovir for those that need it most, especially the teeny tinies.
Treating feline upper respiratory infections: The Big Picture
URI can’t be effectively treated by any drug unless husbandry is addressed. Optimizing housing, air quality, length of stay and infection control, and minimizing stress and crowding, all have far more impact on URI incidence and severity than antiviral or antibacterial drugs. Reducing stress is key.
Shelters can save costs, avoid side effects, and responsibly steward antibiotics, by recognizing that there is no indication for antibacterial treatment for a viral disease. Antibiotics should only be used for URI if there is a strong suspicion that there’s a bacterial infection.
If the cat has yellow-green snot coming out of the nose, antibiotic use is generally recommended and considered to be rational. But is it always? A bad viral infection can also result in seemingly purulent discharge. And even if bacterial infection is present, are antibiotics always needed? Many of us have kids who’ve had cold after cold from school or daycare. How often would we would want a systemic antibiotic for a child with a snotty nose who is happy, bright and running around? So, even with purulent discharge, it’s important to look at overall status and presence or absence of systemic signs.
As for other things that might or might not work or be needed, there is contradictory evidence for the use of L-lysine, so that may not be the best use of shelter resources. There is some evidence suggesting that an intranasal FHV (or FVRCP) vaccine, in addition to the subcutaneous vaccine, might boost a cat’s cell mediated immunity and provide more protection against upper respiratory clinical signs. However, DO NOT substitute the intranasal vaccine for the SQ vaccine in the shelter, as the MLV SQ vaccine is essential to protect against panleukopenia.
When to use antivirals, and how do you know it’s herpes?
When considering an antiviral drug, patient selection is key. Most viral respiratory infections will resolve with time, supportive care and a functioning immune system – as we all know from personal experience. Famciclovir should be reserved for the most vulnerable animals with the most severe clinical signs, particularly those that are not improving on nonspecific treatment. Famciclovir is rational for severely stressed, sick URI cats and for young kittens with severe ocular signs, in which eyes may be lost without aggressive treatment.
Famvir tastes awful, if the cats are to be believed. The stress of dosing could offset the benefits of treatment for some animals.
How do we know it’s herpes? It is very difficult (if not impossible) to confirm that herpes is the cause of disease/clinical signs in client-owned or shelter cats. This is because there are too many false positives and false negatives on common diagnostic tests. The virus can “hide” in many places. And even if FHV DNA is found on PCR, we don’t know if that’s the cause for the clinical signs… could be that a bacterial infection or calicivirus is responsible, and we just happened to find herpes. So outside of research studies with naive cats, PCR has substantial limitations in individual cases. PCR is much more useful in outbreak situations where multiple cats are being tested.
All that said, the expert consensus is that herpes is the most common and important feline ocular and upper respiratory pathogen, so in these really sick kitties, it’s still reasonable to try an antiviral drug.
Famciclovir – does it work and how should we dose it?
From a very nice knowledge summary about the effectiveness of famciclovir for feline herpes infection:
- The number of comparable studies is small. Different studies have looked at ocular, respiratory or dermatological disease, in experimental, shelter and private practice environments, and using different dosage regimens. Confounding variables have included other medications, incomplete histories and lack of confirmed diagnoses.
- Despite this, the consensus is that famciclovir does significantly reduce disease severity and duration in cats with herpes infection.
What is the clinical evidence for different doses?
- Earlier studies suggested a 10, 20, or 30mg/kg dose, q8, 12, or 24, and some reported clinical improvement.
- 40mg/kg TID: Tear concentrations are likely to be effective against herpes.
- 90mg/kg BID: Pharmacologically optimal serum and tear concentrations (this was the study cited in the original blog).
- 90mg/kg TID: Significantly reduced clinical signs, reduced histologic evidence of conjunctivitis, decreased viral shedding and reduced serum FHV-1 titers.
- 40 vs 90mg/kg TID: Cats treated with 90mg/kg had greater and faster improvement, and shorter duration of treatment. This study suggests that 90mg/kg is ultimately more cost-effective.
How about duration of therapy?
There are no clear guidelines. Sources often recommend treatment for 2-3 weeks, which could be excessive, given that the drug is not expected to clear the infection, and that URI often resolves within 7-10 days. It seems reasonable (to me) to treat for 3-5 days and if there is no improvement, stop. If the cat is improving, continue to treat – the duration could be tailored to the cat, based on clinical severity, duration of illness, amount of improvement, and shelter resources.
By LINDA JACOBSON
Thank you to our guest co-author, Dr. Elena Contreras DVM, PhD candidate, Colorado State University, and Graduate of the Maddie’s Shelter Certificate Program, University of Florida.
- Hurley KF, Aziz C (2015) Feline URI – Diagnosis and Treatment. Proceedings, Pacific Veterinary Conference. VIN Proceedings (VIN members only) http://www.vin.com/members/cms/project/defaultadv1.aspx?id=6789821&pid=11768
- Cole J. (2017) In Cats Infected With Feline Herpesvirus Type-1 (FHV-1) Does Treatment With Famciclovir Result in a Reduction of Respiratory and Ocular Clinical Signs? https://www.veterinaryevidence.org/index.php/ve/article/view/105/169
- Thomasy SM, Maggs DJ (2016) A review of antiviral drugs and other compounds with activity against feline herpesvirus type 1. Vet Ophthalmol 19:119-30. Vet Ophthalmol. 2016 Jul; 19(Suppl 1): 119–130.
- Glaze MB (2016) Feline Herpesvirus-1 Ocular Infection. VIN Associate Database (VIN members only) http://www.vin.com/Members/Associate/Associate.plx?from=GetDzInfo&DiseaseId=751