Abstract
Dear Editors,
I found Dr Pedersen’s article ‘Efficacy of a 3C-like protease inhibitor in treating various forms of acquired feline infectious peritonitis’ 1 both interesting and hopeful for a cure of effusive feline infectious peritonitis (FIP), but I was concerned that our studies were deliberately misrepresented in the discussion. Dr Pedersen’s article states, ‘Cats with this form of FIP have been known to undergo spontaneous remissions, indicating that there is a tipping point between immunity and disease’.2,3 What is not said is that all of the cats in both studies cited were receiving Polyprenyl Immunostimulant (PI). That statement, I believe, misleads the reader. I acknowledge that there was not a placebo control group in the studies but it has long been recognized that untreated cats rarely, if ever, survive FIP. There are currently over 100 cats diagnosed with dry form FIP which were treated with PI that have survived for over a year.
Small Animal Clinical Sciences, College of Veterinary Medicine, University of Tennessee, USA
The corresponding author responds:
Dr Legendre’s comment brings up an interesting point. I now realize that most cats exposed to FIP-causing enteric coronavirus mutants develop immunity rather than disease and that, when disease occurs, it resembles tuberculosis in people. A small fraction of cats will progress to disease and death rapidly, some slowly, and some may contain their disease only to have it flare up later in life. The disease course and the form it takes are therefore dependent on the immune response of the host cat and intrinsic and extrinsic immune suppressive influences. Our study raised the possibility that suppressing virus replication may allow some cats to re-establish their immunity, while Dr Legendre suggests in his comment that this can also be done in some cases of FIP by exogenous immune stimulation.
Dr Legendre’s largest and reasonably controlled study testing this postulate involved 60 cats with milder forms of dry FIP treated with the plant extract PI. 3 He found that 8/60 treated cats were still alive at 200 days and 4/60 at 300 days. Historical survival data was available for 59 cats surviving up to 200 days. There was no significant benefit in survival between PI-treated cats and the 59 historical controls up to that time. The number of cats surviving 300 days and longer was too few to confirm a benefit for PI treatment beyond this point.
In his comment, Dr Legendre reports that there are now over 100-PI treated cats that have survived for over a year, rather than the four cats he reported earlier. I would welcome a publication of this cohort confirming a survival advantage of cats with dry FIP that were treated solely with PI, because if non-specific immune stimulation prolongs the life of some cats, it could give credence to combining anti-viral drug therapy with an immune stimulant, such as the past treatment of hepatitis C in people with a combination of ribavirin and interferons. However, just as the case for modern hepatitis C treatment, I believe that highly potent anti-viral drugs will be the sole or main ingredient in treatments for cats with all forms of FIP in the future.