Center for Companion Animal Health

FIP and Polyprenyl immunostimulant (PI)

Based on the results that have been presented to date, the CCAH cannot recommend PI as a means to prolong life in cats with any form of FIP.  

Polyprenyl immunostimulant (PI) is classed as a biologic by the USDA and is manufactured by Sass and Sass in Tennessee (   PI has been given a conditional license by the USDA as therapeutic for “symptoms” of feline herpes virus infection.  According to the Sass and Sass website they are conducting research on its use for FIP and other diseases of companion animals.  Although not currently approved for the treatment of FIP by the USDA, it is being widely used off label for prolonging the life of cats with milder forms of FIP.  It has not shown any benefit for treating or prolonging life of cats with wet FIP or cats with severe disease signs at the onset of therapy.  Although touted on the web and by certain individuals as a way to prolong the life of cats with FIP, in particular the milder dry forms, it is important to review what is known about the efficacy of PI when used for this purpose. PI may cost over $400 a month if used on an average size cat and dosed accordingly and this expense can be magnified by associated veterinary expenses.

The first report on the use of PI for FIP was an article ( published by Dr. Al Legendre and colleagues.  In this study, three cats with the dry form FIP were treated with Polyprenyl Immunostimulant. Two of the three cats were still on treatment and are alive and well 2 years after diagnosis. The third cat survived 14 months but was treated for only 4.5 months.  However, it is important to know that these three cats had disease limited to an intestinal lymph node and that two of the cats were not showing any signs of illness at the time of discovery (one during a spay and one from palpation as a routine health check in preparation to act as a blood donor). The third cat was somewhat scruffy in appearance and had a palpable intestinal lymph node. Blood tests and lymph node biopsies or aspirates were compatible with the diagnosis of FIP.  All three cats were started on PI treatment. One cat treated for 27 months remained clinically normal although it still had blood changes suggestive of subclinical disease. The second cat was treated for 10 months and appeared normal with resolution of the lymph node enlargement. The third cat was treated for the first 3 months and continued to lose weight and the treatment was discontinued. This cat died 11 months later from FIP. The authors believed that these results showed some therapeutic benefit of PI treatment on the presumption that all cats with FIP will die within 200 days or less.  The significance of treatment on these three cats is questionable. Not only were the numbers small, but some cats with no or minimal signs of disease and lesions localized to an abdominal lymph node will spontaneously recover with time ( Moreover, only one of the three cats appeared to have made a complete recovery; one died of FIP and one had signs of subclinical infection at the end of treatment. The authors concluded that “further studies are necessary to assess the potential of the Polyprenyl Immunostimulant,” and only cats with well documented dry form FIP will be considered for inclusion into a subsequent study.

The subsequent study of 102 cats diagnosed with dry FIP was presented in 2012 at the ACVIM forum by Dr. Al Legendre.,d.cGE.  The results of this follow-up study, as summarized by the researcher, were as follows: “Data from cats with suspected FIP was reviewed and cats that met the inclusion criteria were admitted into the study.  From previous experience we did not see any benefit of PI in cats with the effusive or wet form of FIP and these were excluded from the study.  A total of 58 cats with a diagnosis of “dry” form of FIP treated with polyprenyl immunostimulant were included in the data. We started with a total of 102 cats but cats that died before starting the polyprenyl immunostimulant, cats that died before receiving a week of PI treatment and cats from which we did not have a follow up evaluation were eliminated.  There were a wide variety of purebred and domestic short haired cats in the study.  The cats had ocular, neurologic and abdominal forms of the dry FIP with the abdominal form being the most common form.  Nine percent of the cats had vague signs without a specific organ involvement.  Three cats lived longer than 365 days (5%).  Twenty –two percent were alive at least 165 days. The PI appears to improve the wellbeing of the cats that are on treatment.” 

It is noteworthy that 44 of the cats enrolled in the study were eliminated because they either died before treatment could be initiated, died after receiving the drug for less than one week, or were not seen again for follow-up evaluation. Two of the groups that were dropped from the study obviously had severe disease, and it is highly likely that most of the cats lost for follow-up either had died or were not responding to treatment and also had severe disease.  Of the 58 cats with milder dry FIP that remained on the study, 9% (5/58) had vague signs without specific organ involvement, i.e., disease not confirmed.  Even including these cats, only 22% (12/58) were still alive at 165 days and only 5% (3/58) of these cats were alive after 365 days.

Owners contemplating using PI therapy for a cat with FIP should carefully consider these previous findings as well as cost of therapy. Cats with wet FIP or severe dry FIP are not responsive to treatment.  Among cats with a milder form of dry FIP, only 5% will still be alive one year later.  The only claim made by the researchers or manufacturer is that PI treatment may prolong life in a subset of cats with milder dry FIP.  Since no placebo controls were included in this study, and given that fact that a small percentage of cats with dry FIP may live for many months and even years without PI treatment, even this claim must be considered with some skepticism.  Based on the results that have been presented to date, the CCAH cannot recommend PI as a means to treat or even prolong life in cats with any form of FIP.

Polyprenyl immunostimulant as a potential treatment for cats with FIP has an interesting history that extends from Russia to the United States. The research and chemical structure of PI is based on a biologic "plant extract" called Phosprenyl, which is used in Russia to treat a wide range of viral infections in many animal species ( PI may also have a Russian origin. The following web posts were published in Russian, but with the help of Google translator it is possible to access much of this material in English