Polyprenyl Immunostimulant™ and the importance of the immune system in the treatment of viral diseases.

Viral diseases develop when a virus is using the cat’s cells to replicate. The organism protects itself with a system of defenses known as immunity. Because immunity is a barrier for viruses, those evolve in various ways to avoid or eliminate barriers and reduce resistance. Some viruses mimic the host organism’s molecules and get into the cells by evading the immune system. Once inside the immune cells, viruses replicate using the cells and then kill them from within. Other types of viruses defeat immunity by building their own functional molecules and assembling viral particles inside non-immune cells. Because immunity is what protects the cat(s), it is the first target of viruses: one way or another, the invader needs to break through the defenses.

There are three approaches to address/treat/control viral diseases, including in cats:
1 – Vaccination
2 – Direct antiviral drugs
3 – Immunomodulatory drugs


Vaccines are highly specific, large molecules that bind and neutralize a particular virus they recognize. The effectiveness of feline vaccines varies. If vaccine developers miss the viral identity, the vaccine is ineffective to protect the organism from it – that’s what, for example, happens with the flu. The problem with creating FIP vaccines, for example, is likely due to the fact that Feline Infection Peritonitis Virus (FIPV) has a very wide variety of “identities” with two major serotypes, multiple strains, and individual variability.


Antiviral drugs interfere with the entry of the virus into the cell, or replication of viruses, by trying to prevent biosynthesis of viral nucleic acids or proteins, or their assembly into new viral particles. Over the past century, direct antiviral drugs have helped with various feline viral diseases; yet, they often prove insufficient with severe viral diseases and even ineffective with some others.

There’s a reason for that: with most viral infections, antiviral drugs are used to suppress viral replication. However, the majority of viruses cannot be killed or cleared completely, and some of them remain in the organism at any point in time. A course of an antiviral drug may affect the current, ongoing viral activity, but it does not prevent the infection from reoccurring later. Direct antiviral drugs fail to prevent relapses or treat immune-related diseases because they do not address the real reason why cats develop certain viral diseases: the suppression of immunity. These occur due to the cat’s inherited genetic, living environment, and stress-inducing situations for cats, such as major changes in the fur-child family, or crowded living space such as shelters, catteries, and multiple-cat households.

A course of an antiviral drug may affect the current, ongoing viral activity, but it does not prevent the infection from reoccurring later.

This is the main reason why antiviral drugs (when available) often prove insufficient or even ineffective with immunity related veterinary viral diseases such as, for example, Feline Viral Rhinotraheitis (FVR) and related FHV-1 and URI, Feline Leukemia, Feline Immunodeficiency, and Feline Infectious Peritonitis (FIP).

Direct antiviral drugs are designed, by definition, to try to kill viruses and/or reduce viral load, which is only an “effect” of a disease but not a real “cause.” However, an antiviral drug by design is not able to treat a real reason why the disease manifests itself – the immune deficiency. Consequently, a cure for FVR, Feline Leukemia, FIP, and several other viral diseases is not feasible. It is why veterinarian researchers and practitioners are now trying to develop effective treatments that will address and compensate for the true reasons why cats develop such viral diseases in the first place: genetic immunodeficiency and the suppression of immunity.

That is why there are still many feline viral diseases for which effective direct antiviral drugs have not or cannot be made or used. Even if a reasonably effective and safe antiviral drug becomes available, what about the cat’s immunity, which was suppressed or damaged by the virus? Neglecting to rebuild the cat’s immune system means you leave the damage and the ruins. The cat still feels sick and remains susceptible to relapses, other diseases, and viral re-infection. You may have taken care of one aspect of the illness: the ongoing viral replication. Still, you have failed to address the root of the issue: the failure of the immune system that, for one reason or another, allowed the viral infection to develop in the first place.

Direct antiviral drugs fail to prevent relapses in immune-related diseases because they do not address the real reasons why cats develop viral diseases: immunodeficiency and the suppression of immunity.


Immunostimulating (immunomodulatory) medicines are a new, cutting edge development designed to rebuild the damaged immunity and direct it against the invading viruses. In some instances, an antiviral drug may be needed in addition to an immunostimulant drug, but won’t replace it, because suppressing the agent of the disease does not restore the immune system. If immunity has been wiped out, cats feel equally bad regardless of the viral load. Remember how long it takes to get back to your normal self after a head cold or the flu? It’s the same with cats. A medicine that stimulates or modulates immunity helps the body fight the disease; it also regulates the strength of the immune response, aims to correct the damaged immune system and bring it back to a healthy status. Another notable benefit of immunomodulatory therapy is the elimination of the feeling of sickness – a by-product of the disease – As immunity improves, the cat starts feeling better and happier.

Many veterinary specialists agree that immunomodulation is imperative with viral diseases that cause massive damage to immunity. Most viral infections start and end with immunity. A viral disease begins with the virus evading the weakened immunity of an immuno-compromised cat and ends when the restored immunity controls or suppresses the virus.

Examples of such diseases include:
• Feline viral rhinotracheitis (FVR) caused by a herpesvirus;
• Feline infectious peritonitis (FIP), an immune-mediated disease triggered by the mutation of a feline enteric coronavirus (FCoV);
• Feline leukemia virus (FeLV), which impairs multiple immune functions;
• Feline immunodeficiency virus (FIV), to name only a few.

These and some other viral diseases begin with the virus evading the weakened immunity of an immuno-compromised cat and ends when the restored immunity controls or suppresses the virus.

The consensus about an effective treatment for feline infectious peritonitis (FIP) is that while attempts are being made to find antiviral drugs that will slow down the replication of the virus, the most promising approach is to combine an antiviral and an immune response modifier.

Polyprenyl Immunostimulant™ (a.k.a. PI or PPI) triggers an innate immune antiviral response, which is the only defense against the virus in a non-immunized cat. The signal that PI conveys to the cells alerts them to start fighting the invader even if they are blind to the invasion.

In the US, immunity modulation/stimulation using Polyprenyl Immunostimulant™ was pioneered in the early 2000s by Dr. A.M. Legendre, DVM, who tested Polyprenyl Immunostimulant™ in feline viral diseases like feline viral rhinotracheitis (FVR), a common and ubiquitous upper respiratory infection (URI); and feline infectious peritonitis (FIP).

Experts in virology agree that innate cellular immunity is central to the control or elimination of the virus in the body. Polyprenyl Immunostimulant™ belongs to the class of signaling molecules and triggers innate cellular immunity through Toll-receptors 1, which activate defenses by different immunity cells. PI is a unique treatment in veterinary medicine – no similars or analogs exist.

Herpesvirus, which causes feline rhinotracheitis, is a latent infection with periodic flareups. It cannot be cured; it can only be controlled so that the cat does not experience the upper respiratory infection (URI) signs responsible for the feeling and behavior of being sick. All herpesviruses sabotage immune responses. Sickness behavior comes from a lack of proper immune response [Tizad, 2008, Animal Health Res Rev 9; 87-99]. While direct antiviral may lessen the viral load during flareups, cats do not read the viral titer report. If their immunity has been annihilated, cats feel equally bad regardless of viral load.

Polyprenyl Immunostimulant™ is the only therapeutic approved by the USDA for the treatment of feline rhinotracheitis. It reduces the severity of the disease in cats and very safe (zero side-effects), well-tolerated oral liquid designed with cats in mind. Polyprenyl Immunostimulant™ reduces clinical signs and alleviates the feeling of sickness by restoring an adequate immune response.

Feline Infectious Peritonitis (FIP) is a devastating disease caused by a mutated form of feline coronavirus that triggers a global immunity failure in cats. FIP has long been considered an incurable, fatal disease. The way clinical FIP develops as an immune-mediated disease is unique. Interaction between the body’s own immune system and the virus is responsible for the disease.

The 20-year long collaboration between Dr. Legendre, DVM, and Sass & Sass, Inc. and VetImmune® included extensive research, clinical and field trials, ongoing R&D, etc., and delivered the treatment to several thousands of cats in 42 different countries diagnosed with FVR and the dry form of FIP. Consequently, Dr. Legendre also accumulated a list of over 200 cats who are long-term FIP survivors (alive for more than 365 days after being diagnosed with FIP and starting treatment of FIP with Polyprenyl Immunostimulant™). The longest surviving cat on this list has been surviving FIP for over 13 years and is still alive and well.

Our numerous user surveys show Polyprenyl Immunostimulant™ makes cats happy, energetic, and returns them to their healthy selves. You may read a full version of this article [here] and more info on FVR and FIP on our main site

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