This page: FIV is a lentivirus, which means a slow virus - what is often not understood is just how slow it is.

FIV is a Lentivirus, or 'slow' virus

FIV is a 'lentivirus', meaning a 'slow' virus with long incubation periods; any text book will tell you this.

What they don't tell you, however, is just how slow!

To understand just how long it takes for FIV to really affect a cat, it is important to understand the basics of the process.

The main way the virus affects the immune system is by attaching itself to, and destroying certain immune system cells (CD4 T lymphocytes).

These cells control the way the immune system recognises and attacks infections, so they are an important part of the complex immune system.

However, what is important is that the number of these cells in a healthy cat is such that there is a huge safety factor involved; these cells can be reduced in number to a tiny fraction of normal before it really causes immune deficiency.

A normal number of these cells is in the range of 800 - 1500 cells (per cu ml of blood). As the number of these cells reduce, the immune system remains strong and effective, it is only when very low levels are reached that the immunity is really compromised.

In order for the immune system to be suppressed enough to affect the cat, the number of these cells need to be reduced to around 200 or fewer. In fact there have been studies where cats have registered CD4 levels dipping below 100 and the cats still remain symptom free and apparently healthy.

In the human version (HIV) these CD4 numbers are used to determine the stage of the virus. Below 200 is considered to specify the AIDS stage, but above 200, there is minimal actual effect.

So how quickly - or slowly - do these cells decline in number?

The rate of decline of these cells caused by the virus has been shown to vary between 35 - 70 cells per year. So for a normal healthy average of say 1000 cells to be reduced to the danger zone of 200, even at a relatively high rate of say 60 cells per year, would take 10 to 12 years.

Clearly, as far as the human HIV is concerned, that time scale is well within the human life expectancy, so treatment to slow down the decline even more is started in HIV treatment as early as possible. Whereas 10 - 12 years in a cat is the major part of a normal life expectancy anyway, especially when infected as adults. This is why the vast majority of FIV cats never reach the low levels of CD4 cells sufficient to really effect the immune system. - This is how slowly the 'lentivirus' really acts.

FIV name is misleading

It is a shame the name of the virus (Immunodeficiency) gives a false impression that immune deficiency starts as soon as the virus is detected - in fact it rarely effects the cat's immunity at all - technically, CD4 cell levels may decline, but usually not sufficiently to really affect the cat's health, certainly not for several years.

So, why do the text books warn of health risks for FIV cats?

When FIV was first identified, back in 1986, scientists knew it was a slow virus, but their studies had a limited time scale, so they needed to make observations and get results quickly - therein lay their problem.

In order to kick start reactions to the virus, they needed to inject large volumes of highly virulent strains of the virus. This quickly flooded the body with virus, not allowing for any of the natural defence reactions to take place.

It would be like a large storm dropping a month's worth of rain in just a few hours - resulting in dramatic flooding and huge damage. The same month's rain falling normally throughout the month would have none of these damaging effects.

The same difference in results would be true of flooding the cat's system with strong virus, rather than the normal tiny amount from a naturally-infected cat bite, which allows the natural defence system to hold it at bay for years.

Sadly, it is mainly these lab-infected cat studies that the text books have to use to base their statements on. No wonder we are given an un-representatively bleak view of the results of the virus.

Over the last 25 or more years, more and more naturally-infected FIV cats have been allowed to live out their natural lives in homes across the world. The results from this experience shows that the FIV virus does not have the immunosuppressive reactions the text books suggest.

This is all confirmed by the results of the '1000 FIV cats project', which collects the experience of many hundreds of FIV cats living in domestic homes across the world. This shows that FIV cats are predominantly healthy (over 85% of the time) and have no specific ailments that could be classed as common across the total (nothing more than about 20%). See the website for details and results of the project : (opens in new tab).

<back to top>