This page: A simple, non-technical description of what the FIV virus does to the immune system will help you understand what it really means for the FIV cat.

About FIV - What the virus does

How the FIV virus works and affects the immune system


The FIV virus affects the immune system, but does not destroy it, as we are often told!

So, what does the FIV virus actually do to the cat ?

Simple question - not so simple answer, of course.

It helps to understand a little of what is actually happening when a cat is infected with FIV. We are not vets or scientists, so the following is very much our layman's understanding of the way the virus works.

The immune system is very complex, with a range of different cells that work together to provide the protection the body needs. The job of some cells is to identify when an infection arrives in the body, other cells work to destroy the infection, and others work to repair any damage. All these cells need to do their own part and communicate with the others - the ultimate in teamwork!

The FIV virus, once it enters the blood stream, attaches itself to a certain group of these cells, and also affects the communication system between the different cells - which, over time, can confuse the workings of the immune system, and cause it to be less efficient - broadly as follows:

The major cells that operate in the immune system are called T cell lymphocytes, and these divide into several different types, the most relevant here being called CD4+ and CD8+. The method of communication between them uses protein compounds called cytokines. (those are really the only 'technical' terms you need to know.)

Again, in layman's terms, what the various elements do is this: the CD8+ lymphocytes are the main army of cells which go out and destroy invading cells (infections etc.); the CD4+ lymphocytes play a more directing role, by identifying the problems and directing the CD8+ cells to action. They mainly communicate via cytokines, which are proteins that carry the communications between cells.

The FIV virus concentrates mainly on and attaches itself to the CD4+ cells, but does also attach to CD8+, but to a lesser degree.

Gradually, over a period of years, the FIV virus is responsible for the reduction in the number of CD4+ cells. In the human form (HIV) the number of CD4+ cells are measured to give an indication of the progress of the virus (fewer is worse). This is not thought to be a relevant way of measuring FIV, and anyway, the tests to measure them are not normally available outside of research facilities.

Other factors apart from FIV also have an affect on the CD8+ cells, so decline is not necessarily due to FIV. As it is the CD8+ cells that are the ones that really do the destroying of invading infections, it is the other factors (not the FIV) that have the largest effect on how strong the immune system is. However, as it is the CD4+ cells that are most affected by the FIV virus, and it is those cells that do more of the directing of operations, then it is more the speed and efficiency that is affected by FIV than the actual strength of the immune system.

So, as a very generalised statement, as the FIV virus increases, it damages some of the cells that provide the immune function, and can also disrupt the communications between the cells - in effect, making the immune system slightly dyslectic and less efficient than it would otherwise be - a bit like an ambulance with a faulty Sat-Nav! - it gets there in the end and does the business, but not as quickly and efficiently as it might if everything was working at top efficiency.

Just how and at what stage this has a real effect on how the cat is, is not easy to predict, but experience would suggest that the other factors that may have affected the immune system may well be far more important than the FIV in most cases.

Of course, the above is a hugely simplified explanation, and any vet or scientist will recognise the complexities omitted here. However, most of what can only be detected by research testing may have a theoretical influence, whereas in practice, the immune system just keeps on working!

So how far does the reduction in these cells need to go before it has a real effect on the health of the cat?

The immune system is a very strong and robust system, which will keep working even under quite extreme circumstances. It is similar to much of the body's design in that there is a good 'safety factor' designed in to all body organs and systems.

By way of example and for comparison, consider the kidneys. We know that the kidneys can function efficiently despite severe damage - this is why, in humans, it is recognised as quite acceptable to actually donate a healthy kidney to someone else; because we know that we will function perfectly well with only one kidney, and still maintain a sufficient safety factor - so effectively at 50% reduction in kidney power. It is only when that remaining kidney is severely damaged that any problems occur. If the remaining kidney was reduced to 50% efficiency before problems arose, then that would be a total reduction of 75% of full kidney function, so only 25% remaining before problems arose. - Quite a safety factor!

The same is true of the immune system. In fact, particularly the immune system, as it may have to deal with a huge variety of pressures from infections etc. so has to have enormous capacity that is normally not used; so a good safety factor is essential.

With the human virus (HIV) it is normal to take readings of the CD4+ and CD8+ levels, and what is considered 'normal' varies enormously - from 800 to 1500 cells per cubic centimeter of blood. So we could assume an average of say 1000 as the norm for healthy people, yet HIV patients need levels to fall to about 200 before they are considered to be entering the AIDS phase. That is 20% of 'normal' before it is considered a real problem.

Remember, the human HIV is a much stronger virus than the feline FIV; so if HIV needs to reduce the cells down to 20% before it is considered AIDS, then how far must the FIV reduce them before it has a real detrimental affect?

With FIV, whatever the actual numbers, the same proportion would be relevant. When a healthy cat becomes infected with the FIV virus, it takes many years of slow decline before any real effect is noticed, so, as the levels of CD4+s gradually decline, although research scientists may be able to tell, it will not actually affect the cat, due to the major safety factor in the design of the immune system. Mostly, cats with FIV die from any of the usual things cats die from, including old age, long before the CD4+ levels reach a critically low level. - just as patients who donate a kidney, will usually have no problems at all with only 50% of their kidney function for the rest of their lives.

It should be noted that the FIV virus, because it attaches itself to the CD4+ lymphocytes, will be increased if the CD4+ cells increase.

When the body has an infection to deal with, or in other ways the immune system is brought into action, the CD4+ cells will increase and multiply themselves, and in so doing, will also multiply the virus. For this reason, it is important to try and keep anything that may activate the immune system, like stress and infections, to a minimum.

This also is a factor to consider with regard to the standard vaccinations against flu etc, as the purpose of a vaccination is to cause the body to raise an immune response to the vaccine in order to form the protection required to fight the real infection. The very act of raising this immune response could be increasing the amount of FIV virus in the body. For this reason it is important to discuss the implications of vaccination of FIV+ cats with your vet before proceeding (but make sure they really know about FIV!)

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