Immune System and Mutations in cats


We first need to talk a little bit about how our cats immune system work. It’s a very complicated and amazing thing! You will need to keep in mind just a few things:

MHC genes are the ones that lead to the creation of B-cells and T-cells. These cells are responsible for the recognition and killing of existing bacteria and virus. All of them. No exception!

Every MHC gene contains different alleles, each carrying the information for a few specific bacteria, virus, cancer cells or disease.

Here is a very simple and basic example on how an MHC gene may look like (in an extremely simplified schema):

Information to fight Herpesvirus strain 1

Allele1Information to fight Herpesvirus strain 2

Information to fight Herpesvirus strain 3

Information to fight Bartonella

MHCAllele2Information to fight Giardia infection

Information to fight liver Cancer

Allele3Information to fight Coronavirus strain 4

Information to fight Calicivirus

This means we need many different MHC genes to have all the information we need to defeat all possible diseases.

What is really important is not their number, but their diversity.

You can have many MHC genes with a huge diversity (and then the ability to fight and destroy a large amount of diseases) or you can have as many MHC genes with a low diversity (and then the ability to fight and destroy some diseases, but not many others).

When you breed two cats together their immune systems (their MHC genes and their ability to fight diseases) sums up! It’s a very clever way for “mother nature” to guarantee every cat will have a large amount and variety of MHC genes that can fight a large amount of infections. Every cat has a different immune system because the possible combinations of the MHC genes are infinite.

There is another reason why cats have a different immune system from each other (the same is valid for all mammals and humans as well). It’s impossible for a single cat to have all the information to fight all diseases at once. The quantity of genes and information necessary for that is just too huge.

Having different cats with different immune systems guarantees the species will survive extinction in case a lethal disease shows up affecting a large portion of the population. A simple example is the bubonic plague that killed a huge amount of the affected human population. But not all of them. Some had in their immune system the MHC with the information to fight it and survive. Diversity is what makes the immune system such a smart and powerful protection for all species.

There is a very easy connection between the MHC genes and the immune system:
huge MHC diversity = strong immune system

low MHC diversity = weak immune system

When the diversity is low the cat has a weak immune system. In practice that specific cat has lost the ability to fight a certain amount of diseases (larger or smaller depending on how low is the diversity). If we are lucky that cat will never get infected with one of the diseases he lost the ability to fight. While if it happens the cat will have no defence against the infection. If it’s pretty aggressive the cat may even die. As you can imagine this may become a huge problem if the cat loses the ability to fight more and more diseases. A simple herpesvirus can become life threatening. In case of a group of cats sharing the same line (and therefore the same loss in diversity) the herpesvirus can become endemic in a whole group/line and barely impossible to defeat. The same situation can happen with barely any disease as a group/line of cats can become susceptible to any specific disease and therefore unable to fight it as an healthy cat would do.

Something else that is important to remember is the difference between the genotype (the genetic makeup of a specific cat) and the phenotype (the observable characteristics of a specific cat, determined by both genotype and environmental factors).
A cat with a weak immune system may appear healthy and may apparently throw healthy kittens. As breeders we have a tool to know weather a cat has a weak immune system even if apparently healthy and a few more tools to improve it.

Let’s now talk about the Devon Rex!

As we all know the Devon Rex breed has originated with Kirlee, a cat who was very probably the result of a father to daughter or mother to son mating (or another inbreeding form). This conclusion has been made based on the simple fact that the genetic mutation of the devon rex is recessive. Therefore 2 cats carrying that gene had to be bred together in order for it to show up.

Being a specific mutation (like the one that happened in the Devon Rex) a quite unique event we can guess the the cat who originally mutated (carrying the devon rex gene but still not showing this mutation yet) bred a cat from its own progeny, giving birth to Kirlee, who had a pair of the Devon Rex genes and was, therefore, showing the mutation effects and the first Devon Rex!

To select the Devon Rex have been used different breeds of cats to widen the gene-pool. Persian, Cornish Rex, Siamese and other breeds were used. Inbreeding and hybridisation have been the main tools necessary to establish our breed at the very beginning. Hybridisation has been necessary to have more cats to work with and a wider gene-pool, while inbreeding has been necessary in the first place to have several curly cats as a basis for the breed and to fix the traits to have a standard defined.

Due to its origins the Devon Rex is, by definition, a breed with a limited gene-pool, often with high inbreeding percentages, due to the fact that all Devon Rex cats go back to Kirlee and a few more foundation cats.

A mutation made the Devon Rex possible in the first place. Mutations happen all the time in nature and they can be very different from one another. A mutation is basically a change in the DNA sequence. They can happen for different reasons, but the one that matters to us as breeders is a “mistake” in the DNA replication. These errors can lead to no visible change, sometimes to slightly beneficial ones, but most of the time they lead to damaging effects.

Some of these mutations may have no visible effect on our cats. Others may become visible after a few generations, or because of a specific mating.

An example we are all familiar with is Spasticity. It has been a mutation and it happened in a single cat at the beginning of the breed selection. This cat has been overused and the kittens spread all over the world. A recessive is forever, and a recessive mutation is too.

A single mutation in a single cat can be a very serious thing.

Another example is given by the American Burmese breed. It happened many years ago that an unusual cat was born (Good Fortune Fortunatas).

His head type was different and the breeder immediately fell in love with that dome-head type. He became National Winner and was given in stud service to many breeders, his kittens sold to many other breeders, so that his name ended up being in an amazing number of pedigrees in the US. Good Fortune Fortunatas carried the head lethal defect with which burmese breeders are now struggling with. A few other cats were born in the past with that disease, but it was the overuse of this specific cat that led to the huge spreading of the disease in the American Burmese breed.

What do these information mean for us breeders?

In the first place we never have to forget the origins of our breed and the history behind it. We should also never forget that, even if we have our “ideal” Devon Rex in mind and we breed with our own breeding program, the survival of this beautiful breed is on us. What we do, and what we don’t do will end up having a huge weight on the future of the breed.

There are some breeding practices that would help keeping or even improving the health of the breed as a whole. They are no mystery and they are very well explained in most genetics books and not just applied to cats, but also dogs and other animals as it’s based on the “genetics of populations”.

Outcrossing and hybridation

In order for a breed to be healthy we need diversity. It’s something that can be accomplished by outcrossing and hybridization. Both methods are an efficient way to improve diversity (and therefore improve the immune system). To know more about these breeding tools you can read the article dedicated to it.

Work with inbreeding only if you have the knowledge to do it properly

Inbreeding can be a very efficient breeding tool. It has been used by responsible breeders since the breed was established many years ago and thanks to it we enjoy seeing some amazing and extreme Devon Rex.

Still, it’s a very powerful and dangerous tool and should be left in the hands of the most experienced breeders. It should be used only when you really know the lines that are behind your cats. Otherwise you may end up, without knowing it, with spreading a deleterious gene/mutation.

Support diversity with you matings

The biggest damages has been done to the Devon Rex when over-using some “special” cats. These cats may have been considered special for many reasons (ie type or line). What happens in these cases is that by the time you realize this “special” cat is carrying some disease or mutation, it has already been spread so deeply in the breed (or a portion of it) that altering all the offspring is no more an option as it would mean a serious loss in the breed gene-pool. Equally often it happens the information are not shared and many breeders use that line being unaware of the problem.

This is how PKD is now in some lines of Devon Rex (by using not tested Exotics for hybridization) and how HCM has become an increasing worry over the last few years.

The only way to reduce these risks is not to repeat the same breeding too many times, and to sell for breeding only a limited number of kittens from each specific mating. This simple breeding practice will help diversity and contain the spreading of damaging mutations/diseases.

Health must come first

Breeding show quality kittens is one of the goals of most breeders. We all love the feeling of getting “that” very special kitten. We love seeing him/her grow up in the hope of having produced our best kitten ever. The feeling of being more and more close to our “ideal” of the perfect Devon Rex (and we all know how difficult that is!). And of course, the feeling of having the Best Kitten at a show, or a National Winner.

This goal, and feeling, and passion make us successful breeders in the show hall and very good breeders in selecting more and more beautiful cats.

What makes us successful but also responsible breeders is by remembering that the good health of our cats must comes first, even when we have some hard decisions to make in order for it to happen.

What’s the point of that little amazing kitten if he/she will die at 2 weeks because his/her immune system is unable to fight the common bacteria and virus in the environment? What’s the point of producing a bunch of amazing kittens with extreme elfin looks if they may die prematurely because we selected them with severe health issues? And how are we going to really enjoy breeding, and feeling we are doing our best, if we managed to achieve those amazing kittens, but know deep down  that we didn’t select for a good health as well?