Please not that you're reading the old version of this article. You can find the updated article here.

Just like us humans, our favorite pets also have different blood groups. Do you already know what know your feline’s is? Knowing your cat’s blood group can be vital in certain situations, yet rarely does this come up, unless it’s an emergency. Today we will tell you everything you need to know about different blood groups in cats and why you should know your letters!

Blood type systems in cats

Despite the abundance of different blood type systems know in humans, dogs, horses and other species, in domestic cats only one has been established – the AB system. There are three blood groups determined: A, B and AB. A fourth group MiK has also been identified, but not much is yet known about this group and it seems to be very rare (occurring in less than 1% of cats). Frequency of blood groups in different parts of the world significantly
differs, but most figures would fall into these ranges:

1. A (40-90%)

2. B (10-30%)

3. AB (9%)

4. MiK (less than 1%)

For example, in the US 95-99% of cats have A serotype, while in Australia about 63% of cats are A type and 37% are B type.

The frequency of blood types is also associated with the breeds. Surveys show that Siamese, Tonkinese and Oriental shorthairs most commonly have A serotypes, while British shorthairs, Cornish rex and Devon rex have a much higher instance of B serotypes
(over 25%) (Table 1).

Table 1: Frequency of type B in different domestic breeds (data by International Cat Care)

How are blood groups identified in cats?

Blood types in cats are identified by the presence of antigens A and B on feline red blood cells (erythrocytes). Additionally, cats naturally produce allo-antibodies – antibodies against the red blood cell antigens that are not present in their own cells.
Felines of a blood type A have antigens A on their red blood cells and their leukocytes produce anti-B antibodies in their blood. Opposing to this, cats with blood type B have antigens B on their red blood cells and anti-A antibodies in the blood. The rare carriers of both antigens A and B on the red blood cells don’t have either of these antibodies and are marked as AB serotypes (Figure 1). Currently, no cats
have been reported without at least one of these antigens present on their erythrocytes. Antibodies are built to bind to the foreign molecule they can recognize and destroy it. Therefore, if anti-A antibody comes in contact with the antigen A – it
will bind to it and destroy the erythrocyte. This is why the presence of antigens and antibodies and their interactions are vital when it comes to blood transfusions and feline breeding as they are exactly what determines blood compatibility between
cats (more in section “Why do I need to know my feline’s blood type”).

Figure 1: Blood types are identified by the presence of antigens A and B on red blood cells

Genetics and nomenclature of feline blood groups

Blood types are genetically determined. Just like us, cats carry two copies of each gene in their cells. That means they will always carry a pair of two genes determining their blood group. There are three alleles (or variants) of this gene: A,
aab and b. The allele A is dominant, meaning that all cats carrying at least one allele A will always express serotype A regardless of the other present allele. Allele b is recessive, meaning only cats with both alleles
b will be serotype B. And in the end, if a cat has an allele aab combined with or aab or b, it will be AB serotype (Table 2).

Table 2: Serotype in cats

Why is it important to know your cat’s blood group?

Understanding feline blood groups is vital because of the previously described allo-antibodies naturally occurring against red blood cell antigens. Kittens start producing allo-antibodies around two to three months of age, and previous exposure to blood
products is not necessary. Allo-antibodies bind to the corresponding antigens on erythrocytes, causing the lysis of the erythrocytes and leading to their destroyment.

Blood groups in breeding

Blood group clashes can cause neonatal isoerythrolysis and lead to the death of the kitten!

Any of the antibodies present in the cat mom will also be present in her colostrum (newborn kitten milk). When the kitten is first born, its intestine is adapted to absorb mom’s antibodies. This way, mom’s antibodies protect her kittens against diseases
in the early weeks of life, before they can produce their own antibodies. This is normally vital and highly beneficial for newborn kittens.

However, the problem arises if two cats of different blood groups are bred, and some of the kittens inherit a different serotype than their mom. Because mom may be producing antibodies against certain blood types in the colostrum, the kitten’s blood cells
with the corresponding antigens could be destroyed and the kitten could die.

For example, if the queen is serotype B, she will be producing anti-A antibodies. If she is mated with a type A tomcat – some of the kittens in their litter will express serotype A too. When those kittens drink their mom’s milk containing anti-A antibodies,
their erythrocytes will be attacked and destroyed and the kitten will most likely die. This condition is broadly known as NEONATAL ISOERYTHROLYSIS.

However, if the kitten is serotype B, and the queen is type A, this disease will not occur because type A cats don’t produce very high levels of anti-B antibodies.

How to recognize a kitten affected by neonatal isoerythrolysis?

Post birth, all kittens should seem normal and healthy. However, group A and AB kittens in a litter of a type B queen may show symptoms of the disease immediately after they’ve sucked their mom’s milk. The severity of the signs vary depending on the amount
of the absorbed anti-A antibodies:

Some affected kittens may appear “fade”, weak, pale or even yellow. They usually stop nursing over a few days post birth.

Urine red in coloration. When erythrocytes are destroyed, hemoglobin (which is red in color) is released and passed with urine.

Sometimes, the tip of the tail or tips of the ears may slowly die off (healthy circulation to all the body parts is prevented due to the lysis of erythrocytes).

Sudden death with no signs.

No sign. Although rare, it is possible that some incompatible kittens actually remain unaffected.

How can I treat affected kittens?

If the kitten already absorbed the antibodies, the treatment will be very difficult.

If the condition is recognized in within the first few hours of life, the kitten should be taken away from the queen to prevent further absorption of antibodies.

In rare occasions, it may be possible to transfuse the kitten with washed red cells from a group B cat (from the type B queen for instance). The erythrocytes must be washed prior to the transfusion so all the anti-A antibodies present would be removed
and no further harm would be caused to the existing A erythrocytes. This would provide the kitten with the new type B erythrocytes that won’t be destroyed by the anti-A antibodies it absorbed (this is possible as the kitten don’t start producing their
own allo-antibodies in the first weeks of life). Unfortunately, this is often easier said than done as blood transfusions to newborn kittens are practically very difficult.

How can I prevent neonatal isoerythrolysis in my litter?

As the treatment is very difficult and often impossible, preventing the disease is much more important. This disease is more common among pedigree cats, as the frequency of B blood type is higher than in the mixed breeds (Table 1!). It is highly recommended
that the serotypes are always determined beforehand. Afterwards, these rules can be followed:

B queens should always be bred only with tom B!

Avoid breeding B cats in general. This limits the pool and choice, and eliminates the problem altogether.

If you do breed B queen with an A or AB tom, always keep all the A and AB kittens away from their mom in their first 24 hours post-birth. This method was proven to be successful among many breeders. It will require for you to blood type all your kittens
immediately post-birth however.

Blood groups in transfusion therapies

The transfusion therapies are often required in the treatment of different conditions such as anemia, hemorrhage,
hemostatic defects and hypoproteinemia. If the pre-transfusion testing is performed and the donor is carefully selected, cats tend to tolerate and respond to these therapies very successfully.

What is important to keep in mind is that:

There are no universal donors in cats

Unlike humans, there are no universal donors in cats, as no cats have been identified without any antigens on their erythrocytes (such as the null type in humans). All cats should always be donated the blood of their own serotype, otherwise a fetal reaction
may occur resulting in the blood cells being destroyed.

There is a universal recipient however

Serotype AB may receive all blood types, as no allo-antibodies are produced in this type.

Prior to the transfusion, both donor and recipient should be blood typed. Once the blood types are known, cross-matching should be the next performed step. Cross-matching allows us to test the compatibility between the two cats and help us avoid incompatible
transfusions that may be caused by rare and still unknown erythrocyte antigens (such as MiK). Cross-matching is performed by testing donor erythrocytes against recipient allo-antibodies. Once cross-matching confirms that the donor and recipient truly
are compatible, transfusion may be conducted.

What are the results of an unsuccessful blood transfusion?

If the blood transfusion isn’t successful, immune or non-immune reactions can occur from one to two hours after the beginning of the therapy to up 48 hours later. Immune reactions include hemolysis, allergic reactions, and fever. Non-immune reactions
are bacterial contamination, hemolysis, hypocalcemia, hypothermia, hyperammonemia and volume overload. In both types of the reactions, the life of the transfused erythrocytes is shortened.

How can I determine my cat’s blood group?

Two groups of methods can be used to determine blood groups: serological and genetic. Serological methods are based on agglutination reactions and require blood samples. Genetic methods will include tests like Basepaws, but right now can’t distinguish
between A and AB types. Sometimes (in 2-3% of the cases), if the genetic analysis is used to determine the serotype, the blood type can be indeterminable. This is when the genetic results are inconclusive and the serotype B can’t be identified due
to additional unidentified mutations. In such cases, the serologic analysis is recommended for obtainment of a conclusive result.

There are also card typing system tests that offer patient-side testing (such as RapidVet-H, DMS Laboratories, Flemington, NJ etc.).

Frequently asked questions


✔ Can all cats be tested?

Yes, all domestic cats can be tested! Blood typing is not restricted to any specific breeds.

✔ Do I need a blood sample?

Blood samples are usually required if you would like to get a serological test. These tests will only give you the information about the blood type, but they are more precise and can distinguish all three blood types in cats. If you would like to conduct
a genetic test, Basepaws will soon offer this as part of our product, and only hair samples will be needed.

✔ Does the method of sampling matters?

Yes. Proper technique for both blood collection and hair collection is essential for accurate results.

✔ What test should I opt for?

This really depends on what you would like to learn about your cat. If you would solely like to learn its blood type, then patient-side tests such as RapidVet-H, DMS Laboratories, Flemington, NJ and similar could be a good option for you. If you want
precise data about your cat’s blood group, then serological tests are the way to go. If you would like to get to know your cat better overall and find out about its genetic predispositions and ancestry as well as blood type – then you should go with
a full genetic analysis.

✔ Are the same tests used for all animals?

No. Blood type systems are usually species specific and tests work with breeds they were developed for.

We are all striving to get to know our pets as best as we can. Learning about their genetics, health and behaviour is highly important in order for us to be able to provide them with long and healthy lives. The more info we have on things like blood type,
the better life we can offer them.