Martha Cannon from the Oxford Cat Clinic examined the benefits and the dangers of blood transfusions in cats at the FAB Conference 2008. As usual, transfusions are not as straightforward in feline patients as they are in canine or human ones!

A BLOOD TRANSFUSION can be a life-saving procedure. The first documented human-to-human blood transfusion was done as early as 1818, but transfusion medicine really took off in the 1920s and 1930s when anti-coagulants were introduced, allowing blood storage and the development of blood banks. Human transfusion medicine has developed into a very advanced science and blood transfusions are common — in the UK alone the National Blood Service now collects more than two million units of blood every year.

In veterinary medicine, transfusions are undertaken less often, but nonetheless they can have a vital role in both emergency and critical care situations.

Studies looking at the indications and success of feline blood transfusions suggest that the most common reasons for blood transfusion in cats in Germany1 and the USA2,3 are:

• Anaemia caused by blood loss (44%1, 27%2, 52%3)
• Bone marrow failure (38%1,3)
• Anaemia caused by chronic renal failure (20%2)
• Haemolytic anaemia (14%1, 10%3)

The German study reported their success rates — 84% of cats that received a cross-matched blood transfusion were still alive the next day, and 64% were still alive 10 days later. All of the cats that died were thought to have died from their disease rather than from complications arising from their transfusion.

However, feline blood transfusions need to be undertaken with care —there are practical difficulties associated with the collection of the blood. There is also a higher risk of adverse effects for both donor and recipient than in canine blood transfusions. These factors may deter many vets from performing blood transfusions in cats. In a survey of vets attending a major European conference on Feline Medicine (ESFM congress 2006; data on file) only one third
of attendees had performed a blood transfusion in a cat.

When is a blood transfusion useful?
A blood transfusion provides an immediate supply of red blood cells, which are essential for carrying oxygen around the body. A blood transfusion can therefore be life-saving in severe anaemia, especially where there has been a rapid loss of red blood cells, with no time for the body tissues to accommodate to the sudden reduction in oxygen supply. However blood contains many other elements which may also be useful. When blood is transfused the three major elements that can be provided are:

• Red blood cells: it is not possible to generalise, but common recommendations are that a blood transfusion may be indicated if:
  • Anaemia is severe enough to be causing clinical signs, eg, weakness, dyspnoea, ataxia
  • The PCV (packed cell volume) has dropped acutely to
    less than 15%
  • The PCV has dropped slowly but has fallen below 10%
• Plasma: this is the fluid in which the red cells are floating and it contains many proteins with essential functions, for example
  • Albumin; the protein which holds fluid in the circulation. When blood albumin falls too low fluid leaks from the vessels and builds up within the tissues (oedema) or within the chest or abdominal cavities (thoracic and abdominal effusions)
  • Clotting factors; proteins that cause blood to clot when a blood vessel is damaged
  • Inflammatory mediators; some of which help the body to fight infections, but others of which are essential in winding down the inflammatory reaction once the threat has passed
• Platelets: the cells in the blood which work with the clotting factors to allow blood to clot and prevent prolonged bleeding after a minor injury

A single unit of blood can potentially be divided into these three components, so where appropriate they can then be used separately, getting maximum use from each donated unit of blood, and reducing the risk of adverse reactions in the recipient by only giving the elements that are needed. This is now commonplace in human medicine and starting to be available in canine medicine — but currently remains rare in feline medicine.

Blood groups and blood compatibility
The very earliest blood transfusions in people — carried out as early as the 17th century —met with little success, largely because they involved giving sheep blood to humans! Of course we now know that blood must not only be collected from a member of the same species as the intended recipient, but that the two must have compatible blood groups to prevent development of a transfusion reaction.

Blood Groups A, B and AB
For many years we have known that there are three major blood groups in cats — A, B and AB. Group A is the most common, B is quite common in some breeds (see Table 1) but is rare in others, and group AB currently appears to be rare in all breeds.

• If a type B cat is given type A blood, antibodies in its blood will stick to the foreign type A red cells causing their immediate destruction. This haemolytic crisis can be very severe and is often rapidly fatal.
• If a type A cat is given type B blood it may only suffer a milder form of immediate reaction which it will hopefully survive, but it will start to form antibodies against the 'foreign' type B red cells, causing more rapid destruction of the transfused cells. At best this means the benefit of the transfusion will be short-lived (a few days only), and at worst it sensitises the cat to future transfusions, which may then trigger an immediate and fatal transfusion reaction if it needs a second transfusion at a later date.
• Type AB cats are rare. They can receive blood from either A or B cats, but cannot be used as donor cats.
• This same incompatibility of blood types can be a problem if a type B queen produces a type A kitten - which can happen because the gene for type A blood is dominant to the gene for type B blood. The type B queen has antibodies against type A blood, which she passes on to her kitten through her colostrum. The type A kitten therefore absorbs antibodies which attack its own red blood cells, causing jaundice, anaemia and often death.

The Mik antigen
More recently a second blood group has been identified involving the Mik red cell antigen4, and incompatibility here can also cause significant transfusion reactions, even in cats whose A/B blood groups are compatible.

White blood cells, platelets and proteins
While the most severe transfusion reactions occur because of immunological incompatibility of red blood cells, milder reactions can also occur against white blood, cells, platelets and plasma proteins in the transfused blood. These reactions will not be identified by in-house cross- matching; they typically occur within the first hour, and are usually manageable if they are identified early and thetransfusion is stopped. A fatal anaphylactic reaction is possible, but fortunately appears to be very rare. Use of blood components rather than whole blood reduces the risk of these reactions.

Non-immunological reactions
Hopefully the most severe immunological adverse reactions described above can be avoided by performing a cross-match test to assess the compatibility of the donor blood before it is collected. However, even compatible cross-matched blood can trigger adverse reactions in some circumstances, especially if stored blood is used. Problems can arise if the blood is too cold, has been given too quickly, contains damaged red cells due to prolonged storage, improper handling or over-warming, or if the blood has been contaminated during collection, (eg, due to poor aseptic technique).

Ensuring success for the recipient
If a cat is suffering from severe anaemia, or from a disease which has caused loss of platelets (thrombocytopenia) allowing uncontrolled bleeding, a blood transfusion may save its life. However, great care is required to ensure that the transfusion does not do more harm than good:

• Steps must be taken to ensure, as far as possible, that the donor cat's blood is compatible with the recipient. At the very least, both cats' A/B/AB blood group must be checked, and ideally further cross- matching should always be done to identify Mik antigen and other incompatibilities.
• The donor cat must be screened for blood-borne infections such as FeLV and FIV, and would ideally also be screened for Mycoplasma haemofelis (the cause of feline infectious anaemia — see www.fabcats.org for more information on FIA) and other less common infections which can be transmitted via the blood.
• When transfusing the blood, great care is required to avoid overloading the recipient cat's circulation, as this can lead to congestive heart failure.
• Close monitoring for an unexpected transfusion reaction is essential. The first few millilitres of blood should be given very slowly (1 ml/kg/hr for the first 30 minutes) so that any adverse reaction can be identified and the transfusion stopped before large volumes of blood have been given.

Consideration of all these risks is important, and owners must be aware of them too. In the ESFM survey 91 % of vets who had performed a transfusion in a cat had discussed the attendant risks for the recipient with its owner, but this tells us that 9 % of vets had not.

When considering the risks versus benefits of a blood transfusion it is also important to ensure that the volume of blood that will be given will be sufficient to alleviate the recipient's disease. For anaemic cats this can be calculated from formulae involving the donor cat's body size, the severity of the anaemia, the red cell count of the donor blood and the volume to be given.

Ensuring the safety of the blood donor
It is not only the recipient that must receive care and consideration. Collection of blood from a healthy donor cat must be undertaken with enormous care to ensure that health risks to the donor cat are kept to an absolute minimum.

Current recommendations are that the donor cats must be:

• A clinically healthy, fully vaccinated cat, which ideally lives wholly indoors
• Less than 8 years old
• Over 4.5 kg lean bodyweight
• PCV must be over 30% and ideally over 35%
• Free of FeLV/FIV/Mycoplasma haemofelis
• Of compatible blood type and cross-matched to the recipient
• Of suitable temperament not to be overstressed by the visit to the veterinary practice, and fasted to allow safe sedation for stress-free collection of the blood.

Before collecting any blood it is also essential to assess the donor cat's red cell levels (PCV) and to calculate the effect that removing the planned amount of blood will have. The cat's small body size and small circulating blood volume mean that if this is not done it is very easy to induce anaemia in the donor cat!

Even when all these criteria are taken into account, collection of a significant volume of blood is not without its risks.

• Low blood pressure (hypotension) is common if large volumes of blood are collected, especially if sedation is required to allow pain free collection of the planned volume of blood. In the ESFM survey around 12% of the vets had encountered hypotension as a consequence of blood collection.
• 20 % of the circulating volume is the maximum amount of blood that can safely be collected, and then only if the donor's PCV is over 35 %. If more than 10 % of the donor's blood volume is required the volume must be replaced by giving i/v fluids.
• Some cats with significant heart disease (hypertrophic cardiomyopathy) show no signs of ill health and have no heart murmur - the problem can only be identified on an ultrasound scan of the heart (echocardiography). Collection of blood from these cats can be fatal; in one study this occurred in one out of 137 apparently healthy donor cats.

Feline blood banks - a realistic possibility?
Taking all these risks into consideration, and the need for careful and expensive, screening and cross-matching of donor and recipient, we can understand why it is not always easy to find a suitable donor cat at short notice, in an emergency situation. Some practices maintain registers of owners who have signalled their willingness to allow their cats to be used as blood donors and this can be very helpful, but even so it is not always easy to find a compatible cat who is available to be brought into the practice immediately and who has not been fed (so he/she can be sedated), and then running the necessary pre-collection health and infectious disease tests adds additional time and complexity to the situation.

In human, and increasingly in canine medicine, blood banks have been established where blood and blood products are stored for a limited amount of time, and can be ordered for courier delivery when they are needed. This provides an easily accessible source of safe, pre- tested blood of known blood type, and makes blood transfusion a much more practical proposition for veterinary practices. It may be that a feline blood bank will also eventually be developed in the UK, but the concept is not without its ethical dilemmas. Where does the stored blood come from? Collection from pet-owned cats does involve some risk to the cat as we have seen, and the raft of pre-collection tests required might limit the number of owners willing to 'volunteer' their cats, as well as adding significantly to the financial cost of each unit of blood. But if not from pet-owned cats, can we be comfortable with the idea of a colony of cats kept wholly indoors, solely for the purpose of providing blood 'donations'to the blood bank? The potential benefit to the recipient cats, and their owners, is clear - but what of the donors?

Conclusion
Blood transfusions can be life-saving and are a valuable addition to the treatment armoury, especially for cats with severe acute anaemia or thrombocytopenia. As long as they are undertaken with care, and with a good understanding of feline transfusion medicine, they can be a safe and effective treatment option. However, there are very real practical issues associated with the collection of the blood, and with the safe-guarding of the health and safety of the donor cat, which will often limit the availability of a transfusion in an emergency scenario. Nevertheless due attention to these factors is essential before undertaking a feline blood transfusion.

References
1  Weingart et al (2004) Whole blood transfusions in 91 cats: a clinical evaluation. Journal of Feline Medicine and Surgery  6:3 139-148
2  Castellanos  et al (2004) Clinical use of blood products in cats: a retrospective study - 1997-2000. Journal of Veterinary Internal Medicine 18:4 529-532
3  Klaser et al (2005) Red blood cell transfusions in cat: 126 cases - 1999. Journal of the American Veterinary Medical Association 226:6 920-923 (Erratum in 226:7 1094)
4  Weinstein et al (2007) A newly recognized blood group in domestic shorthair cats: the Mik red cell antigen. Journal Veterinary Internal Medicine 21:2 287-29

July 2009