Anemias can be regenerative or non-regenerative. Regenerative anemias show evidence of response by the bone marrow in terms of increase in the number of circulating RBC. This response is measured by the quantity of reticulocytes (immature RBC) present in the circulation.

In nonregenerative anemia, the bone marrow responds poorly and the reticulocyte count is low. Anemias can also be acute or chronic. Most commonly, acute anemias are due to either RBC loss or destruction. Chronic anemias are usually due to lack of RBC production, although slow blood loss can also be a cause.

Laboratory diagnosis of anemia is based on the Heameglobin concentration, the number of RBC and the hematocrit or Packed Cell Volume (PCV).

REGENERATIVE ANEMIAS

Blood Loss

Blood loss may be peracute, acute or chronic and the clinical signs depend on the rapidity of blood volume depletion. Chronic blood loss can result in iron deficiency and a non-regenerative anemia develops. It takes 6 hours for tissue fluids to re-expand (restore?) the circulating blood volume, after which PCV and total protein decrease. Blood loss can be either external or internal. Internal blood loss usually results in a disproportionate decline in the PCV because proteins are readily reabsorbed into the blood vascular system while minimal amounts of intact RBC are reabsorbed.

External parasitism may cause anemia, particularly in young animals. In general, blood-sucking insects cause more irritation than blood loss. Kittens and puppies can become severely anemic due to heavy flea or louse infestations. In endemic areas, blood-sucking ticks infest a wide variety of hosts. The animals should be treated with iron and application of ectoparasiticides.

Internal parasitism may also cause anemia. Hookworm infection may cause profound anemia in young dogs and cats. In severely affected animals that collapse, blood transfusions should precede anthelmintic treatment. Iron lost in intestinal parasitism is not reabsorbed; in chronic cases, response may be better to parenteral iron than to an anthelmintic.

NUTRITIONAL ANEMIAS

Nutritional anemias in animals are rare except for iron-deficiency anemia. This is seen in most commonly in young animals in which iron stores are small, in association with GI parasites or lack of dietary iron.

BLOOD GROUPS

Blood groups are determined by genetically controlled, polymorphic, antigenic components of the RBC membrane.

BLOOD TRANSFUSIONS IN DOGS

Frequently, the need for blood transfusions is acute, as in acute hemolysis or hemorrhage; transfusions are also appropriate in treatment of acute or chronic anemias. Animals with hemostatic disorders often require repeated transfusions of either whole blood, red cells, plasma or platelets. Blood transfusions must be given with care because they have the potential for further compromising the recipient. The diversity of blood groups in animals and the lack of commercially available blood-typing reagents make complete typing and matching difficult but should not preclude the clinical use of transfusions. In horses and dogs, the blood group antigens most commonly implicated in transfusion incompatibilities are known; by selecting donor animals that lack these groups, the prevalence of recipient sensitization can be decreased. Previously sensitized recipients can be detected by cross-matching, which will preclude administration of incompatible blood. In the USA, >99% of cats are of blood group A, so the risk of incompatible transfusions is low. However, certain breeds including Abyssinian, Birman, British Shorthair, Devon rex, Himalayan, Persian, Scottish Fold and Somali have a higher frequency of blood group B. Any incompatible transfusions in cats results in rapid destruction of transfused cells.

Whole blood frequently is not the ideal product to be administered. If the need is to replace the oxygen-carrying capability of the blood, then packed RBC are more appropriate; if replacement of circulatory volume is needed, crystalloid or colloid solutions may be used to replace volume, with packed RBC added as needed. Platelet numbers rise rapidly after hemorrhage, so replacement is not needed. Plasma proteins equilibrate from the interstitial space, so plasma is not needed except in massive hemorrhage (>1 blood volume in 24 hr). Animals that require coagulation factors benefit most from administration of fresh-frozen plasma or cryoprecipitate if the need is specifically for factor VIII, von Willebrand's factor or fibrinogen. Platelet-rich plasma or platelet concentrates may be of value in thrombocytopenia, although immune-mediated thrombocytopenia usually does not respond to administration of platelets because they are removed rapidly by the spleen.

The amount of RBC required to treat anemia is based on the volume necessary to increase the PCV or Hgb concentration to the desired value. All domestic animals have blood volume of ~7% of their body weight except cats, which have a blood volume of 4% of their body weight. By determining the recipient's blood volume and knowing the animal's PCV, the required replacement RBC volume can be calculated. For example, a 25 kg dog has a total blood volume of ~2,000 ml. With a PCV of 15%, the RBC volume is 300 ml; if the PCV is to be increased to 20% that equals an RBC volume of 400 ml. Therefore 100 ml of RBC or 200 ml of whole blood (with PCV of 50%) would be required to increase the recipient's PCV to the desired level. No more than 25% of a donor animal's blood should be collected at one time.

Collection, storage and transfusion of blood must be done aseptically. The anticoagulant of choice is Citrate Phosphate Dextrose Adenine (CPDA-1). Commercial blood bags containing the appropriate amount of anticoagulant are less damaging to blood cells than are vacuum collection bottles. Heparin should not be used as an anticoagulant because it has a longer half-life in the recipient and causes platelet activation; also heparinized blood cannot be stored.

RISK OF TRANSFUSION

The most serious risk of transfusion is acute hemolysis. Fortunately, this is rare in domestic animals. Dogs rarely have clinically significant preformed antibodies, so only those that have received repeated transfusions are at risk. The most common hemolytic reaction in dogs that have received multiple transfusions is delayed hemolysis, seen clinically as shortened survival of transfused RBC and a positive Coombs test. Even crossmatch-compatible RBC given to horses or cattle survive only 2-4 days. Repeated transfusions can cause acute hemolysis. Non-immune causes of hemolysis include improper collection or separation of blood, freezing or over-warming of RBC and infusing under pressure through a small needle.

Other complications include sepsis from contaminated blood, hypocalcemia from too much citrate and hypervolemia (especially in animals with pre-existing heart disease or in very small animals). Urticaria, fever or vomiting are seen occasionally. Transfusions can also spread diseases from donor to recipient, such as RBC parasites (e.g. Haemobartonella, Anaplasma or Babesia and viruses (e.g. retroviruses such as feline or bovine leukemia, equine infectious anemia or other slow viruses). Other diseases, such as those caused by rickettsia or other bacteria, can also be spread if the donor is bacteremic.

BLOOD SUBSTITUTES

An ideal blood substitute would pick up and deliver oxygen and be iso-osmotic, non-antigenic, inexpensive, non-toxic, incapable of transmitting infections and have a half-life of at least several days. Although the ideal blood substitute has yet to be developed, progress is being made, especially in development of stroma-free hemoglobin solutions. Purified polymerized hemoglobin solutions of bovine or human origin are being investigated. Fluorocarbon solutions have also been undergoing testing for many years, but continued problems with efficacy and safety have slowed development.

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