Blood clotting disorders are dysfunctions in the cat’s ability to control blood clotting, which can lead to hemorrhage. Hemostasis is an essential process which prevents the excessive loss of blood when a blood vessel is injured by changing blood from a liquid into a gel to form a blood clot over the break.
There are several key players in the hemostasis;
Platelets – Tiny cell fragments which are made in the bone marrow circulate in the blood. When a blood vessel wall breaks, collagen is exposed, which attracts platelets to the area which stick together to form a plug.
Fibrinogen (clotting factor I) – Soluble proteins which when exposed to chemicals outside the vessel wall convert fibrinogen into sticky fibrin fibres which form a mesh to hold the platelets together.
Clotting factors (II – VIII) – Produced primarily by the liver, but also the platelets and damaged tissue, clotting factors are responsible for the coagulation cascade.
- Factor I – Fibrinogen
- Factor II – Prothrombin
- Factor III – Tissue thromboplastin
- Factor IV – Calcium ions
- Factor V – Labile factor
- Factor VII – Stable factor
- Factor VIII – Antihemophilic factor
- Factor IX – Christmas factor
- Factor X – Stuart-Prower factor
- Factor XI – Plasma thromboplastin antecedent
- Factor XII – Hageman factor
- Factor XIII – Fibrin stabilising factor
Blood clotting process
When an injury occurs to a blood vessel, there are three mechanisms which work together to stop the flow of blood (hemostasis).
- Platelet adhesion: When damage to a blood vessel occurs, circulating platelets form a clump over the vessel to block it off.
- Vasoconstriction – When a blood vessel becomes damaged, vasoconstriction, making the blood vessel smaller, which restricts blood loss from the damaged site.
- Coagulation – Fibrinogen is the first of the 12 clotting factors which are activated by thrombin (a clotting enzyme) to form fibrin strands. These strands help to mesh the platelet plug, strengthening it.
Types of blood clotting disorders
Low blood platelets can be low due to decreased bone marrow production, increased destruction or consumption, and sequestration by the spleen. There are two types of thrombocytopenia; inherited or acquired.
A rare genetic condition in blue-smoke Persian cats, caused by a mutation of a lysosomal trafficking regulator protein. Affected cats have reduced pigment in the skin and eyes, causing photophobia (sensitivity to light), immune deficiency and bleeding disorders due to reduced dense granules and impaired functions of the platelets.
Von Willebrand’s disease (vWD)
A genetic disorder caused by a lack of blood protein known as Von Willebrand’s factor, which is necessary for normal platelet binding. Platelets are normal but are unable to adhere to sites where blood vessels have been injured.
Hemophilia is due to a lack of clotting factors in the blood and can be inherited or acquired.
Hemophilia A (Factor VIII deficiency): The most common form of hemophilia. It occurs as a mutation on the X chromosome, making it sex-linked. Males are most commonly affected because they only have one X chromosome. As females have two, they can carry the faulty chromosome, but won’t develop hemophilia themselves as their normal X chromosome is enough to prevent hemophilia. On rare occasions, a female can have hemophilia if a male with hemophilia mates with a female carrier, and the two X female chromosomes both have the faulty gene.
Factor XII (Hageman factor) deficiency: This form of hemophilia typically does not express a bleeding tendency. It is often an incidental finding when routine blood coagulation testing reveal a very long activated partial thromboplastin time (APTT).
Vitamin K is a cofactor for the enzyme responsible for chemical reactions that maintain blood clotting factors: prothrombin; Factors VII, IX, and X. The most common cause of vitamin K deficiency in cats is due to ingestion of rodenticide.
Clotting factor levels can drop in cats with liver disease as the liver is responsible for the synthesis of several clotting factors.
Disseminated intravascular coagulation
DIC is a condition disease where blood clots form throughout the bloodstream and is tied to systemic or severe inflammation. Microthrombi (tiny blood clots) develop throughout the blood vessels which can become emboli and lodge in small blood vessels which leads to reduced blood flow to the organs, and systemic coagulation uses up the platelets and coagulation factors resulting in severe bleeding.
Symptoms of blood clotting disorders
Symptoms can vary depending on the type of blood clotting disorder involved and can range from asymptomatic to lethal. When the blood loses its ability to clot properly, the following occur:
- Protracted bleeding after an injury or surgery
- Bruising under the skin
- Spots under the skin (petechiae)
- Dark stools due to bleeding in the gastrointestinal tract
- Bleeding from the gums or nose
- Blood in the urine and/or black, tarry stools, vomiting blood
- Pale gums
- Tachycardia (rapid heart rate)
In some cases, asymptomatic cats with mild blood clotting disorders may not be diagnosed until a routine blood panel is performed. Other cats may present with obvious signs (listed above). The veterinarian will take a thorough history, which will include:
- Onset and duration of symptom?
- Underlying medical conditions?
- Is the cat on any medications or supplements?
- Any possible toxin exposures?
Baseline tests – Complete blood count, biochemical profile and urinalysis. Anemia (low red blood cells, thrombocytopenia (low blood platelets) and elevated liver enzymes may be revealed.
Activated Partial Thromboplastin Time (aPTT) – A sample of blood is placed in a vial and reagents are added to make the blood clot. The aPTT test measures the length of time (in seconds) that it takes for clotting to occur.
Prothrombin time (PT) – Also known as INR (International Normalized Ratio), the PT test measures of the amount of time it takes a sample of the cat’s blood plasma to clot. Prothrombin (factor II), is one of the plasma proteins involved in the clotting process.
Activated Clotting Time (ACT) -A measurement of the amount of time it takes for a blood clot to form in a blood sample. Blood is placed a test tube with an activator which stimulates contact activation of the intrinsic coagulation pathway. The activated clotting time tests factors XII, XI, IX and VIII.
Buccal mucosal bleeding time (BMBT) – A small snip is made in the oral mucosa, and a filter paper is held directly under the wound to blot the dripping blood. The length of time it takes for bleeding to stop is measured.
Thrombin Time (TT) – A blood test that measures the time it takes to convert fibrinogen to fibrin in a plasma sample which contains anticoagulant after thrombin has been added.
von Willebrand factor antigen assay (vWF:Ag) – A test to measure measures the amount of vWF in a blood sample.
Individual factor analysis – Coagulation factor test to measure the function the amount of these proteins in the blood.
Bone marrow aspirate or core biopsy – A needle extracts a sample of bone marrow from the humerus (upper bone in front leg), femur (thigh bone) or pelvis. An increase in the number of megakaryocytes suggest increased platelet use, increased platelet destruction or sequestration in the spleen. Decreased megakaryocytes indicate decreased platelet production, which may be due to cancer or viral infection.
The goal of treatment is to address the cause, where found, and manage symptoms.
- Avoid surgery unless absolutely necessary.
- Blood transfusions will be necessary for cats with severe anemia.
- Plasma transfusions for cats with low blood platelets.
- Cryoprecipitate (fresh frozen plasma) or plasma transfusions can replace vWF in cats with Von Willebrand’s disease before surgery. This will decrease the likelihood of abnormal bleeding during surgery and in the recovery period.