Hemophilia A

TL;DR. Hemophilia A is an inherited bleeding disorder in dogs and cats caused by a deficiency of clotting Factor VIII, leading to prolonged bleeding, spontaneous bruising, and internal bleeding that requires careful veterinary management.

Factor VIII is a vital link in the secondary coagulation cascade, helping to form a stable clot.

What is it?

Hemophilia A, also known as Factor VIII deficiency or congenital factor VIII deficiency, is an inherited disorder that impairs the blood's ability to clot. To understand this condition, it helps to look at how a healthy pet's body stops bleeding. When a blood vessel is injured, the body initiates a highly coordinated, multi-step process called the coagulation cascade.

First, tiny cell fragments called platelets rush to the injury site to form a temporary plug. This is known as primary hemostasis. Second, a series of specialized proteins called clotting factors activate one another in a precise sequence, much like a row of falling dominoes. This sequence is called secondary hemostasis. The ultimate goal of this cascade is to produce a strong mesh of protein fibers called fibrin, which binds the platelet plug together and seals the wound.

Factor VIII is a critical link in this clotting chain. In pets with Hemophilia A, the body does not produce enough functional Factor VIII. Without it, the coagulation cascade stalls, and the stable fibrin mesh cannot form. Consequently, even minor bumps, scratches, or routine surgical procedures can lead to prolonged, uncontrolled, and potentially life-threatening bleeding.

Causes & risk factors

Hemophilia A is a hereditary condition, meaning it is passed down from parents to their offspring through their genetic material. The mutation responsible for the disease is located on the X chromosome, making it a sex-linked recessive trait.

Because male dogs and cats have only one X chromosome (XY), inheriting a single defective gene from their mother will cause them to develop the disease. Female pets have two X chromosomes (XX). For a female to show symptoms of Hemophilia A, she must inherit two copies of the defective gene—one from each parent—which is highly uncommon. Instead, female pets with one mutated gene typically become asymptomatic "carriers." They appear perfectly healthy but can pass the mutation on to their puppies or kittens.

According to a leading veterinary internal medicine reference:

"Congenital clotting factor deficiencies, as well as the breeds affected, are listed in Box 85-3 . They are relatively common in dogs but are rare in cats. Most genetic mutations leading to these defects have been well characterized, and some laboratories now offer genetic testing for congenital coagulopathies. Hemophilia A and B are sex-linked traits..." [2]

While the disease is relatively common in dogs compared to cats, it is still classified as an uncommon disorder overall in veterinary medicine. There are no specific breed predispositions listed in the official clinical record for this condition, meaning Hemophilia A can potentially affect any breed of dog or cat.

Signs to watch for

The clinical signs of Hemophilia A can vary widely depending on the severity of the Factor VIII deficiency. Pets with mild deficiencies may show no signs at all until they undergo a routine surgery or experience a trauma. Pets with severe deficiencies often show signs early in life.

Key symptoms to watch for include:

• Prolongation of the PTT (Cardinal): This is a laboratory finding where the blood takes significantly longer than normal to clot in a test tube. It is the hallmark indicator of the disease.
• Spontaneous hematoma formation (Common): Large, soft, fluid-filled swellings under the skin or within muscles, caused by blood pooling outside the blood vessels without any known injury.
• Bleeding into body cavities (Common): Bleeding into the chest, abdomen, or joints. Joint bleeding (hemarthrosis) causes sudden lameness, swelling, and pain. Bleeding into the chest or abdomen can cause weakness, pale gums, and difficulty breathing.
• Intraoperative or delayed postoperative bleeding (Common): Excessive bleeding during a routine procedure (like a spay or neuter) or bleeding that starts hours or days after the surgery has concluded.
• Abortions or stillbirths in the litter (Common): Pregnant carriers or affected females may experience fetal loss due to placental bleeding.
• Protracted umbilical cord bleeding (Occasional): Newborn puppies or kittens may bleed continuously from their belly button after birth.
• Fading puppy syndrome signs (Occasional): Affected newborns may fail to nurse, grow weak, and pass away within their first few weeks of life.

Spontaneous hematomas, or blood-filled swellings under the skin, are a common sign of Hemophilia A.

How vets diagnose it

Diagnosing an inherited bleeding disorder requires a systematic approach. Your vet will begin with a thorough physical examination and a detailed review of your pet's medical history. A history of unexplained bruising, prolonged bleeding after minor cuts, or a family history of bleeding problems will raise immediate red flags.

Your vet will also evaluate your pet's drug history. As noted in a prominent veterinary critical care textbook:

"The drug history should be carefully appraised because numerous drugs can cause or contribute to thrombopathia. Diseases known to affect platelet function should be excluded... If no obvious cause of acquired thrombopathia can be found, a hereditary disorder is suspected." [1]

Once acquired causes are ruled out, your vet will perform specific blood tests:

1. Coagulation Screening Panels: These tests measure how long it takes for your pet's blood to clot. The two primary tests are Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT). In pets with Hemophilia A, the PT is typically normal because it measures a different pathway, but the aPTT will be significantly prolonged (the cardinal sign of this disease).
2. Specific Factor Assays (Gold Standard): If the aPTT is prolonged, your vet will order a specific Factor VIII assay. This test measures the exact activity level of Factor VIII in the blood. A definitive diagnosis of Hemophilia A is made when Factor VIII activity is significantly below normal levels while other clotting factors remain normal.
3. Genetic Testing: DNA testing is increasingly available for many breeds. This can identify the specific genetic mutation, allowing veterinarians to confirm the diagnosis and identify asymptomatic female carriers before they are bred.

Treatment options

There is no cure for Hemophilia A; treatment focuses on managing bleeding episodes, preparing for necessary surgeries, and preventing trauma.

First-Line Therapy: Hormonal Agents

For pets with mild to moderate Hemophilia A, a synthetic hormone called Desmopressin (also known as DDAVP) is the primary medical treatment. Desmopressin is a synthetic analogue of vasopressin. It works by stimulating the cells lining the blood vessels (endothelial cells) to release their stored reserves of Factor VIII and another clotting protein called von Willebrand factor into the bloodstream.

According to a leading veterinary critical care reference:

"DDAVP is pharmacologically altered from vasopressin by substitution of D-arginine for L-arginine, which virtually eliminates the vasopressor activity (via V1 receptors) and significantly enhances antidiuretic activity and the stimulation of endothelial release of factor VIII and vWF (via V2 receptors). The terminal half-life of DDAVP after intravenous administration is 2.5 to 4.4 hours." [3]

Because its effects are temporary, desmopressin is typically administered shortly before a planned minor procedure or immediately following a mild injury to temporarily boost clotting ability. It is important to note that desmopressin is only effective if the pet has some functional stores of Factor VIII to release; it may not be sufficient for pets with severe Hemophilia A.

Supportive Care and Blood Products

For pets experiencing active, severe bleeding, or those with severe Factor VIII deficiency undergoing essential surgery, desmopressin alone is not enough. These pets require the transfusion of blood products to supply the missing clotting factors.

• Fresh Frozen Plasma (FFP) or Cryoprecipitate: These specialized blood products contain high concentrations of active clotting factors, including Factor VIII. Transfusing these products provides immediate, temporary clotting support to stop active bleeding.
• Whole Blood Transfusions: If a pet has lost a significant volume of blood and is anemic, a whole blood transfusion may be necessary to restore both red blood cells and clotting factors.

Prognosis

Long-term prognosis data in dogs and cats with Hemophilia A is limited in veterinary literature. The overall outlook for an affected pet depends heavily on the severity of their specific genetic mutation and how well their environment can be managed.

Pets with mild Hemophilia A often live a normal lifespan with a high quality of life, requiring medical intervention only during surgeries or major traumas. Conversely, pets with severe Hemophilia A face a much more guarded prognosis. These animals are highly susceptible to spontaneous, life-threatening internal bleeding into their chest, abdomen, or brain. Managing a severely affected pet requires strict lifestyle modifications and immediate access to emergency veterinary care.

Prevention

Because Hemophilia A is an inherited genetic disorder, it cannot be prevented through lifestyle changes, vaccines, or diet. Prevention relies entirely on responsible breeding practices.

• Genetic Screening: Breeders should utilize genetic testing to screen potential breeding dogs and cats, especially females, to ensure they are not carriers of the mutated Factor VIII gene.
• Spaying and Neutering: Affected male pets and confirmed female carriers should be spayed or neutered to prevent passing the mutation to future generations. Spaying an affected female must be done with extreme caution, utilizing pre-operative plasma transfusions or desmopressin to prevent hemorrhage.
• Lifestyle Management: For pets already diagnosed with Hemophilia A, owners can prevent bleeding episodes by avoiding rough play, choosing soft toys, using a harness instead of a collar to reduce neck pressure, and keeping the pet's environment free of sharp edges or hazards that could cause bruising or cuts.

When to call your vet

If your pet has been diagnosed with Hemophilia A, or if you suspect they have a bleeding disorder, you must monitor them closely. You should seek immediate emergency veterinary care if you notice any of the following red flags:

• Pale, white, or muddy-colored gums
• Sudden, unexplained weakness, lethargy, or collapse
• Difficulty breathing or rapid, shallow breathing
• Sudden lameness or swollen, painful joints
• Large, spreading bruises or soft swellings under the skin
• Bleeding from the nose, mouth, or urinary tract that does not stop within a few minutes
• Cold extremities (ears and paws) combined with weakness

Sources

• Small Animal Critical Care Medicine, 2nd Edition, page 623, page 980
• Internal Medicine, 5th Edition, page 1290