Caval Syndrome in Dogs

Heartworms are parasites that can live 5 to 7 years in their hosts, generally in the arteries of the lungs. This puts pressure on the pulmonary arteries and the right side of the heart. If heartworms invade the vena cava, the vein that delivers venous blood to the heart from the lower body, or the heart itself, it can result in many dangerous conditions that affect other organs, such as the liver and kidneys. This is why it is sometimes called liver failure syndrome.

Caval syndrome is a progressed condition of heartworm disease. When the heartworm parasite has become too populous in the pulmonary artery of an infected dog, they can migrate into the vena cava, right atrium, and right ventricle of the heart. This is a life threatening condition that causes issues ranging from jaundice and anemia to liver dysfunction, heart failure, and death.

Symptoms of caval syndrome are often not present in many dogs with the condition. Though caused by the heartworm parasite, symptoms of caval syndrome are different than those of a typical heartworm infestation, and can include:

• Decrease in activity
• Lethargy
• Coughing
• Increased breathing rate, or tachypnea
• Breathing distress
• Fainting when stressed
• Weight loss
• Weakness 
• Loss of appetite
• Fever
• Jaundice 
• Pale mucous membranes
• Weak pulse
• Blood in the urine
• High level of hemoglobin in the urine, or hemoglobinuria
• Hepatosplenomegaly, or swelling of the liver and spleen
• Anemia
• Irregular gallop or murmur in heartbeat
• Pulmonary hypertension, or high blood pressure in the lungs
• Heart failure

Caval syndrome is a progression of heartworm disease. Symptoms of heartworm disease can be seen most often following exercise, and can include:

• Soft, dry cough
• Shortness of breath
• Decreased stamina
• Nervousness
• Weakness
• Disorientation
• Fainting spells

The one and only cause of caval syndrome is:

• Heartworms that migrate into the vena cava and heart 

The heartworm parasite is spread from animal to animal through a bite from an infected mosquito. Caval syndrome is most common in male dogs showing no previous signs of a heartworm infestation, and it occurs in dogs with a moderate to large number of heartworms.

Often, the symptoms of caval syndrome are not present, or only symptoms of heartworm disease are seen. Routine tests can show a heartworm infestation.

If heartworm is suspected, caval syndrome can be detected with a blood test. Two blood tests are used for this, the Knott’s test and the ELISA test. If your dog tests positive for heartworms, then your veterinarian will use chest x-rays, echocardiography, electrocardiography, and ultrasounds to assess the extent of the heartworm migration.

Before any treatments are started, routine lab work is done to make sure the liver and kidneys are functioning properly, as they can be adversely affected by the drug used to kill the adult heartworms.

Caval syndrome is an advanced stage of heartworm disease and has a poor rate of survival. It carries a 14% to 42% risk of death, even with heartworm removal. Without treatment, death generally occurs within 24 to 72 hours. Organ failure leading to death can occur before or after treatment. Heartworm removal from the vena cava, right atrium, and right ventricle will need to be undertaken immediately for your dog to have the best chance of survival.

Before the heartworms are treated, any heart failure, liver disease or kidney disease present is treated first. Shock is treated with supportive care, such as intravenous fluids that can be given before or after surgery.

Then, surgical removal of the heartworms is recommended before any other treatments. This entails the administering of sedation or anesthesia, then using forceps to grasp and pull the heartworms out of the heart manually through the jugular vein. This is guided by an echocardiograph. This treatment can fail if hypothermia, fluid accumulation in the abdominal cavity, or an elevation in pressure in the central vein occurs after surgery. 

Corticosteroids, heparin, and antibiotics may be administered after surgery. Urine color, blood urea nitrogen, and the level of red blood cells are closely monitored after heartworm removal. Some dogs may need oxygen therapy. 

Once the blood urea nitrogen level, blood cell counts, breathing rate and appetite of your dog has returned to normal, an adulticide is often recommended to kill any worms still remaining. Common drugs are melarsomine. milbemycin, and selamectin. Ivermectin may be administered, but has some medical risks, such as pulmonary embolism.

If your dog has undergone surgical heartworm removal, he will be released to go home once he has stabilized. With successful surgery, improvement in cardiac output, a reduction in vein pressure, a decrease in anemia, and a resolved hemoglobinuria can be seen, usually within several days.

Your veterinarian will most likely recommend an adulticide to treat the remaining heartworms, and this can be given at home for up to 18 months. Often, a visit for a blood test will be scheduled 4 to 6 months after treatment to check on the level of adult heartworms still present. If positive, the adulticide may be continued.

With treatment, recovery can vary from a return to normal activity to persisting liver or kidney problems, irreversible lung changes, and a lifetime of medication. This will depend on the severity of the condition prior to treatment.  

Preventing a heartworm infestation is an important part of recovery, and of protecting your dog from becoming infected at all. Since mosquitoes need to be present to deliver an infected bite, keep your dog away from mosquito infested areas. Use a mosquito prevention, such as a spot treatment, and give your dog monthly heartworm pills to prevent an infestation.