Heartworm testing in the past has depended on finding microfilaria in the blood stream of infected dogs. These tests may miss 30 to 45 percent of the dogs that have heartworms. Three microfilarial tests are commonly used. The blood smear test uses one drop of blood on a slide covered with a coverslip. The slide is examined under a microscope for microfilaria. The Knotts Test uses 1cc of blood that is fixed with formalin and centrifuged to concentrate the microfilaria. The Difil test is a filter test that filters 1cc of lysed blood through a clear plastic filter. The filter is placed on a slide, stained and examined for microfilaria. The Knotts Test and the Difil Tests are more likely to find microfilaria due to the larger blood sample that is examined. The blood smear test is diagnostic when microfilaria are found, but may miss infections with fewer numbers of microfilaria. Since these tests check only for microfilaria, they are limited in their ability to detect heartworm infection. Adult heartworms may be present in the heart without microfilaria being present in the blood. This is called an occult heartworm infection. Occult heartworm disease can be caused by the following: 1) All the adult heartworms are of one sex. 2) The adult worms have been made sterile by medication that the dog may have been previously given (ivermectin or D.E.C.). 3) An antibody reaction in the lungs has killed all the microfilaria. 4) heartworms in heart are too young to have produced microfilaria. The most common reason for occult infections is an antibody mediated reaction in the blood vessels of the lungs that kills the microfilaria just as they are born. The number of microfilaria in the blood does not correlate with either the number of heartworms or the severity of the heartworm disease. Microfilaria tests are inaccurate and do not help determine the severity of infection therefore they should be replaced with the newer tests.
Microfilaria testing also has the possibility of diagnosing heartworm disease when there are no adult heartworms. Microfilaria can be confused with another parasite that is similar in appearance. Dipetalonema reconditum is very similar to Dirofilaria immitis in appearance but it is not harmful to the dog. Below is a table that contrasts these two filarid parasites. Microfilaria may also be present in a dog that had a blood transfusion from an infected dog. Puppies that have been born from infected mothers have also been reported to have microfilaria without adult heartworms.
Characteristics of Infection By Dirofilaria immitis and Dipetalonema reconditum
|Characteristic||D. Immitis||D. reconditum|
|Hosts||Dogs, Cats, ferrets, humans,Marine mammals, etc.||Dog|
|Adult sites||Heart, pulmonary arteries||Connective tissue|
|Thickness||Thicker than RBC||Thinner than RBC|
|Length||Longer than 290 microns||Shorter than 275 u|
Occult Heartworm Tests
Occult heartworm disease literally means “hidden” heartworm disease. Heartworm infections in the heart that are not producing microfilaria are considered hidden heartworm infections or occult heartworm infections. Adult heartworms can be diagnosed by a serum test that detects chemical, fecal, and uterine discharges of adult heartworms. This test uses a technology called Enzyme Linked Immuno Sero Assay (ELISA.) This test uses a antibody that has been manufactured against the heartworm secretions. This antibody is attached to an enzyme. When the antibody comes in contact with the heartworm secretion it changes shape. This shape change causes the enzyme to glow blue. A blue color change is a positive reaction for heartworm secretions. These secretions come only from mature, adult, female heartworms. Immature heartworms, male heartworms or young L5 stage heartworms will not be detected.
Radiographs of the chest in animals that have no microfilaria but are suspected to have heartworms have long been the diagnostic test relied upon to determine occult heartworm disease. Radiographs are helpful but the actual heartworms cannot be seen. The signs of heartworm infection can be seen but the infection may be advanced prior to these signs developing. Ultrasound can detect heartworms if the infection is severe but it is not a good test for screening dogs for occult heartworms.
ELISA technology makes a “Heartworm test a Heartworm test”. Our clients deserve the best and when they bring their pet in for a heartworm test they expect the results to be accurate. Too many pets have died with heartworm disease that tested negative on microfilaria tests. The ELISA test is more expensive and time consuming. The client must be made aware of the additional security afforded with our testing and the reason for the increased cost. Heartworm testing is recommended every six months in this area due to the high incidence of heartworms in the Gulf Coast.
Heartworm treatment is a carefully managed process that matches the needs of the heartworm infected pet with the medications available to help insure a safe effective treatment. We first evaluate the pet for the severity of the disease prior to the treatment. This evaluation includes a through physical examination, blood tests, radiographs, and ultrasound to determine the severity of the infection and the likelihood of a successful treatment. These tests will determine if an allergic reaction to the heartworms is present, if the liver function and kidney function is adequate, if the lung tissue is normal, and if the heartworms have migrated from the main pulmonary artery to the right ventricle or vena-cava. The findings of this evaluation are discussed with the owner of the pet so that a plan can be made for the treatment of their pet.
The success rate of the heartworm treatments here at Safari is high. We cure 95% of all pets that present with minimal clinical signs. We are able to cure 85% of the dogs that have severe clinical signs due to heartworm infection. This group of pets require additional therapy that is more involved than the normal heartworm treatment. The compound we use is an arsenic derivative. There are currently two arsenic compounds for treatment Caparsolate and melarsomine .
Initial Evaluation For Treatment
Day one of pre-treatment evaluation: This includes collection of blood to be sent to the lab, placement of an intravenous catheter and general anesthesia (isoflurane) for chest radiographs and ultrasound. The dog is started on H/D diet and ascriptin. (the aspirin may be started as far as two weeks in advance of the treatment and have beneficial effects)
Day two of Pre-treatment evaluation: Evaluation of test results and consultation with the owner regarding the treatment and prognosis.
Class 1: Patients in this category are characterized as having no symptoms of heartworm disease or mild symptoms of heartworm disease. No radiographic signs or signs of anemia are evident. Patients with mild disease may have subjective signs such as a general loss of condition, fatigue on exercise, or occasional cough; however, no objective radiograph or other abnormal laboratory parameters will be present.
Class 2: Patients in this category are characterized as having moderate heartworm disease. Radiographic signs or signs of anemia (PCV less than 30% but greater than 20%) are evident. Mild protein leakage into the urine (+2) may be present. radiographs show right heart enlargement and slight pulmonary artery enlargement. Lung densities may be present. Patients have clinical signs such as general loss of condition, fatigue on exercise or occasional cough.
Class 3: Patients in this category are characterized as having severe heartworm disease. Guarded prognosis is given for the treatment success. Subjective signs include wasting of the body due to poor heart function, constant fatigue, persistent cough, difficulty breathing and/or other signs of heart failure such as ascites, and or a jugular pulse. Radiographs show right heart enlargement, severe pulmonary artery enlargement, and pulmonary lesions typical for thromboembolism. Signs of significant anemia < 20% and other blood test abnormalities may be present. Proteinuria is usually present.