With the advent of serological testing, the detection of heartworms has taken a dramatic step forward but has yet to achieve optimum sensitivity. In the following NEWSLETTER, we examine the causative agent, clinical symptoms, diagnosis, prophylaxis and treatment of heartworm disease.

Dirofilaria immitis is endemic in all parts of the United States, but is particularly common in the Atlantic and Gulf Coast marshes where salt water mosquitoes are prevalent and the area has high moisture content. The lifecycle of the heartworm parasite starts with an infected mosquito biting a healthy dog, cat, or ferret and depositing larvae in the puncture wound. The larvae migrate to either muscle or fat tissue, molt, and then develop into the young adult stage within 60 to 90 days post-infection. Migration to the right ventricle and pulmonary arteries follows, and the young adult matures within the next 3 to 4 months. Females are gravid by 5 to 6 months with microfilaria being detected (patent period) 6 to 7 months after the initial infection.

Dogs with typical heartworm disease fatigue easily, suffer a gradual loss of condition, and may have a chronic soft cough. The disease progresses to right sided heart failure, and chronic congestion with hepatic cirrhosis and ascites. Damage to the lung is more serious when heartworm infected dogs receive regular exercis (even a fast 1/4 mile walk each day). Pulmonary embolisms precipitate episodes of respiratory distress, and post-caval occlusions can cause sudden collapse and death. Most affected dogs are more than 1 year old with a mean age of 5 years. Dogs with no clinical signs may have an average worm burden of up to 25, while 50 worms are associated with moderate to severe heartworm disease.

Nonspecific blood tests, modified Knott’s Test and heartworm antibody detection have been largely replaced by the adult heartworm antigen assay. A direct relationship exists between the heartworm antigen titer (OD reading) and the degree of worm burden.

Studies performed by Drs. Courtney and Zeng at the University of Florida and published in CANINE PRACTICE, Vol. 20, pages 15-17, September/October 1995, gave the following data for the two leading manufacturers of heartworm testing kits—Synbiotics and IDEXX:

Number of dogs in ench category with percentages in parentheses.

DIROCHEK is a trademark of Synbiotics Corporation. PETCHEK-FF is a trademark of IDEXX Corporation.

From this study, it is clear that the specificity of both assays is 100% (all negatives tested negative), but the sensitivity for each was approximately equal at around 75%. Therefore, one out of four infected animals tested negative. The reason for less than satisfactory sensitivity is probably related to cases with relatively low infective worm burden. Accuracy (which depends on both sensitivity and specificity) is the most important single criterion when selecting a diagnostic test, especially when the prevalence of heartworm infection is unknown (i.e., nonendemic areas).

Where does this leave the practicing veterinarian? A positive result is diagnostic, but a negative result may be a true negative or a false negative. A negative obtained during a routine screen on a healthy animal does not warrant a follow up until next season, but if a negative result is obtained on a symptomatic dog additional confirmatory testing is indicated. This includes: Knott’s test, microfilarial testing of young adults not receiving macrolide preventives, chest radiographs, and ultrasound or echocardiography. Microfilarial tests are not reliable for screening dogs receiving heartworm macrolides (ivermectin and milbemycin oxime), because they are likely to be negative after 6-9 months on these preventives.

Important variables affecting the interpretation of tests results include: likelihood of heartworm disease in the locale and patient, accuracy of the test used, and prognosis with or without treatment.