Current Advice on Parasite Control:
Heartworm - Feline Heartworm
Life Cycle and Stages
Cat Life Cycle
The heartworm life cycle is demonstrated in the figure below:
- Microfilariae (307 to 322 µm by 6.8 to 7.0 µm) are ingested by female mosquitoes when they feed on infected canids. (Felids are seldom microfilaremic; consequently, they are less likely sources of microfilariae for mosquitoes). After development through two molts (approximately 2 weeks), infective L3 larvae are present in mosquito mouth parts. Development may require a longer period at cooler temperatures.
- Third-stage larvae (L3) are 1,000 µm by 40 µm. They are deposited by mosquitoes on the skin surface during subsequent feeding(s) and then enter the new host via the mosquito bite wound. Most L3 larvae molt to fourth-stage larvae (L4) within 1 to 3 days in the subcutaneous, adipose, and muscle tissues.
- The final molt to the juvenile worm occurs approximately 2 months (50 to 70 days) after infection.
- Juvenile worms (1 to 3 cm in length) enter the vascular system and are carried to the heart and pulmonary arteries; they arrive as early as 70 days after infection. Most feline infections are aborted at this time, resulting in inflammation in the pulmonary vessels, pulmonary parenchyma, and airways. In a small percentage of infections, further development to mature adult worms and mating may occur. Because of the typically small worm burdens in cats (often one to three worms), single-sex infections (i.e., male or female only) are common.
- Maturation to adult worms occurs at about 6 months; development and release of microfilariae occurs about 7 or 8 months after initial infection.
- As noted previously, microfilaremia is infrequent (typically less than 20% of naturally infected cats) and short-lived; the number of circulating microfilariae typically is low. Microfilariae are seen in small numbers in experimentally infected cats and persist no more than a few weeks to months.
- Microfilaria of D. immitis are shown in the image below. Their structure is similar to microfilariae in dogs.
- Mature heartworms in cats are smaller than those in dogs; 12-month-old female worms in cats average 21 cm in length, compared with longer than 26 cm in dogs.
- Adult heartworms are believed to live for 2 to 4 years in cats.
Heartworm disease in the cat may involve some or all of the following:
- Pulmonary arterial, bronchial, and alveolar disease—Heartworm Associated Respiratory Disease (HARD)—is associated with the death of developing juvenile worms. Cats may present with cough, dyspnea, and/or wheezing.
- Death of adult heartworms (if present) can potentiate HARD signs. Sudden death occurs in approximately 10 to 20% of diagnosed cases. Pathogenesis is unclear, but a condition (similar to acute respiratory distress syndrome [ARDS]) caused by the release of antigenic moieties from injured or dying adult worms is suspected.
- Vomiting unrelated to eating may be present.
- Pulmonary thromboemboli (fragments from dead adult worms) may cause acute vascular and interstitial inflammatory events that lead to dyspnea and death.
- Hematological abnormalities may include anemia, hyperglobulinemia, basophilia, and eosinophilia.
- Neurological signs may indicate aberrant migration of the worm to the brain, eye, or spinal cord.
- Heartworm disease in the cat may involve some or all of the following:
- Geographic prevalence of feline heartworm infection (adult worms) generally follows canine infections, but infection occurs at approximately 10% of the prevalence rate for dogs (see map below for canine prevalence). Cats are infected with juvenile worms at a much higher rate than with adult worms; estimates based on necropsy and antibody studies suggest that cats are infected with juvenile worms at an infection rate similar to that of dogs. The greatest number of cases is seen in the southeastern U.S., the Mississippi River Valley, and Texas.
- Necropsy-based data (percentage infected) for cats with adult heartworms in shelters is shown in the figure below.
- A study involving more than 2,000 largely asymptomatic cats was conducted in 19 states comprising 21geographic regions (see figure below). For pet cats (excepting North Carolina, which surveyed shelter animals), the nationwide exposure rates (i.e., percentage of cats that were antibody-positive) were approximately 12%. Many of these cats were from areas not highly endemic for heartworm infection. Results do not confirm mature heartworm infection but imply infection with immature worms and increased risk of HARD.
- A second study of 25,000 cats tested nationwide reported a 15.9% antibody positive rate.
Host Association and Transmission Between Hosts
- More than 70 species of mosquito are capable of transmitting D. immitis. However, only about 22 species are common and important vectors.
- Differences in biological behavior and host preferences by mosquitoes can affect the epidemiology of heartworms in a particular location.
- Transmission can occur anytime infected mosquitoes are active and feeding.
- Although L3 larvae taken from mosquitoes that had fed on experimentally infected cats were capable of infecting puppies, the infrequency, low numbers, and transient nature of microfilaremia in cats support that in many cases, cats are a dead-end host for D. immitis.
Site of Infection and Pathogenesis
- In dogs, adult heartworms reside largely in the pulmonary arteries. However, if numerous worms are present, worms may be found in the right ventricle and pulmonary trunk. In cats, the relative size of the adult heartworm is such that it is much more likely that a portion of the worm can be observed in the right side of the heart.
- Migration of immature heartworms to other anatomic sites can occur in cats, sometimes resulting in death.
- Pulmonary intravascular macrophages—the primary component of the cat’s reticuloendothelial system—have been implicated in the unique pulmonary pathology found in cats.
- Because of potentially lower worm burden, single-sex infections, and infrequency of microfilaremia, heartworm infection is more difficult to diagnose in cats than in dogs.
- Fewer than 20% of cats with mature heartworm infection are microfilaremic. Because of the frequent absence of microfilariae, microfilarial testing (e.g., direct smear, microhematocrit tube, modified Knott test, millipore filter test) is less useful in the cat than in the dog. Nevertheless, a positive microfilariae test is more likely to confirm active adult heartworm infection in the cat.
- Eosinophilia (and possibly basophilia), although nonspecific, can support a diagnosis of heartworm infection. Eosinophilia, often transient, is most frequently observed 4 to 7 months after infection.
Antibody tests (available as both send-off and point-of-care tests) do not require the presence of circulating antigen produced by mature female worms for a positive test result. Different antibody tests may detect migrating larvae of different ages. If an exposed cat eliminates the infection at the juvenile worm stage, HARD may still result.
- Antibody testing may not be effective for diagnosis of feline heartworm disease. Approximately 15 to 25% of cats with adult infection are antibody-negative. Conversely, 80 to 90% of antibody-positive cats do not harbor mature heartworms.
- Depending on the preventive product used, up to 30% of cats on preventives that are exposed to heartworms will convert to an antibody-positive state without mature infection or heartworm-related disease.
- In a limited number of experimental studies, 50% of experimentally infected cats with confirmed pulmonary pathologic lesions (HARD) seroconverted to a negative antibody status within 8 months after infection. Within 16 months after infection, 100% of experimentally infected cats (HARD) were negative for detectable antibodies. However, after initial infection and induction of HARD lesions, these cats were restricted from further potential exposure to heartworm-infected mosquitoes.
- Positive antibody tests may or may not support heartworms as a cause of respiratory signs or lesions. Additionally, a negative antibody test does not rule out current or previous infection.
- Antigen testing is available as both send-off and point-of-care tests.
- Only mature female worm infections (typically 7 to 8 months post-infection) are detected. Rarely, in experimental infections, female heartworms may be detected as early as 6 months after infection. A positive antigen test indicates that the cat harbors mature female heartworms or that mature female heartworms were present but died recently.
- One third of adult heartworm infections in cats consist only of male worms and will not be detected using available antigen tests.
- Studies of naturally infected cats examined at necropsy have shown that some antigen tests are capable of detecting a single mature female heartworm.
- A negative antigen test does not conclusively rule out adult heartworm infection nor will it detect juvenile worms often responsible for pulmonary disease (HARD).
- The most common radiographic findings in feline heartworm disease are enlargement of the right caudal lobar artery and a bronchointerstitial inflammatory pattern in the caudal lung lobes.
- Ultrasound in the hands of a skilled ultrasonographer may detect 68% of naturally infected cats. However, the caudal pulmonary arteries must be examined to their point of bifurcation within the lung fields.
- At present, removal of adult worms from cats using melarsomine dihydrochloride is not recommended.
- Early data indicate that concurrent ivermectin and doxycycline therapy for Wolbachia reduces inflammation associated with worm death in dogs. It is not yet known whether this same result can be achieved in cats.
- Surgical removal of heartworms via the jugular vein can be performed. However, deaths have been reported during the use of this procedure. In addition, jugular venotomy is expensive and requires the use of a fluoroscope for anatomic guidance.
- Microfilaricidal therapy is not recommended in cats.
- At present, no specific therapy is recommended for asymptomatic cats with confirmed heartworm infections. However, infected cats Cats with symptomatic feline dirofilariasis should be treated with corticosteroids in decreasing dosages to minimize dyspnea, coughing, and wheezing caused by death of either juvenile (HARD) or adult worms. Antileukotrienes also may be beneficial in reducing the risks associated with adult worm death. Bronchodilators may be useful if there is radiographic evidence of air-trapping. Specific therapies should be used if vomiting and neurological signs are present.
- Emergency treatment of symptomatic cats may include parenteral corticosteroid therapy, oxygen therapy via an oxygen cage or nasal insufflation, and furosemide in ARDS-like cases. If available, ventilator therapy may be utilized. Xanthine bronchodilators (theophylline or aminophylline) may be used to dilate bronchioles and support the muscles of respiration.
- The published life expectancy for cats with adult dirofilariasis is 1.5 years (median).
- For cats surviving acute disease beyond day 1, the median survival is 4 years.
- Current information suggests that more than 80% of cats diagnosed with heartworm infection survive the infection.
Control and Prevention
- All cats with potential exposure to heartworm-infected mosquitoes should be on year-round heartworm prevention. Available preventives for cats include oral ivermectin, topical moxidectin/imidacloprid, and topical selamectin. Topical moxidectin/imidacloprid is also approved for use in ferrets.
- Although cats housed indoors have a lower risk for heartworm infection, studies have shown that 25 to 30% of heartworm-infected cats were characterized by their owners as “strictly” indoor cats. Furthermore, certain mosquitoes that typically are found indoors will feed on cats.
- Available broad-spectrum feline and ferret heartworm preventives also have label claims for other important feline internal or external parasites.
Public Health Considerations
- Because cats typically are amicrofilaremic, heartworm-infected cats pose little to no public health risk.