Life Cycle of Dirofilaria Immitis

A basic knowledge of the life cycle of the heartworm parasite is imperative for understanding strategies for prevention, diagnosis, and therapy. The mosquito is a requisite intermediate host for transmission of this disease. The first-stage larvae (L1) or microfilariae are ingested by the female mosquito following a blood meal from an infected animal. The parasite molts twice within the mosquito, and these molts are required for the parasite to develop into the infective third stage (L3). The parasite maturation within the mosquito from L1 to L3 requires approximately 1 to 3 weeks depending on environmental conditions, most notably an ambient temperature of at least 14° C (57° F).

The L3 larvae are the infective stage of the parasite and are transmitted to a susceptible host through a bite from an infected mosquito. The third larval stage travels within the subcutaneous tissues of the host, molting to the fourth larval stage (L4) in approximately 1 to 2 weeks and becoming a young adult (L5) 30 to 60 days after initial inoculation. These young adults migrate and enter the systemic venous system where they are carried to the lungs approximately 100 days after infection. Under ideal conditions the infection may become patent (adult worms producing microfilariae) in 5 months. More often, however, this complete cycle requires in excess of 7 months.

Establishment of a Diagnosis

The American Heartworm Society (AHS; www.heartwormsociety.org) recommends the use of a test that detects adult heartworm antigen (Ag) as the primary screening test for HWD in dogs. A number of manufacturers supply heartworm Ag tests, and readers are referred to the excellent articles by Atkins (2003) and Courtney and Zeng (2001) for data about the relative sensitivity and specificity of these tests. Ag tests are substantially more sensitive than microfilarial concentration tests for detection of heartworm infection. In addition, the widespread use of macrocyclic lactone (ML) or macrolide preventive medications further decreases the sensitivity of the microfilarial tests for diagnosis by rendering the vast majority of infections amicrofilaremic (occult). In contrast, these drugs have no effect on the sensitivity of the Ag tests. Circulating Ag generally is undetectable until 6 to 7 months following infection, and this interval can be as long as 9 months after infection in dogs intermittently receiving MLs. As a general rule, weakly positive test results should be rechecked either by repeating the original test or preferably by submitting a sample for evaluation by a different Ag test at a reference laboratory. False-positive results occur infrequently and almost always are associated with technical errors. False-negative test results usually are the result of immature infections, infections with low numbers of female worms, or all-male infections. Although once thought uncommon, complexing of circulating Ag with specific or nonspecific host antibody may leave insufficient concentrations of free Ag for detection and subsequently lead to a false-negative test result.

Blood smear examination or use of a concentration test for the detection of circulating microfilariae is not recommended for routine screening of dogs, even when infection is suspected. The simultaneous use of both an adult heartworm Ag test and a concentration test for microfilariae has been advocated by some as the optimal screening protocol. However, addition of a concentration test probably increases the likelihood of diagnosis in a negligible percentage (≈1%) of cases, such as when a very low number of gravid females is present or when the circulating Ag is complexed to host antibody. Once a positive Ag test result is obtained, however, a concentration test should be performed to identify any circulating microfilariae. This information is important because it helps confirm the accuracy of the positive Ag test result and also may influence the preventive medication used in that patient.

Some preventive medications, most notably those containing the active ingredient milbemycin, are potent microfilaricides at the suggested preventive dosage and may be associated with substantial risk of a shocklike reaction when administered to dogs with a heavy burden of circulating microfilaria. The risk of a rapid microfilaria kill is much lower with ML preventives containing ivermectin, selamectin, or moxidectin. Dogs with severe microfilaremia optimally should be kept in the clinic when the first dose of any preventive medication is given. It also is prudent to recommend that the client observe a patient without microfilaremia for the entire day following administration of the first dose of preventive medication, regardless of which agent is used or what the concentration of circulating microfilariae.

Classification and Staging

It has been suggested that dogs with documented adult heartworm infection be classified, optimally based on radiographic findings and laboratory studies, before initiation of adulticidal therapy. In the vast majority of dogs with heartworm infection, the disease is diagnosed during routine wellness examinations. These dogs typically do not have demonstrable clinical signs nor do they commonly display radiographic or biochemical alterations associated with the infection. When HWD causes clinical signs, these generally are attributable to pulmonary parenchymal injury, pulmonary arterial injury, or cardiac dysfunction. Less frequently encountered clinical syndromes include severe glomerular disease, caval syndrome (see later), and disseminated intravascular coagulopathy.

There are a number of ways to classify patients with positive heartworm test results. One approach is related to the use of the Immiticide brand of melarsomine and is based on the U.S. Food and Drug Administration (FDA)–approved label. The drug package insert for Immiticide provides very detailed and specific information about this classification system, although there is some potential for overlapping of clinical and laboratory findings when this three-class system is used. Another method is to consider heartworm patients in terms of clinical syndromes or presentations, an approach that considers the aforementioned classification system but also provides additional perspective regarding treatment approaches. Dogs can be characterized clinically as follows:

Role of Wolbachia

Like all filarial nematodes that use an arthropod intermediate host and vector, D. immitis harbors obligate, intracellular, gram-negative bacteria belonging to the genus Wolbachia (Rickettsiales). Studies of non–D. immitis filariae demonstrated that treatment with tetracyclines during the first month of infection was lethal to some Wolbachia-harboring filariae but not to a filaria that did not harbor Wolbachia. In addition, treatment of Wolbachia-harboring filariae suppressed microfilaremia and made surviving microfilariae noninfectious. There is strong evidence to show that doxycycline at a dosage of 10 to 20 mg/kg/day PO in dogs substantially reduces the number of adult parasites and in some cases results in the gradual elimination of D. immitis when coadministered with ivermectin. In addition, with this combination of drugs there is loss of uterine content in adult female heartworms, microfilariae (L1 stage) are removed from the circulation, and although microfilariae still can molt within the mosquito from L2 to L3, these L3 organisms are rendered noninfectious. Ongoing studies suggest that doxycycline combined with ivermectin may be a feasible alternative adulticidal therapy for some dogs unable to be treated with melarsomine and may render the majority of dogs heartworm Ag negative after 36 to 45 weeks of therapy. However, it remains to be determined whether some combination of doxycycline and an ML can serve as a safe and effective alternative adulticidal option. Concerns with this approach include the potential for ongoing pulmonary and vascular injury during the relatively long period required to eliminate some infections as well as the possibility that not all infections will be completely resolved by this strategy.

It has been suggested that Wolbachia contributes to pulmonary and renal inflammation through its surface protein WSP, independently from its endotoxin component; however, the data supporting this supposition are not strong. Nevertheless, there is keen interest in studies to determine the role of Wolbachia in the pathogenesis of HWD, and there is hope that suppressing this symbiote with doxycycline before initiating adulticidal therapy may reduce the severity of lung and vascular injury after adulticide administration. Based on available data, the AHS suggests pretreatment of animals with doxycycline for 30 days before adulticidal therapy to reduce Wolbachia numbers. This preadulticide protocol offers the potential benefits of time to allow juvenile worms to mature fully to the susceptible adult stage and increased death of adult worms, higher levels of seroconversion, and reduction in the severity of clinical signs associated with the inevitable pulmonary embolization of dead and dying adult parasites. If there are shortages of doxycycline, minocycline can be substituted at a dose of 5 to 10 mg/kg q12h PO. There are no studies with minocycline in canine heartworm disease, but it has similar antibacterial activity.

Melarsomine Dihydrochloride Therapy

The only drug currently approved for adulticidal therapy of canine HWD is melarsomine dihydrochloride (Immiticide). Three specific dosing regimens are listed as approved protocols on the package insert. The most commonly employed regimens are the first two approaches.