Important heartworm basics for the practicing veterinarian

Although heartworms (Dirofilaria immitis) have been prevalent and important parasites in companion animals for decades now, there is still some misunderstanding about the biology and behavior of this parasite. The basic roles of dogs and mosquitoes in the life cycle of heartworm are easily understood and usually effectively communicated. It is the details, such as the importance of the capability of mosquitoes to serve as competent heartworm vectors, their geographic distribution, habitats, preferences and feeding habits that are often ignored or underemphasized. Likewise, certain aspects of the life cycle in the dog, such as where worms are located at specific times during the life cycle, longevity of adult worms, when microfilariae appear in blood, what is required for them to be produced, and what influences their presence or absence, are assumed to be less important biological features of heartworm infections. Understanding these features of heartworm infections is necessary if we are to employ valid and reliable diagnostic, treatment and prevention strategies. In this brief article, I will review some aspects of the biology and life cycle of heartworm infections that are important in developing these strategies and assessing their efficacies.

Photo 1: Male and female heartworms. The male heartworm (center) is identified by its smaller size and spiral tail. Female worms can grow to 12 inches in length.

Life cycle basics

Dirofilaria immitis is transmitted, of course, by mosquitoes. What few realize is that among the approximately 3,000 species, 70 or fewer actually can transmit the parasite to dogs. In the United States that number decreases to about 22 different mosquito species from which heartworm larvae have actually been recovered. Both the numbers of different mosquito species and their percentages of the total population may vary from region to region. Consequently, the prevalence of heartworm in dogs in a particular area is determined to a great extent by the numbers and kinds of mosquitoes, their density, how often they feed, and the length of the mosquito season.

Transmission of heartworms requires ingestion of microfilariae from an infected dog when a susceptible mosquito vector takes a blood meal. Micro-filariae migrate to the malpighian tubules (the mosquito's excretory system) and undergo development via two molts to the infective third-stage larva. These larvae migrate back to the mouthparts and await subsequent feeding on a suitable host. Development of D. immitis microfilariae to infective larvae within mosquitoes is highly temperature dependent. The infective stage can be reached in as few as eight days at 30 degrees C or it may require as long as 28 days at 8 degrees C.

When infected mosquitoes feed, they deposit larvae in a pool of saliva at the skin surface. Larvae actively enter the mosquito bite wound and begin migration to their preferred location in the dog, the right ventricle of the heart and main pulmonary arteries. Molt from the third- to the fourth-stage larva occurs as early as three to four days after larvae enter into the subcutaneous tissues. During their migration toward the heart, fourth-stage larvae have increasing capabilities to penetrate tissues and are found with greatest frequency in muscles of the abdomen and thorax. The molt to the fifth-stage larva (immature adult) occurs as early as 50 days, but this may vary between 50 and 70 days.

Table 1

Research suggests that the route of migration of larvae to the cardiopulmonary tissues is via the systemic veins. When fifth-stage larvae arrive in the heart, they are small worms measuring little more than 2.5 cm (1 inch), and are quickly carried to the more distal extremities of the pulmonary arteries. As these worms mature, they increase in size dramatically and tend to "grow" upward toward the proximal pulmonary arteries, the pulmonary trunk and the right ventricle. As worm burdens increase, greater numbers of worms are found in closer proximity to the heart. My experience has been that the vast majority of the worms that are found in the pulmonary vessels tend to inhabit the main pulmonary arteries.

Although it can occur, few worms seem to find their way into the lateral branches of the pulmonary arteries. Adult heartworms, most of which reach sexual maturity at 6-6.5 months, can be quite large. Female worms attain lengths of 23-31 cm (up to 12 inches). Males are smaller at 15-19 cm and have a characteristic spiral or coiled tail (Photo 1, p. 12). Greater than 75 percent of the mature length of male and female worms is achieved prior to production of microfilariae by female worms. Microfilaraie are unensheathed embryos that are released by females only after mating and fertilization. There is considerable variation in the time required for maturation, production and release of microfilariae by female worms (Figure 1, p. 14).

Table 2

Occasionally microfilariae are retained in the uterus for weeks prior to their release into the general circulation. The reproductive tract comprises a significant percentage of the mass of female worms as would be suspected. In addition to microfilariae, numerous proteins, protein conjugates and other molecules are also produced in the reproductive tract of female worms. The release and subsequent circulation of certain of these molecules (antigens) serves as the basis for the commercially available antigen detection tests. The production of certain of these molecules in much greater quantities in female worms, and therefore our inability to detect male worms in most cases, is the only hindrance to the exquisitely sensitive and specific concept of antigen detection. Appearance of detectable reproductive tract antigens may or may not coincide with release of microfilariae. The appearance of these antigens may preceed the release of microfilariae by female worms, or proceed it as is indicated in Figure 1.

Photo 2: Microfilariae of Dipetalonema reconditum (top) and Dirofilaria immitis from a Modified Knott's procedure. See Table 3, p. 15 for descriptions of structural features.

Heartworms are known to survive for long periods in dogs. Some reports indicate that female worms can live and continue to produce microfilariae for up to seven years. Most sources quote their survival at between five and seven years. Microfilariae can survive in infected dogs for up to two years.

Heartworm disease

Disease in dogs resulting from heartworm infections can be inapparent or severe, depending upon several factors. Factors include the stage of the parasite and its location, the numbers of parasites present and the severity of the host reaction. Early arrival of the immature adult (L5) in the pulmonary vessels can result in eosinophilic pneumonitis and coughing associated with the presence of these young worms. Maturation of worms and their occupation of the pulmonary arteries can result in inflammation and proliferation of the arterial wall (villous endarteritis). Death of worm adults and resulting embolic worm fragments can trigger a cascade of inflammatory events leading to thrombosis and decreased blood flow.

Important Heartworm Developmental time Points

Some have emphasized that heartworm disease, in most cases, is inappropriately named. The observed disease syndrome, particularly during early infections, is more a disease of the lungs than of the heart. In severe, long-standing infections, ventricular hypertrophy and classical right heart failure with accompanying liver disease and abdominal ascites are observed. Fortunately, this component of the disease is not seen as frequently as it once was due to the availability and use of safe, effective and convenient preventatives (Table 1, p. 13). Remember that these disease syndromes can be more severe in dogs whose exercise patterns place additional physiological stress and burdens on the heart and lungs. Microfilariae are not usually responsible for observable disease, although they have been implicated by some as causes of dermatitis, vasculitis and even glomerulonephritis.

Important life cycle stages and developmental features

Microfilariae

Microfilariae are produced only when sexually mature male and female heartworms are present. Infections with either sex alone will not result in production and release of microfilariae by female worms (Occult or hidden infections). Production of microfilariae (with some exceptions) begins after worms have achieved ages of 6 to 6.5 months (Figure 1). Recall that female worms may produce sufficient amounts of detectable antigen in the absence of male worms, therefore certain dogs may be positive when tested for antigen, but will test negative for microfilariae. It is also possible to produce a positive microfilaremia by transfusing microfilariae infected blood into a negative dog. This is unlikely since most blood donor animals are monitored for heartworm infection. Equally rare is the transplacental transmission of microfilariae from infected bitches to pups. Of course, in neither of these cases can microfilariae mature to adult worms. Microfilariae may persist in circulation after adult worms either have died or have been removed with effective adulticides. Recall that circulating microfilariae can survive (if not exposed to microfilaricidal drugs) for two years in infected dogs. All preventative medications, except perhaps diethylcarbamazine, are capable of eliminating microfilariae if administered either at monthly or six-month intervals (Table 1). The rate at which microfilariae disappear following use of preventative products varies with product, dosage and formulation. A syndrome also has been described in which the dog's immune system may effectively sequester microfilariae in certain organs, or destroy them by producing microfilariae-specific antibodies. In this case, microfilariae cannot be detected in circulating blood (immune-mediated occult syndrome). These immune-mediated events can result in severe pneumonitis in the very few dogs that develop this syndrome. Lastly, dogs in different regions of the world are host to filarial parasites that produce micro-filariae that could be confused with those of D. immitis (Table 2). The most common of these filarial parasites in the North America is Dipetalonema reconditum. Adults of Dipetalonema reside in the subcutaneous and peri-renal tissues. Their microfilariae are smaller that those of D. immitis and have structural features that are different (Table 3, Photo 2, p. 14). I should probably mention that any attempt to detect microfilariae should utilize a concentration procedure such as the Modified Knott's test or a filtration test. Direct examination of blood, without concentrating the sample, can result in failure to recover microfilariae. This is particularly important if numbers of circulating microfilariae levels are low.

Table 3

Other life cycle stages

Heartworm stages other than mature female heartworms (generally > six months) are usually not detectable with available testing methodologies. This includes third- and fourth-stage larvae that are en route to the heart. Subcutaneous larvae, those in muscle or those traveling to the lungs do not betray their presence or migrations in the host. Early fifth-stage larvae (immature adults) that arrive in the heart between 70 and 110 days may induce clinical signs such by coughing, or pulmonary lesions that may be observable on radiographs. However, until either microfilariae are produced or sufficient amounts of circulating antigen (produced by female worms) is present, heartworm infections cannot be detected. Developing heartworms also may migrate to abnormal (aberrant) locations. Such sites may include the subcutaneous nodules, the anterior chamber of the eye, or systemic arteries. Unless they are accompanied by clinical signs or visible lesions referable to these develop-mental locations, these aberrant infections usually go unnoticed. Most worms that are found at these sites are solitary and immature. Thus, it is unlikely that they would either produce microfilariae or enough antigen (female worms only) to detect. The migration of heartworm, either during the course of its normal life cycle, or during its migration to aberrant sites, do likely induce detectable antibody responses. These antibody responses, unlike their counterparts in the cat, do not serve a useful purpose in the dog. Given the prevalence of heartworms in most regions of the country, virtually all dogs would test positive for heartworm-specific antibodies. Also, other available tests are more reliable in the dog than they are in the cat.

Suggested Reading

The complexity of the heartworm developmental cycle in the dog, and other factors such as vector biology, climatology, attributes of preventative and adulticidal medications, and efficacies and performance of diagnostic tests can interact to create difficult diagnostic and/or treatment situations. A thorough knowledge of the life cycle and habits of heartworms often can help us to understand and explain infection status and test results that, at first, seem confusing. It can also help devise and implement more effective testing, treatment and prevention strategies.