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
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
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.
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).
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
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.