Since human dirofilariasis infection is rare and generally occurs in isolated cases, the epidemiology of human dirofilariasis is not well understood.  However, several factors are known to influence the frequency and distribution of infection, including: 1-the size of the natural host population in a particular area, 2-the prevalence of infection in the natural host population, 3-the density of the vector population (mosquitoes of various species), and 4-the degree of human exposure to the bite of infected mosquitoes (Villanueva, Rodriguez-Perez).

• D. immitis:  D. immitisis the causal agent of heartworm in dogs, and to a much lesser extent cats, in tropical and subtropical areas of the world.  Thus, the frequency and distribution of human D. immitis infection is related to the prevalence of canine heartworm (Global Distribution of Canine Heartworm).  In the United States, the majority of human cases have been clustered along the Gulf and Atlantic coast, where the prevalence of canine dirofilariasis reaches levels as high as 40% (Distribution of Canine Heartworm in the United States).  Canine transmission rates in the Northern Hemisphere are greatest during the warmer, humid months of July and August (Bielawski, et al).  Frequency and distribution of human infection is independent of dog ownership.  In a review pulblished in 1999, human pulmonary dirofilariasis caused by D. immitis infection was reported to occur at the same frequency in men and women, and most often in adults in their 50s and 60s (Table).  Racial statistics also show a higher prevalence in whites than in blacks (Shah).

• D. repens: D. repens infection is found in dogs, cats, and other animals in Europe, Africa, and Asia.  Many human infections have been reported in the last 100 years, especially in Italy.  Italy has the highest number of cases in the world, followed by France, Sri Lanka, the Ukraine, Russia, Greece, Turkey and Uzbekistan (Pampiglione, et al).  One retrospecitve study on the distribution of human dirofilariasis infection in the European Union published in 1999 reported that 66% of the cases studied were from Italy, 21.7% were from France, 8% were from Greece, and 4% were from Spain (Muro, et al).

• D. tenuis:  D. tenuis infection is found only in raccoons in the southeastern United States, in the region between Florida and Texas, south of Arkansas.  About 75% of all infections have come from Florida (Gutierrez).

• D. ursi:  D. ursi infection is found in the northern United States and Canada in the perirenal fat and mediastinum of bears.

• D. subdermata:  D. subdermata infection is found in the northern United States and Canada in porcupines.

• D. striata:  D. striata infection is found in deep tissues and muscular fascia of some species of wild cats on the American continent.

• D. lutrae:  D. lutrae infection is found in American otters.