D. immitis is a filarial nematode in the superfamily Filarioidea (order Spirurida, class Secernentea). The males are 12 to 20 cm in length and 0.7 to 0.9 mm in diameter, with a spirally coiled posterior end. The females are 25 to 31 cm in length and 1.0 to 1.3 mm in diameter. D. immitis was first found in the United States, but it occurs globally, with a tendency for increased prevalence in humid warm regions conducive to abundant mosquito populations. In the United States, the prevalence can be as great as 45% in dogs within 150 miles of the Gulf of Mexico coast, Atlantic coast north to New Jersey, and Mississippi River and its tributaries. The prevalence elsewhere in the United States is generally less than 5%. Although dogs are the primary host, D. immitis has been found in coyotes, wolves, dingoes, foxes, sea lions, harbor seals, wolverines, ferrets, and cats. The number of nematodes per dog is variable, ranging from single nematodes to as many as 250. Cats are less tolerant, with maximum parasite loads of 1 to 3 nematodes. D. immitis can persist for 5 to 7 years in dogs and 2 to 3 years in cats.
Mature female nematodes reproduce ovoviparously and the microfilaria are released from the uterus of the worm via the vaginal opening into the host blood. Microfilaria are 220 to 330 |lm in length and 5 to 7 |lm in width. Under laboratory conditions, over 60 species of mosquitoes are competent hosts for D. immitis. When mosquitoes take a blood meal, microfilaria are ingested and reside in the mosquito midgut briefly and then migrate to the Malpighian tubules, where they enter the cells and shorten. After a short developmental period, the juvenile nematodes leave the cells day 6 to 7 postinfection and enter the lumen of the tubules where the first molt (day 10) takes place, with the formation of the second-state juveniles (J2). After further growth and differentiation, the J2 molts (days 10-14 postinfection) to the infective stage (J3), which reaches a length of 1.3 mm. The J3 migrate through the hemocoel to the proboscis sheath of the mosquito. Development of the juvenile stages ceases if ambient temperatures decline below 15°C, which constitutes a constraint on the distribution of D. immitis. When the infected mosquito takes a blood meal, the J3 escape from the proboscis sheath, dropping onto the host in a droplet of hemolymph; they enter the host through the wound made by the piercing mouthparts of the mosquito. The J3 enter the subcutaneous tissue where they undergo the third molt to the fourth-stage juveniles, which reside in subcutaneous tissues or muscle of the abdomen or thorax for about 60 days, at which time the last molt to the adult stage occurs. The nematodes are now 12 to 15 mm long and enter the pulmonary arteries and attain lengths of 3.2 to 11 cm by 85 to 120 days postinfection. Fertilized females can be found 120 days postinfection and microfilaria enter the blood 6 to 9 months postinfection. Microfilaria can survive in the blood for 21/2 years. Host treatment with tetracycline leads to the loss of the endosymbionts and a concomitant reduction in survival and reproduction of D. immitis.
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