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Filariasis, Dirofilaria spp.(Pathogen – Tissue Nematode)

Currently, there are two subgenera, Dirofilaria Dirofilaria and Dirofilaria Nochtiella, based on the presence or absence of external longitudinal cuticular ridges.  D. Dirofilaria immitis will be referred to as D. immitis.  The others in the Dirofilaria Nochtiella species will be designated at Dirofilaria and the species name.  A number of these species, such as D. tenuis, D. repens, D. ursi, D. striata, D. subdermata, and Brugia spp., are animal filaria, cause subcutaneous or conjunctival nodules in humans.
D. immitis, the dog heartworm, causes a common zoonotic filarial infection in dogs in the tropical, subtropical, and warm temperate regions of the world. The infection was prevalent in the southern coastal and southeastern United States; however, the zone of endemicity has been increasing northward and to the west. Pulmonary lesions are the most common findings in humans infected with D. immitis.  Dirofilaria tenuis is a parasite of the raccoon within the United States; Dirofilaria repens is found in dogs and cats in Europe, Africa, and Asia and is the cause of human infections in these geographic areas; Dirofilaria ursi infects bears in Canada, the northern United States and Japan; Dirofilaria subdermata infects porcupines in North America; and Dirofilaria striata infects wild cats such as bobcats, Florida panthers, ocelots, and margays in North and South America.      

Dirofilaria pulmonary nodules 
Dirofilaria in eye
Intraocular Dirofilaria           Dirofilaria in dog heart

Adult female and male worms reside in the right heart of dogs, and microfilariae are found in the blood, where they are ingested by mosquitoes, or in the case of D. ursi, Simulium black flies. After biological development in the mosquito, dogs and humans may be infected with the infective larvae when the mosquito takes a blood meal. Development to a mature adult takes approximately 180 days in the dog. In humans, the worms do not reach maturity and no microfilariae can be detected.

Infection in humans is acquired through the bite of infected mosquitoes.

Although at least 15 species of mosquitoes can carry the infective forms and the infection is considered an emerging zoonosis, probably less than 1,000 cases of human infection have been described. However, it is quite possible that many infections go undiagnosed and the number of cases is higher than documented.  Canines, cats, foxes, muskrats, otters, and sea lions are natural hosts. The infection in animals has been noted in the United States, Africa, Mediterranean countries, Southeast Asia, and the Pacific Islands. Infections in dogs in the southeastern United States may be as high as 40%.

Clinical Features:
Accidental infection of man results in lung nodules, a subcutaneous mass anywhere in the body, or an ocular lesion that may be subconjunctival or periorbital. Many patients are asymptomatic, and pulmonary nodules (coin lesions) are noted on routine X‑ray examinations or at autopsy. Symptomatic patients exhibit signs and symptoms leading to chest examination, with common findings of chest discomfort, cough, fever, chills, malaise, and occasional hemoptysis. Infections tend to be more frequent in males than in females, and most individuals are 40 to 60 years old.  When the worms lodge in the pulmonary artery branches, they cause an infarct. These lesions are usually on the periphery of the lungs and are sharply defined (coin lesion). There is a central necrotic area surrounded by a granulomatous inflammation and a fibrous wall. Dead or dying worms may be found in the lesion.  Only 10% of the patients have a peripheral eosinophilia.  Most lesions (76%) are seen in the right lung, with a predilection for the lower lobe.
In the case of eye infections, Dirofilaria spp. worms are often motile, and noticed by the patient because of interference with vision. Removal is curative and full visual acuity is generally restored with no long term sequelae.

Clinical Specimen:
Biopsy, autopsy:  Multiple sections are recommended; larvae may be detected in only a few microscopic sections. 

Laboratory Diagnosis:
Histology:  Microfilariae cannot be found in the blood or tissues, and serologic results are of little value because of the lack of sensitivity and specificity of this method. Some patients have a moderate eosinophilia. Diagnosis can be confirmed by the identification of worms in surgical or autopsy specimens. Because the immature larvae may be detected in only a few microscopic sections, careful histologic examination is necessary. The cuticle of nematodes contains chitin, which can be stained with nonspecific whiteners such as calcofluor white. Dirofilaria larvae stained with calcofluor can be easily recognized in tissue sections, whereas the parasite may be difficult to identify using routine histologic stains.

Organism Description:
Adult: Approximately 12-13 cm long and, 0.5 mm thick and white in color. 

Laboratory Report:
Worms identified in histologic sections OR an adult worm is removed and identified from the eye.

Surgical removal of the worms is the only known treatment.  No chemotherapeutic agents are used, since they appear to be ineffective.       
Garcia, L.S. 2007.  Diagnostic Medical Parasitology, 5th ed., ASM Press, Washington, D.C.

Canines, cats, foxes, muskrats, otters, and sea lions are natural hosts. The infection in animals has been noted in the United States, Africa, Mediterranean countries, Southeast Asia, and the Pacific Islands. Infections in dogs in the southeastern United States may be as high as 40%.  Periodic treatment of dogs is recommended.