Blood and Tissue (Filarial) Nematodes

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Blood and Tissue (Filarial) Nematodes

Blood and tissue filarial nematodes are roundworms that infect humans. These organisms are transmitted via a blood-sucking arthropod vector such as a mosquito, midge, or fly. The filarial nematodes infect the subcutaneous tissues, deep connective tissues, body cavities, and lymphatic system. The life cycles of the filarial nematodes are complex (Figure 53-1). The infective larval stage resides in the insect vector with the adult worm stage, which is the pathogenic form in humans. When the arthropod vector feeds on a human blood meal, the infective larvae are injected into the bloodstream. The larvae are motile and migrate to the lymphatic vessels. The infective larvae grow and develop into the adult gravid worm in the human host over a period of months. The male and female adult worms mate in the definitive human host. The female worm produces large numbers of larvae called microfilariae. Depending on the species, the microfilariae may maintain the egg membrane as a sheath or may rupture the egg membrane, resulting in an unsheathed form. These parasites can reside in the host for many years and cause chronic, debilitating conditions and severe inflammatory responses. Identification of the various species is based on the morphology of the microfilaria, the periodicity (defined circadian rhythm), and the location within the human host. Microfilariae morphologic characteristics are important in the identification and include the presence or absence of the sheath and the presence and arrangement of the nuclei in the tail of the worm (Figure 53-2). A comparison of the morphologic characteristics of the pathogenic filarial worms is depicted in Figure 53-3. Diagnosis of infection is based on the identification of the microfilariae in the blood or tissue of the host.

Wuchereria Bancrofti

General Characteristics

Wuchereria bancrofti is transmitted in a mosquito, the Culex fatigans, Anopheles, or Aedes spp. The adult worm has a sheath that stains faintly or not at all. It may grow to approximately 298 μm in length by 2.5 μm to 10 μm wide. The tail is pointed with no nuclei present (Figure 53-4).

Epidemiology

W. bancrofti is the most common identified species of filarial worms that infect humans. It is widely distributed in the tropical and subtropics including Africa, South America, Asia, the Pacific Islands, and the Caribbean. The mosquito vectors have complex life cycles that include laying eggs and developing larvae on the surface of a water source. When the larvae mature into adult mosquitos, the male and females will swarm in the evening and mate. The female requires feeding on a blood meal in order to reproduce. The mosquito becomes the intermediate host for the microfilaria parasite. Humans are the definitive host and the reservoir for W. bancrofti. The parasite has two forms that demonstrate different periodicities. The nocturnal periodic form is found in the peripheral blood during the night between 10 pm and 4 am. The second form is found only in the Pacific Islands and is present in the blood at all times, but more frequently during the day in the afternoon hours.

Pathogenesis and Spectrum of Disease

Microfilaria clinical disease varies geographically based on the species of nematode causing the infection. The disease may present as acute or asymptomatic for many years. W. bancrofti causes bancroftian filariasis and elephantiasis. The adult worm resides in the lymphatic vessels distal to the lymph nodes. The presence of the organisms within the host results in an immunologic response including inflammation, hyperplasia, lymphedema, and hyperplasia. Lymphedema most often occurs in the lower extremities. Elephantiasis is a crippling condition that results from extended periods of filarial infection. The obstruction of the lymphatic vessels causes fibrosis and proliferation of dermal and connective tissue, resulting in the wrinkled, dry appearance of an “elephant” extremity. Lymphedema may also occur in the arms, female breasts, and scrotum of infected males.

Acute lymphatic filariasis results from worms residing within the lymph nodes. The lymph nodes swell and lymphangitis may appear peripherally from the infected node. Hydrocele formation, a fluid filled sac within the scrotum, may occur when adult worms block the retroperitoneal or subdiaphragmatic lymphatic vessels. Obstruction of the lymphatic vessels may result in a condition referred to as chyluria. Chyluria is a result of the lymphatic rupture and fluid entering the urine. The urine will appear milky white. Resulting infection and changes in the skin may result in increased bacterial infections.

Patients residing in endemic tropical regions for filarial parasites may present with a syndrome referred to as tropical pulmonary eosinophilia (TPE). The microfilariae migrate through the pulmonary blood vessels, causing an allergic hypersensitivity in the host. The patients develop a strong immune response to the presence of the parasites with an elevated serum immunoglobulin E (IgE) level. Symptoms of TPE include weight loss, low-grade fever, cough and wheezing at night, and lymphadenopathy. Without treatment, patients may develop chronic and progressive respiratory complications resulting in death.

Laboratory Diagnosis

Direct Detection

Definitive laboratory diagnosis is based on the identification of the parasites in blood, fluids, or tissue. Blood samples should be drawn in accordance with the periodicity of the infection to optimize the likelihood of isolating the infecting organism. Direct examination of blood, urine, hydrocele fluid, or chyle (milky fluid produced in the small intestine for fat digestion and taken up by the lymphatic system) may be used for identification of the parasite. The fluid is placed on a slide and air-dried to prevent distortion of the parasite. The specimen should be stained with Giemsa, Wright’s, or hematoxylin stain and examined microscopically. Ultrasound may be used to visualize the organisms within the tissues.

Nucleopore filtration or Knott’s concentration may be used to increase the likelihood of isolating a filarial parasite from blood. The blood is passed through a polycarbonate filter that contains a 2-µm pore. Distilled water is passed through the filter, lysing the red blood cells and improving the visualization of the parasites. The filter is then air-dried, stained with Giemsa, and examined for the presence of microfilaria. Knott’s concentration uses centrifugation to concentrate the organisms to a slide. One milliliter of anticoagulated blood is placed in 9 mL of 2% formalin, centrifuged at 500× g for 1 minute, and then applied to a microscope slide. The slide is then stained and examined microscopically. Sometimes adult worms may be visualized moving within the lymphatics, using high-frequency ultrasound.

Serologic Detection

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