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 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 2). A comparison of the morphologic characteristics of the pathogenic filarial worms is depicted in Figure 3. Diagnosis of infection is based on the identification of the microfilariae in the blood or tissue of the host.
Fig1. Life cycle of human filarial worms.
Fig2. Identification of microfilariae.
Fig3. Anterior and posterior ends of microfilariae found in humans. A, Wuchereria bancrofti. B, Brugia malayi. C, Loa loa. D, Onchocerca volvulus. E, Mansonella perstans. F, Mansonella streptocerca. G, Mansonella ozzardi.
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 4).
Fig4. Microfilaria of Wuchereria bancrofti in thick blood film.
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, lymph edema, 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 retro peritoneal 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.
Endosymbiont
W. bancrofti, Brugia spp., and Onchocerca volvulus harbor an endosymbiotic alpha-proteobacterium, Wolbachia sp. Wolbachia is an obligate intracellular organism.
The parasites require the endosymbiont for larval development, viability, and fertility. The bacteria have also been implicated in the pathogenesis of the infection with filarial parasites. The bacterial antigens enhance the host inflammatory response, leading to increased scar ring and damage within the host lymphatic system. The bacterium is sensitive to tetracycline, azithromycin, and rifampin. Combination antibiotic treatment in conjunction with treatment for the parasite infection improves clearance of the filarial parasite.
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 para site 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
Serologic assays that measure antibody response have limited utility in the diagnosis of infections with microfilariae. The antibodies tend to demonstrate a high cross reactivity with other antibodies made in response to a wide variety of parasitic worm infections. The absence of an antibody reaction would, however, indicate the lack of infection by a microfilaria species. Laboratory detection of W. bancrofti-circulating antigens has demonstrated high specificity and sensitivity in detecting parasitic infections. However, the commercial testing formats available are not FDA-approved.
Molecular Diagnostics
Polymerase chain reaction (PCR) amplification is available in reference laboratories for the rapid diagnosis of an infection with blood microfilariae including W. bancrofti and Brugia spp. Multiplex PCR has been developed to differentiate W. bancrofti and Brugia malayi in blood and mosquitos. The test results indicated that the reaction was highly sensitive and more efficient than traditional microscopic detection.
BRUGIA MALAYI AND BRUGIA TIMORI
GENERAL CHARACTERISTICS
The Brugia spp. are lymphatic filarial parasites resembling W. bancrofti. The adult parasites are somewhat smaller, (B. timori, 300 μm long and 5-6 μm wide; B. malayi, 270 μm long and 5-6 μm wide) have a different geographic distribution, and do not typically cause lymphadenitis in the genital regions.
EPIDEMIOLOGY
The Brugia spp. are distributed throughout the Far East including China, Indonesia, Korea, Malaysia, Japan, India, and the Philippines. The distribution of B. timori is limited to the two islands of Timor, an island of Indonesia. The organism is transmitted via mosquitos included in the genus Anopheles and Mansonia.
PATHOGENESIS AND SPECTRUM OF DISEASE
As in infections with W. bancrofti, two periodic forms exist. The nocturnal form is the most common and located near areas of coastal rice fields, whereas the nonperiodic form is associated with infections in areas near swampy forests. The pathogenesis and spectrum of disease is essentially the same as for W. bancrofti, with the exception that involvement of the genital lymphatic vessels is pre dominantly associated with W. bancrofti. Clinical disease progresses faster following infection with B. malayi than with W. bancrofti. Microfilariae may appear in the blood in as little as 3 to 4 months.
Brugia spp. have been implicated in zoonotic infections of dogs, cats, rabbits, and raccoons worldwide. Cases of human infection have occurred in the United States in the northeastern region. Clinical disease is typically asymptomatic but may present with a tender region in the cervical, axillary, or inguinal region. The lymphatic mass may contain either a live or a dead worm. If the worm is no longer viable, the mass may be surrounded by a granulomatous reaction.
LABORATORY DIAGNOSIS
Definitive diagnosis is generally by the identification of the adult worms in the blood of infected individuals. The adult worms can be distinguished from W. bancrofti morphologically. The B. malayi microfilariae are sheathed and contain 4 to 5 subterminal and 2 terminal nuclei in the tail. B. timori also contains 5 to 8 subterminal and terminal nuclei in the tail, but they are much larger than B. malayi. The B. malayi sheath will stain bright pink with Giemsa, whereas the B. timori sheath does not stain. The microfilariae of B. timori tend to be somewhat longer. High-frequency ultrasound has been useful in identifying adult worms in various locations within the patient, such as lymphatic vessels of the legs, inguinal area (groin or lower abdomen), lymph nodes, and female breasts. Nucleic acid-based methods have been developed but are not widely used in clinical laboratories.
THERAPY
Diethylcarbamazine (DEC) is the treatment of choice for lymphatic filarial parasites including W. bancrofti and Brugia spp. Additionally, ivermectin and albendazole may be used. Death of the microfilarial worms may result in an increased hypersensitive reaction requiring the need for treatment with antihistamines to limit the inflammatory symptoms.
PREVENTION The use of insect repellent is recommended for travellers in areas where the parasites are endemic. DEC has also been used for prophylactic treatment before travel. Vector control studies in combination with mass drug administration of DEC and ivermectin have successfully decreased the population of the arthropod vectors and decreased filarial infection in the human hosts.
LOA LOA
GENERAL CHARACTERISTICS
Loa loa, commonly referred to as the eye worm, is a microfilaria that circulates in the bloodstream and resides in the subcutaneous tissue in the human host. The worm may grow up to 300 μm. EPIDEMIOLOGY The parasite is found within the rain forests of West and Central Africa. The organism is transmitted through a bite of the tabanid fly or deer fly of the genus Chrysops. The female lays her eggs on the leaves of small plants near the water. The larvae feed on small insects and develop in wet soil. The male fly feeds on pollen and the female feeds on a blood meal. PATHOGENESIS AND SPECTRUM OF DISEASE
The organism is often associated with asymptomatic infection. The larvae develop into adult worms in approximately 6 to 12 months, but can persist in the human host for up to 17 years. The infection is typically identified when the adult worm is seen migrating within the sub conjunctiva of the host. Symptoms associated with infection include episodic “calabar swelling,” which are localized areas of transient angioedema in response to the production of parasitic metabolic products. Predominant swelling on the extremities with inflammation of nearby joints and peripheral nerves may occur. Immune mediated encephalopathy, nephropathy, and cardiomyopathy may occur.
LABORATORY DIAGNOSIS Infections with Loa loa may be asymptomatic for many years before the appearance of microfilariae in the peripheral blood. Therefore, patient diagnosis is often made on the basis of the patient’s clinical symptoms including calabar swelling, eosinophilia, and travel or residency in an endemic area.
Direct Detection
Definitive diagnosis is made by identification of the adult worm from the eye, in tissue or in the peripheral blood. The organism contains a sheath that does not stain with Giemsa. The adult females are larger than the adult males. The nuclei extend to the tail in an irregularly arranged fashion.
Serologic Detection
As with other filarial infections, serologic assays have limited use for diagnosis. A Loa-specific recombinant protein has been used in the development of an enzyme linked immunosorbent assay (ELISA) and has demonstrated improved specificity but limited sensitivity.
Molecular Diagnostics
PCR assays are currently available but are limited to research laboratories.
THERAPY
DEC is the treatment of choice. In heavy infections, inflammation and allergic reactions may occur, requiring the administration of antiinflammatory medications. Allergic responses can result in central nervous system damage, encephalitis, coma, and death.
PREVENTION
Prophylactic treatment with DEC has been used to prevent infection.
ONCHOCERCA VOLVULUS
GENERAL CHARACTERISTICS
Onchocerca volvulus predominantly resides in tissue nodules within the host. Adult worms measure approximately 300 μm long by 5-9 μm wide.
EPIDEMIOLOGY
O. volvulus is found throughout Africa, Central America, and South America. The parasite is transmitted by the black fly, Simulium spp. The black fly lays its eggs in running water where the larvae attach to the rocks. The larvae feed on algae and bacteria. The adults emerge as a flying insect. The females require a blood meal, whereas the males are nectar feeders. The flies feed predominantly during the day.
PATHOGENESIS AND SPECTRUM OF DISEASE
Onchocerciasis, commonly referred to as river blindness, is a result of subcutaneous infection with the parasite. The infections are typically localized to the skin, lymph nodes, and eyes. Skin infections result in pruritus, edema, and erythema. Hypo- or hyperpigmentation can occur following a lengthy infection. Nodules, containing the adult worm, vary in size and are firm and tender. Lymph adenopathy may be found in the inguinal or femoral regions. Enlargement of the lymph node may result in a condition referred to as “hanging groin” that may result in a hernia. Onchocercal eye disease may be seen in moderate to heavy infections. Infections of the eye may lead to serious damage and blindness. Mortality increases in adults that experience blindness and systemic infection.
LABORATORY DIAGNOSIS
Direct Detection
Definitive diagnosis is made from the identification of the adult worm from tissue such as in a nodule or skin snip. Skin samples are placed in physiologic buffered saline for up to 24 hours. Following incubation, the worms will emerge from the tissue and can be visualized microscopically. Occasionally the adult worms may be found in blood or urine following treatment. Microfilariae may also be visible in the cornea of the eye.
The microfilariae lack a sheath. The tail is tapered, appears bent or flexed, and does not include nuclei (Figure 5).
Fig5. Microfilaria of Onchocerca volvulus. (Courtesy Dr. Henry Travers, Sioux Falls, S.D.)
Serologic Detection
Although serologic tests generally lack specificity, recombinant ELISAs using multiple antigens have demonstrated increased sensitivity and specificity for the diagnosis of onchocerciasis.
Molecular Diagnostics
PCR amplification assays have been developed and are currently limited to research laboratories.
THERAPY
Ivermectin is the recommended treatment. However, in Africa, where O. volvulus and L. loa are coendemic, ivermectin treatment is often associated with encephalopa thy in patients with heavy microfilaria infections. Surgical excision of nodules containing adult worms is recommended when they are located on the head. PREVENTION Mosquito control using insecticides in endemic areas has been used to assist in the control of transmission of O. volvulus. In addition, a mass-treatment program with ivermectin is effective in preventing infection.
MANSONELLA SPP. (M. OZZARDI, M. STREPTOCERCA, M. PERSTANS)
GENERAL CHARACTERISTICS
Mansonella spp. are generally not associated with serious infections. The adult worms of all species are very similar in size, ranging from approximately 200-225 μm long and 4-6 μm wide.
EPIDEMIOLOGY
Mansonella spp. are distributed throughout varied geo graphic regions in Africa and South America. M. ozzardi is limited to Central and South America and the Carib bean islands. The parasites are transmitted by biting midges of the genus Culicoides. The female requires a blood meal for the maturation of eggs and typically bites in the early evening or morning hours. Transmission of M. ozzardi has also been associated with bites from the black fly (Simulium amazonicum).
PATHOGENESIS AND SPECTRUM OF DISEASE
M. streptocerca may be found in the skin; however, most infected individuals appear asymptomatic. Patients may present with a pruritic rash and pigmentation changes. In addition, lymphadenitis may occur. M. perstans resides in the pericardial, pleural, and peritoneal cavities. Symptomatic patients present with swelling of the arms or face similar to infection with L. loa. M. ozzardi and M. perstans are found in the blood.
M. perstans and M. ozzardi do not demonstrate periodicity when circulating within the bloodstream. M. ozzardi infections are not well characterized.
LABORATORY DIAGNOSIS
Mansonella spp. microfilariae do not possess sheaths. M. streptocerca and M. perstans tails contain nuclei that extend to the end of the tail. The tail of M. streptocerca is often referred to as a “shepherd’s crook.” M. ozzardi have tails with nuclei that do not extend to the tip.
THERAPY
Ivermectin is effective in the treatment of M. streptocerca and M. ozzardi infections. Treatment of M. perstans infections has not been effective in most cases.
PREVENTION
Prevention relies on the use of insect repellents and adequate clothing.
