Diagnosis

The diagnosis of filarial diseases can be problematic, because these infections require para-sitologic techniques to demonstrate the offending organisms (Figure 18a.4). In addition, satisfactory methods for the definitive diagnosis in amicrofi-laremic states can be difficult. The definitive diagnosis of filariasis can only be made by the demonstration of the offending parasites. Micro-filariae can be found in the blood, hydrocele fluid or, occasionally, in another body fluid. These fluids can be examined microscopically, either directly or, for greater sensitivity, after concentration of the parasites by the passage of fluid through a polycarbonate cylindrical filter (pore size, 3 or by the centrifugation of fluid fixed in 2% formalin (Knott's concentration technique) or 2% formalin/10% Teepol (Dickerson et al., 1990). The blood sample can be stored in this condition for several months at room temperature prior to filtration in the standard fashion. The use of formalin inactivates any blood-borne infectious agents. Speciation of the parasite can be undertaken by morphologic examination (Figure 18a.4).

Occasionally, diagnostic aspiration for cyto-logic examination (e.g. hydrocele or breast lump) results in the identification of microfilariae in the aspirate (Bapat and Pandit, 1992; Varghese et al., 1996; Kapila and Verma, 1996).

Bancrofti Malayi

Fig. 18a.4 Differential characterizations of microfilariae. (A) Brugia malayi. (B) Brugia timori. (C) Wuchereria bancrofti. (D) Onchocerca volvulus. (E) Mansonella streptocerca. (F) Loa loa. (G) Mansonella perstans. (H) Mansonella ozzardi. Redrawn after Craig and Faust (1964). Clinical Parasitology, 7th edn, Lea and Febiger: Philadelphia, PA; with permission

Fig. 18a.4 Differential characterizations of microfilariae. (A) Brugia malayi. (B) Brugia timori. (C) Wuchereria bancrofti. (D) Onchocerca volvulus. (E) Mansonella streptocerca. (F) Loa loa. (G) Mansonella perstans. (H) Mansonella ozzardi. Redrawn after Craig and Faust (1964). Clinical Parasitology, 7th edn, Lea and Febiger: Philadelphia, PA; with permission

Parasitologic Diagnosis by Identification of the Adult Parasite

Surgical removal of a nodule or mass for diagnostic purposes may lead to the discovery of a nematode within lymphatic tissue. Speciation of the parasite can be undertaken by morphologic examination (Ash and Orihel, 1987) or by PCR (see below).

Detection of Circulating Parasite Antigen

Assays for circulating antigens of W. bancrofti permit the diagnosis of microfilaremic and cryptic (amicrofilaremic) infection (More and Copeman, 1990; Lammie et al., 1994; Chanteau et al., 1994; Rocha et al., 1996; Weil et al., 1997). There are currently two commercially available tests, one in an ELISA fomat (Trop-Ag W. bancrofti, manufactured by JCU Tropical Biotechnology Pty. Ltd, Townsville, Queensland, Australia) and the other a rapid-format card test (marketed by Binax, Portland, ME, USA). Both assays have reported sensitivities in the range 96-100% and specificities that approach 100% (More and Copeman, 1990; Lammie et al., 1994; Chanteau et al., 1994; Rocha et al., 1996; Weil et al., 1997). There are currently no tests for circulating antigens in brugian filariasis.

Serodiagnosis Using Parasite Extract

The development of serodiagnostic assays of sufficient sensitivity and specificity for routine use has proved problematic (Ambroise-Thomas, 1974; Voller and deSavigny, 1981; Speiser, 1980), primarily because of their poor specificity. Extensive cross-reactivity is found in the sera of individuals infected with closely related helminth parasites and even certain protozoal parasites (Maizels et al., 1985; Lal and Ottesen, 1988). Further, as is the case for serodiagnosis of most infectious diseases, it is difficult to differentiate previous infection or exposure to the parasite

(aborted infection) from current active infection. Indeed, most residents of filariasis-endemic regions are antibody-positive (Ottesen et al., 1982). Nevertheless, such serologic assays have a definite place in diagnosis, as a negative assay result effectively exludes past or present infection.

The prominent role of antifilarial antibodies of the IgG4 subclass in active filarial infection (Ottesen et al., 1985) has led to the development of serodiagnostic assays based on antibodies of this subclass. Antifilarial IgG4 antibodies have improved specificity, but positive assays may still be seen in uninfected individuals living in endemic areas (Chanteau et al., 1994) and in those infected with other filarial species (e.g. onchocerciasis, loiasis, mansonelliasis).

Molecular Diagnostics

PCR-based assays for DNA of W. bancrofti and B. malayi in blood have also been developed. In a number of studies evaluating PCR-based diagnosis, the method is of equivalent or greater sensitivity compared with parasitologic methods, detecting patent infection in almost all infected subjects. In addition, the technique is able to detect cryptic infection (amicrofilaremic, circulating antigen-positive infection) in some subjects (Abbasi et al., 1996; Lizotte et al., 1994; Williams et al., 1996), and parasite DNA can be detected in the saliva of microfilaremic individuals (Abbasi et al., 1996). The technique is, in addition, useful for the speciation of parasite material removed at surgery, especially when morphologic diagnosis is not possible.

Imaging Studies

In cases of suspected lymphatic filariasis, examination of the scrotum (Amaral et al., 1994) or female breast using high-frequency ultrasound in conjunction with Doppler techniques may result in the identification of motile adult worms within dilated lymphatics. Worms may be visualized in the lymphatics of the spermatic cord in up to 80% of infected men (Noroes et al., 1996b; Dreyer et al., 1995a). Live adult worms have a distinctive pattern of movement within the lymphatic vessels (termed the 'filaria dance sign') (Amaral et al., 1994). Worms appear to remain in a constant location, in so-called 'nests' (Noroes et al., 1996b) within lymphatic vessels. This technique may be useful to monitor the success of antifilarial chemotherapy, by observing for the disappearance of the dance sign (Dreyer et al., 1995a,b; Noroes et al., 1997).

Evaluation of lymphatic function by lympho-scintigraphy can provide useful information in lymphatic filariasis. This technique involves injection of 99Tc-radiolabeled albumin or dextran into the dermis, followed by sequential imaging with a gamma camera (Suresh et al., 1997; Dissanyake et al., 1995; Freedman et al., 1994). Radionuclide lymphoscintographic imaging of the limbs reliably demonstrates widespread lymphatic abnormalities in both asymptomatic and microfilaremic persons, as well as in those with clinical manifestations of lymphatic pathology (Dreyer and Noroes, 1997). While of potential utility in the delineation of anatomical changes associated with infection, lymphoscinti-graphy is unlikely to assume first place in the diagnostic evaluation of individuals with suspected infection.

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