Diagnosis of Schistosomiasis
For a rapid and inexpensive epidemiological assessment of S. haematobium infection to identify high-risk communities in need of treatment, questionnaires have been proven to be a valuable tool.[147,148] The proportion of schoolchildren reporting to have had blood in urine (macrohematuria) or to have had schistosomiasis in the past 2–4 weeks showed a correlation with the S. haematobium prevalence in a multicountry study.[149,150]
Reagent strips to assess microhematuria in urine are also considered as good diagnostic indicator for heavy S. haematobium infections in the community and can be used for the identification of high-transmission areas.[151,152] Recent work has shown that microhematuria, identified by reagent strips, is still a useful indicator of S. haematobium in a setting with a history of two decades of preventive chemotherapy. Hematuria levels decrease after treatment against schistosomiasis, and hence reagent strips can also be applied as indirect measure to assess drug efficacy, with the most appropriate assessment time ~6 weeks after treatment.
For the identification and mapping of S. mansoni-endemic areas, the recently developed point-of-care circulating cathodic antigen (POC-CCA) dipstick or cassette test in urine is recommended as noninvasive, sensitive and rapid diagnostic tool.[93,154] A recent multicountry study revealed that a single POC-CCA, applied on urine samples, is more sensitive than multiple Kato–Katz thick smears using stool samples. The POC-CCA is particularly useful in low-infection intensity settings, without showing cross-reactivity with S. haematobium infections.[154,155] Due to its rapid test cassette format, it is easy to apply and a single person can, after a short introduction and training, perform hundreds of tests per day without the need of an equipped laboratory. The POC-CCA assay is therefore a promising tool for rapid mapping, large-scale epidemiological surveys and individual patient management in remote health facilities without trained laboratory technicians or sophisticated equipment. The cost of the POC-CCA assay (above US$ 1.00 per test) is an issue though.
The color grading on the hematuria reagent strips or the color intensity on the POC-CCA dipstick and cassette test band are correlated with the number of eggs excreted in urine or stool,[23,155,156] and hence allows a semiquantification of the intensity of infection. Although reagent strips for hematuria assessment have a color chart that helps to classify the intensity of hematuria, the POC-CCA tests currently lack this useful grading option and the color intensity of the test band has to be interpreted by the reader.
Of note, the intensity of an infection is linked with morbidity. Hence, diagnostic methods allowing a quantification of the intensity help to assess the level of morbidity. Schistosoma haematobium infection is quantified by means of filtering 10 ml of a vigorously shaken mid-day urine sample through a polycarbonate filter and by subsequently counting the eggs trapped on the filter using microscopy. An infection is considered as heavy, if 50 or more eggs per 10 ml of urine are counted on the filter. The current cutoff to differentiate between low and heavy infection intensities was only set a decade ago, whereas studies conducted in the 1980s and 1990s used considerably higher egg count thresholds.
The method most widely used for the diagnosis and quantification of S. mansoni and S. japonicum infections in epidemiological studies is the Kato–Katz technique.[161–163] A thick smear containing 41.7 mg of filtered stool is examined under a microscope. Eggs from S. mansoni, S. japonicum and other intestinal helminths are counted and, after multiplication with a factor of 24, reveal an estimate of eggs per 1 g of stool (EPG). Thresholds for moderate and heavy S. mansoni and S. japonicum infection intensities are 100 and 400 EPG, respectively. The recently developed FLOTAC technique allows examination of up to 1 g of stool. The FLOTAC technique is based on the flotation of helminth eggs by using a flotation solution with a specific gravity. This method requires more sophisticated laboratory equipment than the Kato–Katz technique, such as a centrifuge and different chemicals, and is relatively low throughput. While Kato–Katz thick smears are prepared from fresh stool samples, and require rapid work-up, FLOTAC can be done on fixed stool samples many weeks after stool collection. A recent study in Côte d'Ivoire compared the accuracy of Kato–Katz and FLOTAC for S. mansoni diagnosis and concluded that FLOTAC is as sensitive as Kato–Katz, but egg counts using FLOTAC are considerably lower compared with Kato–Katz. The influence of the diagnostic accuracy of Kato–Katz and FLOTAC for assessing drug efficacy against soil-transmitted helminth infections has been studied[166,167] and needs to be done for S. mansoni. Additional direct parasitological methods for the detection of Schistosoma infections are summarized in Table 3.
Detection of Schistosoma spp. DNA via polymerase chain reaction (PCR) is reported to have a high sensitivity and specificity, particularly in low-transmission settings. However, the PCR cycle thresholds are correlated with microscopic egg counts for both S. haematobium and S. mansoni, and in view of a decrease in sensitivity when infection intensities drop, applicability of PCR in areas where people have mostly light-intensity infections must be promoted with care. The advantages of PCR are that parasitic DNA can be extracted from serum, plasma, feces and urine,[169,171–173] with particularly the latter being a patient- and technician-friendly noninvasive approach; samples can be frozen or stored on filter papers without the need for direct processing; there is potential for a high throughput of patients (i.e., one well-trained technician can extract DNA from at least 96 urine samples and process all within 1 day); and the isolated DNA can be simultaneously screened for other infections.[168,169,174] Disadvantages of a PCR approach are that the methodology requires highly skilled laboratory personnel and well-equipped laboratory infrastructure. Costs for a single test are therefore considerably higher than for more conventional diagnostic assays. The potential of the application of PCR in resource-constrained countries is therefore limited. Recent publications identified the loop-mediated isothermal amplification (LAMP) of S. japonicum DNA in human sera as more sensitive than PCR and highlighted that due to its lower price and ease of application it might be a valuable tool for field diagnosis and disease surveillance in schistosomiasis- endemic areas. The LAMP assay is also used for the detection of S. haematobium and S. mansoni DNA in intermediate host snails, and considered as an easily applicable tool, which could be used in low-technology parasitology laboratories in areas where interruption of schistosomiasis transmission or elimination of the diseases needs verification.[176,177]
Although antibody-based detection assays have a limited applicability for confirmation of cases and success of treatment, because they cannot readily distinguish past from present infections, they are very useful for the purpose of confirming interruption of transmission, because they are able to show the contact of people with Schistosoma parasites. For example, in the People's Republic of China, where integrated schistosomiasis control efforts reduced S. japonicum infections to very low levels, indirect immunodiagnostic assays such as the circumoval precipitin test, dipstick dye immunoassay, indirect hemagglutination assay or enzyme-linked immunosorbent assay are applied for population screenings to detect potential cases, which are then confirmed by subsequent stool examination.[178–180]
A promising diagnostic approach is the detection of a Schistosoma worm antigen in serum or urine applying the circulating anodic antigen (CAA) test procedure via the up converting phosphor (UCP) technology lateral flow assay.[181,182] The CAA test is highly sensitive and able to diagnose light infections and might, if designed in a high-throughput rapid test format, be a suitable tool for the verification of schistosomiasis transmission interruption and surveillance in formerly endemic areas. The CAA–UCP technology needs validation in endemic settings, particularly in areas where prevalence and intensities of infections are very low.
Additional new diagnostic paradigms, which still need further evaluation are, for example, a modified version of the miracidium hatching test, which can be used for the diagnosis of any schistosome species,[183,184] or the saline gradient technique and the Mini-FLOTAC for the detection of S. mansoni or S. japonicum.[186,187] Moreover, new rapid diagnostic tests (RDTs) have been developed that are visualizing human IgG attached to S. haematobium eggs in filtered urine or detecting antischistosome antibodies in fingerprick blood samples that are binding to S. mansoni cercarial transformation fluid suitable for the detection of S. haematobium and S. mansoni.[189–191] A proof of concept for mobile phone microscopy has recently been established for soil-transmitted helminth infection diagnosis and hence warrants validation for schistosomiasis diagnosis. Other mobile diagnostic tools have been assessed and their strengths and limitations discussed. A sampling strategy well established in the veterinary field but only recently adapted to human parasitology is the pooling of stool or urine samples for a rapid assessment of infection intensity and drug efficacy.[187,194]
The typical age prevalence and intensity curves for Schistosoma infections rise from early age and peak in SAC. Since most direct parasitological methods are not sensitive enough to detect very light infections, they have a limited suitability to detect schistosomiasis in young children. Moreover, stool samples from infants and PSAC are difficult to obtain and are often not of a consistency that allows adequate examination with, for example, the Kato–Katz technique. Therefore, serological examinations using either antibody detection assays to determine the contact of the child with the parasite, or the POC-CCA or CAA-UCP tests that can be applied on urine samples might reveal more reliable results on Schistosoma infection in children. The highlighted insensitivity of direct parasitological methods can generally result in an underestimation of prevalence or overestimation of treatment efficacy, which have important ramifications on treatment decisions in helminth control programs. It is crucial to adapt the diagnostic approach to the epidemiological situation in the area, the current stage of control and population under investigation. Hence, while in population subgroups with high-intensity infection potential (i.e., SAC in a high-transmission area not yet targeted by schistosomiasis control), simple methods such as the Kato–Katz technique for S. mansoni or S. japonicum, or the urine filtration for S. haematobium, will reveal an accurate picture of the prevalence and transmission potential, other more sensitive methods are needed to detect light infections with high sensitivity. Of note, light infections might not only occur in PSAC, but also in individuals with single exposures to the infective agent such as returning travelers or tourists,[3,196] or among individuals who developed partial immunity by regular and frequent exposure in a high-transmission area. Mostly, light infection levels are also expected in populations that are periodically treated against schistosomiasis in the current era of intensified control. Hence, over the course of helminth control programs, it is important to change the diagnostic approaches from simple low-cost methods applied in high-transmission settings to highly sensitive and specific methods that identify infections in areas targeted for transmission control, surveillance or elimination.[195,198] It is clear that, particularly for the latter three points, new diagnostic tools and strategies, supported with standard protocols need to be developed.[187,199] The ultimate goal is to get away from testing multiple samples, increasing the amount or volume of tested samples or combining the results of different diagnostic methods, which are approaches currently recommended in the absence of tools that meet predefined target product profiles.[200,201]
Expert Rev Anti Infect Ther. 2013;11(11):1237-1258. © 2013 Expert Reviews Ltd.