Elsevier

Acta Tropica

Volume 85, Issue 2, February 2003, Pages 203-209
Acta Tropica

Review article
Performance characteristics and quality control of community based ultrasound surveys for cystic and alveolar echinococcosis

https://doi.org/10.1016/S0001-706X(02)00224-3Get rights and content

Abstract

The probability of disease given the results of a test, is called the predictive value of the test. The predictive value of a test is not a property of the test itself but will vary according to the prevalence of the disease in the studied population. The positive predictive value (PPV) is the probability that the subject tested has the disease given that a positive result is obtained. The negative predictive value (NPV) is the probability that the subject tested is normal given that a negative result is obtained. As the prevalence of a disease in a population approaches zero so does the PPV and most of the positive cases will be ‘false positives’. Conversely the NPV will be very high at low prevalences and there will be few ‘false negative’ results. The sensitivity and specificity of a test are properties of the test and do not vary with prevalence. The higher the sensitivity and specificity of a particular test the greater the predictive values will be at any given prevalence of the disease. Ultrasound (US) is increasingly used for detecting lesions due to cystic and alveolar echinococcosis (CE and AE) and portable US scanners facilitate community based mass screening surveys in remote rural communities. Screening is justified with AE and CE in endemic areas as diagnosis at an early stage can lead to a better prognosis following treatment. The sensitivity and specificity of US has been reported to be between 88–98% and 95–100% respectively for CE and the sensitivity is a little higher for AE. Both species have pathognomonic signs on US and the technique is considered to be the ‘gold standard’ although it is still an imperfect test. Clinical, laboratory and epidemiological data also play an important role in the diagnosis of CE and AE. US results where possible, should be evaluated in relation to these findings. Suspected CE and AE images, may benefit from the use of other imaging techniques such as magnetic resonance imaging, computerised tomography and in the case of AE angiography or cholangiography. Immunological tests or molecular biological techniques also provide a useful back up, especially for AE. As sensitivity and specificity are properties of the US diagnostic test they should not vary if the case mix reported in different studies remains the same. The use of the WHO standardised US classifications for CE and AE should be used so that the properties of the test are standardised. Quality control of field based studies will depend on geographical variations in the case mix and the relative proportions of cyst types without pathognomonic signs. The latter will have the most bearing on variations in specificity, as would the use of different classifications. Inter- and intra-observer variability and differences in prevalence will affect the performance of US in different endemic settings. Community based surveys must adhere to the highest ethical standards and the outcome of surveys should result in appropriate treatment and follow-up strategies for all infected individuals and suspected cases found during the surveys.

Introduction

Cystic and alveolar echinococcosis (CE and AE) are due to infection with the metacestode stage of Echinococcus granulosus and Echinococcus multilocularis respectively. AE is limited to the northern hemisphere whilst CE has a global distribution. Both species are most prevalent amongst low socio-economic groups where safe piped water is generally unavailable, hygiene is poor and where hospital, veterinary and educational facilities are lacking or of a low standard. Diagnostic and treatment facilities are therefore often poor and access to appropriate drugs and treatment limited (Macpherson, 1994). This introduces a diagnostic access bias and only obvious, severe and the wealthy few seek diagnosis with the majority of infections remaining undiagnosed. This is compounded by the fact that early and most long-term infections are completely asymptomatic and clinical diagnosis is difficult and not normally possible. In such areas the prevalence of CE and AE is underestimated and the public health importance of these zoonoses remains unappreciated. Since the mid 1980's (for CE) (WHO, in press) and early 1990's (for AE) (Craig et al., 1992) mass community based surveys using portable ultrasound (US) scanners have been conducted in many remote, rural areas of the world, including Tunisia (Mlika et al., 1986, Bchir et al., 1987, Bchir et al., 1991) Libya (Shambesh et al., 1992, Shambesh et al., 1999), East Africa (including Tanzania, Kenya, Sudan, Ethiopia) (Macpherson et al., 1987, Macpherson et al., 1989), Argentina (Frider et al., 1985, Frider et al., 1988, Saint Martin et al., 1988) Uruguay (Perdomo et al., 1997) and China (Wang et al., 1991, Chai, 1992, Jiang, 1991). Community based screening surveys for AE have been conducted in China (Craig et al., 1992, Bartholomot et al., 2002). Such surveys have demonstrated, for the first time, the true extent of the disease in these low socio-economic areas. Screening is justified and desirable in endemic areas as diagnosis at an early stage of infection can lead to a better prognosis following treatment. Standardised US classifications have recently been developed for CE (WHO, in press) and by the European Network for the Concerted Surveillance for AE (Pawlowski et al., 2001). These classifications should be used in conjunction with the recommended treatment protocols for different cyst types (Pawlowski et al., 2001), thus minimising potential ethical complications. The increasing use of US surveys (WHO, in press) in disparate geographical areas, where the case mix and prevalence of the parasite species varies, prompts this examination of the basic properties of US as a diagnostic test.

Section snippets

Ultrasound as a diagnostic test for CE and AE

US has been increasingly used, since the late 1970's, for detecting pathological lesions due to a number of parasitic infections including CE and more recently AE (Macpherson, 1992). Each diagnostic test per se is inexpensive (the equipment, however, is expensive), non-invasive, painless, gives instant results which can be easily recorded by a variety of means and is well accepted by communities. Today, US is considered to be the best generally available diagnostic test and is the ‘gold

Pathognomic signs

The prevalence of various space occupying lesions will vary in different parts of the world providing unique challenges to the interpretation of CE and AE lesions. Complicating space occupying lesions may include: simple cysts, biliary cysts, polycystic liver disease, haemangiomas which may be single or multiple, pyogenic abscesses, subphrenic abscesses, amoebic liver abscesses due to Entamoeba histolytica, hydatid cysts due to Echinococcus spp. including CE and AE and those due to E. vogeli,

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