Many surveys exist in the literature and of course the estimates vary according to the sensitivity and specificity of the test used. Before the introduction of the enzyme-linked immunoelectrotransfer blot (EITB), the low sensitivity and specificity of the existing tests made it difficult to assess the extent of infection in endemic regions. Woodhouse et al. (1982) reported an overall 1 % of seropositivity in 20,000 samples from Mexico; with some communities reaching 6 %, the sensitivity and specificity of the test used in that survey—immunoelectrophoresis—were estimated to be around 50 %. Thereafter, some population-based studies used the enzyme-linked immunosorbent assay (ELISA) with varied results. Coker-Vann et al. (1981) reported seroprevalence rates ranging from 0.2 to 8.4 % in asymptomatic individuals living in different regions of Indonesia, which contrasted with the 16 % seroprevalence rate of asymptomatic individuals living around the Wissel Lakes area, where cysticercosis was highly endemic. In the latter, patients with diagnosis of neurocysticercosis had a seroprevalence of 61 %. In another study performed in rural Madagascar, 18 % of healthy individuals versus 36 % of neurocysticercosis patients had a positive ELISA in serum (Michel et al. 1993). Posterior studies, showing the disappointing reliability of the serum ELISA, make these surveys of historical importance only.

Once the highly specific EITB using purified lentil lectin glycoprotein antigens was available, multiple surveys have demonstrated that from 8 to 25 % of “healthy” persons living in endemic villages have antibodies to Taenia solium (García et al. 1991; Sarti et al. 1992). Asymptomatic seropositive individuals mostly present weak reactions to only one to three antibody bands (GP50, GP42-39, and GP24) (Schantz et al. 1994). In field conditions, antibody seroprevalence rates may overestimate the actual prevalence of infection because persons with anticysticercal antibodies resulting from exposure or from past infections are also detected. Nevertheless, antibody prevalence can give a general assessment of the levels of transmission and help to orientate control strategies. At the individual level, detection of specific anticysticercal antibodies in an asymptomatic person has limited clinical use.

More recently, some population-based data using circulating cysticercal antigen detection has become available. The prevalence of circulating antigen ranges from 1 to 22 % (Dorny et al. 2012; Mwanjali et al. 2013; Mwape et al. 2013; Nguekam et al. 2003; Praet et al. 2010). Antibody prevalence is usually much higher than antigen prevalence in these studies. There is no information about the predictive value of a positive antigen test in asymptomatic individuals or in the general population, neither is it known whether the use of a higher cutoff would improve its predictive value for neurocysticercosis. As for community-based antibody surveys, the application of antigen detection in an asymptomatic individual would likely be of no practical use, although its use for early detection of extraparenchymal infections has been suggested (García et al. 2012).

The proportion of asymptomatic individuals with neurocysticercosis detected by computed tomography (CT) in endemic populations ranges from 10 to 25 % (Del Brutto et al. 2005; Fleury et al. 2003; Garcia-Noval et al. 1996, 2001; Medina et al. 2005; Montano et al. 2005; Sánchez et al. 1999) (Table 4.2). A single study in a high-risk population using magnetic resonance imaging (MRI) reported 5 % of people harboring small viable cysts, usually a single one (Prasad et al. 2011)

A closer view to the results showed in Table 4.2 provides useful insights. Field studies using CT consistently demonstrate a significant proportion of villagers harboring one or a few brain calcifications, more rarely multiple calcifications or even more rarely viable cysts. In these same field conditions, most asymptomatic, antibody-positive people show weak reactions. The positive predictive value of a positive antibody test in asymptomatic individuals is around 35 %, and the imaging correlate in such cases is usually one or a few parenchymal brain calcifications. The relative frequency of viable cysts in general population is low. Not all cases of neurocysticercosis are detected by an antibody assay, since a proportion of seronegative villagers will also show calcifications on brain CT (Montano et al. 2005). Overall, approximately 50 % of asymptomatic individuals with parenchymal brain calcifications are EITB negative. A proportion of villagers will have a positive antigen test, apparently fewer than those who are antibody positive. The positive predictive value of a positive antigen test in an asymptomatic individual is not well known and has been reported to be from 14 to 53 % (Mwanjali et al. 2013; Nguekam et al. 2003). The neuroimaging correlation for a positive result in antigen detection is usually that of multiple viable parenchymal brain cysts. It is unclear whether the remaining antigen-positive, neuroimaging-negative individuals may have viable brain cysts not detected by imaging or cysts elsewhere in their body.

Diverse epidemiological studies and clinical series have demonstrated associations between positive tests and a history of intestinal taeniasis, pig raising, having seen cysticercosis in their pigs, and other more generic factors such as living in a rural area and poor sanitation. A history of intestinal taeniasis in the person or in the household is a very strong risk factor. The accuracy of questioning for an antecedent of taeniasis could be improved by showing villagers a proglottid, since patients who had expelled nematodes may assume that the question refers to this antecedent and answer affirmatively. Pig raising is a poorly specific indicator since in many villages more than half of the villagers raise pigs (Diaz et al. 1992; Garcia-Noval et al. 1996; Sarti et al. 1994). Having seen cysticercosis cysts in pigs (or in their own pigs) could provide a more specific marker, but we have not been able to find comparative assessments of the predictive value of these questions. There seems not to be particularly clear differences in the rates of infection be gender. In contrast, the prevalence of antibody-positive individuals in the general population begins to rise between 6 and 10 years of age and increase progressively with higher rates between 20 and 50 years of age (Diaz et al. 1992; Schantz et al. 1994).

Clustering of human and porcine cases around a tapeworm carrier was first described by Sarti-Gutierrez et al. (1988). Using GPS, Lescano et al. (2007) confirmed this and described that a stronger concentrically gradient is found for porcine seroincidence as an indicator of more recent transmission. Seizure cases, however, are not clustered around the tapeworm carrier, likely reflecting a clinically silent period between infection and seizures longer than the average life span of the adult tapeworm (Lescano et al. 2009). Familiar aggregation of cases is likely due to common source exposure, although some sort of genetic predisposition to infection or disease cannot be ruled out. The polymorphism of toll-like receptor 4 has been shown to be associated to symptomatic disease in human neurocysticercosis (Verma et al. 2010), and differences in some HLA determinants between patients and controls were demonstrated long ago in Mexico (Del Brutto et al. 1991).