A systematic review of the frequency of neurocyticercosis with a focus on people with epilepsy

Abstract

Background: The objective of this study is to conduct a systematic review of studies reporting the frequency of neurocysticercosis (NCC) worldwide.

Methods/principal findings: PubMed, Commonwealth Agricultural Bureau (CAB) abstracts and 23 international databases were systematically searched for articles published from January 1, 1990 to June 1, 2008. Articles were evaluated for inclusion by at least two researchers focusing on study design and methods. Data were extracted independently using standardized forms. A random-effects binomial model was used to estimate the proportion of NCC among people with epilepsy (PWE). Overall, 565 articles were retrieved and 290 (51%) selected for further analysis. After a second analytic phase, only 4.5% of articles, all of which used neuroimaging for the diagnosis of NCC, were reviewed. Only two studies, both from the US, estimated an incidence rate of NCC using hospital discharge data. The prevalence of NCC in a random sample of village residents was reported from one study where 9.1% of the population harboured brain lesions of NCC. The proportion of NCC among different study populations varied widely. However, the proportion of NCC in PWE was a lot more consistent. The pooled estimate for this population was 29.0% (95%CI: 22.9%-35.5%). These results were not sensitive to the inclusion or exclusion of any particular study.

Conclusion/significance: Only one study has estimated the prevalence of NCC in a random sample of all residents. Hence, the prevalence of NCC worldwide remains unknown. However, the pooled estimate for the proportion of NCC among PWE was very robust and could be used, in conjunction with estimates of the prevalence and incidence of epilepsy, to estimate this component of the burden of NCC in endemic areas. The previously recommended guidelines for the diagnostic process and for declaring NCC an international reportable disease would improve the knowledge on the global frequency of NCC.

Figure 1. Life cycle of Taenia solium cysticercosis (source: CDC-DPDx).

Cysticercosis is an infection of both humans and pigs with the larval stages of the parasitic cestode, Taenia solium. This infection is caused by ingestion of eggs shed in the feces of a human tapeworm carrier (1). Pigs and humans become infected by ingesting eggs or gravid proglottids (2), (7). Humans are infected either by ingestion of food contaminated with feces, or by autoinfection. In the latter case, a human infected with adult T. solium can ingest eggs produced by that tapeworm, either through fecal contamination or, possibly, from proglottids carried into the stomach by reverse peristalsis. Once eggs are ingested, oncospheres hatch in the intestine (3), (8) invade the intestinal wall, and migrate to striated muscles, as well as the brain, liver, and other tissues, where they develop into cysticerci (9). In humans, cysts can cause serious sequellae if they localize in the brain, resulting in neurocysticercosis. The parasite life cycle is completed, resulting in human tapeworm infection, when humans ingest undercooked pork containing cysticerci (4). Cysts evaginate and attach to the small intestine by their scolex (5). Adult tapeworms develop, (up to 2 to 7 m in length and produce less than 1000 proglottids, each with approximately 50,000 eggs) and reside in the small intestine for years (6). (This life cycle is available online at URL: http://www.dpd.cdc.gov/dpdx/HTML/Cysticercosis.htm).

Figure 2. Flowchart describing the number of papers remaining at different phases of the study.

Figure 3. Distribution of documents identified during a systematic search of the literature from 1990 to 2008 on the frequency of neurocysticercosis which were included in the three phases of the review.

a) Phase I (n = 565), b) Phase II (n = 290) and c) Phase III (n = 26).

Figure 4. Forest plots of the proportion of NCC (95% CI) in people with epilepsy from 12 studies reporting from cases in all age groups.

*Indicates studies among people with epilepsy and seizures. ** Indicates studies among people with active epilepsy only.

Figure 5. Forest plots of the proportion of NCC (95% CI) among people with epilepsy in children and adults.

a) people aged between 0 and 19 years old and b) aged 20 years old or more. *Indicates studies among people with epilepsy and seizures. ** Indicates studies among people with active epilepsy only.