Human Alveolar Echinococcosis-A Neglected Zoonotic Disease Requiring Urgent Attention

Abstract

Alveolar echinococcosis (AE) in humans is caused by the larval (metacestode) stage of Echinococcus multilocularis, commonly known as the 'fox tapeworm'. This disease predominantly targets the liver and has an invasive growth pattern, allowing it to spread to adjacent and distant tissues. Due to its gradual progression and tumour-like characteristics, early diagnosis and prompt intervention are crucial, particularly as there are currently no highly effective vaccines or chemotherapeutics against AE. Current estimates suggest that ~10,500 new infections occur annually worldwide; however, more research is required to refine the prevalence and incidence data for both human and animal hosts in endemic areas of the world. This article discusses the biology of E. multilocularis, outlines aspects of the pathogenesis, diagnosis, treatment, and management of AE, reviews its global distribution, annual incidence, and prevalence, highlights the role of molecular parasitology in advancing therapeutic strategies, and presents recommendations for improving the prevention and control of AE in human populations.

Keywords: Echinococcus multilocularis; alveolar echinococcosis; biology; diagnosis; humans; incidence; pathogenesis; prevalence; treatment.

Figure 1

Life cycle of Echinococcus multilocularis. In its adult form, Echinococcus multilocularis—measuring about 1.2 to 4.5 mm—lives in the small intestine of its definitive host (1). Mature (gravid) proglottids release eggs that exit the host via faeces and are immediately infective (2). Once a suitable intermediate host ingests these eggs, they hatch in the small intestine, releasing six-hooked oncospheres (3). These oncospheres break through the intestinal wall and travel through the bloodstream to various organs—most often the liver. There, the oncospheres develop into thin-walled (alveolar) cysts that are multilocular (having multiple chambers) and expand outward by budding (4). Each of these cysts can generate numerous protoscoleces. A definitive (canid) host becomes infected when it consumes an intermediate host (or tissues thereof) that harbours these cysts and protoscoleces (5). After being swallowed, the protoscoleces emerge, attach to the lining of the small intestine (6) and mature into adults over a period of 32 to 80 days. Humans are considered accidental intermediate hosts. Infection occurs when a person ingests the eggs, which then release oncospheres in the small intestine (3); these oncospheres enter blood vessels and are passively transported via the blood stream primarily to the liver where they form cysts, causing alveolar echinococcosis (AE) (4). If protoscoleces are freed from the cysts, they may spread (metastasize) to other organs, such as the lungs, brain, heart, and/or bones, in a process sometimes referred to as secondary AE. Figure image originates from https://www.cdc.gov/dpdx/echinococcosis/index.html (accessed on 15 January 2025).

Figure 2

Pictorial summary of clinical and surgical aspects of alveolar echinococcosis (AE) in humans. (A) Computed tomography (CT) scan showing AE in the liver, spleen, and extraperitoneal areas. (B) An atypical laparoscopic resection of a lesion caused by AE from the liver dome. (C) AE in the left liver prior to left hemi-hepatectomy.

Figure 3

Estimated number of new cases of human alveolar echinococcosis (AE) annually by region and country. (A) Global distribution of new cases of AE in humans in known endemic regions in the Northern Hemisphere. Globally, the annual number of AE cases is estimated at 10,489 per year (range: 8191–14,409), with the majority of cases in Eastern Asia (predominantly in China), Eastern Europe (mainly in Russia), Central Asia (mainly in Kyrgyzstan), and Western Europe (mainly Switzerland, Germany, and France), as well as ~50 new cases reported annually in other endemic regions; (B) estimated numbers of new cases of human AE annually by country for endemic regions. A logarithmic scale is used for the y-axis to account for differences between high- and low-incidence areas. The highest estimated case numbers are in China and Russia, while some European and North American countries report lower numbers. Data are summarised from ref. [25].