**3. Cysticerci formation**

NCC is characterized by the formation of cysticerci, which are larval stages of the pork tapeworm, *T. solium*, within the central nervous system. These cysticerci can be found in various locations, such as the brain parenchyma, ventricles, or subarachnoid spaces [3, 4, 17, 18]. Cysticercosis formation is a crucial aspect of NCC, the CNS manifestation of the parasitic infection caused by the larval stage of the pork tapeworm, *T. solium.*

#### **3.1 Characteristics of cysticerci**

Cysticerci are the larval stage of *T. solium* and exhibit distinct features [3, 4, 17–19].

#### *3.1.1 Fluid-filled cysts*

Cysticerci are bladder-like structures filled with clear or turbid fluid. The cysts vary in size, ranging from a few millimeters to several centimeters in diameter. Their size can influence the clinical presentation and potential complications [17–19].

#### *3.1.2 Nodular stage*

Over time, the cysticerci undergo a process of degeneration and transition into a nodular stage. In this stage, the cysts become more inflammatory and calcify, developing a thick fibrous capsule around the scolex. The nodular stage is associated with increased inflammatory response and clinical manifestations [3, 4, 18, 19].

#### *3.1.3 Vesicular stage*

In the early vesicular stage, the cysticerci are small and surrounded by a translucent membrane. These cysts are often asymptomatic and may go unnoticed. Histologically,

#### *Neurocysticercosis: An Overview of Pathology and Pathogenesis DOI: http://dx.doi.org/10.5772/intechopen.113237*

the cysticerci in this stage are characterized by a thin eosinophilic membrane, a fluidfilled cavity, and a scolex (tapeworm's head) with hooklets. Initially, the cysticerci are in a vesicular stage, characterized by a thin-walled cyst containing a small fluid-filled bladder with an invaginated scolex (tapeworm's head). This stage is less inflammatory and may be asymptomatic [3, 4, 17–19].

In NCC, irregularly shaped large fluid cysts are a hallmark of NCC and are particularly common in areas of high endemicity. These cysts are typically spherical to oval in shape and vary in size, usually several centimeters in diameter. Their appearance in neuroimaging such as computed tomography (CT) or magnetic resonance imaging (MRI) is characterized by a well-defined, thin-walled, translucent structure filled with clear or slightly turbid fluid.

Cysts without scolex or protoscolex are also present. A distinctive feature of these cysts is the absence of a scolex or protoscolex within the cystic lümen [20–22]. Unlike other stages of cysticercosis where the scolex is identifiable and plays a critical role in attachment and growth, these large liquid cysts lack this vital structure. Although the cyst wall is thin, it can elicit an immune response in the host. Inflammation surrounding the cyst is a common feature and leads to the formation of a pericystic-enhancing edge on neuroimaging ([23, 24], Sotelo et al. 1985). This enhancing edge represents an inflammatory reaction to the presence of the cyst and is often used as a diagnostic criterion for NCC [21, 23, 25–27].

Large irregularly shaped liquid cysts are particularly common in hyperendemic areas of NCC, such as Mexico and Argentina [21, 24, 27]. The high prevalence in these regions is often attributed to factors such as pork consumption, sanitation, and cultural practices that influence the transmission of *T. solium*, the causative agent of NCC. Understanding the characteristics of irregularly shaped large liquid cysts in NCC is crucial for accurate diagnosis and management ([23–25], Sotelo et al. 1985). Treatment strategies may include antiparasitic drugs such as albendazole or praziquantel in combination with corticosteroids to reduce inflammation ([21, 27], Sotelo et al. 1985).

Consequently, irregularly shaped large liquid cysts without scolex are a hallmark of NCC in hyperendemic areas. These cysts present unique challenges in diagnosis and treatment due to their potential to elicit inflammatory responses and neurological symptoms. Advanced imaging techniques and a deep understanding of the epidemiology of the disease are crucial to addressing this particular presentation of NCC in clinical practice and research in regions such as Mexico and Argentina.

### *3.1.4 Colloidal stage*

As the cysticercus matures, it enters the colloidal stage. The cysticercus undergoes degenerative changes during this stage, leading to a granulomatous reaction around the parasite. The granuloma comprises a central necrotic area, surrounding edema, a fibrous capsule, and an outer layer of inflammatory cells. The colloidal stage is commonly associated with clinical symptoms such as seizures, headaches, and focal neurological deficits [4, 17–19].

#### *3.1.5 Calcified stage*

Over time, some cysticerci may undergo calcification, resulting in the calcified stage. Calcified lesions can be detected through neuroimaging studies like CT or MRI. Calcification represents a host response aimed at containing the infection. In the calcified stage, the cysticercus is no longer viable [3, 4, 18, 19].
