Cestodes

Taenia saginata is acquired by humans, the only definitive host, through ingestion of undercooked or raw beef containing larval cysts (cysticerci). Cattle are the intermediate host. They become infected after grazing on pasture contaminated by human faeces containing eggs.

Within the human small intestine, the ingested cysticercus evaginates its scolex and attaches to the intestinal mucosa using four muscular suckers. Taenia saginata characteristically lacks hooks on its scolex, an important distinguishing feature from Taenia solium. Over roughly two to three months it matures into an adult worm that can reach several metres in length, composed of hundreds to over a thousand proglottids.

Gravid proglottids, each containing tens of thousands of eggs, detach from the distal end of the worm. They are characteristically capable of active motility and migrate independently out through the anus (sometimes noticed by patients as a crawling sensation), rather than being passed solely with stool. This distinguishing behavioural feature is useful in clinical history-taking.

Taeniasis, the adult intestinal infection, is usually mild or asymptomatic, occasionally producing nonspecific abdominal discomfort. Humans do not develop the larval (cysticercus) tissue disease from Taenia saginata, since humans are not a suitable intermediate host for this species. This is a critical distinction from Taenia solium.

Taenia solium shares a broadly similar basic life cycle to Taenia saginata. Humans acquire adult intestinal infection (taeniasis) by ingesting undercooked pork containing cysticerci. Pigs are the natural intermediate host after ingesting eggs from human faeces.

The scolex of Taenia solium bears both suckers and a crown of hooks (armed scolex), distinguishing it morphologically from the unarmed Taenia saginata.

The critical distinction, and the reason Taenia solium is of far greater medical importance than Taenia saginata, is that humans are also capable of serving as an intermediate host for Taenia solium. This gives rise to a second, entirely distinct and far more serious disease.

Taeniasis refers to adult intestinal worm infection, acquired by eating undercooked pork containing cysticerci. It is generally mild or asymptomatic, in the same manner as Taenia saginata taeniasis. It is of particular importance because a human harbouring an adult Taenia solium worm becomes a source of infective eggs.

Cysticercosis results when a human ingests Taenia solium eggs (rather than larval cysts in meat). The human then acts as an intermediate host, in the same biological role normally occupied by the pig. Eggs can be ingested through:

• faecal-oral transmission from another infected individual
• autoinfection from one's own intestinal adult worm, via reverse peristalsis carrying eggs from the intestine to the stomach
• faecal-oral self-contamination

Ingested eggs hatch in the small intestine, releasing oncospheres that penetrate the intestinal wall and disseminate via the bloodstream to tissues throughout the body, most importantly skeletal muscle, subcutaneous tissue, the eye, and the central nervous system. There they develop into cysticerci (fluid-filled larval cysts containing an invaginated scolex). Cysticerci can remain viable for years before eventually degenerating and calcifying, a process that itself can provoke significant inflammation.

Neurocysticercosis refers specifically to cysticercosis involving the central nervous system. It is the single most important clinical manifestation of Taenia solium infection worldwide and among the leading causes of acquired epilepsy in endemic regions.

Cysts can lodge in:

• brain parenchyma
• the ventricular system, where they can obstruct cerebrospinal fluid flow and cause hydrocephalus
• the subarachnoid space, where a distinctive racemose form (large, multilobulated, non-scolex-bearing cysts) can develop and cause chronic basal meningitis

Clinical presentation depends heavily on cyst location, number, and stage of degeneration. Seizures are the most common presenting feature. They typically occur or intensify as a cyst begins to degenerate, since the associated inflammatory host response, rather than the presence of a viable, immunologically quiescent cyst, is the principal trigger for symptomatic disease.

Other presentations include raised intracranial pressure, hydrocephalus, focal neurological deficits, and, less commonly, cognitive decline or psychiatric symptoms with heavy cerebral cyst burden.

Echinococcus granulosus is the cause of cystic echinococcosis (hydatid disease). Its life cycle is centred on a predator-prey relationship between canids and herbivores. Humans are incidentally drawn into the cycle as an accidental intermediate host.

Dog definitive host. Dogs (and other canids, including wolves and foxes in sylvatic cycles) serve as the definitive host. They harbour small adult tapeworms, only a few millimetres in length and far smaller than Taenia species, attached to the intestinal mucosa. The worms produce eggs passed in canine faeces.

Intermediate hosts. Sheep are the classic and most epidemiologically important natural intermediate host. The cycle is maintained through dogs feeding on the viscera of infected sheep at slaughter, a practice specifically targeted by control programmes. Cattle, goats, and other livestock can also serve this role.

Humans become an accidental intermediate host through ingestion of eggs from contaminated soil, water, or food, or via direct contact with an infected dog's contaminated fur or faeces. No onward transmission occurs from an infected human, who is a dead-end for the parasite.

Hydatid cyst. Following ingestion of eggs, oncospheres hatch in the small intestine, penetrate the intestinal wall, and disseminate via the portal circulation. They most commonly lodge in the liver (the most frequently affected organ) and lungs, though any organ can be involved.

The larval structure that develops, the hydatid cyst, is a complex, slowly enlarging, fluid-filled structure with a distinctive laminated outer membrane and an inner germinal layer. Numerous protoscolices (each capable of developing into an adult worm if ingested by a definitive host) and daughter cysts bud internally from the germinal layer. The cyst can grow slowly over years, often remaining clinically silent until it reaches sufficient size to cause mass effect on the affected organ or its surrounding structures, or until cyst rupture occurs.

Rupture can occur spontaneously, from trauma, or during surgical or percutaneous intervention. It can produce acute anaphylaxis due to the highly antigenic cyst fluid. It can also seed secondary cysts throughout the peritoneal or pleural cavity, depending on rupture site. This is a serious and specifically feared complication that governs careful surgical technique when hydatid cysts are managed operatively.

Echinococcus multilocularis, a related species with a fox-rodent sylvatic cycle, causes alveolar echinococcosis. This is a distinct and more aggressive disease pattern in which the larval mass grows in an infiltrative, tumour-like fashion rather than forming a discrete cyst. It is capable of local invasion and even metastasis-like spread to distant organs, and carries a substantially worse prognosis than cystic echinococcosis if untreated.

Diphyllobothrium latum, the fish tapeworm, is the largest tapeworm infecting humans, potentially reaching over ten metres in length. It is acquired through ingestion of raw or undercooked freshwater fish (or anadromous fish that spend part of their life cycle in freshwater) containing the larval plerocercoid stage.

The life cycle involves two intermediate hosts. A copepod ingests free-swimming coracidium larvae hatched from eggs passed into freshwater in human stool. A fish then ingests the infected copepod, and the larva develops into the infective plerocercoid within the fish's tissue. Larger predatory fish can also become infected by eating smaller infected fish, concentrating larvae further up the food chain.

Adult worms attach to the small intestinal mucosa. Most infections are asymptomatic or produce only mild gastrointestinal symptoms.

The clinically distinctive feature of Diphyllobothrium latum infection is its capacity to cause vitamin B12 deficiency. The worm directly competes with and avidly absorbs dietary vitamin B12 (and to a lesser extent folate) from the intestinal lumen. This occasionally produces megaloblastic anaemia and, in longstanding severe deficiency, neurological manifestations analogous to those of dietary or pernicious vitamin B12 deficiency. This mechanism and consequence is unique among the intestinal cestodes.

Hymenolepis nana, the dwarf tapeworm, is the smallest tapeworm infecting humans and the most common cestode infection worldwide. It is the only medically important tapeworm capable of completing its entire life cycle within a single human host, without requiring an intermediate host or the external environment. This is a direct parallel in concept (though not mechanism) to the autoinfective capacity of Strongyloides among the nematodes.

Infection is acquired by direct ingestion of eggs, via faecal-oral transmission, contaminated food or water, or autoinfection from one's own perianal contamination. The eggs hatch in the small intestine and penetrate the intestinal villi briefly. They develop into a cysticercoid larval stage within the mucosa itself, then re-emerge into the lumen and attach to mature into adult worms. This can be completed entirely within the human intestine.

An indirect cycle involving arthropod intermediate hosts (fleas, grain beetles) also exists and contributes to transmission in some settings.

The capacity for direct, intra-host completion of the life cycle, combined with the potential for internal autoinfection, allows heavy, self-sustaining infection to develop. Internal autoinfection is egg-to-larva-to-adult occurring repeatedly within the same host, analogous in outcome, though not in underlying larval biology, to Strongyloides autoinfection. Heavy infection occurs particularly in children in crowded or poor-hygiene settings.

Most infections are light and asymptomatic. Heavier infection can produce abdominal pain, diarrhoea, and, in children, may contribute to growth and nutritional impairment. Severe complications are uncommon relative to the other cestodes.