Cysticercose

Donald P McManus, Molecular Parasitology Unit, Tropical Health Program, Australian Centre for International and Tropical Health and Nutrition, The Queensland Institute of Medical Research, Brisbane, Queensland, Australia

Copyright © 1998 Elsevier Ltd. All Rights Reserved.

Characteristics of the organisms and their antigens

The term 'cysticercosis' is a general one applied to the occurrence of cystic (larval) stages of cestodes (tapeworm parasites) in humans and domestic and laboratory animals. Cysticercosis due to the cys-ticercus (known by veterinarians as Cysticercus cellulosae) of the pig tapeworm, Taenia solium, is the most common simple cystic form in humans. The life cycle of T. solium is depicted in Figure 1. Pigs act as the intermediate host and the adult worms occur in the human gut. In pigs the cysticercus may lodge anywhere in the body but shows predilection for muscle. When undercooked or raw pork infected with cysticerci (measly pork) is eaten by humans, the scolex (activated by bile) evaginates and becomes attached to the duodenum, the bladder of the cysticercus is digested and the worm differentiates into an adult. The fully developed adult (up to several meters long) sheds single segments or proglottids (proglottis = single segment) each containing 50 000 eggs and these are voided in the feces. On ingestion by a pig, the egg hatches in the gut to release the oncosphere which bores its way through the mucosa and into the general circulation, finally localizing and developing in body muscle.

Neurocysticercosis

The eggs of T. solium, when ingested by humans, can also develop into cysticerci. Their presence causes a serious and often fatal disease due to the frequent involvement of the nervous system (neurocysticercosis), resulting in 'cysticercus epilepsy'. T. solium occurs in most pork-eating countries where sanitary conditions are primitive and scavenging pigs have access to human fecal material. Neurocysticercosis is now the most prevalent disease of the central nervous system in the USA, although most of the patients appear to have originated from Mexico or Latin America. The disease is also a major health hazard, and has been reported from Africa, China, Japan, Indonesia, Poland, Italy, Spain and Central and South America but has virtually disappeared from most other areas of Europe.

Other cysticercoses

Other important cysticercoses are caused by T. saginata in cattle (adult in humans), T. oi'is. T. bydatigena and T. multiceps in sheep ladults in dogs). Another human Taenia has recently been identified in South-East Asia; new genetic evidence and earlier biological studies suggest this 'Asian Taenia'' is a distinct entity, but that it is closely related to, and should best be regarded taxonomically as, a subspecies or strain of T. saginata. Valuable experimental models for immunologic studies of cysticercosis include T. taeniaeformis in rats, 7 . cras-siceps in mice and T. pisiformis in rabbits. This group of worms collectively belong to the family Taeniidae (the taeniids) but the account given here has especial emphasis on T. solium because of its medical importance. Hydatid disease, caused by dog tapeworms of the genus Echinococcus is, strictly-speaking, also a cysticercosis.

Antigens

Few antigens have been characterized from tapeworms and much work has been aimed at providing reagents suitable for specific and sensitive immuno-diagnosis. Antigen B (95-105 kDa) (paramyosin, see below), an immunodominant antigen of T. solium, believed previously to be specific, has now been shown to cross-react with antibodies produced to a number of other human helminth infections. Two low molecular weight antigens (26 kDa and 8 kDa) have been identified as specific for T. solium and appear to provide requisite diagnostic sensitivity. Most encouragingly, seven major lentil lectin-bind-ing glycoprotein antigens (13-50 kDa) have been identified in T. solium and these have been applied very successfully in a highly specific and sensitive western blot assay for diagnosing human cysticercosis, and for immunodetection of infected pigs. ELISA tests have also been developed for detection of Taenia antigens in the feces of human carriers. Some protective antigens have been partially characterized and these include molecules from T. taeniaeformis, T. saginata and T. ovis (see below). Very few of these antigens have been localized and further work in this area is clearly warranted.

Cysticerei In brair

Eggs aceidenflally Ingested by human; cysteeicl divirtop in skin and brain

Taenia sujVmtt The 'port' lapcvíQim

Human [Definitive hut]

'Measly' pgrKliam

Human «als underCTiolted ^^ meal

Adult worm dcvcbps in gut

Pfoq(oltis passed in íceos

Egg hatches Hnd released oncosphere migrates lo muscles and forms ce#i/fosa9

Eqcts released Irom proglottis

Cysticercus cetfobsue [in musçlél

Oncosphere

Taenia sujVmtt The 'port' lapcvíQim

[Intermedíale host]

Human [Definitive hut]

Adult worm dcvcbps in gut

Neuroeystlecreosœ nnd epilepsy

Causing

Cysticerei In brair

Oncosphere

Cysticercus cetfobsue [in musçlél

'Measly' pgrKliam

Human «als underCTiolted ^^ meal

Pfoq(oltis passed in íceos

Egg hatches Hnd released oncosphere migrates lo muscles and forms ce#i/fosa9

Eqcts released Irom proglottis

Eggs aceidenflally Ingested by human; cysteeicl divirtop in skin and brain

Figure 1 Life cycle of the 'pork' tapeworm, Taenia solium. Note that the larval stage (Cysticercus) can develop in humans, causing the dangerous and often fatal disease neurocysticercosis. (After Smyth (1994), with permission.)

Immune response of the host

T. solium, similar to other larval cestodes in mammals makes intimate contact with host tissue at two sites (Figure 2): 1) during the early phase of infection, when the recently hatched oncosphere pentrates the intestine; and 2) at the final encystment site, commonly in muscles, viscera and the nervous system. The immune response at the oncosphere penetration site is often referred to as 'early' or 'pre-encystment' immunity, and that at the final establishment site as 'late', 'postoncospheral', or 'postencystment' immunity (Figure 2).

Secretory immunoglobulin A (IgA) in gut secretions and (where relevant) in colostrum prob ably plays an important role in attacking invading oncospheres, a situation related to the fact that this antibody may be resistant to intestinal enzymes. Thus infection of mice with T. taeniaeformis results in the production of intestinal IgG antibodies protective against challenge infection, and oral immunization results in a protective immune response against this species. An IgA antibody response has also been reported following T. taeniaeformis infection in rats and T. pisiformis infection in rabbits.

Mast cells accumulate around both invading oncospheres and developing larvae and it is possible that IgE may react with antigen, causing degranu-lation of these cells with a consequent increase in vascular permeability; this would allow IgCi anti-

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