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TAENIASIS-CISTICERCOSIS: A BIBLIOGRAPHIC REVIEW

 GENERALITIES

Porcine cisticercosis is a zoonotic parasitic disease caused by the larval form of Taenia solium called Cysticercus cellulosae.  The intermediate host is the pig and humans are the definitive hosts (Evans et al., 2000). It is important to stress that currently the scientific literature does not consider cysticercosis and taeniasis as two separate entities, but rather refers to the taeniasis/cisticercosis complex (Schantz, 1999). 

The background of Cysticercosis as a disease of pigs goes back in ancient times. In the 4th century BC, Aristophanes XE “Aristophanes”, in his work “The history of animals”, describes the presence of cysticercosis XE “cysticercosis” in the tongue  and muscles of pigs (Lasso, 1994).  Afterwards Plinio (25-79 BC) names the adult form of the parasite taenia (from the Greek tainia, which means “lace” or “strip”) (Cordero e Hidalgo, 1999). 

Scientific research and knowledge about the relationship between Cysticercus cellulosae in pigs and Taenia solium in humans is owed to Kuchenmeister and Leuckart. In 1895 and 1896 they demonstrated experimentally the development of Taenia solium in humans by making ingest alive cysticerci to prisoners condemned to death, finding more or less developed taeniae in their intestines (Grove, 1990). 

LIFE CICLE OF THE PARASITE

The natural life cycle of the taeniasis /cysticercosis complex involves the human being as the only definitive host of Taenia solium. The pig, hosting Cysticercus cellulosae, is the intermediate host in this cycle and the source of infection for humans (Quiroz, 1997).  However, humans can act as an incidental intermediate host by ingesting eggs of Taenia solium accidentally (Nash and Neva, 1984; Matías et al., 1983).

Humans infected by Taenia solium, eliminate eggs or pregnant  proglotids with the faeces.  Once the eggs and/or proglotids are ingested by the pigs they arrive to the digestive track where the oncospheres hatch thanks to the action of the digestive juices. Once free, the oncospheres adhere to the mucose and then penetrate the intestinal wall to reach the blood or lymphatic vessels.  Thanks to this way, the oncospheres disperse in the whole organism, reaching the pig’s muscles, a localisation of major importance for the life cicle.  Here the larva or cysticercus develops after 8-10 weeks (Náquira, 1999; Nash y Neva, 1984; Quiroz, 1997).

Humans get infected and develop taenia ingesting poorly cooked  pork meat infected with viable cysticerci.  The cysticercus gets to the stomach and then to the small intestine, where, thanks to the action of  the gastric and biliary juices, the scolex envaginates and gets fixed in the intestinal mucose. Here starts the development of the adult taenia, which can arrive to measure 2-5 metres and some times up to 8 metres. (Matías et al., 1983;Tagle, 1984; Quiroz, 1997).

Cysticercosis can happen in humans when accidentally ingesting food or water contaminated with feces of infected people, or by fecal-oral way due to lack of hygiene in the hands of people infected with adult taenias (Cordero and Hidalgo, 1999; Quiroz, 1997).  Oncospheres activate going through the digestive track, they then penetrate the intestinal wall and through the blood or linfatic vassels arrive to different organs where they develop to cysticerci.  This process takes about 2-3 months.  Neurocysticercosis develops when the cysticerci locate in the CNS (Náquira, 1999).

 

EPIDEMIOLOGY

Epidemiology of Teniasis

Infection with Taenia solium is important in countries consuming pork meat and it is mainly restricted to regions mainly to regions with low socioeconomic development. The illness is endemic in Latin America, South Africa, Southeast Asia and the Indian subcontinent.  The infection is frequent in areas where people lack adequate sanitary infrastructures and pigs search for food on the streets having an easy access to human feces (Soulsby, 1987).

Epidemiology of Human Cisticercosis

Human Cysticercosis has preference for the CNS and is usually known as NCC .  This illness represents a serious health problem in the majority of underdeveloped countries, including Peru and represents the main cause of epilepsy in adults (García et al., 1993).

NCC  is recognised as a common cause of a neurological illness in developed countries such as the United States.  It is caused by immigration of people from endemic zones (Clinton, 2000). This parasitic disease represents a serious public health problem and it involves up to 10% of the acute neurological patients (Herrera,1999).

Epidemiology of Porcine Cysticercosis

The problem of cysticercosis in pigs is not anymore considered  solely as a parasitic disease but rather as an important social problem. In underdeveloped countries breeding pigs is a common practice and generally it is being carried out under bad hygienic conditions which allow the pigs access to any sort of organic debris or even to human feces.  In rural zones where there is an absence of latrines, the environment is contaminated with feces and in areas where pigs are bred and commercialized, more than 1% of human population is carrier  of Taenia solium XE “Taenia solium” or Taenia saginata and  porcine cysticercosis XE “cysticercosis” affects more than 20% of pigs (Náquira, 1999).

The economic dependency of pig farming has caused the creation of systems of breeding and commercialization XE “commercialization” that favour the spread of Taenia solium XE “Taenia solum”. This is mainly due to the fact that official slaughterhouses are not longer being used and have been replaced by clandestine or backyard slaughterhouses, avoiding in this way the confiscation of infected meat (González et al., 1996b).

CLINIC SIGNS OF DISEASE

Taeniasis and Cysticercosis in humans

Taeniasis is a disease exclusively of humans.  Humans are usually carriers of one taenia and for this reason it has been called “solitary taenia” (Borchert, 1981), even if some researchers have observed cases of multiple infections.  Biotic potential of this taenia is high, with its 40,000 eggs/proglotids (Cordero e Hidalgo, 1999).

Signs and symptoms of this parasitic disease are common and not very specific.  Adult taenia causes irritation in the mucose of the small intestine (Lapage, 1983) and the clinic signs are variable.  It is possible to observe nervousness, insomnia, anorexia, weight loss and abdominal pain together with digestive upsetting (Chin, 2001) such as diarrhoea, constipation and epigastric pain (Schantz et al., 1999).

            The most important consequence of the accidental ingestion of eggs of Taenia solium by humans is NCC . Clinic signs of the illness are associated with the number, size, location and form (cystic or racemose) of the cisticerci and to the host’s immune response.  A variety of factors associated to this disease give rise to a complex range of responses that covers almost the entire range of neurologic symptoms, from asymptomatic forms up to sudden death as a consequence of blockage of the ventricular system (Martínez et al., 2000).

In the CNS  the cisticeri can stay for decades without causing  any symptoms nor inflammatory response. In some cases, however, the host’s inmune system starts with an inflammatory reaction that produces changes inside the parasite and in the adjacent nervous system (Del Bruto y Sotelo, 1988). This inflammatory reaction brings the cisticercus to a degenerative process that will finish with its death and its subsequent calcification (Del Bruto, 1999).

In the CNS two types of cysticercosis can be distinguished: vesicular and racemose. The vesicular form shows round or oval cysts or vesiculae with a translucent wall and a diameter varying from 0.5-1.5cm. Inside it contains a fluid containing the scolex.  The cisticercus  is surrounded by a thin layer of fibrous tissue that separates it from the surrounding tissue.  Racemose cysticercosis is a variety that appears as a long and lobar vesicula similar to a grape raceme.  This vesicula can measure up to 10cm and can contain various ml of fluid.  It is generally observed in the ventricular cavities and the cisternae located in the base of the skull.  The scolex can be shown only histiologically (Náquira, 1999).

In cases with moderate and severe infection, as well as in crises of epileptic type, deficiency signs can be observed.  Many cases of severe cysticercosis infection have initially shown signs of intracranial hypertension and confused hallucinatory signs.  In some of them, hemiparesia and focal epileptic crisis have occurred.

When the cysts are located in the cerebral parenchyima and/or ventriculae, depending on their size and the degree of damage produced, “internal hidrocephalia”, with intermittent symptoms can occur.  Cysts inside the ventricular spaces are fixed in the ependimary coating  or are freely floating in the cerebrospinal fluid.  This may cause  a valve effect that in some occasions can produce fatal and acute crises of intracranial hypertension.  In other cases these cysts cause wavy and sudden crises that disappear after shaking the head (Lobato et al., 1981). The third and fourth ventriculae are those more often hosting single cycticerci  (Proaño et al., 1997).

Cisticercosis in the Pig

Porcine cisticercosis  is located preferably in skeleton muscles, brain, tongue and heart.  When the infection is very intense it is possible to find cysts in the eye globe, liver, kidney, lungs, spinal cord, lymphatic glands and subcutaneous conjunctive tissue (Quiroz, 1997; Lapage, 1983).

Clinic signs of Cisticercosis in pigs are difficult to record, except in those cases in which the infection is very intense.  In this case paralysis of the tongue and lower jaw or difficulty in walking are observed.  If the brain has a great number of cysts, turning and encephalitis and finally death can occur.  Due to the short life span of domestic pigs, it is difficult to register the signs as domestic pigs do not live long enough to record them (Borchert, 1981).

 

ECONOMIC IMPACT OF THE TAENIASIS/CISTICERCOSIS COMPLEX

Cisticercosis is the cause of 40-50 dollars loss for each animal in Peru.  Animals diagnosed positive to the tongue test, are rejected by the buyers and owners find themselves obliged to sell the animal for up to a third of its original value so as not to loose the investment (González et al., 1993).  It has been estimated that adding  all the countries in Latin America that are affected by this parasitic disease, the annual economic loss would be 164 millions American dollars approximately (Schantz et al., 1999).

A study on pig commercialisation carried out in Peru in 1989 has demonstrated that from the 65,000 tons of pork meat  consumed during that year, 29,250 tons (45%) came from clandestine butchering, of which 11,700 tons were infected with cysticercosis.  Again, taking into account that between half to two thirds of infected meat is lost in Peru, this means a loss of 5 million dollars p.a. due to cysticercosis (González et al., 1993).

In the case of human cysticercosis, economic loss is high due to the seriousness of neurologic signs, long evolution of the illness as well as the physical and mental unfitness that the disease causes.  NCC implies high losses due to treatment, surgical operation and hospitalization (Acha y Szyfres, 1996; Flisser et al., 1999; Velazco-Suárez et al., 1982).  In Mexico it has been estimated that cysticercosis costs to the nation a total of 20 million American dollars p.a. in hospitalization and treatment of neurocysticercosis (Flisser, 1988).

 
DIAGNOSIS

Diagnosis of Taeniasis in Humans

Standard techniques of diagnosis of taeniasis depend on the detection of materials of the parasite (progloctids or eggs) in the faeces (Allan et al., 2003).  Diagnostic tests, as the coproparasitologic test, are simple and relatively cheap, but they are not very sensitive nor specific (Schantz y Sarti, 1989).  Moreover, eggs of Taenia solium and Taenia saginata appear identical under the microscope, implying problems with the diagnosis of the species (Allan et al., 2003).

ELISA tests  for the detection of the coproantigen, developed on the basis of antibodies of Taenia solium and Taenia saginata, have demonstrated to be specific, as they do not show cross reactions with other gastrointestinal parasites. The problem of this test is that it does not discriminate among the two taenias (Allan, 1999).  Serologic diagnosis of intestinal infection of Taenia solium has also been described and shows that is possible to detect species specific circulating antibodies through EITB (Wilkins et al.,1999).

Diagnosis of Cysticercosis in Humans

Clinic Diagnosis

Ophtalmocysticercosis – Using an ophtalmoscope, an examination test of the bottom of the eye can be carried out to see the rising of the retina, the tear and the bleeding, the inflammtory process and occasionally the cisticercus itself alive, moving free, occasionally evaginating  the  scolex when it is located in the vitreous (OPS/OMS, 1992).

Subcutaneous and muscular cysticercosis.  Parasitoscopic analysis allows to diagnose the illness, by surgical extraction of the tumor and by examination through the compression method or by  histopathologic study (OPS/OMS, 1992).

 

Diagnosis by images (CAT)

Both the Computerized Axial Tomography (CAT) and Magnetic Resonance (MR) have been, in the last few years, the most useful techniques for the diagnosis of neurocysticercosis (OPS/OMS, 1992).

CAT allows to visualise cystic forms, generally spread in the cerebral parenchima.  Hydrocephalia and reactive cerebral oedema can also be observed with CAT. Findings with this scanning technique are usually different depending on the  developmental stage in which the parasite is. Even though CAT is a valuable resource for the diagnosis of neurocysticercosis, it is advisable to use it together with a serological test such as Electroimmunotransfer Blot (EITB).  Then results can be correlated with the patient’s general health (Atías, 1994, García, 1994).

In the last years it has been proven that MR has some advantages compared to CAT, such as: a) detection of early phases, when the embryos are developing in the tissues, b) identification of isointense cysts, which are not easily identified by CAT, c) occasional visualization of scolex and intra-ventricular cysts and information about viability of cysts, depending on the intensity of the image of cystic content (OPS/OMS, 1992).  However, it must be pointed out that, unfortunately, the high cost of CAT and MR equipment limits its accessibility, especially in developing countries  (Nash y Neva, 1984).

Serological diagnosis

Serological tests represent a help in the diagnosis by images of human cerebral cysticercosis (Escalante, 1999).  Serological tests are considered the main tool for the clinic diagnosis in many endemic regions in developing countries  (Nash y Neva, 1984) as through them differencing diagnosis between a damage by cysticercosis and others looking similarly can be carried out (Escalante, 1999). Among the main serological tests there are:

·         Immunoenzymatic assay (ELISA) – It is the most used technique for the sero-diagnosis of cysticercosis.  This test has 70% of sensitivity and 73% of specificity due to the cross reaction with the larval form of Equinococcus granulosus (Pathak et al., 1994).

·         Electroinmunotransfer blot (EITB) – This test uses a proteinaceous antigene purified  by its chromatographic affinity with lectins. Seven glycoproteic strips, usually recognised by the antibodies of the patient suffering with cysticercosis, are considered.  These glycoproteic bands have a molecular weight of 50, 42-39, 24, 21, 18, 14 and 13 Kd. A reaction to one or all bands is considered indicative of cysticercosis or al least indicative of exposition to it.  The test has a sensitivity of 98% and a 10% specificity in detecting antibodies of Taenia solium in serum and CRL (Tsang et al., 1989a).

·         ELISA for the detection of antigens – various tests have been developed to detect antigens of Taenia solium but only  those using monoclonal antibodies can assure specificity (Harrison et al., 1999).  Detection of antigenes can be carried out in serum as well as in the the encephalic liquid (García et al., 1998, García et al., 2000).

One problem that can appear In serological tests which are meant to detect antibodies is the fact that a positive result can indicate only the exposure to infection but  not necessarily an active infection (Harrison et al.,1989).  In addition, antibodies can persist for a long period of time after parasites have been eliminated by immune mechanisms and/or anti-parasitic therapy (Harrison et al.,1989; García et al., 1997).

Diagnosis of cysticercosis in pigs

Tongue examination

This test is usually carried out in zones where the illness is present.  It is used to discriminate the presence of cysticercosis in pigs, prior to a commercial transaction.  The test consists in the palpation of nodes and/or visual examination of cysticerci.  For this, the animal is restrained, a stick is introduced in its snout to keep it open and the tongue is pulled out using a piece of cloth. Criteria applied in the diagnosis are: a) observing the presence of cists on the surface of the tongue, b) Palpation of the tongue and its base, c) observation of cysticerci or tears that can suggest that they were extracted (a common practice).  This method is relatively sensitive (87%) and highly specific (99%) to detect cysticercosis in pigs.  The advantage of this test is that it is easy to learn and of great usefulness as an aproximate method in countries with cysticercosis in pigs (González et al., 1993). 

Inspection of the slaughterhouses:

Diagnosis in slaughterhouses is carried out by cutting the muscles (serratus dorsalis, psoas, gracilis, masseter, diaphragm and heart) and viscera of the pig (lungs, liver) searching for cisticerci. The problem is that if the exam is not done with care, cisticerci may pass unnoticed, especially in the case of slight infections (González et al., 1990).

An additional problem is the fact that breeders who suffered confiscation of their infected slaughterhouses, do not go back to the slaughterhouses and afterwards process in artisanal ways with the consequence that the animals, infected or not, are commercialized informally  or in clandestine form. For this reason, it can be concluded that the diagnosis of cysticercosis in slaughterhouses is of limited epidemiological usefulness (González et al., 1999).

Serological Diagnosis

Cysticercosis serological tests for pigs like ELISA present cross reactions with other parasites (Flisser et al., 1999).  In addition they offer low sensitivity and specificity (89 ± 8% and 61 ± 10%) (González et al., 1993).  On the contrary, EITB test has no cross reactions and it is 100% specific and quite sensitive (98%) (Tsang et al., 1989a; Tsang y García, 1999; Flisser et al., 1999).  Both tests allow the detection of exposure to eggs of Taenia solium more than the infection.  For this reason research has been focused to detect the antigene of Cysticercus cellulosae with the aim of testing viable or not viable infection in pigs.

Diagnosis of cisticercosis in pigs through the detection of the antigene has given good results using monoclonal antibodies directed against Cysticercus bovis, meta-cestode of Taenia saginata (Harrison et al., 1989)..

Monoclonal antibodies have been developed against the secretion-excretion antigens of Cysticercus bovis.  These have demonstrated to present more sensitivity in detecting most of the specific antibodies in comparison to the somatic antibodies and cystic  liquid (Brandt et al., 2001).  These monoclonal antibodies did not present cross reaction when they were  presented to serum in infected animals with hydatid cysts and Cysticercus tenuicollis (larvae forms of Equinococcus granulosus and Taenia hydatigena, respectively) (Harrison et al., 1989).

The ELISA -Ag assay uses the monoclonal antibodies of the isotype Ig G and has a sensitivity of 92.3% and a specificity of 98.7% to diagnose bovine Cysticercosis (Van Kerekhoven et al., 1998).  These values have stayed standard for the diagnosis of cysticercosis in pigs in Peru.  This  highlights its high discriminatory ability between exposure and infection with larvae of Taenia solium.

CYSTICERCOSIS TREATMENT

Human Cysticercosis

For a long period treatment of neurocysticercosis has been focused only in the  control of the syntomatology produced by the illness, using anticonvulsive drugs, steroids and derivates of the cephalo rachidian liquid or decompressive craniotomies which were curative only in the case of cysticercosis of the 4th ventriculus. From the beginning of the current decade various specific treatments focused on the death of the parasite have been applied.  Praziquantel and albendazole are at the moment the main cysticide drugs used in humans (Del Brutto et al., 1993; García et al., 1997).

 

Clinic and experimental evidence confirm the effectiveness of praziquantel in all cases of neurocysticercosis, as the only therapy or   associated with surgery.  Average recommended dosis is 50mg daily for each kg of body weight during 15 days.  To avoid aggravation of the syntomatology which can happen at the beginning of treatment, it is recommended to start this treatment with  hospitalised patients.  For praziquantel it is recommended to associate the administration of dexametazone In daily doses of 4-16mg (Martínez, 1984).

Following studies with praziquantel, albendazol has been tried.  A dosis of 15mg/kg/day during 30 days has been administered to 20 patients with a diagnosis of  cystic parenchymatose neurocystiercosis detected by CAT. A disappearing of 96% of cysts has been obtained and the side effects have been kept under control  (Ramos, 1991).  Other types of treatments with praziquantel have been tried with good results.

         The hospitalization under strict care  is recommended in any treatment against cysticercosis in humans.  This is recommended at least during the first week of treatment, when side effects usually appear. In this case the use of steroids is recommended especially when there are signs of cysticercotic encephalitis and subaracnoidal cysts (Botero 1999).  Standard treatment at the moment is 15mg/kg/day of albendazole for 8 days in three doses, with the simultaneous  administration of dexamethasone (Del Brutto y Sotelo 1988).

Porcine Cysticercosis

Searching for an efficient treatment for cysticercosis in pigs, a series of paraciticide drugs have been tried out. Among these,  oxfendazole has been shown to be the ideal drug for this illness.   It is effective with one dosis and it is cheap for the treatment of muscular cysticercosis (González et al., 1996a).

A dosis of 30mg/kg of oxfendazole eliminates 100% of muscular cysts starting from the 4th week after treatment and at 12 weeks cysts have reabsorbed leaving a completely hygenic  situation (González et al., 1998).  It has also been noted that animals treated with oxfendazole are resistant to new infections of cysticercosis (González et al., 2001).

PREVENTIVE MEASURES AND PORCINE CYSTICERCOSIS CONTROL

Controls of meat at the slaughterhouse has been for years the effective way to avoid the spread of cysticercosis.  This strategy has been applied to eradicate the disease in various countries more than 100 years ago.  Unfortunately our country has not reached the same results due to the idiosycracy and poor cultural and economic development of their inhabitants.  The situation forced those who suffered confiscation (loosing the whole production lacking any compensation)  to divert their  production to clandestine or artisanal slaughterhouses, creating in this way paralel clandestine markets for the commercialization and distribution (González, 1993).

Treatment with a one dosis of 30mg/kg of live weight with oxfendazole resulted in an innovative and effective alternative. The problem is that the drug is not available on the national market in th concentration and presentation which would ease its commercialization.   In addition the treated animal must stay at least 2 months under care before being considered clean, a situation that does not always take place in the field, which makes its immediate applicability difficult. (González et al., 1998).

Alternative measures to block the biological cycle of Taenia solium exist.  Thew inactivation of the Cysticercus cellulosae of porcine infected meat can be achieved by keeping the infected pork meat refrigerated at –24ºC for a day, at -15ºC for 3 days or at -5ºC for 4 days and by cooking it up to a boiling point before frying small pieces of 5cm maximum (Quiroz, 1997).  Health education XE “health education” \b \i  is as well an important factor in the prevention and control of the disease, avoiding the closure of the human-pig circle of Taenia solium.

At present,  research is being carried out to protect pigs through  mechanisms of immunization against the eggs of Taenia solium.  In this way, vaccination turns out to be an efficient alternative to control and/or eradicate the taeniasis/cysticercosis complex.

 
 
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