The journey towards elimination of gambiense human African trypanosomiasis: not far, nor easy.

Considering the epidemic situation of gambiense human African trypanosomiasis (HAT) at the end of the twentieth century, the World Health Organization (WHO) and partners strengthened disease control and surveillance. Over the last 15 years, the activities implemented through the National Control Programmes have brought gambiense HAT under control and now its elimination is deemed as an achievable goal. In 2012, WHO targeted gambiense HAT for elimination as a public health problem by 2020. The final goal will be the sustainable disease elimination by 2030, defined as the interruption of the transmission of gambiense HAT. The elimination is considered feasible, because of the epidemiological vulnerability of the disease, the current state of control, the availability of strategies and tools and international commitment and political will. Integration of activities in the health system is needed to ensure the sustainability of the elimination. The development of user-friendly diagnostic and treatment tools will facilitate the integration process. Adequate funding is needed to implement activities, but also to support research that will make the elimination sustainable. A long-term commitment by donors is needed and ownership of the process by endemic countries is critical.

Update on field use of the available drugs for the chemotherapy of human African trypanosomiasis.

Despite the fact that eflornithine was considered as the safer drug to treat human African trypanosomiasis (HAT) and has been freely available since 2001, the difficulties in logistics and cost burden associated with this drug meant that the toxic melarsoprol remained the drug of choice. The World Health Organization responded to the situation by designing a medical kit containing all the materials needed to use eflornithine, and by implementing a training and drugs distribution programme which has allowed a transition to this much safer treatment. The introduction of the combination of nifurtimox and eflornithine (NECT) has accelerated the shift from melarsoprol to the best treatment available, due to reduced dosage and treatment time for eflornithine that has significantly lessened the cost and improved the burden of logistics encountered during treatment and distribution. The decrease in the use of more dangerous but cheaper melarsoprol has meant a rise in the per patient cost of treating HAT. Although NECT is cheaper than eflornithine monotherapy, an unexpected consequence has been a continuing rise in the per patient cost of treating HAT. The ethical decision of shifting to the best available treatment imposes a financial burden on HAT control programmes that might render long-term application unsustainable. These factors call for continuing research to provide new safer and more effective drugs that are simple to administer and cheaper when compared to current drugs.

Controlling sleeping sickness - a review.

Following a period characterized by severe epidemics of sleeping sickness, restoration of effective control and surveillance systems has raised the question of eliminating the disease from sub-Saharan Africa. Given sufficient political and financial support, elimination is now considered a reasonable aim in countries reporting zero or less than 100 cases per year. This success may lead health authorities across the affected region to downgrade the disease from 'neglected' to simply being ignored. In view of the significant levels of under-reporting of sleeping sickness mortality in rural communities, this could be a short-sighted policy. Loss of capacity to deal with new epidemics, which can arise as a consequence of loss of commitment or civil upheaval, would have serious consequences. The present period should be seen as a clear opportunity for public-private partnerships to develop simpler and more cost-effective tools and strategies for sustainable sleeping sickness control and surveillance, including diagnostics, treatment and vector control.

Mapping sleeping sickness in Western Africa in a context of demographic transition and climate change.

Human population growth, climate change and economic development are causing major environmental modifications in Western Africa, which will have important repercussions on the epidemiology of sleeping sickness. A new initiative, the Atlas of human African trypanosomiasis (HAT), aims at assembling and geo-referencing all epidemiological data derived from both active screening activities and passive surveillance. A geographic database enables to generate up-to-date disease maps at a range of scales and of unprecedented spatial accuracy. We present preliminary results for seven West African countries (Benin, Burkina Faso, Côte d'Ivoire, Ghana, Guinea, Mali and Togo) and briefly discuss the relevance of the Atlas for future monitoring, control and research activities.

[The Mandoul human African trypanosomiasis focus in Chad: from evaluation to control]. / Le foyer de trypanosomiase humaine africaine du Mandoul au Tchad : de l'evaluation au contrôle.

The Mandoul focus of human African trypanosomiasis in southern Chad was first described by Gaston Muraz in the 1920s. After 40 years of control measures, case reports became rare and the focus was forgotten. However the number of cases began to increase in 1993 and coordinated control measures were implemented in 2002. The first phase of control consisted of mapping out the focus that was shown to involve 45 villages and camps on both sides of the Mandoul River. The estimated number of inhabitants in the area is 20.000 and the endemic prevalence was 3.78%. Dynamic passive screening and regular active screening undertaken in the framework of the Chadian human African trypanosomiasis control program with the assistance of expert technicians from the subregion reduced the prevalence to 0.77% in 2006. Although this reduction is encouraging, control measures must be maintained and greater involvement of the health care system will be needed to achieve sustainable control of the disease and ultimately to eliminate human African trypanosomiasis as a public health problem.

[Organizing an active screening campaign for human African trypanosomiasis due to Trypanosoma brucei gambiense]. / Organisation d'une campagne de dépistage actif de la trypanosomiase humaine africaine à Trypanosoma brucei gambiense.

Organization of an active screening program for human African trypanosomiasis in an outbreak area is subject to strict guidelines that must take into account the size of the population, the specificity and sensitivity of the diagnostic techniques used, and the cost of screening. Numerous parameters can affect the outcome including accessibility of the outbreak area (road conditions, rainy season); awareness of village populations and of local administrative, traditional, and religious personalities; quality of local health-care facilities and personnel; possibility of referring patients to a health care institution able to provide treatment, etc. For these reasons the cost of screening programs can be high in terms of human, physical, and financial resources. Careful planning and preparation is necessary to ensure worthwhile results. The model described in this article allows screening of 300 to 600 persons a day in areas in which the endemic disease prevalence is higher than 1%. A variant for areas with lower endemicity allows screening of up to 1500 persons a day.

Study of resistance to anti-tuberculosis drugs in five districts of Equatorial Guinea: rates, risk factors, genotyping of gene mutations and molecular epidemiology.

SETTING: Five districts in Equatorial Guinea, March 1999 to February 2001. OBJECTIVES: To determine tuberculosis drug resistance among new and previously treated cases, the risk factors associated with resistance, and the mutations associated with isoniazid and rifampicin (katG, inhA and rpoB genes) resistance, and to genotype resistant strains. RESULTS: A positive culture identified as Mycobacterium tuberculosis complex was obtained in 240/499 patients. Susceptibility testing was performed in 236 strains. The overall resistance rate in new cases was 16.9% compared to 41.6% in previously treated cases. Isoniazid resistance was the most frequent (respectively 12.5% and 16.6%) in the two groups, while multidrug resistance was observed in 1.7% and 25% of new and previously treated cases, respectively. Female sex was statistically associated with resistance in new cases. Of 41 isoniazid-resistant strains, 33 (80.5%) had mutations in the inhA gene; none had mutations in the katG gene and eight had no mutations in either gene. All strains had low-level isoniazid resistance. Of eight strains resistant to rifampicin, six had mutations in the rpoB gene. Genotyping defined seven clusters. CONCLUSIONS: Moderate resistance was found in new cases. Low-level isoniazid resistance predominated among mutations in the inhA gene, with a high percentage of clustering in resistant strains.

Molecular epidemiology of tuberculosis in the Bata and Malabo districts of Equatorial Guinea.

SETTING: Bata and Malabo districts, Equatorial Guinea, 1 March 1999 to 28 February 2001. OBJECTIVE: To study the molecular epidemiology of tuberculosis (TB). RESULTS: During the study period, 429 patients were diagnosed with TB in the Bata and Malabo districts. A positive culture was obtained in 206 (48%) TB patients, with RFLP analysis being performed in 185 (89.8%). Ninety-two different patterns were identified. Single patterns were found in 71 strains (38.3%) and the remaining 114 strains (61.6%) were classified into 21 clusters (of 2 to 25 patients). In addition, 37 of the typing strains were resistant to one or more anti-tuberculosis drugs, and 30 were included in clusters (81%), with 21 low level isoniazid (MIC < or = 1 microg/ml) resistance strains in the same cluster. Statistical analysis showed that resistance to anti-tuberculosis drugs (OR 3.1; 95% CI 1.2-7.6; P = 0.014), and positive smear results (4+ grade smear) (OR 4.3; 95% CI 1.5-12; P = 0.005), were significantly more frequent among patients with clustered strains. No epidemiological links were related to clustering. CONCLUSIONS: The level of clustering (61.6%) observed suggests a high degree of recent transmission and a predominance of determined patterns of Mycobacterium tuberculosis strains among the population of Equatorial Guinea.

Quantitative determination of circulating anodic and cathodic antigens in serum and urine of individuals infected with Schistosoma intercalatum.

Circulating anodic and cathodic Schistosoma antigens (CAA and CCA) have been determined by enzyme immunoassays in serum and urine of 60 individuals infected with S. intercalatum in Equatorial Guinea. The median egg output was 29 eggs/g of faeces (range 3-840). The egg output strongly correlated with concentrations of serum CAA (p = 0.47) and urine CAA (p = 0.42) (P < 0.001 for both); the later 2 quantities were also correlated with each other (p = 0.44, P < 0.001). All except 3 infected individuals had detectable amounts of serum CAA and/or urine CCA, a sensitivity of 95% for these 2 tests combined. Urine CAA was detected in 43% of patients. Serum CCA was detected in all infected individuals; however, no significant correlation was obtained between serum CCA levels and egg output in the stools of individual patients. This is the first study to demonstrate CCA in specimens of patients infected with S. intercalatum. The detection of CCA in urine is a new, non-invasive diagnostic method for S. intercalatum infection.

[Sleeping sickness: one hundred years of control strategy evolution]. / Maladie du sommeil: cent ans d'évolution des stratégies de lutte.

Sleeping sickness has been known since the fifteenth century but the real progress in the knowledge of the disease occurred in the nineteenth century with the development of microscopy. From 1841 to 1901 the parasites and their vectors have been identified, the symptomatology and the epidemiology have been described. However, due to absence of any effective cure, the campaign against the disease was still based on the isolation of the patients and the transfer of exposed populations. The discovery of atoxyl in 1905 provided doctors with their first therapeutic weapon and, in 1910, the first action of vector control was undertaken with success in the Island of Principe. Between the two world conflicts, Jamot published the rules to fight against major outbreaks. Their application in Oubangui-Chari, in Cameroon and in French Occidental Africa brought tremendous results and signed the triumph of the mobile unit concept. Success which will not be denied until the sixties when the disease was believed to be eradicated. From the sixties to the nineties, the concept of the integration of prevention and care added to the exclusion of any vertical system will result in a progressive reniewed outbreak of the sleeping sickness in the known foci. As a paradox, it is a time rich in discovery as regards diagnosis, treatment and entomology. In 1994, the World Health Organisation got concerned with the situation of the disease in Central Africa where the outbreak of the disease reinforced. A second paradox appeared; it is the next to total disinterest from the politics and fund raisers which will save the disease. Today, sleeping sickness is the typical example of the orphan disease, a show case brandished by all the good souls. In 2001, an agreement between the WHO and the pharmaceutical industry brings back the financial funds required to fight the disease. Basically, it is a matter of resuming the action by using what is still existing and by creating new strategies considering the extreme lack of human and logistical resources. The objective is to eradicate the sleeping sickness as a public health problem. The challenge is huge, but is on the way to success.

[Human African trypanosomiasis in an urban area: an emerging problem?]. / Trypanosomose humaine africaine en milieu urbain: une problématique émergente?

The human African trypanosomiasis is essentially a rural disease. The notification of cases in urban area has always been incidental; either a diagnosis made in town revealed a disease contracted in rural environment or it meant the preservation of a complete epidemiological cycle in a remaining urban micro-focus. In Kinshasa, in Democratic Republic of Congo, about forty cases have been notified each year. All of them came from the nearby foci of Bandundu, Lower Congo and Kasaï. In 1996 the number of cases reached suddenly 254 and today the average annual number comes up to 500 in spite of all the efforts undertaken to fight the disease. A study of cases in 1998 and 1999 shows that patients are essentially distributed in suburbs and that the most affected by the disease are the 15-49 year old ones whose job is related with agricultural or fishing activities. Two phenomena seem to explain this sudden increase: the massive inflow of refugees in outskirts of town coming from provinces where trypanosomiasis is endemic and a major economic crisis throwing out urban population in suburbs living on a subsistence micro-agriculture. These concomitant factors have contributed to the setting up of a trypanosomiasis belt around the capital. Today a strategy has to be reconsidered in order to fight against the disease in the capital itself and to make the medical staff aware of the diagnosis of a disease still unknown in their sanitary district.

[The focus of human trypanosomiasis in Campo (Cameroon). History and endemic situation in 1998]. / Le foyer de trypanosomose humaine de Campo (Cameroun). Historique et situation de l'endémie en 1998.

For the first time in the last thirteen years, the human sleeping sickness focus at Campo, spanning the Cameroon-Equatorial Guinea border areas, has been prospected. The screening was carried out simultaneously on both sides of the border. This focus has been known since the beginning of the century but, contrary to what took place in other well-known foci in bordering countries south of Cameroon, either in the 1920s or the 1980s--there has never been an epidemic outbreak in that area. Such an epidemiological situation makes this focus particularly interesting. Though still active, trypanosomiasis is not very manifest. According to passive screening carried out in recent years, the estimated prevalence ranges between 0.2 and 0.5%. For this screening, 5,255 persons were examined on the Cameroonian side of the focus (90.6% of the census population). The serological screenings were carried out with the CATT 1.3, which is the CATT generally used in screening, and with the latex CATT which associates LiTat 1.3, 1.5 and 1.6. The search for trypanosomes was made by testing the lymph nod juice in presence of adenopathy and in the blood by Quantitative Buffy Coat (QBC), the mini anion exchange centrifugation (mAEC), as well as the in vitro culture using the kit for in vitro isolation of trypanosomes (KIVI) for individuals suspected to be serologically positive. 16 patients were identified in Cameroon but none in Equatorial Guinea. The results show that the Campo focus is active only on the Cameroonian side, centred on the village of Ipono with a limited prevalence (0.3%). The persisting epidemic is most likely to be associated with the presence of pigs carrying the Trypanosoma brucei gambiense which was identified during the study in Ipono. The strain that we isolated was studied by isoenzyme electrophoresis on cellulose acetate. Its zymodeme is the same as that of the human strain isolated in Campo. With the collected epidemiological data, a concerted medical and entomological action could be planned within the limits of the village of Ipono to eradicate the disease. This action may be organised by the existing local health structures. During this study, the latex CATT proved to be more cost-effective than the CATT 1.3 since a similar result was reached requiring eight times less work at a lower cost. This remains to be confirmed in a hyperendemic focus.

[The concept of neglected disease]. / Le concept de maladie négligée.

"Neglected disease", "neglected population" and more generally "public health negligence" are emerging concepts being put forward by numerous humanitarian groups. Sleeping sickness provides a typical example to illustrate these concepts. After causing a major epidemic in the 1930s, sleeping sickness had been practically eradicated by the end of decolonization. Because of more urgent priorities, independent national governments relinquished control activities thus allowing the disease to return insidiously. By the beginning of the 1990s the situation was comparable to that prevailing in 1930 without inciting a response commensurate with the extent of the problem. Sleeping sickness is currently not a priority and, more simply, is not given proper attention because it affects only a few people living in regions presenting little economic interest. This point underlines the especially devastating combination of neglected disease and neglected population. As early as 1999 the World Health Organization with the determined support of Medecins Sans Frontieres launched a campaign not only to promote control measures for sleeping sickness at the international level but also to use initiatives in the domain to illustrate the enormous potential for progress against neglected disease. The effects of this campaign are now beginning to be felt.

[Status of sleeping sickness in 2003]. / Situation de la maladie du sommeil en 2003.

On May 3, 2001, the World Health Organization signed a major agreement with the pharmaceutical industry for the supply of drugs necessary for treatment of sleeping sickness. At that time Dr. Gro Harlem Brutland, director of the WHO, announced, "We can now look forward to halting the spread of sleeping sickness...". The purpose of this article is to take a look at the situation two years later. A first assessment showed that most national programs for the control of human African trypanosomiasis (NPCHAT) had practically become inoperative. One of the first steps in the new eradication campaign consisted of reviving these NPCHAT teams. However this goal could be achieved only insofar as awareness of the severity of the disease and of the need to act was felt at every level of decision-making. In 2001 the Pan-African Tse-Tse Trypanosomiasis Eradication Campaign (PATTEC) initiative was launched by African State leaders to promote special attention at the ministerial level, high-level training, and international cooperation sometimes involving several NPCHAT teams. Actions in the field include trials using new strategies, expert assistance for personnel throughout the duration of prospection, and screening and immediate treatment of numerous patients in outbreak areas where the disease was thought to be extinct. Although progress has not always been measurable in concrete terms, the dynamics have shifted almost everywhere.

[Sleeping sickness, forgotten illness: what are the consequences in the field?]. / Maladie du sommeil, maladie oubliée: quelles conséquences sur le terrain?

For nearly 25 years, sleeping sickness was forgotten and increasingly neglected. Research programs and control activities against human African trypanosomiasis were discontinued. Statistical studies show a constant decrease in the number of people screened and cases detected and little change in the ratios of actively versus passively diagnosed cases and of the early (blood and lymph involvement) versus late (cerebral involvement) stage cases. In the field neglect of the disease led to deterioration not only physical facilities but also human resources. As teams aged, senior members were often replaced by less than fully qualified people resulting in a decline in efficiency and organization. Many basic notions were lost and the albeit scarce innovations in diagnosis and therapy were often overlooked. When the fight against sleeping sickness was finally resumed, these factors had to be taken into account. Efforts in the field have been focused on four areas: renovation of equipment, didactic and practical training for health care personnel, development of a decision-making algorithm based on diagnostic findings, and implementation of new therapeutic protocols.

[Schistosoma intercalatum bilharziasis: clinical and epidemiological considerations]. / La bilharziose à Schistosoma intercalatum: considèrations cliniques et èpidèmiologiques.

Schistosoma intercalatum bilharziasis continues to raise numerous questions regarding pathogenicity and gravity. The parasite was identified recently and the last fully described outbreak occurred 10 years ago in the city of Bata, Equatorial Guinea. Geographically Schistosoma intercalatum biharziasis is limited to one part of the African continent but has shown a tendency to spread. Hybridization of Schistosoma intercalatum and Schistosoma haematobium has been observed. The main clinical manifestation of Schistosoma intercalatum is rectal bleeding. The endoscopic appearance of lesions is variable and non-specific ranging from granulomas or polyps to ulcerations. Complications include severe rectitis or genital involvement such as salpingitis with secondary sterility. Spontaneous abortion has also been reported. Association with salmonella and klebsiella infection has been confirmed and can lead to life-threatening situations. Few studies have been performed to assess the value of diagnostic tests. The sensitivity of stool smears and urinary sedimentation testing is 81.7% and 56.3% respectively using the two examinations as references for one another. The sensitivity of immunological tests is generally good but varies depending on the reference technique used. Specificity can be affected by cross-reaction with other schistosomas or trematodes and even with nematodes and hematozoons. Treatment with a single dose of Biltricide has proven to be effective. Prevention requires education of the population at risk and use of molluscacides. The control strategy must be adapted in function of the epidemiology of the disease, diagnostic data, cost and effectiveness of screening and treatment.

[Control of human African trypanosomiasis: back to square one]. / Lutte contre la trypanosomiase humaine africaine: la boucle bouclée?

The natural history of sleeping sickness is cyclic. The first epidemic outbreak in the 19th century devastated the population and resolved spontaneously for lack of victims. Intensive development during the colonial period and the movement of population that it spawned led to another epidemic in the early 1920s that reached such severe proportions that drastic steps had to be taken. At that time, Jamot was given complete political, administrative, and financial freedom to combat the disease. This program led to the development of the mobile team concept and so-called vertically structured vector control strategy and was so successful that sleeping sickness ceased to be considered as a major public health problem at the beginning of the 1960s. In the ensuing years sleeping sickness was largely neglected. Monitoring the disease required specialized teams that were no longer considered as cost-effective. One by one the measures that had been implemented to control the disease disappeared, thus setting the scene for a new outbreak grew. In 1995, the incidence of sleeping sickness reached the same levels as in the 1920s. The current situation is a classic example of a neglected disease with a paucity of competent specialists, diagnostic tests, effective drugs, and operational facilities. It was not until 2001 that new hope appeared thanks to a combined public- and private-sector initiative allowing restructuring of treatment teams, renovation of facilities, free distribution of drugs, and research to develop new therapeutic agents. Also thanks to the PATTEC initiative, the governments of the African affected nations are showing new in interest in sleeping sickness. However the battle is far from won and much effort will be required. Time is running out and the stakes are high.

[Has the focus of human African trypanosomiasis in Luba, Equatorial Guinea been eradicated?]. / Le foyer de trypanosomiase humaine africaine de Luba en Guinée Equatoriale a-t-il été éradique?

Human African trypanosomiasis was considered a major public health problem in Luba, Equatorial Guinea in 1985. Because of the lack of qualified personnel, the emergency response consisted of a simple control strategy based on serological screening without parasitological confirmation and staging by lumbar puncture. This strategy was highly effective since the outbreak seems to have stopped. The authors discuss implications of this strategy which raises the risk of misdiagnosis and unwarranted treatment of trypanosomiasis. Other points of discussion in this article include the concept of sterilization of the disease area and need for continued low-grade surveillance.

[Cost of a national program to control human African trypanosomiasis in the high Mbomou region, Central African Republic]. / Cout d'un projet d'appui au programme national de lutte contre la trypanosomiase humaine africaine dans le foyer du Haut Mbomou, Republique Centrafricaine.

An outbreak of human African trypanosiaisis is ongoing in the High Mbomou area of the Central African Republic. This area is located on the Sudanese border approximately 1,100 kilometers from the capital city of Bangui. According to current estimates, the cost of implementing the National Human African Trypanosomiasis Program is 754,000 United States Dollars, i.e., 4.1 dollars per protected inhabitant. However actual conditions in the field suggest that this estimate should be revised. Special field conditions include constant refugee movement across the border, lack of accurate epidemiological data concerning neighboring Haut Zaire, and low participation of village residents in mass screening operations (less than 50%). In response to these problems, the authors recommend the organization of more exploratory missions to allow better targeting of screening and therapy. In the initial plan, exploratory missions were to account for 1% of the total cost. This proportion will probably require upward adjustment.

[Historical aspects of the risk factors of Schistosoma intercalatum schistosomiasis]. / Historique des facteurs de risque de la bilharziose à Schistosoma intercalatum.

Bilharziosis is a considerable public health problem. It is caused by many species of schistosoma, four of which have wide geographical distribution: Schistosoma mansoni, S. haematobium, S. japonicum and S. intercalatum. The recently discovered S. intercalatum is limited to central and west Africa. Its spread is progressive and its pathogenicity is not completely known. S. intercalatum bilharziosis is usually manifested in the form of dysentery. The physiopathologic explanation of this clinical manifestation is less clear. Immunopathologically, the formation of an inflammatory granuloma constitutes the origin of its symptoms. This is due to many biological factors including delayed hypersensitivity reactions. All cellular immunity changes will facilitate the appearance of symptoms. Our aim has been to show the importance of malnutrition as a pathogenic factor of S. intercalatum bilharziosis. The initial research hypothesis was as follows: malnutrition plays a role in the evolution of a patient from an asymptomatic state of infection to a symptomatic state of illness. We carried out the study in the suburbs of Bata, in Equatorial Guinea. The inhabitants of Ncolombong, essentially rural immigrants, comprised our study population. Following their consent, we recruited individuals less than 45 years of age who had not taken praziquantel during the last 12 months. We included a total of 297 patients. Our study was a case-control, matching on sex and age. A case was defined as an infected patient with acute or chronic diarrhea occurring within the last month' preceding the stool sample analysis. All cases were retained after exhaustive screening of the study population. Each case (group 1) was matched with one or several asymptomatic infected patients (group 2) and two or several asymptomatic noninfected patients chosen at random (group 3). The definition of malnutrition was as follows: weight/height < or = 90% for children less than 15 years of age or weight/height < or = 90% with a corporal mass index < or = 20 for children more than 15 years of age. Two logistical regression models were performed in order to distinguish pathogenic from infection factors. Among the confusion bias identified, none of the helminthiasis in Bata are risk factors. The risk factors of the infection have been searched with an interrogatory. The bias caused by the interviewer is minimized because all the team staff were trained for a week before the beginning of the study. Apart from malnutrition, the other causes of cellular immunodeficiency do not seem to have any relationship with the development of symptoms. The logistical model of infection identified the classical risk factors of infection: river leisures (OR = 3.97, CI 95%: 1.86-8.47), poor or average quality of walls of the house (OR = 2.53, CI 95%: 1.15-5.58), lack of water well (OR = 2.08, CI 95%: 1.08-4). Our study could not show any relationship between malnutrition and bilharziosis. The nutritional state does not play a significative role in the infection or development of the disease. Nevertheless, the nutritional state of the host probably influences other host or parasite factors. As a result, we still don't know its influence on ADCC (Antibody Dependent Cellular Mediated Cytotoxicity) mechanisms, on adult parasite adaptation and the efficiency of laying of eggs which affects the parasitological charge. We haven't found any relationship between parasitological load and appearance of symptoms. The parasitological load indirectly reflects the efficiency of the laying and nothing proves that it is correlated with the intensity of delayed type hypersensibility reactions. In the logistical model of the disease, a stay of more than 2 months in an endemic area (OR = 0.14, CI 95%: 0.03-0.76) and a poor or average quality of walls of the house decreased the risk (OR = 0.31, CI 95%: 0.11-0.85). This result permits us to suppose that there is a tolerance to schistosomian antigens by cellular immunity