A New Approach to Helminthiasis Control: the Use of an Organophosphorous Compound *

Schistosomiasis Consortium for Operational Research and Evaluation: Mission Accomplished

The Schistosomiasis Consortium for Operational Research and Evaluation (SCORE), a program focusing on schistosomiasis control in sub-Saharan Africa between 2008 and 2019, investigated ways to improve coverage and efficacy of ongoing chemotherapy programs and concluded that because of continued transmission, mass distribution of praziquantel cannot eliminate the disease without complementary control activities. Schistosomiasis Consortium for Operational Research and Evaluation’s activities comprised large-scale, multicountry field studies comparing various mass drug administration strategies and some specific research avenues, such as assessment of high-sensitivity diagnostics, identification of hotspots, quantification of the role of the snail host, predictive modeling, and changes in schistosome population genetics under drug pressure. The discoveries made and the insights gained regarding cost-effective strategies for delivering preventive chemotherapy should assist policy makers to develop guidelines for the control and ultimate elimination of schistosomiasis.

Effects of cholinesterase inhibitor metrifonate on naive rats and rats with a model of hypoxia-induced impaired memory

Cholinesterase inhibitors are currently used in the therapy of different kind of dementia to improve brain memory functions. The acetylcholinesterase inhibitor metrifonate was studied in naive rats and in rats with a model of sodium nitrite-induced hypoxia. One active avoidance test and in two passive avoidance tests were used. In the active avoidance test metrifonate increased the number of avoidances during the learning session only. In both passive avoidance tests, metrifonate prolonged latency differently during the learning session and in short-term or in long-term memory retention. Hypoxic rats showed lower numbers of avoidances in learning and memory retention sessions. Metrifonate increased the number of avoidances during the learning session for hypoxic rats. In the step-through passive avoidance test, metrifonate increased the latency of reactions in the learning session and in long-term memory retention tests. In the step-down passive avoidance test, the groups with hypoxia and metrifonate did not change the latency of reaction in the learning and long-term memory retention sessions, but increased the latency of reactions in the short-term memory retention test. Morphological data showed a significant impaired neuronal structure in a CA1 zone of the hippocampus in hypoxic rats and a tendency to preserving in rats treated with metrifonate. Our results suggest that metrifonate improves cognitive functions in naive and in hypoxic rats.

Antischistosomal drugs: past, present ... and future?

The major antischistosomal drugs that have been or still are in use against infections with schistosomes are considered here together with some compounds that have not been in clinical use, but show interesting characteristics. Each individual compound presents aspects that may be enlightening about parasite biochemistry, parasite biology, and host-parasite relationships. Special attention is given to the mechanisms of action, an understanding of which is seen here as a major factor of progress in chemotherapy. Three compounds are currently in use, i.e., metrifonate, oxamniquine, and praziquantel, and all three are included in the World Health Organization list of essential drugs. They are analyzed in some detail, as each one presents advantages and disadvantages in antischistosomal therapy. The reported occurrence of drug-resistant schistosomes after treatment with oxamniquine and praziquantel suggests strict monitoring of such phenomena and encourages renewed efforts toward the development of multiple drugs against this human parasite.

Drug resistance to schistosomicides and other anthelmintics of medical significance

It is usual for people to be infected for some period in life with parasitic worms, which may cause morbidity or even kill. Anthelmintics are used for the treatment and control of the human helminthiases, since no vaccines are yet available. Despite the widespread use of these compounds, drug resistance has become apparent only with antischistosomal chemotherapy, in contrast to the situation with other anti-infective agents in human medicine and with veterinary anthelmintics, where resistance is widespread. This paper reviews research on drug resistance in human helminthiasis with emphasis on schistosomicidal drugs.

Sister-chromatid exchange frequencies induced by metrifonate in mammalian in vivo and in vitro systems

Metrifonate is a drug widely used to cure several helminthiases in man including cysticercosis. In this study we determine its capacity to induce SCEs in vivo and in vitro using the fluorescence-plus-Giemsa technique. In the human lymphocyte system we tested 10, 20, 30, 40, 50 and 60 micrograms/ml added at 24 h of culture with a duration of 72 h. The results showed a SCE inducing effect with the exception of the first tested dose (10 micrograms/ml), but without a dose-response expression. A cell cycle progression delay was seen with the three highest dosages. In the mouse bone marrow we tested 30, 60 and 120 mg/kg in a 24-h experiment, and the results indicated an SCE increase only with the highest dose. No modification of the cell proliferation kinetics was observed.

Field trial of the efficacy of a simplified and standard metrifonate treatments of Schistosoma haematobium

In a randomized double-blind study, the percentage egg reduction and cure rate after the standard schedule of metrifonate treatment of Schistosoma haematobium (3 doses of 7.5 mg.kg-1 at two-weekly intervals; A) and an abbreviated regimen (3 doses of 5 mg.kg-1 in one day; B) were compared in five villages in Somalia. 300 patients who were excreting 20 or more eggs of S. haematobium in 10 ml urine were recruited. The patients were classified according to their home villages and were then, randomly allocated to treatment A or B. They had similar ages, weights and egg output. Each patient received 3 doses of metrifonate and 2 doses of identical appearing placebo. Group A received metrifonate on the 1st, 4th and 5th dosing occasions and placebo on the 2nd and 3rd times. Group B received metrifonate on the 1st, 2nd and 3rd dosing times and placebo on the 4th and 5th times. Two hundred and one patients were followed up from 1 to 6 months. The remaining 99 (33%) patients either did not complete treatment or were lost during follow up. Egg reduction in the groups 1, 2, 3 and 6 months after treatment were 97, 97, 95 and 93% in Group A and 96, 96, 94 and 92% in Group B (NS). Corresponding cure rates for Group A were 52, 50, 48 and 44%, and in Group B they were 47, 48, 43 and 40% (NS). Seven patients from Group A and 9 from Group B complained of minor side-effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Parasitic infections, anaemia and nutritional status: a study of their interrelationships and the effect of prophylaxis and treatment on workers in Kwale District, Kenya

A study was conducted between January and August 1979 on 150 male roadworkers in Kwale District in the coastal lowlands of Kenya. At the baseline examination 59% had hookworm (mean of 227 epg +/- 541), 38% had infections of Schistosoma haematobium (mean 52 +/- 82 ova per 10 ml urine), 23% had a blood film positive for malarial parasites, 47% had anaemia (Hb below 13g/dl), and 31% had a percentage weight for height below 80% of a reference value for healthy men. It was found that anaemia was significantly associated with hookworm infections and egg counts. Three interventions were evaluated. These consisted of providing pyrantel pamoate for hookworm to all men, treating the S. haematobium infections with metrifonate, and giving weekly chloroquine prophylaxis for suppression of malaria to one group of men. A final examination conducted 16 weeks later showed a significant improvement in haemoglobin levels in anaemic men treated for hookworm and in those receiving chloroquine compared with those receiving placebos. Multiple regression analyses revealed that both hookworm and S. haematobium egg counts were associated with anaemia at the baseline examinations, that factors related to the treatment of hookworm and prophylaxis for suppression of malaria were related to haemoglobin increases in anaemic men, and that successful treatment of urinary schistosomiasis was associated with weight gain. The feasibility of the interventions was good and the costs moderately low.

The neuropharmacology of schistosomes

The schistosomal nervous system uses some of the same neurotransmitters as the vertebrate system, but significant differences exist with respect to neurotransmitter metabolism and receptor specificity. The cholinergic system has been most extensively studied, and the serotoninergic system has also been examined carefully; catecholamines and benzodiazepines also seem to affect nervous system function. The objective of this work has been to determine whether neuroactive drugs could be used to disrupt the parasites' physiology sufficiently to have therapeutic impact, while avoiding toxic effects on the host nervous system. Promising findings have been made, suggesting that the unique properties of the schistosomal nervous system might be exploited to therapeutic advantage.

Metrifonate. Summary of toxicological and pharmacological information available

The organophosphorus compound 0,0-dimethyl-(1-hydroxy-2,2,2-trichloroethyl)-phosphonate was introduced as an insecticide, trichlorfon, in 1952 (Lorenz et al., 1955) and as a drug, metrifonate, in the treatment of schistosomiasis in 1960 (Lebrun and Cerf, 1960). This organophosphorus compound is unique in that it has been claimed not to be a direct acting cholinesterase inhibitor but being transformed nonenzymatically into an active component dichlorvos, 2, 2-dichlorovinyl dimethyl phosphate (DDVP). The evidence for this transformation has mostly been indirect. Recently it has been proved chemically and quantitatively that this transformation occurs in the animal body (Nordgren et al., 1978). Metrifonate is the sole organophosphorus compound currently studied clinically in schistosomiasis. A substantial therapeutic effect is obtained only in Schistosoma haematobium infections. In this review on available data of metrifonate it is suggested that further more detailed studies of both S. haematobium and S. mansoni are necessary. This should include studies of the enzymic properties of the worms and the reaction of their esterases towards both metrifonate and DDVP as well as the pharmacokinetics of these compounds in man. In addition there are still unsolved discrepancies reported regarding organ toxicity of the compound which may, however, be due to different grades of parity of the test material.

Evaluation of schistosomicides against experimentally established Schistosoma mansoni infections in mice and hamsters

The procedures employed at the Sterling Winthrop Research Institute for the maintenance of Schistosoma mansoni, method of infection, and measurement of efficacy are discussed. Data are presented illustrating the activities of a number of known antischistosomal agents in mice and hamsters against a Puerto Rican strain of the parasite with a well-documented origin.

Contribuição à profilaxia das geohelmintíases

O autor faz uma revisão sumária sobre diferentes ensaios visando ao controle das geohelmintíases e apresenta o resultado de sua experiência na tentativa de controle da ancilostomíase e da ascaridíase em população rural no Estado do Rio de Janeiro e em um orfanato no Estado da Guanabara. Conclui que os tratamentos em massa mantêm as parasitoses sob controle durante os períodos em que são administrados, tomando-se necessário, para melhores e mais persistentes resultados, medidas gerais visando à melhoria das condições sanitárias e sócio-econômicas das populações expostas ao risco.

Inhibition by metrifonate and dichlorvos of cholinesterases in schistosomes

1. No species differences between Schistosoma haematobium and Schistosoma mansoni were detected when the I50 of metrifonate for the acetylcholinesterases (AChE) and the cholinesterases (ChE) of these two trematodes were determined in isolated enzyme preparations or following exposure of the intact worms to this drug in vitro.2. S. haematobium appeared to be more affected by AChE inhibition because, after administration of metrifonate to hamsters, a hepatic shift of the parasites was observed with a dose of metrifonate (150 mg (0.6 mmol) per kg) which produced no shift of S. mansoni, although AChE inhibition was comparable in both species.3. Administration of a possible metabolite of metrifonate, dichlorvos, to hamsters resulted in a greater inhibition of AChE and ChE activities of S. haematobium than those of S. mansoni. Furthermore, when schistosomes were incubated with dichlorvos, inhibition of AChE activity of female S. haematobium was significantly greater (P<0.005) than that of both sexes of S. mansoni and of male S. haematobium.4. The discrepancy between the lack of a significant chemotherapeutic effect of metrifonate in hamsters infected with S. haematobium and the clinical results obtained with this organophosphorus compound in human schistosomiasis haematobium is discussed, and the need to conduct similar studies in primates is pointed out.

Experimental chemotherapy of Schistosomiasis III: laboratory and clinical trials with trichlorphone, an organophosphorus compound

Oogram studies have been carried out on mice, hamsters, and Cebus morikeys experimentally infected with Schistosoma mansoni and treated with trichlorphone (0,0-dimethyl 1-hydroxy-2, 2, 2-trichloroethylphosphonate). In mice, despite a slight hepatic shift of schistosomes, all animais presented oogram changes when dosed, per os, at the schedules of 200, and 100 mg/kg/day × 7. In hamsters, antischistosomal activity could be detected only at toxic leveis. In monkeys, trichlorphone showed insignificant action even after oral administration of 30 mg/kg/day for 10 consecutive days. In 5 volunteers, a sharp drop in cholinesterase plasma level was observed 24 hours after a single oral dose of 7.5 mg/kg. However, cholinesterase levels returned to the initial values within a period of 11 to 27 days. Trichlorphone was then administered to 12 schistosome patients (7.5 mg/kg/day, every fort- night, × 5). One month after therapy, interruption of egg laying was observed in 6 patients. Late parasitological control showed that all treated patients continued to pass viable S. mansoni eggs with their stools.