The rapid development of drug-resistance by Trypanosoma evansi in immunosuppressed mice

Drug resistance in animal trypanosomiases: Epidemiology, mechanisms and control strategies

Animal trypanosomiasis (AT) is a complex of veterinary diseases known under various names such as nagana, surra, dourine and mal de caderas, depending on the country, the infecting trypanosome species and the host. AT is caused by parasites of the genus Trypanosoma, and the main species infecting domesticated animals are T. brucei brucei, T. b. rhodesiense, T. congolense, T. simiae, T. vivax, T. evansi and T. equiperdum. AT transmission, again depending on species, is through tsetse flies or common Stomoxys and tabanid flies or through copulation. Therefore, the geographical spread of all forms of AT together is not restricted to the habitat of a single vector like the tsetse fly and currently includes almost all of Africa, and most of South America and Asia. The disease is a threat to millions of companion and farm animals in these regions, creating a financial burden in the billions of dollars to developing economies as well as serious impacts on livestock rearing and food production. Despite the scale of these impacts, control of AT is neglected and under-resourced, with diagnosis and treatments being woefully inadequate and not improving for decades. As a result, neither the incidence of the disease, nor the effectiveness of treatment is documented in most endemic countries, although it is clear that there are serious issues of resistance to the few old drugs that are available. In this review we particularly look at the drugs, their application to the various forms of AT, and their mechanisms of action and resistance. We also discuss the spread of veterinary trypanocide resistance and its drivers, and highlight current and future strategies to combat it.

Immunomodulatory potential of Sarcophaga argyostoma larval hemolymph as a natural alternative to berenil in treating Trypanosoma evansi in vivo

This study compared effects of diminazene aceturate (berenil), commonly used to treat domestic animals infected with Trypanosoma evansi, with the hemolymph of Sarcophaga argyostoma larva. The hemolymph may be acting as a possible natural alternative to berenil, based on immunomodulation mediated inflammatory response. Inflammatory mediators and histopathological changes in liver, kidney, and spleen of albino mice experimentally infected with T. evansi were studied. Mice were divided into five groups: G1, uninfected, untreated (negative control); G2, T. evansi infected (positive control); G3, infected and treated with berenil; G4, infected and treated with hemolymph; G5, infected and treated with hemolymph 3 days before infection (prophylactic group). Animals in (G4) and (G5) exhibited a significant overall reduction in serum levels of IFN-γ. However, the reduction in TNF-α and IL-6 levels was more limited compared to (G2) and (G3). Notably, an elevation in IL-10 levels was observed compared to animals in other groups. Furthermore, the groups treated with hemolymph demonstrated an alleviation of T. evansi infection in contrast to the other groups. This study highlights that the administration of Sarcophaga argyostoma larval hemolymph at a dosage of 0.5 ml/kg significantly inhibited T. evansi organisms in vivo, showcasing a pronounced trypanocidal effect.

Variation of sensitivity of Trypanosoma evansi isolates from Isiolo and Marsabit counties of Kenya to locally available trypanocidal drugs

Trypanocidal resistance is a major cause of treatment failure. This study evaluated the sensitivity of Trypanosoma evansi field isolates collected from Marsabit and Isiolo counties, Kenya. A total of 2,750 camels were screened using parasitological tests for trypanosomes. Of the screened camels, 113 tested positive from which 40 T. evansi isolates were tested using the single dose mice sensitivity test. Five treatment groups each comprising of 6 mice were inoculated intraperitoneally with 1x105 trypanosomes of each isolate and treated 24 hours later with isometamidium chloride at 1 mg/kg, homidium chloride at 1mg/kg, diminazene aceturate at 20 mg/kg and quinapyramine sulphate & chloride at 1 mg/kg. The fifth group was left untreated (positive control). The mice were monitored daily for 60 days. A survey on camel owners' practices that influence development of resistance to trypanocidal drugs was then conducted. Results indicated presence of drug resistance in all the 7 study sites that had infected camels. Seven of the isolates tested were resistant to diminazene aceturate whereas, 28, 33 and 34 were resistant to isometamidium chloride, quinapyramine sulphate & chloride and homidium chloride, respectively. Seven (17.5%) isolates of the 40 tested were sensitive to all 4 drugs, whereas, 7.5%, 10%,55% and 10% were resistant to 1,2,3 and 4 drugs, respectively. The prevalence of multiple drug resistance was 75%. Survey data indicated that camel management practices influenced the prevalence and degree of drug resistance. In conclusion, the multiple drug resistance observed in the two counties may not be an indication of total trypanocidal drug failure. Judicious treatment of confirmed trypanosomiasis cases with correct dosage would still be effective in controlling the disease since the observed resistance was at the population and not clonal level. However, integrated control of the disease and the vectors using available alternative methods is recommended to reduce drug use.

African animal trypanocide resistance: A systematic review and meta-analysis

Background

African animal trypanocide resistance (AATr) continues to undermine global efforts to eliminate the transmission of African trypanosomiasis in endemic communities. The continued lack of new trypanocides has precipitated drug misuse and overuse, thus contributing to the development of the AATr phenotype. In this study, we investigated the threat associated with AATr by using the major globally available chemotherapeutical agents.

Methods

A total of seven electronic databases were screened for an article on trypanocide resistance in AATr by using keywords on preclinical and clinical trials with the number of animals with treatment relapse, days taken to relapse, and resistant gene markers using the PRISMA checklist. Data were cleaned using the SR deduplicator and covidence and analyzed using Cochrane RevMan®. Dichotomous outputs were presented using risk ratio (RR), while continuous data were presented using the standardized mean difference (SMD) at a 95% confidence interval.

Results

A total of eight publications in which diminazene aceturate (DA), isometamidium chloride (ISM), and homidium chloride/bromide (HB) were identified as the major trypanocides were used. In all preclinical studies, the development of resistance was in the order of HB > ISM > DA. DA vs. ISM (SMD = 0.15, 95% CI: -0.54, 0.83; I ² = 46%, P = 0.05), DA vs. HB (SMD = 0.96, 95% CI: 0.47, 1.45; I ² = 0%, P = 0.86), and HB vs. ISM (SMD = -0.41, 95% CI: -0.96, 0.14; I ² = 5%, P = 0.38) showed multiple cross-resistance. Clinical studies also showed evidence of multi-drug resistance on DA and ISM (RR = 1.01, 95% CI: 0.71-1.43; I ² = 46%, P = 0.16). To address resistance, most preclinical studies increased the dosage and the treatment time, and this failed to improve the patient's prognosis. Major markers of resistance explored include TbAT1, P1/P2 transporters, folate transporters, such as F-I, F-II, F-III, and polyamine biosynthesis inhibitors. In addition, immunosuppressed hosts favor the development of AATr.

Conclusion

AATr is a threat that requires a shift in the current disease control strategies in most developing nations due to inter-species transmission. Multi-drug cross-resistance against the only accessible trypanocides is a major public health risk, justifying the need to revise the policy in developing countries to promote control of African trypanosomiasis.

Immunosuppression of Syrian golden hamsters accelerates relapse but not the emergence of resistance in Leishmania infantum following recurrent miltefosine pressure

Although miltefosine (MIL) has only been approved for the treatment of visceral leishmaniasis (VL) in 2002, its application in monotherapy already led to the development of two confirmed MIL-resistant isolates by 2009. Although liposomal amphotericin B is recommended as first-line treatment in Europe, MIL is still occasionally used in HIV co-infected patients. Since their immune system is incapable of controlling the infection, high parasite burdens and post-treatment relapses are common. Linked to the particular pharmacokinetic profile of MIL, successive treatment of recurrent relapses could in principle facilitate the emergence of drug resistance. This study evaluated the effect of immunosuppression (cyclophosphamide 150 mg/kg once weekly) on the development of MIL-resistance in Syrian golden hamsters infected with Leishmania infantum. The hamsters were treated with MIL (20 mg/kg orally for 5 days) whenever clinical signs of infection or relapse were observed. The immunosuppression resulted in a significant depletion of CD4⁺ lymphocytes and MHCII-expressing cells in peripheral blood, and a concomitant increase in tissue parasite burdens and shorter time to relapse, but the strain's susceptibility upon repeated MIL exposure remained unaltered. This study demonstrates that immunosuppression accelerates the occurrence of relapse without expediting MIL resistance development.

Genomic analysis of Isometamidium Chloride resistance in Trypanosoma congolense

Isometamidium Chloride (ISM) is one of the principal drugs used to counteract Trypanosoma congolense infection in livestock, both as a prophylactic as well as a curative treatment. However, numerous cases of ISM resistance have been reported in different African regions, representing a significant constraint in the battle against Animal African Trypanosomiasis. In order to identify genetic signatures associated with ISM resistance in T. congolense, the sensitive strain MSOROM7 was selected for induction of ISM resistance in a murine host. Administered ISM concentrations in immune-suppressed mice were gradually increased from 0.001 mg/kg to 1 mg/kg, the maximal dose used in livestock. As a result, three independent MSOROM7 lines acquired full resistance to this concentration after five months of induction, and retained this full resistant phenotype following a six months period without drug pressure. In contrast, parasites did not acquire ISM resistance in immune-competent animals, even after more than two years under ISM pressure, suggesting that the development of full ISM resistance is strongly enhanced when the host immune response is compromised. Genomic analyses comparing the ISM resistant lines with the parental sensitive line identified shifts in read depth at heterozygous loci in genes coding for different transporters and transmembrane products, and several of these shifts were also found within natural ISM resistant isolates. These findings suggested that the transport and accumulation of ISM inside the resistant parasites may be modified, which was confirmed by flow cytometry and ex vivo ISM uptake assays that showed a decrease in the accumulation of ISM in the resistant parasites.

The animal trypanosomiases and their chemotherapy: a review

Pathogenic animal trypanosomes affecting livestock have represented a major constraint to agricultural development in Africa for centuries, and their negative economic impact is increasing in South America and Asia. Chemotherapy and chemoprophylaxis represent the main means of control. However, research into new trypanocides has remained inadequate for decades, leading to a situation where the few compounds available are losing efficacy due to the emergence of drug-resistant parasites. In this review, we provide a comprehensive overview of the current options available for the treatment and prophylaxis of the animal trypanosomiases, with a special focus on the problem of resistance. The key issues surrounding the main economically important animal trypanosome species and the diseases they cause are also presented. As new investment becomes available to develop improved tools to control the animal trypanosomiases, we stress that efforts should be directed towards a better understanding of the biology of the relevant parasite species and strains, to identify new drug targets and interrogate resistance mechanisms.

Resource competition may lead to effective treatment of antibiotic resistant infections

Drug resistance is a common problem in the fight against infectious diseases. Recent studies have shown conditions (which we call antiR) that select against resistant strains. However, no specific drug administration strategies based on this property exist yet. Here, we mathematically compare growth of resistant versus sensitive strains under different treatments (no drugs, antibiotic, and antiR), and show how a precisely timed combination of treatments may help defeat resistant strains. Our analysis is based on a previously developed model of infection and immunity in which a costly plasmid confers antibiotic resistance. As expected, antibiotic treatment increases the frequency of the resistant strain, while the plasmid cost causes a reduction of resistance in the absence of antibiotic selection. Our analysis suggests that this reduction occurs under competition for limited resources. Based on this model, we estimate treatment schedules that would lead to a complete elimination of both sensitive and resistant strains. In particular, we derive an analytical expression for the rate of resistance loss, and hence for the time necessary to turn a resistant infection into sensitive (tclear). This time depends on the experimentally measurable rates of pathogen division, growth and plasmid loss. Finally, we estimated tclear for a specific case, using available empirical data, and found that resistance may be lost up to 15 times faster under antiR treatment when compared to a no treatment regime. This strategy may be particularly suitable to treat chronic infection. Finally, our analysis suggests that accounting explicitly for a resistance-decaying rate may drastically change predicted outcomes in host-population models.

Trypanosoma evansi and surra: a review and perspectives on origin, history, distribution, taxonomy, morphology, hosts, and pathogenic effects

Trypanosoma evansi, the agent of "surra," is a salivarian trypanosome, originating from Africa. It is thought to derive from Trypanosoma brucei by deletion of the maxicircle kinetoplastic DNA (genetic material required for cyclical development in tsetse flies). It is mostly mechanically transmitted by tabanids and stomoxes, initially to camels, in sub-Saharan area. The disease spread from North Africa towards the Middle East, Turkey, India, up to 53° North in Russia, across all South-East Asia, down to Indonesia and the Philippines, and it was also introduced by the conquistadores into Latin America. It can affect a very large range of domestic and wild hosts including camelids, equines, cattle, buffaloes, sheep, goats, pigs, dogs and other carnivores, deer, gazelles, and elephants. It found a new large range of wild and domestic hosts in Latin America, including reservoirs (capybaras) and biological vectors (vampire bats). Surra is a major disease in camels, equines, and dogs, in which it can often be fatal in the absence of treatment, and exhibits nonspecific clinical signs (anaemia, loss of weight, abortion, and death), which are variable from one host and one place to another; however, its immunosuppressive effects interfering with intercurrent diseases or vaccination campaigns might be its most significant and questionable aspect.

Studies of quinapyramine-resistance of Trypanosoma brucei evansi in China

In the present article, we summarize our studies of antrycide-resistance of Trypanosoma brucei evansi in four aspects in the last recent several years, the analysis of quinapyramine-sensitive situation of T. b. evansi in China, biological characteristics of T. b. evansi population in quinapyramine-resistance and biological materials of quinapyramine-resistance in T. b. evansi population. Firstly, the correlative assays of effective dosage of quinapyramine on T. b. evansi disease between in vivo and in vitro methods showed that their relationship was parabolic with positive correlation. On the other hand, the IC(50) and CD(100) values of 12 T. b. evansi isolates, AHB, GDB1, GDB2, HNB, JSB1, JSB2, YNB, ZJB, GDH, GXM, HBM and XJCA, collected from buffaloes, horses, mules and camels across nine provinces of China were examined using the two methods, respectively. Among them, the nine isolates, AHB, GDB1, GDB2, HNB, JSB1, JSB2, YNB, ZJB and GDH, became quinapyramine-sensitive T. b. evansi. Secondly, T. evansi populations could rapidly obtain antrycide-resistance when they were passed through immunosuppressed mice treated with low doses of the drug. But, the replication rate of trypanosomes with antrycide-resistance decreases as the level of drug-resistance increases. Thirdly, the analysis of the HK, G6PDH, ALAT and ASAT isoenzymes showed that they were not involved in the quinapyramine-resistance of T. b. evansi. But the protein bands of 15.79kDa and 19.76kDa might be involved in the antrycide-resistance of T. b. evansi population. At genetic level, the gene, TbTA1, could be amplified from the T. b. evansi isolate sensitive to quinapyramine-sensitivity but the T. b. evansi isolate with quinapyramine-resistance using not only the RT-PCR technique, but also PCR technique. We used the SSH (Suppression Subtractive Hybridization) to clone highly or low expressed cDNA fragments caused by production of antrycide-resistance in T. b. evansi. The 5 low and 9 high expressed new cDNA fragments were amplified. Among them, the 3 low expressed cDNA fragments had the same sequence of 65 amino acids and the 3 high expressed cDNA fragments were located in chromosome VI, like T. brucei. Lastly, more work needs to be done in order to elucidate the mechanism of quinapyramine-resistance of T. b. evansi.

Melarsoprol- and pentamidine-resistant Trypanosoma brucei rhodesiense populations and their cross-resistance

Resistance to melarsoprol and pentamidine was induced in bloodstream-form Trypanosoma brucei rhodesiense STIB 900 in vitro, and drug sensitivity was determined for melarsoprol, pentamidine and furamidine. The resistant populations were also inoculated into immunosuppressed mice to verify infectivity and to monitor whether rodent passage selects for clones with altered drug sensitivity. After proliferation in the mouse, trypanosomes were isolated and their IC(50) values to the three drugs were determined. To assess the stability of drug-induced resistance, drug pressure was ceased for 2 months and the drug sensitivity was determined again. Resistance was stable, with a few exceptions that are discussed. Drug IC(50)s indicated cross-resistance among all drugs, but to varying extents: resistance of the melarsoprol-selected and pentamidine-selected trypanosomes to pentamidine was the same, but the pentamidine-selected trypanosome population showed lower resistance to melarsoprol than the melarsoprol-selected trypanosomes. Interestingly, both resistant populations revealed the same intermediate cross-resistance to furamidine. Resistant trypanosome populations were characterised by molecular means, referring to the status of the TbAT1 gene. The melarsoprol-selected population apparently had lost TbAT1, whereas in the pentamidine-selected trypanosome population it was still present.

Resistance to clinical drugs in African trypanosomes

Drug resistance in African trypanosomes continues to confound clinicians and to stymy development o f equatorial Africa, taking its toll in lives and economic development. Drugs in current, widespread use have been employed continuously for over 60 years in some instances. The recent studies of Fairlamb and colleagues have outlined a defective purine-transport system in drug-resistant trypanosomes, which appears to explain resistance to several established tryponocides and suggests a guide for the development of new drugs. The recently developed agent dl-alpha-di fluoromethylornithine (DFMO) is effective against West African, but not East African, disease and its activity may be the result of the unregulated synthesis of S-odenosylmethionine in tryponosomes. In this report, Cyrus Bacchi outlines recent developments in the elucidation of mechanisms of resistance to established drugs and naturally occurring resistance to DFMO.

Drug resistance in pathogenic African trypanosomes: what hopes for the future?

Trypanosomosis is a serious threat to both man and animals mostly in Africa. Although the first pathogenic trypanosome was discovered over a hundred years ago, there is still no prospect for effective control or eradication of the disease through the development and use of vaccines because of the phenomenon of antigenic variation. Control continues to rely heavily on chemotherapy and vector control strategies. This therapy and prophylaxis depends on the use of drugs which, apart from having been developed over 5 decades ago, suffer from such limitations as toxicity and with their continued use, drug resistance. Resistance to currently used drugs is a serious problem in most fields of anti-microbial chemotherapy, particularly in the case of trypanosomosis where resistance and cross-resistance in animals and man have been developing rapidly. The frequently and widely reported decreasing efficiency of available trypanocides, difficulties of sustaining tsetse control and little hope that a conventional, anti-trypanosome vaccine will be produced in the near future, increase the imperative need for new drugs and alternative effective ways for the control of trypanosomosis. This review examines aspects of drug resistance in pathogenic trypanosomes, measures to minimise it, areas of future research in new drug targets and alternative control strategies. Based on these, it is our opinion that for now the management and control of trypanosomosis will continue to depend on proper usage of the few available trypanocides, especially strategic deployment of the sanative drugs in order to reduce the development of drug resistance, in addition to the continued use of environmentally friendly vector control programmes such tsetse trapping.

African bovine trypanosomiasis: the problem of drug resistance

The three trypanocides used to control tsetse-transmitted trypanosomiasis in domestic animals in Africa have been in use for over 40 years and, not surprisingly, resistance of trypanosomes to these drugs has emerged. Because of the relatively limited market in Africa and the high costs of developing and licensing new drugs, international pharmaceutical companies have shown little interest in the development of new trypanocides for use in either animals or humans. Therefore, the current challenge is to achieve optimal use of the relatively old existing drugs, and it is in this context that the problem of drug resistance has to be quantified--as discussed here by Stanny Geerts, Peter Holmes, Oumar Diall and Mark Eisler.

Development of a decision support system for trypanocidal drug control of bovine trypanosomosis in Africa

During this century livestock production has increased massively through the improved ability to diagnose, treat, control and prevent certain diseases. Despite these advances, disease remains a major constraint to livestock production and welfare throughout the world. In many instances, this is a result of failure to properly apply methods that are already available. TrypsChemo is an expert system that attempts to aid the application of veterinary knowledge to disease management. It has been designed to maximise the effectiveness and cost efficiency of the different trypanocidal drug regimens currently available for prophylaxis and treatment of tsetse-transmitted bovine trypanosomosis in Africa. This paper describes the design of TrypsChemo, the properties of the system, and illustrates how it can be used to support decision making for trypanocidal drug control. The system is currently undergoing a structured evaluation by potential users in Africa.

Uptake of diamidine drugs by the P2 nucleoside transporter in melarsen-sensitive and -resistant Trypanosoma brucei brucei

The African trypanosome, Trypanosoma brucei brucei, possesses at least two nucleoside transporter systems designated P1 and P2, the latter being implicated in the selective uptake of melaminophenyl arsenical drugs. Since arsenical-resistant trypanosomes show cross-resistance in vivo to aromatic diamidines, we have investigated whether these drugs are also substrates for the P2 nucleoside transporter. In melarsen-sensitive T. b. brucei, the diamidines, including the commonly used trypanocides, pentamidine and berenil, were found to abrogate lysis induced by the P2 transport of melarsen oxide in vitro. Measurement of [ring-3H]pentamidine transport in melarsen-sensitive T. b. brucei, demonstrated that uptake is carrier-mediated, with a Km of 0.84 microM and a Vmax of 9.35 pmol s-1 (10(8) cells)-1. Pentamidine transport appears to be P2-mediated in these cells, as pentamidine strongly inhibited uptake of [2',5',8-3H]adenosine by the P2 transporter, with a Ki of 0.56 microM. Furthermore, [ring-3H]pentamidine transport was blocked by a number of P2 transporter substrates and inhibitors, as well as by other diamidine drugs. Analysis of the uptake of pentamidine and other diamidines in melarsen-resistant trypanosomes in vitro and in vivo, which also show differential levels of resistance to these compounds in vivo, indicated that P2 transport was altered in these cells and that accumulation of these drugs was markedly reduced.

Induction of resistance to melarsenoxide cysteamine (Mel Cy) in Trypanosoma brucei brucei

A population of Trypanosoma brucei brucei with reduced sensitivity to melarsenoxide cysteamine (Mel Cy) was produced in immunosuppresed mice using subcurative drug treatment. Melarsenoxide cysteamine resistance was stable after cyclical transmission through Glossina morsitans centralis. In vitro, the blood-stream forms showed 15-fold higher values for the minimal inhibitory concentration as compared with the parental clone. Cross-resistance could be determined with another arsenical drug, melarsoprol (14-fold) and to two different diamidines (diminazene aceturate: 47-fold; pentamidine methanesulphonate: 34-fold), but not to suramin. When cells were transformed to procyclic forms and tested in vitro, the sensitivity of the resistant population to melarsenoxide cysteamine was only 6-fold lower than that of the parent, but comparatively high cross-resistance could be shown to other drugs (melarsoprol; 85-fold; pentamidine methanesulphonate: 17-fold; quinapyramine sulphate: 40-fold). Selection of the resistant trypanosomes from non-resistant ones was possible under pentamidine methanesulphonate pressure in cell culture.

Chemotherapy and delivery systems: haemoparasites

Chemotherapy of haemoparasitic diseases in domestic animals is dependent on a limited number of compounds, many of which are chemically closely related. In this review, a summary is given of each of the drugs currently available for treatment and prophylaxis of trypanosomosis and the tick-borne diseases theileriosis, babesiosis, anaplasmosis and cowdriosis. In contrast to the situation with the drugs used for tick-borne diseases, drug resistance appears to be becoming an increasing problem associated with the compounds used for trypanosomosis. The literature that has been reviewed, therefore, is that which relates to the methods used to identify and quantify drug resistance in trypanosome populations, reports of resistance to trypanocides, and cross-resistance between trypanosome populations, reports of resistance to trypanocides, and cross-resistance between trypanocides. The possible reason(s) for the apparent lack of development of resistance to the compounds used for treatment of tick-borne diseases is also discussed. Local toxicity at the site of injection is a problem that is particularly associated with many of the trypanocides when used on a long-term basis in individual animals. Various alternative preparations of the currently used trypanocides therefore have been evaluated in an attempt to reduce this toxicity, and are summarised. Finally, future developments in haemoparasitic chemotherapy are considered and, for trypanosomosis, highlight the importance of integrating chemotherapeutic and chemoprophylactic programmes with control of the vector when drug resistance becomes a significant constraint.

Pharmacology of diminazene: a review

Chemotherapy for trypanosomiasis in domestic livestock depends on only a few compounds, of which several are chemically closely related. Of these compounds, the most widely used therapeutic agent in cattle, sheep and goats is diminazene aceturate. Diminazene was first described in 1955. Subsequently, a substantial body of data has been generated on various pharmacological aspects of the compound. In this review, we consider the current status of knowledge concerning the therapeutic spectrum of diminazene, resistance to diminazene in trypanosomes, and combination therapeutic regimens in which diminazene has been administered together with other compounds. Analytical techniques for diminazene, the pharmacokinetics of diminazene, data concerning diminazene's toxicity, and the different molecular mechanisms by which diminazene may exhibit trypanocidal action are also considered.

Preface: positive interactions between anti-infection drugs and the immune response: an emerging paradigm

In the third and fourth decades of this century chemotherapy began to be established as one of the greatest success stories in medicine. Now unfortunately severe problems compromise the efficacy of drugs used to treat infectious diseases, two of the most serious handicaps being the rapidity with which target pathogens can develop drug-resistance and the slow rate at which replacement products are appearing on the market. Increased understanding of the ways in which existing drugs act may help both to prolong their usefulness and to generate novel therapeutic strategies.

Hypothesis: impaired immunity as a factor which contributes to the spread of drug-resistance

Evidence has accrued to indicate that host defence mechanisms enhance the efficacy of many of the drugs used to treat infectious diseases. Because of this, and also because of the likelihood of increased pathogen loads in immunoincompetent hosts, some infections are less likely to be completely cured by normal regimens of chemotherapy in individuals with drastically impaired immune responsiveness. In such circumstances natural selection could result in the accelerated emergence of drug-resistance pathogens.

Interactions between immunity and chemotherapy in the treatment of the trypanosomiases and leishmaniases

The immune status of a host infected with Trypanosoma spp. or Leishmania spp. can play an important role in successful chemotherapy. In animal models, treatment of African trypanosomiasis with difluoromethylornithine or melarsoprol requires an appropriate antibody-mediated immune response. An intact immune system is also necessary for rapid clearance of trypanosomes from the bloodstream following treatment with suramin or quinapyramine. Similarly, an efficient cell-mediated immune responses is required for maximal efficacy of pentavalent antimonials in the treatment of leishmaniasis. However, the potential relationship between parasite-induced or acquired immunosuppression and effective chemotherapy has been poorly studied. Macrophages which have been activated by bacterial cell wall components or gamma-interferon are known to display increased activity against Leishmania donovani or Trypanosoma cruzi. In experimental and clinical visceral leishmaniasis, use of macrophage activators together with pentavalent antimonials has lowered the dose of antimony required to cure the infection.