Pathogenic protozoa and helminths are among the most important causes of morbidity and mortality in the world. An estimated 700 million people suffer from malaria, filariasis, and schistosomiasis alone, and two-thirds of the world's population lives in conditions in which parasitic diseases are unavoidable.
Some parasitic diseases were among the first to be treated by specific remedies; indeed, cures for malaria, amoebic dysentery, and tapeworm infection have been known for centuries. Nevertheless, the therapeutic armamentarium for parasitic infection remains severely restricted. Many of the antiparasitic drugs that are available leave much to be desired in terms of efficacy and safety, and a few parasitic infections remain for which there is no effective remedy at all.
Protozoa
Protozoa are unicellular organisms. Those of medical importance are conveniently classified into four groups: amoebae, flagellates, sporozoa, and ‘others’ (Table 5.1). The agents used for treatment are very varied and often specific to the particular organism involved. Consequently, antiprotozoal agents defy formal classification and are best considered in the context of the organisms against which they are used. The mode of action is often poorly characterized.
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Table 5.1 Principal pathogenic protozoa infecting man, and the drugs commonly used in treatment |
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Amoebae
Parasitic amoebae
Several species of parasitic amoebae are found in man. Only one, Entamoeba histolytica, the causative parasite of amoebic dysentery and amoebic liver abscess, is commonly incriminated in disease. Invasive infection is caused by the motile trophozoite form, but the disease is transmitted by non-motile cysts, which represent
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a more resistant resting phase. Occurrence of a morphologically identical, but non-pathogenic, species, E. dispar, makes laboratory identification of cyst excreters problematic.
An effective treatment for amoebiasis has been available for many years in the form of emetine, an alkaloid of ipecacuanha root. Emetine, and its less toxic derivative dehydroemetine, act by inhibiting protein synthesis in amoebae. These drugs have been superseded by metronidazole (or one of the other 5-nitroimi-dazoles) (p. 53), which is very effective in acute amoebiasis and amoebic liver abscess.
Other useful drugs include diloxanide furoate, which is particularly effective for the elimination of cysts from symptomless excreters, and chloroquine. Two antibiotics, tetracycline and the aminoglycoside paromomycin, also have some activity against amoebae. Di-iodohydroxyquinoline (diodoquin; iodoquinol) is no longer recommended because of its toxicity.
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Free-living amoebae
Although E. histolytica is, to all intents and purposes, the only parasitic amoeba pathogenic to man, certain species of free-living amoebae (especially Naegleria spp.) occasionally cause primary amoebic meningoencephalitis. The condition is almost invariably fatal and only amphotericin B, an antifungal agent (p. 61), has shown any useful activity in tests in vitro. Free-living amoebae of the Acanthamoeba group are occasionally involved in serious eye infections (amoebic keratitis) following contamination of contact lenses. Suitable antimicrobial chemotherapy for this condition remains to be defined, but local application of propamidine and neomycin is often used.
Flagellates
Trypanosomes
Trypanosomes are transmitted by insects, and their distribution is restricted to areas where the vector is found. In tropical Africa, tsetse flies transmit Trypano-soma brucei, of which two subspecies (T. brucei gambiense and T. brucei rhode-siense) cause ‘sleeping sickness’, a disease that is inexorably fatal if untreated.
Organic arsenicals, though extremely toxic, have been used for many years to treat all forms of sleeping sickness. Melarsoprol (Mel B) is the derivative usually used. Melarsoprol affects glycolysis, and also forms a complex with trypanothione, which replaces glutathione in these organisms. Resistance may occur and is thought to be due to reduced uptake of the drug.
If treatment can be started before the trypanosomes invade the central nervous system, suramin—which, like melarsoprol, interferes with glycolysis—may be curative. The diamidine pentamidine is also effective in the early stages of T. brucei gambiense infection. Various targets have been proposed for this drug, but its primary site of action has proved elusive.
South American trypanosomiasis, Chagas' disease, is a chronic condition affecting heart muscle and other organs. It is caused by T. cruziand is transmitted by reduviid bugs, nicknamed ‘kissing bugs’ because of their predilection for feeding round the mouths of sleeping persons. There is presently no drug of proven efficacy in Chagas' disease, although modest success has been obtained with the nitrofuran derivative nifurtimox, and the imidazole benznidazole.
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Leishmania
Leishmania, which are related to the trypanosomes, also cause a variety of clinical conditions and are also transmitted by biting insects, in this case sandflies. Cutaneous leishmaniasis (oriental sore) caused by Leishmania tropica or L. major is usually self limiting, but visceral leishmaniasis (kala azar) in which the reticulo-endothelial system is infected by L. donovani or L. infantum, is potentially fatal. Leishmaniasis occurs in the Middle East, India, parts of Africa, and countries on the south European coast of the Mediterranean. Various species of Leishmania have also been incriminated in disease in Central and South America, including L. braziliensis (mucocutaneous leishmaniasis, or espundia), L. mexicana (Chiclero's ulcer, a form of cutaneous leishmaniasis), and L. chagasi (visceral leishmaniasis).
Two pentavalent antimonials, sodium stibogluconate and meglumine antimonate, are traditionally used for the treatment of leishmaniasis. Resistance has, however, become a problem in many parts of the world and alternatives have been sought. Most success has been achieved with the antifungal polyene ampho-tericin B (p. 61), especially when administered in a liposomal formulation that carries the drug into macrophages. The antifungal azoles (p. 62), the aminogly-coside, paromomycin, and pentamidine also exhibit some activity against leishmania and offer alternatives in recalcitrant cases. A new oral phosphocholine analogue, miltefosine, is presently in clinical trial.
Other flagellates
Two other flagellates cause disease in man: Giardia lamblia (also known as G. intestinalis)—a common cause of diarrhoea, abdominal pain, and steatorrhoea—and Trichomonas vaginalis, a common cause of vaginitis or, more rarely, urethritis. G. lamblia is transmitted in the cyst form, often in infected water; T. vaginalis is transmitted venereally. Both of these parasites are susceptible to nitroimidazoles such as metronidazole (p. 53). Resistance is uncommon. Mepacrine (known in the US as quinacrine), and the anthelminthic benzimida-zole albendazole also exhibit effective antigiardial activity, but there are few alternatives for refractory trichomoniasis—except, perhaps, polyenes such as natamycin and trichomycin (p. 61), which are used in some countries.
Sporozoa
The sporozoa are all parasitic; they have a complex life cycle involving alternate sexual and asexual phases. Among important human parasites are the malaria parasites and the coccidia.
Malaria parasites
Malaria is the most important of all parasitic diseases. It remains the commonest cause of fever in the world and is a major cause of morbidity and mortality in areas of high endemicity throughout the tropical belt. Four species infect man.
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Plasmodium falciparum is the most dangerous, since primary infections are often rapidly fatal if left untreated. P. vivax and P. ovale, which cause benign tertian malaria, and P. malariae, which causes quartan malaria, rarely kill but give rise to debilitating infections. The most common species world-wide is P. falciparum, which accounts for over 90 per cent of infections in tropical Africa; in some parts of the world, notably the Indian subcontinent, P. vivax is the dominant species.
The malaria parasite is transmitted by the bite of infected female Anopheles mosquitoes. The parasites first infect liver cells; then after 1–2 weeks the liver parasites mature and infect circulating red blood cells to commence the cycle of erythrocytic schizogony which is responsible for the overt signs of disease. P. vivax and P. ovale can also set up a cryptic infection in the liver, which may cause the relapse of symptoms up to 2 years after the infection is acquired. A proportion of erythrocytic parasites differentiate into male and female gameto-cytes, which do not develop further in the mammalian host but complete the sexual phase of development in the anopheline vector when ingested during a blood meal.
The traditional mainstay of the treatment of malaria is quinine, originally derived from the bark of the cinchona tree in Peru, but various other effective antimalarials have been developed. These include the 4-aminoquinolines chloro-quine and amodiaquine, which act on erythrocytic parasites, and the 8-aminoquinoline primaquine, which is selectively active against the liver forms. Quinine and the 4-aminoquinolines are thought to achieve their effect by preventing the polymerization of haem (ferriprotoporphyrin IX), a reaction which is needed to detoxify this product of parasite metabolism within red blood cells. Primaquine appears to act in a different way, possibly by interfering with mito-chondrial enzymes.
Pyrimethamine, a dihydrofolate reductase inhibitor related to trimethoprim (p. 48), exhibits a selectively high affinity for the plasmodial form of the enzyme. It has been used alone, or with dapsone (p. 58), for treatment and prophylaxis. An alternative combination of pyrimethamine with sulfadoxine has been associated with some fatal reactions and is no longer recommended as a prophylactic agent. Surprisingly, some pyrimethamine-resistant mutants retain susceptibility to proguanil and chlorproguanil, biguanides that are metabolized in the body to compounds closely related to pyrimethamine, and having an identical mode of action. Proguanil is used solely for prophylaxis. A combination with atovaquone (a hydroxynaphthoquinone that acts on the respiratory chain of some protozoa), is available for treatment and prophylaxis. Chlorproguanil (Lapudrine) combined with dapsone—‘lapdap’— is also under investigation in Africa.
Since the 1960s, resistance to chloroquine and other antimalarial agents has become common in P. falciparum in many parts of the world. Quinine is the most reliable alternative, and this old compound has resumed its place as the drug of choice for the treatment of falciparum malaria (see p. 357). Chloroquine-resistant strains usually remain susceptible to mefloquine (a quinolinemethanol) and
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halofantrine (a phenanthrenemethanol), but resistance to these is also beginning to appear. Derivatives of artemisinin, the active principle of an ancient Chinese herbal remedy, qinghaosu, are being successfully used in parts of south-east Asia and Africa. Formulations include artemether and the water-soluble artesunate (hemisuccinate), which are suitable for injection. Some antibiotics, notably tetra-cyclines and clindamycin, have antimalarial activity and are used as adjuncts to quinine therapy in chloroquine-resistant falciparum malaria.
Coccidia
Phylogenetically related to the malaria parasites are the coccidia, which share many features of the complex life cycle, but are not transmitted by insect vectors. Several species, including Cryptosporidium parvum, Isospora belli, and Cyclospora cayetanensis cause diarrhoea in man. Infection is usually self-limiting, but in immunocompromised patients, especially those suffering from AIDS, they may cause severe and protracted symptoms. C. parvum in particular may cause a profuse, intractable diarrhoea for which various drugs, including macrolides such as azithromycin and the aminoglycoside paromomycin, have been used (alone or in combination) with modest success. Nitazoxanide, a nitrothiazole derivative that exhibits activity against protozoa as well as certain helminths and bacteria, also shows some promise in cryptosporidiosis. Infections with I. belli and Cyclo. cayetanensis respond to co-trimoxazole, though antimicrobial treatment is not usually necessary.
The most important human coccidian parasite is Toxoplasma gondii. Intrauterine infections with this organism are an important cause of congenital malformations and stillbirth throughout the world. AIDS sufferers may develop toxoplasma encephalitis, apparently by reactivation of latent infection. Cats often harbour the parasite and liberate the infectious oocysts in their faeces; this probably represents a major reservoir of infection, although undercooked meat is also a recognized source. Pyrimethamine, in combination with a sulphonamide (usually sulphadiazine) is the treatment of choice in toxoplasmosis. Clindamycin and the macrolide antibiotic spiramycin have also been successfully use, especially in combination with pyrimethamine. Spiramycin has been recommended during pregnancy, when antifolates are best avoided.
Other protozoa
Balantidium coli is a ciliate, cosmopolitan in distribution, which is a rare cause of severe diarrhoea. Treatment has not been properly defined, but tetracyclines and metronidazole appear to be effective.
Babesia spp., like malaria parasites, infect red blood cells, but they are unrelated. They are predominantly animal parasites that are occasionally transmitted to man by the bite of ixodid ticks. Recorded European cases have mostly been in splenectomized patients and have usually been caused by B. divergens. Infection in these patients is life-threatening and requires urgent treatment, but
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experience is limited. There is evidence that the combination of clindamycin and quinine is useful, but exchange transfusion may also be needed. Infection with B. microti occurs in previously healthy persons in parts of North America. It is usually self-limiting, but clindamycin and quinine can be used if therapy is warranted.
Encephalitozoon cuniculi and some other microsporidia occasionally cause infection, usually in immunocompromised patients. Although these protozoa have ribosomes of the prokaryotic type, inhibitors of bacterial protein synthesis do not seem to work. There is anecdotal evidence that albendazole may be useful.
Helminths
Helminths are parasitic worms. They often have a complex life cycle involving a period of development outside the definitive host either in soil or in an intermediate host. Helminths of medical importance fall into three major groups: nematodes (roundworms), trematodes (flukes), and cestodes (tapeworms) (Table 5.2).
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Table 5.2 Principal helminth parasites of man and the drugs commonly used in treatment |
Little work is done on the development of anthelminthic agents for use in man, and most agents in present use have emerged through application of drugs originally intended for the treatment of animals. Despite this, a wide variety of compounds is available for use in worm infections and, astonishingly, there are now three compounds—ivermectin, praziquantel, and the benzimidazole derivative albendazole—which between them cover virtually the whole helminthic spectrum. Resistance to these agents is known to occur in animals, and there are reports of failures of treatment in man, but difficulties in testing limit knowledge of the prevalence of resistance or its mechanisms.
Information on the mechanisms by which anthelminthic agents achieve their effect is also relatively limited. The neuromuscular system of worms seems to be peculiarly susceptible to chemotherapeutic attack: many important anthel-minthic agents, including piperazine, praziquantel, ivermectin, levamisole, and pyrantel appear to achieve their effect by paralysing the worms. In contrast, the benzimidazoles interfere with the polymerization of tubulin in the formation of microtubules in the cytoskeleton of the worm.
Nematodes
The most important group of nematodes are the filarial worms, among which are Wuchereria bancrofti, which causes elephantiasis throughout the tropics, Onchocerca volvulus, the cause of river blindness in West Africa, and Loa loa, the African ‘eye worm’. Infection is transmitted by biting insects.
Less important, but extremely common world-wide, are the intestinal nematodes, which include the hookworms Ancylostoma duodenale andNecator americanus, the common roundworm Ascaris lumbricoides, the threadworm Enterobius vermicularis, and the whipworm Trichuris trichiura.
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Filarial worms
Diethylcarbamazine (DEC), a relative of piperazine, is most widely used in filar-iasis, although albendazole is also effective. The mode of action of DEC is something of a mystery since it appears to have no effect on the viability of microfilariae (the larval forms found in blood or skin) in vitro. One possibility is that the drug has a relatively trivial effect on the surface integument of the worms, which then succumb to immune clearance mechanisms.
Ivermectin, a derivative of avermectin B, a macrocyclic lactone antibiotic produced by Streptomyces avermitilis, has activity against a number of helminths. It has replaced DEC as the drug of choice for the treatment of onchocerciasis. Use of ivermectin does not seem to be accompanied by the severe side-effects (Mazzotti reaction) associated with the administration of DEC and a further bonus is the concomitant expulsion of some intestinal worms. Ivermectin was originally developed for use in animals, and it is still used extensively in veterinary practice, together with other derivatives, including doramectin. It is active against many of the arthropod ectoparasites that cause problems in animal husbandry as well as helminths, and is reported to be effective in the treatment of human scabies.
DEC and ivermectin do not reliably kill the adult worms. This can be achieved with the antitrypanosomal drug suramin, but it is too toxic for routine use. Roundworms other than filariae that invade tissues include Trichinella spiralis, now an uncommon cause of human infection, and the Guinea worm, Dracunculus medinensis. Anthelminthic therapy is of unproven benefit in these infections, but benzimidazoles have some activity against both parasites.
Guinea worm infection, lymphatic filariasis, and onchocerciasis are being targeted by the World Health Organization (WHO) for elimination. Guinea worm, which is transmitted via the water flea Cyclops, has been much reduced in prevalence by health education and the provision of clean water in infected areas. The other two campaigns are being facilitated by drug donation programmes from the pharmaceutical industry: albendazole (alone or in combination with iver-mectin) for lymphatic filariasis; and ivermectin in onchocerciasis.
Intestinal nematodes
With the exception of E. vermicularis, intestinal worms are most common in countries with poor standards of hygiene. Treatment has traditionally relied on a variety of compounds of variable efficacy, including piperazine, levamisole, and pyrantel pamoate (see Table 31.1,p. 365). More reliable, but also more expensive, are benzimidazoles, which are active against most intestinal roundworms. Albendazole exhibits the broadest spectrum of activity and is the drug of choice for the elimination of intestinal roundworms. Mebendazole is also sometimes used, but the first of the benzimidazoles to be introduced into human medicine, thiabendazole, has now been largely abandoned because of its side-effects.
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Larvae of the dog ascarid, Toxocara canis, sometimes infect children who come into contact with dogs. Thiabendazole and DEC have been used in treatment, but there is some evidence that albendazole or ivermectin may be more effective. Piperazine or, preferably, fenbendazole are used to de-worm infected dogs.
Trematodes
Trematodes (flukes) generally have a complex life cycle involving a stage of development in a snail and, usually, a secondary intermediate host, as well as in the definitive host in which the mature adult forms develop.
Most important are the schistosomes, which cause human infection in many parts of Africa as well as the Far East, the West Indies, and South America. Infection is acquired when cercaria (the infective form) penetrate the skin following exposure to water inhabited by infected snails. Mature adults develop in the portal vessels from where they migrate to the small veins of the rectum (Schistosoma mansoni and S. japonicum) or the bladder (S. haematobium). Eggs are then passed through the rectal or bladder mucosa into the faeces or urine.
For many years, treatment relied on trivalent antimonials of which sodium (or potassium) antimony tartrate (tartar emetic) was the mainstay. Other compounds were later introduced. They include metriphonate, which was originally developed as an organophosphate insecticide and is active against S. haematobium, and oxamniquine, a quinolinemethanol used to treat S. mansoni infection. These drugs have been generally superseded by praziquantel, which is highly active against all three species of Schistosoma that commonly infect man. Praziquantel appears to cause schistosomes to release their hold within the venules, and to expose surface antigens that are normally protected from host attack.
Praziquantel is also the drug of choice for most other trematode infections. These include Clonorchis sinensis (Chinese liver fluke) infection, which is acquired from eating uncooked freshwater fish and is extremely common in parts of the Far East where raw fish is widely eaten, andParagonimus westermani (lung fluke) infection, which is acquired from raw or undercooked crabs and crayfish.
Praziquantel is not, however, effective against the liver fluke encountered in the UK, Fasciola hepatica. This is predominantly a parasite of sheep, and human infections can often be traced to wild watercress from sheep pastures. Bithionol was formerly used in treatment, but the veterinary anthelminthic, triclabendazole is now preferred.
Cestodes
By far the most important tapeworms are Echinococcus granulosus and the closely related E. multilocularis (the hydatid worms). These parasites are unusual in that man is an intermediate host, harbouring the larval form in hydatid cysts which arise, usually in the liver, following ingestion of eggs from an infected
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dog. The internal wall of the hydatid cyst consists of a germinal layer from which ‘brood capsules’ containing protoscolices develop. Hydatid disease occurs in many countries, including parts of the UK, where sheep and sheepdogs maintain the cycle of infection.
There is no reliable chemotherapy for hydatid disease, although some success has been obtained with albendazole or praziquantel. However, chemotherapy remains an adjunct to surgical removal, which is not without risk from the spillage of viable protoscolices into the peritoneal cavity. This can be minimized by introduction of a scolicidal agent such as formalin or cetrimide into the cyst. Surgery is not usually feasible in E. multilocularis infection since the cyst infil-trates surrounding tissue.
Other tapeworms infecting man include Taenia saginata (the beef tapeworm), T. solium (the pork tapeworm), Hymenolepis nana (the dwarf tapeworm), and Diphyllobothrium latum (the fish tapeworm). Despite their reputation, none of these well-adapted parasites causes much mischief under normal circumstances, although autoinfection with the larval form of T. solium can cause an epilepti-form condition known as cerebral cysticercosis.
The ancient, and effective treatment for tapeworm infection is extract of male fern (Dryopteris filix-mas); the acridine dye mepacrine is also effective. These old drugs have now been replaced by niclosamide or praziquantel, both of which are highly effective. Niclosamide treatment causes disruption of the worm and in T. solium infection there is a theoretical risk of cysticercosis caused by autoin-fection with liberated eggs. Cerebral cysticercosis may respond to treatment with praziquantel given together with steroids. Albendazole is also effective and, since it penetrates better into the CSF, it may be preferable.