Plasmodium spp.

Malaria has been well documented as an ancient disease in Egyptian and Chinese writing beginning in 2700 bc. By 200 bc, malaria was identified in Rome, spread throughout Europe during the twelfth century, and arrived in England by the fourteenth century. By the early 1800s malaria was found worldwide.

Malaria has played a tremendous role in world history, influencing the outcome of wars, the movement of populations, and the development and decline of various nations. Before the American Civil War, malaria was found as far north as southern Canada; but it was no longer endemic within the United States by the 1950s.

It is estimated that more than 500 million individuals worldwide are infected with Plasmodium spp., and as many as 2.7 million people a year, most of whom are children, die from the infection. Malaria is endemic in more than 90 countries with a population of 2400 million people, representing 40% of the world’s population. At least 90% of deaths caused by malaria occur in Africa. Plasmodium falciparum is the major species associated with deadly infections throughout the world. Unfortunately, prevention remains a complex problem, and no drug is universally effective for all Plasmodium species.

In addition, of the five species that infect humans, P. vivax and P. falciparum cause 95% of infections. P. vivax may be responsible for 80% of the infections, because this species has the widest distribution in the tropics, subtropics, and temperate zones. P. falciparum is generally confined to the tropics, P. malariae is sporadically distributed, and P. ovale is confined mainly to central West Africa and some South Pacific islands. The fifth human malaria, Plasmodium knowlesi, a malaria parasite of long-tailed macaque monkeys, has been confirmed in human cases from Malaysian Borneo, Thailand, Myanmar, and the Philippines.

The vector for malaria is the female anopheline mosquito. When the vector takes a blood meal, sporozoites contained in the salivary glands of the mosquito are discharged into the puncture wound (Figure 49-1). Within an hour, these infective sporozoites are carried via the blood to the liver, where they penetrate hepatocytes and begin to grow, initiating the preerythrocytic or primary exoerythrocytic cycle. The sporozoites become round or oval and begin dividing repeatedly. Schizogony results in large numbers of exoerythrocytic merozoites. Once these merozoites leave the liver, they invade the red blood cells (RBCs), initiating the erythrocytic cycle. A dormant schizogony may occur in P. vivax and P. ovale organisms, which remain quiescent in the liver. These resting stages have been termed hypnozoites and lead to a true relapse, often within 1 year or up to more than 5 years later. Delayed schizogony does not occur in P. falciparum, P. malariae, or P. knowlesi.

Once the RBCs and reticulocytes have been invaded, the parasites grow and feed on hemoglobin. Within the RBC, the merozoite (or young trophozoite) is vacuolated, ring shaped, more or less ameboid, and uninucleate. The excess protein and hematin present from the metabolism of hemoglobin combine to form malarial pigment. Once the nucleus begins to divide, the trophozoite is called a developing schizont. The mature schizont contains merozoites (whose number depends on the species), which are released into the bloodstream. Many of the merozoites are destroyed by the immune system, but others invade RBCs and initiate a new cycle of erythrocytic schizogony. After several erythrocytic generations, some of the merozoites begin to undergo development into the male and female gametocytes.

Although malaria is often associated with travelers to endemic areas, other situations resulting in infection include blood transfusions, use of contaminated hypodermic needles, bone marrow transplantation, congenital infection, and transmission within the United States by indigenous mosquitoes that acquired the parasites from imported infections.

Plasmodium Vivax (Benign Tertian Malaria)

General Characteristics

P. vivax infects only the reticulocytes; thus, the parasitemia is limited to approximately 2% to 5% of the available RBCs (Tables 49-1 to 49-3, Figures 49-2 and 49-3). Splenomegaly occurs during the first few weeks of infection, and the spleen will progress from being soft and palpable to hard, with continued enlargement during a chronic infection. If the infection is treated during the early phases, the spleen will return to its normal size. A secondary or dormant schizogony occurs in P. vivax and P. ovale, which remain quiescent in the liver. These resting stages have been termed hypnozoites.

TABLE 49-1

Plasmodium spp.: Clinical Characteristics of the Five Human Infections

Infection	P. vivax	P. ovale	P. malariae	P. falciparum	P. knowlesi	Comments
Incubation period	8-17 days	10-17 days	18-40 days	8-11 days	9-12 days	All may be extended for months to years
Prodromal symptoms  Severity  Initial fever pattern	Mild to moderate Irregular (48 hr)	Mild Irregular (48 hr)	Mild to moderate Regular (72 hr)	Mild Continuous remittent	Mild to moderate Regular (24 hr)	All may mimic influenza symptoms Early symptoms may reflect lack of regular periodicity
Symptom periodicity	48 hr	48 hr	72 hr	36-48 hr	24-27 hr
Initial paroxysm  Severity  Mean duration	Moderate to severe 10 h	Mild 10 h	Moderate to severe 11 h	Severe 16-36 h	Moderate to severe Not available	P. knowlesi might increase/lose virulence on passage in humans
Duration of untreated primary attack	3-8+ wk	2-3 wk	3-24 wk	2-3 wk	Not available
Duration of untreated infection	5-7 yr	12 mo	20+ yr	6-17 mo	Not available
Parasitemia limitations	Young RBCs	Young RBCs	Old RBCs	All RBCs	All RBCs
Anemia	Mild to moderate	Mild	Mild to moderate	Severe	Moderate to severe	P. knowlesi can be as dangerous as P. falciparum
CNS involvement	Rare	Possible	Rare	Very common	Possible
Nephrotic syndrome	Possible	Rare	Very common	Rare	Probably common

TABLE 49-2

Plasmodia in Giemsa-Stained Thin Blood Smears

	Plasmodium vivax	Plasmodium malariae	Plasmodium falciparum	Plasmodium ovale	Plasmodium knowlesi
Persistence of exoerythrocytic cycle	Yes	No	No	Yes	No
Relapses	Yes	No, but long-term recrudescence is recognized	No long-term relapses	Possible, but usually spontaneous recovery	No
Time of cycle	44-48 hr	72 hr	36-48 hr	48 hr	24 hr
Appearance of parasitized RBCs; size and shape	1.5-2 times larger than normal; oval to normal; may be normal size until ring fills half of cell	Normal shape; size may be normal or slightly smaller	Both normal	60% of cells larger than normal and oval; 20% have irregular, frayed edges	Normal shape, size
Schüffner’s dots (eosinophilic stippling)	Usually present in all cells except early ring forms	None	None; occasionally comma-like red dots are present (Maurer’s dots)	Present in all stages including early ring forms; dots may be larger and darker than in P. vivax	No true stippling; occasional faint dots
Color of cytoplasm	Decolorized, pale	Normal	Normal, bluish tinge at times	Decolorized, pale	Normal
Multiple rings/cell	Occasional	Rare	Common	Occasional	Common
All developmental stages present in peripheral blood	All stages present	Ring forms few, since ring stage brief; mostly growing and mature trophozoites and schizonts	Young ring forms and no older stages; few gametocytes	All stages present	All stages present
Appearance of parasite; young trophozoite (early ring form)	Ring is diameter of cell, cytoplasmic circle around vacuole; heavy chromatin dot	Ring often smaller than in P. vivax, occupying of cell; heavy chromatin dot; vacuole at times “filled in”; pigment forms early	Delicate, small ring with small chromatin dot (frequently 2); scanty cytoplasm around small vacuoles; sometimes at edge of red cell (appliqué form) or filamentous slender form; may have multiple rings per cell	Ring is larger and more ameboid than in P. vivax; otherwise similar to P. vivax	Rings to diameter of RBC; double chromatin dots; appliqué forms rare; multiple rings per RBC
Growing trophozoite	Multishaped irregular ameboid parasite; streamers of cytoplasm close to large chromatin dot; vacuole retained until close to maturity; increasing amounts of brown pigment	Non-ameboid rounded or band-shaped solid forms; chromatin may be hidden by coarse dark brown pigment	Heavy ring forms; fine pigment grains	Ring shape maintained until late in development; non-ameboid compared to P. vivax	Slightly ameboid and irregular; band forms seen; very little pigment
Mature trophozoite	Irregular ameboid mass; 1 or more small vacuoles retained until schizont stage; fills almost entire cell; fine brown pigment	Vacuoles disappear early; cytoplasm compact, oval, band shaped, or nearly round and almost filling cell; chromatin may be hidden by peripheral coarse dark brown pigment	Not seen in peripheral blood (except in severe infections); development of all phases following ring form occurs in capillaries of viscera	Compact; vacuoles disappear; pigment dark brown, less than in P. malariae	Denser cytoplasm (slightly ameboid) band forms seen; little to no malaria pigment (scattered, fine brown grains)
Schizont (pre-segmenter)	Progressive chromatin division; cytoplasmic bands containing clumps of brown pigment	Similar to P. vivax except smaller; darker, larger pigment granules peripheral or central	Not seen in peripheral blood (see above)	Smaller and more compact than P. vivax	Between 2 and 5 divided nuclear chromatin masses; abundant pigment granules occupy of RBC
Mature schizont	16 (12-24) merozoites, each with chromatin and cytoplasm, filling entire red cell, which can hardly be seen	8 (6-12) merozoites in rosettes or irregular clusters filling normal-sized cells, which can hardly be seen; central arrangement of brown-green pigment	Not seen in peripheral blood	of cells occupied by 8 (8-12) merozoites in rosettes or irregular clusters	RBCs normal size; distorted/fimbriated RBCs very rare; occupy whole RBC; maximum of 16 merozoites; no rosettes; grapelike clusters
Macrogametocyte	Rounded or oval homogeneous cytoplasm; diffuse delicate light brown pigment throughout parasite; eccentric compact chromatin	Similar to P. vivax, but fewer in number; pigment darker and more coarse	Gender differentiation difficult; “crescent” or “sausage” shapes characteristic; may appear in “showers” with black pigment near chromatin dot, which is often central	Smaller than P. vivax	Occupy most of RBC; bluish cytoplasm; dense pink chromatin at periphery of parasite
Microgametocyte	Large pink to purple chromatin mass surrounded by pale or colorless halo; evenly distributed pigment	Similar to P. vivax, but fewer in number; pigment darker and more coarse	Same as macrogametocyte (described above)	Smaller than P. vivax	Occupy most of RBC; cytoplasm pinkish purple; early forms similar to mature trophozoite
Main criteria	Large pale red cell; trophozoite irregular; pigment usually present; Schüffner’s dots not always present; several phases of growth seen in one smear; gametocytes appear as early as third day	Red cell normal in size and color; trophozoites compact, stain usually intense, band forms not always seen; coarse pigment; no stippling of red cells; gametocytes appear after a few weeks	Development following ring stage takes place in blood vessels of internal organs; delicate ring forms and crescent-shaped gametocytes are only forms normally seen in peripheral blood; gametocytes appear after 7-10 days	Red cell enlarged, oval, with fimbriated edges; Schüffner’s dots seen in all stages; gametocytes appear after 4 days or as late as 18 days	Ring forms compact; single/double chromatin dots, appliqué forms, multiple rings/RBC (mimic P. falciparum); overall RBCs not enlarged; developing stages mimic P. malariae (band forms, 16 merozoites in mature schizont, but no rosettes)

TABLE 49-3

Malaria Characteristics with Fresh Blood or Blood Collected Using EDTA with No Extended Lag Time*

Type of Malaria	Characteristics
Plasmodium vivax (benign tertian malaria)	1. 48-hour cycle 2. Tends to infect young cells 3. Enlarged RBCs 4. Schüffner’s dots (true stippling) after 8-10 hours 5. Delicate ring 6. Very ameboid trophozoite 7. Mature schizont contains 12-24 merozoites

Early stages mimic P. falciparum; later stages mimic P. malariae

Pathogenesis and Spectrum of Disease

In patients who have never been exposed to malaria, symptoms such as headache, photophobia, muscle aches, anorexia, nausea, and sometimes vomiting may occur before organisms can be detected in the bloodstream. In other patients with prior exposure to the malaria, the parasites can be found in the bloodstream several days before symptoms appear.

Severe complications are uncommon in P. vivax infections, although coma and sudden death or other symptoms of cerebral involvement have been reported, particularly in patients with varying degrees of primaquine resistance. These patients can exhibit cerebral malaria, renal failure, circulatory collapse, severe anemia, hemoglobinuria, abnormal bleeding, acute respiratory distress syndrome, and jaundice. Acute cerebral malaria involves changes in mental status and if untreated may result in fatality within 3 days.

Although P. ovale and P. vivax infections are clinically similar, P. ovale malaria is usually less severe, tends to relapse less frequently, and usually ends with spontaneous recovery, often after no more than 6 to 10 paroxysms (see Tables 49-1 to 49-3, Figures 49-2 and 49-3). Like P. vivax, P. ovale infects only the reticulocytes, so that the parasitemia is limited to approximately 2% to 5% of the available RBCs. For many years the literature has stated that as with P. vivax, a secondary or dormant schizogony occurs in P. ovale, which remain quiescent in the liver. However, newer findings indicate that hypnozoites have never been demonstrated by biologic experiments.

After a few days of irregular periodicity, a regular 48-hour cycle is established. Over time, the paroxysms become less severe and more irregular in frequency and then stop altogether. In some patients infected with P. ovale, relapses occur after weeks, months, or up to 1 year or more. The RBCs tend to be enlarged (young RBCs), Schüffner’s dots (also known as James stippling) are present from the beginning of the cycle, the developing rings are less ameboid than those of P. vivax, and the mature schizont contains an average of eight merozoites.

P. malariae invades primarily the older RBCs, limiting the number of infected cells (see Tables 49-1 to 49-3, Figures 49-2 and 49-3). The incubation period between infection and symptoms may be much longer than that for P. vivax or P. ovale malaria, ranging from about 27 to 40 days. A regular periodicity is seen from the beginning, with a more severe paroxysm, including a longer cold stage and more severe symptoms during the hot stage. Collapse during the sweating phase is not uncommon.

A regular periodicity of 72 hours is seen from the beginning of the erythrocytic cycle. The infection may end with spontaneous recovery, or there may be a recrudescence or series of recrudescence (recurrence of symptoms) over many years. These patients are left with a latent infection and persisting low-grade parasitemia for many, many years. The RBCs tend to be normal to small (old RBCs), there is no true stippling, the RBCs may have fimbriated edges, the developing rings tend to demonstrate “band” forms, and the mature schizont contains an average of 6 to 12 merozoites.

Plasmodium falciparum invades all ages of RBCs, and the number of infected cells may exceed 50% (see Tables 49-1 to 49-3, Figures 49-2 to 49-4). Schizogony occurs in the spleen, liver, and bone marrow rather than in the circulating blood. Ischemia caused by the obstruction of vessels within these organs by parasitized RBCs will produce various symptoms, depending on the organ involved. A decrease in the ability of the RBCs to change shape when passing through capillaries or the splenic filter may lead to plugging of the vessels Also, only P. falciparum causes cytoadherence, a feature that is associated with severe malaria.

The asexual and sexual forms circulate in the bloodstream during infections by four of the Plasmodium species. However, in P. falciparum infections, as the parasite grows, the RBC membrane becomes sticky and the cells adhere to the endothelial lining of the capillaries of the internal organs. Thus, only the ring forms and the gametocytes (occasionally mature schizonts) normally appear in the peripheral blood. Periodicity of the cycle will not be established during the early stages, and the presumptive diagnosis may be totally unrelated to a possible malaria infection. If the fever does develop a synchronous cycle, it is usually a cycle of 36 to 48 hours. Because P. falciparum infects young and old RBCs, very heavy parasitemia can occur. The RBCs are all sizes; there is no true stippling, but Maurer’s dots (coarse granulation in the cytoplasm of RBCs) are sometimes present; often there are multiple rings per RBC and the rings are delicate and often have two dots of chromatin, appliqué or accolé forms (ring forms identified within the marginal regions of the erythrocytes); and the gametocytes are crescent-shaped.

P. knowlesi invades all ages of RBCs, and the number of infected cells can be significantly more than seen in P. vivax, P. ovale, and P. malariae. P. knowlesi infection should be considered in patients with a travel history to forested areas of Southeast Asia, especially if P. malariae is diagnosed, unusual forms are seen with microscopy, or if a mixed infection with P. falciparum/P. malariae is diagnosed. Because the disease is potentially fatal, proper identification to the species level is critical.

The early blood stages of P. knowlesi resemble those of P. falciparum, whereas the mature blood stages and gametocytes resemble those of P. malariae (see Tables 49-1 to 49-3, Figures 49-2 to 49-4). Unfortunately, these infections are often misdiagnosed as the relatively benign P. malariae; however, infections with P. knowlesi can be fatal. The RBCs are all sizes, there is no true stippling (fine, granular, blue stippling in RBCs stained with Wright’s stain or red when using eosin hematoxylin as seen in Figure 49-3 (P. vivax photo, third from the top), often there are multiple rings per RBC (there may be 2 to 3 rings), the rings are delicate and often have 2 to 3 dots of chromatin, band forms are typically seen with the developing trophozoites, and the mature schizont contains 16 merozoites. The early stages mimic P. falciparum, whereas the later stages mimic P. malariae.

Because of different levels of parasitemia, low organism densities, and confusion among various morphologic criteria for identification, detection of mixed infections can be quite difficult. Even if a mixed infection is suspected, identification to the species level may not be possible using routine microscopy methods. However, using polymerase chain reaction (PCR) methods, it is likely that higher detection and identification rates of chronic and mixed malarial infections will be possible.

Malaria is considered to be immediately life threatening, and a patient with the diagnosis of P. falciparum or P. knowlesi malaria should be considered a medical emergency because the disease can be rapidly fatal. This approach to the patient is also recommended in situations where P. falciparum or P. knowlesi cannot be ruled out as a possible diagnosis. Any laboratory providing the expertise to identify malarial parasites should do so on a 24-hour basis, 7 days a week.

Examination of a single blood specimen is not sufficient to exclude the diagnosis of malaria, especially when the patient has received partial prophylaxis or therapy and has a low number of organisms in the blood. Patients with a relapse case or an early primary case may also have few organisms in the blood smear. Regardless of the presence or absence of any fever periodicity, both thick (Figure 49-5) and thin blood films should be prepared immediately, and at least 200 to 300 oil immersion fields should be examined on both films before a negative report is issued. If the initial specimen is negative, additional blood specimens should be examined over a 36-hour time frame. Although Giemsa stain is recommended for all parasitic blood work, the organisms can also be seen with other blood stains, such as Wright’s stain. Using any of the blood stains, the white blood cells (WBCs) serve as the built-in quality control; if the WBCs look good, any parasites present will also look good. Figure 49-6 compares the multinucleated stages (schizont) of Plasmodium malariae and Plasmodium vivax. Fluorescent nucleic acid stains, such as acridine orange, may also be used to identify organisms in infected RBCs. However, this may be more difficult to interpret because of the presence of white blood cell nuclei or RBC Howell-Jolly bodies.

Blood collected using ethylenediaminetetraacetic acid (EDTA) anticoagulant is preferred; however, if the blood remains in the tube for any length of time before blood film preparation, Schüffner’s dots may not be visible after staining (P. vivax, as an example) and other morphologic changes in the parasites will be seen. Also, the proper ratio between blood and anticoagulant is required for good organism morphology, so each collection tube should be filled to the top. Finger-stick blood is recommended, particularly when the volume of blood required is minimal (i.e., when no other hematologic procedures have been ordered). The blood should be free flowing when taken for smear preparation, and should not be contaminated with alcohol used to clean the finger before the stick. However, the use of finger-stick blood is currently much less common, and venipuncture blood is the normal specimen collected for the laboratory. Identification to the species level is highly desirable, because this information determines which drug(s) is (are) recommended. In early infections, patients with P. falciparum infections may not have the crescent-shaped gametocytes in the blood. Also, low parasitemia with the delicate ring forms may be missed; consequently, oil immersion examination at 1000× is mandatory.

Therapy

Malaria has become a more serious health problem, both in residents of endemic areas and in travelers returning to nonendemic areas. Therapy has become more complex as a result of increased resistance of P. falciparum to a variety of drugs, resistance problems with P. vivax, and the need to treat severe disease complications. Antimalarial drugs are classified according to the stage of malaria against which they are targeted. These drugs are referred to as tissue schizonticides (which kill tissue schizonts), blood schizonticides (which kill blood schizonts), gametocytocides (which kill gametocytes), and sporonticides (which prevent formation of sporozoites within the mosquito). It is important for the clinician to know the species of Plasmodium involved in the infection, the estimated parasitemia, and the geographic and patient travel history to assess the possibility of drug resistance related to the organism and geographic area.

Chloroquine-resistant P. falciparum is present in almost all endemic areas other than Central America and the Caribbean. Increasing resistance to sulfadoxine-pyrimethamine and mefloquine has also been identified in P. falciparum. Therefore, treatment with ACT—including artesunate-mefloquine, artemether-lumefantrine (Coartem), and artesunate-amodiaquine—has been instituted against P. falciparum. However, resistance to artesunate-mefloquine has already appeared in Southeast Asia. Current information on the distribution of drug-resistant P. falciparum is available from the Centers for Disease Control and Prevention (CDC) malaria hotline in Atlanta, Georgia [phone (770) 488-7788]. Therapy for chloroquine-resistant P. falciparum remains very complex with continual changes; thus consultation with an infectious disease specialist is highly recommended. Current treatment guidelines from the CDC are available at www.cdc.gov/malaria/pdf/clinicalguidance.pdf. Chloroquine resistance continues to evolve and spread. Primaquine tolerance has also been documented.

Babesia Spp.

The genus Babesia includes approximately 100 species transmitted by ticks of the genus Ixodes. In addition to humans, these blood parasites infect a variety of wild and domestic animals. Cases of babesiosis have been documented worldwide, and several outbreaks in humans have occurred in the northeastern United States, particularly in Long Island, Cape Cod, and the islands off the East Coast (Homer). Although there are many species of Babesia, Babesia microti is the cause of most human infections in the United States, whereas B. divergens tends to be more common in Europe, is often found in splenectomized patients, and causes a more serious form of the disease.

General Characteristics

Organism

Although the life cycle of Babesia spp. is similar to that of Plasmodium spp., no exoerythrocytic stage has been described; also, sporozoites injected by the bite of an infected tick invade erythrocytes directly. Once inside the erythrocytes, the trophozoites reproduce by binary fission rather than schizogony. Once the tick begins to take a blood meal; the sporozoites are injected into the host with the tick’s saliva.

The trophozoites of Babesia can mimic P. falciparum rings; however, there are differences that can help differentiate the two organisms (Figure 49-7). Babesia trophozoites vary in size from 1 to 5 µm; the smallest are smaller than P. falciparum rings. Also, ring forms outside of the RBCs and two to three rings per RBC are much more common in Babesia. The ring forms of Babesia tend to be very pleomorphic and range in size, even within a single RBC. The diagnostic tetrads, the Maltese Cross, though not seen in every specimen or species, may be present (see Figure 49-7).

Trypanosoma Spp.

Trypanosoma spp. are hemoflagellate protozoa that live in the blood and tissue of the human host (Tables 49-4 and 49-5, Figures 49-8 to 49-10). African trypanosomiasis (sleeping sickness) is caused by Trypanosoma brucei gambiense and T. brucei rhodesiense species belonging to the family Trypanozoon and is confined to the central belt of Africa. American trypanosomiasis (Chagas’ disease) is produced by Trypanosoma cruzi, which belongs to the family Schizotrypanum and is confined to the American continent. Trypanosoma rangeli belongs to the family Tejaria, produces an asymptomatic infection, and is also present only on the American continent. African trypanosomes and T. rangeli are transmitted directly into the bite wound by salivary secretions from the insect vector, whereas T. cruzi is transmitted through contamination of the bite wound with the feces from the reduviid bug.

Leishmania Spp.

Leishmaniasis is caused by more than 20 species of the protozoan genus Leishmania, with a disease spectrum ranging from self-healing cutaneous lesions to debilitating mucocutaneous infections, subclinical viscerotropic dissemination, and fatal visceral involvement. Published disease burden estimates place leishmaniasis second in mortality and fourth in morbidity among all tropical diseases. Leishmaniasis is classified as one of the “most neglected diseases,” based on its association with poverty and on the limited resources invested in diagnosis, treatment, and control. The World Health Organization (WHO) estimates that 1.5 million cases of cutaneous leishmaniasis (CL) and 500,000 cases of visceral leishmaniasis (VL) occur every year in 88 countries. Estimates indicate that approximately 350 million people are at risk for acquiring leishmaniasis, with 12 million currently infected.

Cases of leishmaniasis are seen each year in the United States and can be attributed to immigrants from countries with endemic infection, military personnel, and American travelers. Another concern is the potential for more infections occurring in areas of endemic infection in Texas and Arizona.

General Characteristics

The parasite has two distinct phases in its life cycle: amastigote and promastigote (Table 49-6, Figure 49-11). The amastigote form is an intracellular parasite in the cells of the reticuloendothelial system and is oval, measuring 1.5 to 5 µm, and contains a nucleus and kinetoplast. Leishmania spp. exist as the amastigote in humans and as the promastigote in the insect host. As the vector takes a blood meal, promastigotes are introduced into the human host. Depending on the species, the parasites then move from the bite site to the organs within the reticuloendothelial system (bone marrow, spleen, liver) or to the macrophages of the skin or mucous membranes.

TABLE 49-6

Features of Human Leishmanial Infections^a

Species	Disease Type	Humoral Antibodies	Delayed Hypersensitivity	Parasite Quantity	Self-Cure	Recommended Specimen
Leishmania donovani	VL	Abundant	Absent	Absent	Rare	Bone marrow, spleen
	CL	Variable	Present	Present	Yes	Skin macrophages
	DL	Variable	Variable	Variable	Variable	Skin macrophages
L. tropica	CL	Variable	Present	Present	Yes	Skin macrophages
L. major	CL	Present	Present	Present	Rapid	Skin macrophages
L. aethiopica	CL	Variable	Weak	Present	Slow	Skin macrophages
	DCL	Variable	Absent	Abundant	No	Skin macrophages
L. mexicana	CL	Variable	Present	Present	Yes	Skin macrophages
	DCL	Variable	Absent	Abundant	No	Skin macrophages
L. braziliensis	CL	Present	Present	Present	Yes	Skin macrophages
	MCL	Present	Present	Scant	No	Skin macrophages

CL, Cutaneous leishmaniasis; DCL, diffuse cutaneous leishmaniasis; DL, dermal leishmanoid; MCL, mucocutaneous leishmaniasis; VL, visceral leishmaniasis.

^aFor culture, specimens must be collected aseptically; in older lesions, the number of parasites may be scant and difficult to recover. Isolation: Hamsters; culture media (Novy-MacNeal-Nicolle medium and Schneider’s Drosophila medium with 30% fetal bovine serum). Serology: Most suitable for visceral leishmaniasis; little value for cutaneous leishmaniasis; limited value for mucocutaneous leishmaniasis. Montenegro test: Delayed hypersensitivity reaction to intradermal injection of cultured parasites.

More than 90% of cutaneous leishmaniasis cases occur in Afghanistan, Algeria, Iran, Iraq, Saudi Arabia, Syria, Brazil, and Peru. There has been an increase in the number of cases among military personnel deployed in Afghanistan, Iraq, and Kuwait. Autochthonous (native to origin) human infections have been described in Texas. Most of the cases of mucocutaneous leishmaniasis occur in Bolivia, Brazil, and Peru. More than 90% of the cases of visceral leishmaniasis are found in Bangladesh, Brazil, India, Nepal, and Sudan.

Depending on the species involved, infection with Leishmania spp. can result in cutaneous, diffuse cutaneous, mucocutaneous, or visceral disease. In endemic areas with leishmaniasis, co-infection with human immunodeficiency virus (HIV)-positive patients is common. If co-infected patients are severely immunocompromised, up to 25% will die shortly after being diagnosed. The use of highly active antiretroviral therapy (HAART) has dramatically improved the prognosis of these co-infected patients.