Page 2555 - Hematology_ Basic Principles and Practice ( PDFDrive )

C H A P T E R 158

HEMATOLOGIC ASPECTS OF PARASITIC DISEASES

David J. Roberts

Parasitic diseases are not common in medical, let alone hematologic, substantially through the Global Fund and the World Health Orga-
practice in North America or Europe. However, much of the world’s nization’s “Roll Back Malaria” campaign (www.rbm.who.int). The
population is infected by and becomes symptomatic as a result of a current estimated annual death total from malaria in Africa is
plethora of parasites, and many of these infections represent global 630,000. 6
public health problems. The distribution of malaria is determined by features of host,
Although some significant parasitic diseases are transmitted in vector, and parasite. In summary, the global distribution of autoch-
temperate climates, the majority of parasites of significance to human thonous or endogenous malaria is limited by the lower temperature
health are endemic in the tropical world. This reflects not only limits for development of the parasite in the mosquito (sporogony)
socioeconomic circumstances but also the origin of our species in of 20°C for P. falciparum and 15°C for other human malarias. Within
tropical Africa, where the human host, parasites, and also vectors have these limits, transmission does not occur above 1500 m in arid
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established complex relationships over evolutionary timescales. regions or in the Central and South Pacific (because of the absence
Notwithstanding such geographic variation in the incidence of para- of suitable vectors). In addition, P. vivax malaria is rare in Africa,
sitic disease, both travelers and recent immigrants now present to where the population frequency of the blood group Duffy negative
−
−
hematology clinics and laboratories all over the world with increasing (Fya Fyb ) is high. P. ovale requires a lengthy period of sporogony
frequency. Even in those circumstances, there are marked variations and is confined to areas of Africa and Southeast Asia with a high
in practice in North America as well as in Europe, where the United density of susceptible Anopheles spp. P. knowlesi is transmitted from
Kingdom reports more cases of imported malaria than the United macaque monkeys in forest areas of Borneo, Malaysia, Thailand, and
States and indeed has a 10-fold greater incidence of malaria per Vietnam. 5
capita, reflecting the increased frequency of travel to and from In some malarious areas the seasonal pattern of clinical malaria is
endemic areas compared with North American populations. determined by the increase in vector density after rainfall, leading to
Patients with malaria, leishmaniasis, trypanosomiasis, and babesio- an increase in new infections as transmission increases. In naive
sis may present directly or indirectly to hematologists. This chapter individuals, parasites can cause chronic infection lasting many
is concentrated on the biologic, clinical, and hematologic features of months.
these infections and the hematologic aspects or complications of their The intensity of transmission determines the distribution of clini-
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treatment. Comprehensive accounts of the general medical aspects of cal symptoms in different age groups. In general, in areas of high
these diseases are provided in many other recent textbooks. 2–4 transmission, younger children experience severe disease. Where
transmission is less intense, older children experience severe disease.
Finally, if the rate of transmission is very low, few cases of malaria are
MALARIA seen in any age group, and such populations would have little natural
immunity. In such areas, a sudden increase in vectorial capacity
Malaria is a major public health problem in tropical areas, and it is (through the accidental introduction of efficient vectors or higher
estimated that it is responsible for 600,000 to 900,000 deaths annu- density, biting, or survival of the resident vectors), more rapid parasite
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ally and 150 to 300 million infections. The vast majority of morbid- sporogony, or migration of infected or nonimmune populations can
ity and mortality caused by malaria is caused by infection with result in epidemics where large numbers fall ill in all age groups. The
Plasmodium falciparum, although Plasmodium vivax, Plasmodium transition from high to low transmission has been classified by
ovale, and Plasmodium malariae are also responsible for human infec- holoendemicity, hyperendemicity, mesoendemicity, and hypoende-
tions. A fifth species, Plasmodium knowlesi, has been shown to cause micity. These categories can be related epidemiologically to age-
human infection in some parts of Southeast Asia (for review, see the specific rates of parasite prevalence or splenomegaly and theoretically
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article by Millar and Cox-Singh ). In endemic areas, a significant to the reproductive ratio of malarial infection. 8
proportion of the mortality and morbidity is from anemia. In Europe Malaria exerts a substantial selection for human traits that protect
and North America, malaria is not infrequently a clinical problem in from infection. Sickle cell trait and thalassemia traits protect from
travelers or recent arrivals from malaria-endemic areas, and hematolo- infection and are truly polymorphic characteristics in many parts of
gists may be involved in the diagnosis and management of the disease. the world. Understanding genetic epidemiology has provided the
Moreover, in nonendemic areas, malaria may cause a fatal transfusion- foundation of population genetics and has provided classic examples
transmitted infection, and detection of blood donors who may be of principles of genetic selection in vivo—for example, balancing
carrying the disease represents a major challenge for blood services. selection for sickle cell trait and negative epistasis for sickle cell trait
and α-thalassemia. The homozygous forms of these characteristics
cause significant clinical disease, such as sickle cell disease,
Epidemiology β-thalassemia, and glucose-6-phosphate dehydrogenase (G6PDH)
deficiency. In endemic areas these genetic diseases represent major
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Approximately 1 billion people live in areas of endemic or epidemic public health problems (for review, see Williams and Luzzatto
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malaria. The global mortality and morbidity were revised to 350 et al ).
million cases and 1 million deaths per year, respectively, following an
evaluation of the prevalence of infection in Southeast Asia (Fig.
158.1). There is, however, substantial evidence that the incidence of Parasitology
severe disease is now falling, sometimes spectacularly, in many parts
of Africa following the widespread introduction of artemisinin In P. falciparum (see later for a discussion of the other human
combination treatment, impregnated bed nets, and residual spraying parasites) the infective sporozoite forms are inoculated into the
because the resources available for malaria control have increased bloodstream from the salivary glands of a female Anopheles mosquito

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