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p. 1692

MALARIA

Malaria in humans is caused by protozoan parasites of the genus

Plasmodium: Plasmodium falciparum, P. vivax, P. ovale, or P. malariae.

Early symptoms of malaria are nonspecific with fever present in the

majority of patients, and approximately two-thirds of infected persons

may experience symptoms such as headache, muscle aches and pains,

and malaise.

Case 81-1 (Question 1)

Travelers with symptoms of malaria should seek medical evaluation as

soon as possible. Falciparum malaria is the most severe form of malaria

and has the highest mortality. Mortality is higher in those with no

immunity such as travelers from non-endemic areas, while adults living

permanently in the country and exposed to malaria do develop semiimmunity. Suspected or confirmed malaria, especially P. falciparum, is a

medical emergency necessitating early intervention. In instances when

the traveler develops a febrile illness consistent with malaria and access

to medical care is not readily available, a reliable supply of malaria

treatment can be self-administered presumptively as a temporary

measure.

Case 81-1 (Questions 1, 2, 3,

5)

Travelers to malaria-endemic areas need to understand the risk of

malaria and know how to best prevent it with mosquito-avoidance

measures and chemoprophylaxis (where appropriate). Pregnant women

are at greater risk for malaria and its associated complications. Women

who are pregnant or likely to become pregnant should be advised to

avoid travel to endemic areas if possible. If travel cannot be deferred,

use of an effective chemoprophylaxis regimen is essential.

Case 81-1 (Question 4),

Case 81-2 (Questions 1–3),

Case 81-3 (Question 1),

Tables 81-1 and 81-2

Several factors must be considered when selecting a chemoprophylactic

drug regimen, including drug resistance, patient-specific factors, adverse

effects, precautions, and contraindications.

Case 81-1 (Question 4),

Case 81-2 (Questions 1–3),

Case 81-3 (Question 1),

Tables 81-1 and 81-2

AMEBIASIS

Amebiasis is caused by Entamoeba histolytica and implicated in amebic

dysentery and hepatic abscesses. Diagnosis of amebic dysentery may

Case 81-4 (Questions 1, 2),

Case 81-5 (Questions 1, 2),

include the following information: history of travel, stool and biopsy

specimens, and ultrasound to exclude liver abscesses. Treatment for

amebiasis calls for a combination of agents with both luminal-acting and

extraintestinal effects. Amebic cyst passers and pregnant women must

be treated to avoid invasive disease and transmission of the infection.

Case 81-6 (Question 1),

Case 81-7 (Question 1),

Table 81-3

GIARDIASIS

The signs and symptoms of giardiasis could be insidious and vague, but

patients usually present with diarrhea and bulky, foul-smelling stools

along with laboratory confirmation of Giardia lamblia.

Case 81-8 (Question 1)

The major treatments for giardiasis are metronidazole and tinidazole or

nitazoxanide.

Case 81-8 (Question 2),

Table 81-3

p. 1693

p. 1694

ENTEROBIASIS

The signs and symptoms of the infection may be subtle, but the specific

diagnostic test for Enterobius vermicularis is the cellophane tape swab.

Therapy for enterobiasis includes the antihelminthic agents: albendazole,

pyrantel pamoate, or mebendazole (not available in the United States).

Household measures to eradicate the infection need to be addressed.

Case 81-9 (Questions 1, 2),

Table 81-3

CESTODIASIS

Primarily this includes Taenia saginata and Taenia solium, and symptoms

of infections are nonspecific. It is critical to differentiate these two

infections from other cestode infections. Praziquantel remains an

effective agent for allspecies of Cestodes. Cysticercosis, a

complication caused by the larval cysts of T. solium that may be

associated with central nervous system infection (neurocysticercosis), is

serious and requires separate diagnostic testing and controversial

therapy. Praziquantel, which is indicated for most cestodiasis, is well

tolerated.

Case 81-10 (Questions 1, 2),

Table 81-3

PEDICULOSIS

Head and body lice are associated with dermatologic reactions, which

can be treated with a number of agents: permethrin, pyrethrin,

malathion, and ivermectin. Special attention needs to be focused on

correct application procedures, drug resistance, and lice

decontamination measures.

Case 81-11 (Questions 1, 2),

Table 81-3 and 81-4

SCABIES

Scabies produces pruritic rash and excoriations of the interdigital area of

the upper and lower limbs. Treatment includes lindane, permethrin, and

crotamiton. Clothes and personal items of the infected person and

family members need to laundered (>50°C) to avoid reinfection.

Case 81-12 (Question 1),

Table 81-3

MALARIA

Epidemiology

According to World Health Organization (WHO) estimates, malaria was responsible

for an estimated 214 million cases of malaria in 2015 with an estimated 438,000

deaths, which reflects an overall 60% reduction in malaria mortality rates since

2000.

1 The global malaria burden continues to be disproportionately high with

approximately 89% of malaria cases and 91% of deaths in sub-Saharan Africa.

1

Within regions of high malaria transmission, children less than 5 years of age are

particularly susceptible with more than two-thirds (70%) of all malaria deaths

occurring in this age group.

1 Globally in children under the age of 5 years, the

malaria death rate fell by 65% between 2000 and 2015. While malaria is no longer

an endemic infectious disease in the industrialized countries of North America,

Europe, Australia, New Zealand, and Japan, travel to malaria-endemic regions, as

well as immigration and refugee migrations into non-endemic areas, have resulted in

increases in the numbers of imported cases of malaria in industrialized countries.

2,3

Millions of US travelers visit malaria-endemic areas each year and approximately

1,500 cases of malaria are diagnosed in the United States annually, mostly in returned

travelers.

4

Distribution of Malaria Species

Malaria in humans is caused by protozoan parasites of the genus Plasmodium:

Plasmodium falciparum, P. vivax, P. ovale, or P. malariae.

5 The distribution of the

four Plasmodium species of malaria varies worldwide. However, the vast majority

of imported cases are caused by P. falciparum or P. vivax

2

; the proportion of

imported cases caused by P. falciparum or P. vivax within a country reflects both the

destinations that travelers select and, to a larger degree, the nature of immigrant

communities.

2

In most countries, only 5% or less of malaria cases are caused by P.

ovale and P. malariae.

2

In addition, P. knowlesi, a species normally found in

monkeys, has been documented as a cause of human infections and some deaths in

forest fringe areas of Southeast Asia.

5

Life Cycle of the Malaria Parasite and Transmission

The life cycle of the malaria parasites in the human host is complex. All species are

transmitted by the bite of an infective female Anopheles mosquito that carries

malaria-causing parasites and injects the asexual forms or sporozoites into the human

bloodstream. Over a period of 5 to 16 days (depending on the species), the

sporozoites grow, divide, and produce daughter cells, or merozoites, per liver cell.

Infections caused by P. vivax and P. ovale may remain in a dormant stage for

extended periods of time within the liver (hypnozoites) and cause relapses by

invading the bloodstream for months or years later. The merozoites exit the liver

cells and reenter the bloodstream and begin a cycle of invading red blood cells and

asexual replication, thus producing and releasing newly formed merozoites from the

red blood cells. Through this process symptoms can develop 1 week after being

bitten by an infected mosquito, though in falciparum malaria this can occur up to 3

months or occasionally longer. The characteristic malarial paroxysms of chills and

fever usually coincide with the periodic release of merozoites and other pyrogens in

the blood. In P. falciparum infections, this periodicity may not always be apparent. A

subpopulation of merozoites differentiate into sexual forms of the parasite, resulting

in male and female gametocytes that circulate in the bloodstream (Fig. 81-1). If the

gametocytes in the host blood are ingested by a female Anopheles mosquito during a

blood meal, fertilization and an asexual division in the mosquito midgut occurs

where they develop into oocysts. Each oocyst grows and divides, producing

thousands of active haploid forms, called sporozoites; once mature the oocyte bursts,

releasing sporozoites, making their way to the mosquitos’s salivary glands.

Inoculation of the sporozoites into a new human host perpetuates the malaria life

cycle.

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Figure 81-1 Plasmodium falciparum gametocytes.

While the majority of cases of malaria are transmitted by the bite of an infective

femal e Anopheles mosquito, occasional transmission has occurred by blood

transfusion, needle sharing, organ transplantation, or congenitally from mother to

fetus.

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