Malaria is the number one killer of all the parasitic diseases. More than 90% of the deaths worldwide occur in sub-Saharan Africa. In 2016, it was estimated that there were 216 million malaria cases and an estimated 445,000 deaths (WHO World Malaria Report, 2017).

The Organisms

There are four main species of Plasmodium that cause malaria in humans: Plasmodium vivax, Plasmodium falciparum, Plasmodium malariae, and Plasmodium ovale. Plasmodium knowlesi, which normally infects macaques, is known to cause zoonotic malaria in Southeast Asia. The two most common species are P. vivax and P. falciparum, with falciparum being the most pathogenic of all. Transmission to humans is by the bloodsucking bite of female Anopheles mosquitoes (Figure 1). The morphology and other characteristics of these species are summarized in Table 1 and illustrated in Figures 2 and 3 A–C.

Fig1. Life cycle of malaria parasites. Continuous cycling or delayed multiplication in the liver may cause periodic relapse over several years (1–2 years in P. ovale, 3–5 years in P. vivax). Relapse does not occur with P. falciparum, although a long prepatent period may occur, resulting in initial symptoms appearing up to 6 months or more after exposure.

Table1. Some Characteristic Features of the Malarial Parasites of Humans (Romanowsky-Stained Preparations)

Fig2. Morphologic characteristics of developmental stages of malarial parasites in the red blood cell. Note cytoplasmic Schüffner dots and enlarged host cells in P. vivax and P. ovale infections, the band-shaped trophozoite often seen in P. malariae infection, and the small, often multiply infected rings and the banana-shaped gametocytes in P. falciparum infections. Rings and gametocytes are typically seen in peripheral blood smears from patients with P. falciparum infections. (Reproduced with permission from Goldsmith R, Heyneman D: Tropical Medicine and Parasitology. McGraw-Hill, 1989. © McGraw-Hill Education.)

Fig3. Distinguishing features between the two most common malarial parasites: A: P. vivax trophozoite inside a red blood cell with Schüffner dots. B: Double rings and C: banana-shaped gametocytes are typically seen in P. falciparum infections. D: Insecticide impregnated bed nets are an important means of protection against mosquitoes transmitting malaria. (A–C: Used with permission from Sullivan J: A Color Atlas of Parasitology, 8th ed. 2009. D: Used with permission from WHO/TDR image library/Crump.)

Human infection results from the bite of an infected female Anopheles mosquito, through which the sporozoites are injected into the bloodstream. The sporozoites rap idly (usually within 1 hour) enter parenchymal cells of the liver, where the first stage of development in humans takes place (exoerythrocytic phase of the life cycle). Subsequently, numerous asexual progeny, the merozoites, rupture and leave the liver cells, enter the bloodstream, and invade erythrocytes. The merozoites do not return from red blood cells to liver cells.

Parasites in the red cells multiply in a species-characteristic fashion, breaking out of their host cells synchronously. This is the erythrocytic cycle, with successive broods of merozoites appearing at 48-hour intervals (P. vivax, P. falciparum, and P. ovale) or every 72 hours (P. malariae). During the erythrocytic cycles, certain merozoites enter red cells and become differentiated as male or female gametocytes. The sexual cycle therefore begins in the vertebrate host, but for its continuation into the sporogonic phase, the gametocytes must be taken up and ingested by bloodsucking female Anopheles.

P. vivax and P. ovale may persist as dormant forms, or hypnozoites, after the parasites have disappeared from the peripheral blood. Resurgence of an erythrocytic infection (relapse) occurs when merozoites from hypnozoites in the liver break out, are not phagocytosed in the bloodstream, and succeed in reestablishing a red cell infection. Without treatment, P. vivax and P. ovale infections may persist as periodic relapses for up to 5 years.

Pathology and Pathogenesis

The incubation period for malaria is usually between 9 and 30 days, depending on the infecting species. For P. vivax and P. falciparum, this period is usually 10–15 days, but it may be weeks or months. The incubation period of P. malariae averages about 28 days. Falciparum malaria, which can be fatal, must always be suspected if fever, with or without other symptoms, develops at any time between 1 week after the first possible exposure and 2 months (or even longer) after the last possible exposure. Travelers to endemic areas should be advised that if they become ill with a fever or flulike illness while traveling or after returning home, they should seek immediate medical attention and tell their physician of their travel history.

P. vivax, P. malariae, and P. ovale parasitemias are relatively low grade, primarily because the parasites favor either young or old red cells but not both; P. falciparum invades red cells of all ages, including the erythropoietic stem cells in bone marrow, so parasitemia may be very high. P. falciparum also causes parasitized red cells to adhere to the endothelial lining of blood vessels, with resulting obstruction, thrombosis, and local ischemia (Maier et al, 2008). P. falciparum infections are therefore far more serious than the others, with a much higher rate of severe and frequently fatal complications (cerebral malaria, malarial hyperpyrexia, gastrointestinal disorders, algid malaria, blackwater fever). Consideration of malaria in the differential diagnosis in patients with a suggestive presentation and history of travel to an endemic area is critical because delays in therapy can lead to severe illness or death with falciparum malaria.

Periodic paroxysms of malaria are closely related to events in the bloodstream. An initial chill, lasting from 15 minutes to 1 hour, begins as a synchronously dividing generation of parasites rupture their host red cells and escape into the blood. Nausea, vomiting, and headache are common at this time. The succeeding febrile stage, lasting several hours, is characterized by a spiking fever that frequently reaches 40°C or more. During this stage, the parasites invade new red cells. The third, or sweating, stage concludes the episode. The fever subsides, and the patient falls asleep and later awakes feeling relatively well. In the early stages of infection, the cycles are frequently asynchronous and the fever pattern is irregular; later, paroxysms may recur at regular 48- or 72-hour intervals, although P. falciparum pyrexia may last 8 hours or longer and may exceed 41°C. As the disease progresses, splenomegaly and, to a lesser extent, hepatomegaly appear. A normocytic anemia also develops, particularly in P. falciparum infections.

Normocytic anemia of variable severity may be detected. During the paroxysms, there may be transient leukocytosis; subsequently, leukopenia develops, with a relative increase in large mononuclear cells. Liver function tests may give abnormal results during attacks but revert to normal with treatment or spontaneous recovery. In severe P. falciparum infections, renal damage may cause oliguria and the appearance of casts, protein, and red cells in the urine.

Epidemiology and Control

 P. vivax and P. falciparum are the most common species found throughout the tropics and subtropics, with P. falciparum found predominately in Africa. P. vivax has a wider distribution than P. falciparum since it is able to survive at higher altitudes and in cooler climates in the mosquito vector. Although P. vivax can occur throughout Africa, the risk of infection is considerably less due to the low frequency of the Duffy receptor on red blood cells among many African populations (Mendes et al, 2011).

In the United States, CDC reported approximately 1500 cases of malaria in 2015, with P. falciparum and P. vivax comprising the majority of infections imported from countries in which malaria is endemic (CDC Morbidity and Mortality Weekly Report, 2018).

All forms of malaria can be transmitted transplacentally, by blood transfusion or by needles shared among drug misusers when one is infected. Such cases do not develop a liver infection; thus, relapse does not occur. Natural infection (other than transplacental transmission) takes place only through the bite of an infected female Anopheles mosquito.

Malaria control depends on vector control, for example, elimination of mosquito breeding places and insecticides; personal protection against mosquitoes, for example, screens, insecticide-treated netting (Figure 3D), protective clothing with sleeves and long trousers, and repellents; diagnostic testing; disease surveillance; adequate treatment with quality-assured artemisinin-based combination therapies (ACTs); and preventative therapies especially in areas of drug resistance (CDC Treatment Guidelines, 2013). A successful malaria vaccine used in conjunction with other interventions will greatly reduce the burden of disease and offers hope for eventual interruption and elimination in defined areas. Two vaccines undergoing clinical trials are (1) P. falciparum sporozoite (PfSPZ) vaccine that is an attenuated, aseptic, purified, cryopreserved vaccine of irradiated P. falciparum sporozoites (Seder et al, 2013; Sissoko et al, 2017) and (2) the RTS, S/AS01 malaria vaccine that is a recombinant circumsporozoite protein (Gosling and von Seidlein, 2016; Olotu et al, 2016).

For information on both prophylaxis and treatment, referral to the CDC for current recommendations is advised (http:// www.cdc.gov/malaria/travelers/index.html; CDC Malaria Hot Line at 770-488-7788 or toll-free at 855-856-4713). Starting on April 1, 2019, clinicians treating patients with severe malaria should call CDC to obtain IV artesunate (https://www.cdc. gov/malaria/new_info/2019/artesunate_2019.html).

اخر الاخبار

اخبار العتبة العباسية المقدسة

المجمع العلمي يقيم برنامجًا قرآنيًّا رمضانيًّا في جامعة الكوفة

الحلقة 21 من جائزة العميد القرآنية تشهد تأهل القارئ محمد حسن زاده من أفغانستان إلى المرحلة النهائية

موكب أهالي كربلاء يحيي ذكرى شهادة الإمام علي (عليه السلام) عند مرقده الطاهر

لليوم الثالث.. شعبة الخطابة تنظم مجالس العزاء لإحياء ذكرى شهادة الإمام علي (عليه السلام)

المزيد

الآخبار الصحية

خمسة مؤشرات صحية مهملة قد تنقذ حياتك

متى تكون الحمى طبيعية؟

التدخين الإلكتروني يهدد قلبك.. دراسة جديدة تكشف مخاطر ارتفاع ضغط الدم

دراسة تكشف فائدة بعض أنواع الشاي في محاربة الاكتئاب

المزيد

الآخبار التكنلوجية

الجهاز المناعي يظهر دورا غير متوقع في صحة العظام

ابتكار من جامعة الشارقة منخفض التكلفة للحماية من الزلازل

اكتشاف تأثير غير متوقع للكاكاو على وظائف الدماغ

باحثون يرصدون أثر تغيّر المناخ في دم البشر

المزيد

مواضيع ذات صلة

Trichuris trichiura (Whipworm— Intestinal Nematode)

Enterobius vermicularis (Pinworm—Intestinal Nematode)

Toxoplasma gondii (Tissue Sporozoa)

Leishmania Species (Blood Flagellates)

Trypanosoma cruzi (Blood Flagellate)

Trypanosoma brucei Rhodesiense and Trypanosoma brucei gambiense (Blood Flagellates)

Cryptosporidium (Intestinal Sporozoa)

Entamoeba histolytica (Intestinal and Tissue Ameba)

Giardia lamblia (Intestinal Flagellate)

Classification of Parasites

Toxoplasmosis

The Lung Flukes