Malaria Parasites

Author: MBLOSGTU

Introduction

Malaria is one of the major causes of disease for people living in tropical and subtropical areas.

Despite intensive control efforts during the twentieth century, approximately 40% of the world’s population still remain at risk of infection.

Globally, it is estimated that there are 300–500 million new Plasmodium infections and 1.5–2.7 million deaths annually due to malaria.

Most morbidity and mortality is caused by Plasmodium falciparum, and the greatest disease burden is in African children under 5 years of age.

Unfortunately, the impact of malaria infections on health is increasing as financial constraints continue to hamper malaria control programmes. Mosquitoes have become resistant to insecticides and drug-resistant parasites have spread through many endemic areas.

Most recently, global warming has the potential to expand the extent of anopheline-susceptible areas and to put even more people at risk.

Epidemiology

The prevalence of malaria has increased at an alarming rate during the last decade.

There are now an estimated 300–500 million cases annually, occurring in some 101 countries and territories – almost half of which are situated in Africa south of the Sahara.

It is estimated that 3,000 children under the age of 5 die from malaria every day.

Frequent international air travel has led to increasing numbers of imported cases and deaths in returned travellers and visitors to previously malaria-free, developed countries.

Factors contributing to the resurgence of malaria include breakdown of control programmes, rapid spread of resistance of malaria parasites to chloroquine and other quinolines, and migration of non-immune populations (for agriculture, commerce, or trade) from malaria-free areas to regions with high transmission.

Armed conflicts causing displacement of large populations to areas with difficult living conditions, changing rainfall patterns and land use (leading to new mosquito breeding sites), and changes in vector behaviour have further compounded the problem. Governments have generally responded slowly due to adverse socioeconomic conditions and limited health resources.

Life Cycle

The slide heading "Life cycle" indicates that the malaria parasite’s life cycle is crucial to its transmission; however, detailed cycle information is integrated in subsequent sections.

Pathogenesis

The non-specific attachment of the merozoites to the surface of erythrocytes is followed by their apical reorientation.

The paroxysms of fever and chills that characterize acute malaria are related to the rupture of erythrocytes and the subsequent release of merozoites and parasite products into the circulation.

P. falciparum has two distinguishing features contributing to its increased pathogenicity: it can amplify to high parasitaemia (sometimes in excess of 30%) because red blood cells of any age may be invaded, and mature forms alter the surface of infected erythrocytes, causing them to sequester in particular vascular beds.

Sequestration

The hallmark of P. falciparum malaria is the sequestration of infected erythrocytes within the capillaries and postcapillary venules in organs such as the brain, lung, heart, bone marrow, kidney, liver, pancreas, intestine, and even in the intervillous spaces of the placenta.

Cytoadherence

Adherence of trophozoite and schizont-infected erythrocytes in target organs is a major feature of the pathophysiology of P. falciparum malaria.

Rosetting

Rosetting refers to the adherence of uninfected erythrocytes to those containing mature forms of some (but not all) isolates of P. falciparum.

Clinical Manifestations

Many factors influence the severity of malaria including host genetics, immune responses (and circulating cytokines), metabolic disturbances, vascular factors, and variations in parasite adhesion capabilities.

Cerebral Malaria

The main histopathological feature of cerebral malaria is widespread sequestration of infected erythrocytes in the cerebral microvasculature. The capillaries and postcapillary venules become dilated, congested, and often obstructed by parasitized erythrocytes.

Respiratory Distress

Respiratory distress in severe malaria is well described. It has been attributed to pulmonary oedema or the adult respiratory distress syndrome, especially in adults, and may also result from coexistent pneumonia, sequestration of parasites in the lungs, or central respiratory drive changes associated with cerebral malaria.

Anaemia

Infection with P. falciparum causes changes in the erythrocyte membrane which contribute to anaemia.

Thrombocytopenia and Coagulation

Moderate thrombocytopenia is commonly seen in all human malaria infections. Its cause may be due to decreased platelet survival, enhanced aggregation with sequestration from activated cells, or antibody-mediated clearance. Disseminated intravascular coagulation is observed in about 5% of patients with severe malaria.

Renal Failure

P. falciparum is the only species that causes acute renal failure. Sequestration of parasitized erythrocytes in glomerular and interstitial vessels, along with reduced renal blood flow and oxygen delivery, contribute significantly to acute renal insufficiency.

Hypoglycaemia

Hypoglycaemia (blood glucose concentration ≤2.2 mmol/l or 40 mg/dl) is generally associated with quinine infusion in adults, primarily due to quinine-induced hyperinsulinaemia. Other mechanisms, such as the influence of circulating cytokines, may also be involved. In children, pretreatment to avoid hypoglycaemia is crucial.

Pregnancy

In areas of intense transmission, women develop clinical immunity during childhood. However, during the first pregnancy, marked proliferation of P. falciparum in the placenta may overcome this immunity.

Heart and Liver

Sequestration of erythrocytes also occurs in the heart, and although jaundice and abnormal liver function tests are common, hepatic failure is rare.

Laboratory Diagnosis

Malaria should be suspected in patients presenting with febrile illness (or a history of malaria) in endemic areas, or in febrile patients who have travelled to endemic regions (particularly within the past 12 months).

Laboratory abnormalities that heighten suspicion include thrombocytopenia with a normal white cell count, the presence of malaria pigment in macrophages and other white blood cells, abnormal liver function tests, elevated lactate dehydrogenase, or haemoglobinuria. Blood slides should be sent to a reference laboratory for confirmation.

Microscopy

The diagnosis of malaria is usually made by examining Giemsa-stained thick and thin blood smears under an oil immersion lens for intraerythrocytic ring-stage parasites.

Thick Films: Made from a drop of blood dried on a slide and stained with a water-based Giemsa stain. They allow concentration of parasites (with lysis of red cells) and are 20–40 times more sensitive than thin films in samples with low parasitaemia.

Thin Smears: Fixed with anhydrous methanol to preserve parasite and erythrocyte morphology. They are used to differentiate parasite species and to quantify the percentage of infected erythrocytes.

Identification depends on the skill of the microscopist and the parasitaemia level. In cases of synchronous replication, there may be very few or even no parasites present despite severe complications from sequestered parasites.

Alternative microscopy techniques employing fluorochromes such as Acridine Orange can allow rapid screening (in less than 1 minute) at lower magnifications (×400). The Acridine dye-stained buffy coat examination (the ‘QBC’ technique) increases sensitivity, though cost and technical issues may limit its use.

RAPID DIAGNOSTIC TEST (RDT)

Malaria RDTs are qualitative immunochromatographic lateral flow tests (available in dipstick, cassette, or card formats) that detect malaria antigens in peripheral blood.

Antigens Detected by Malaria RDTs

• HRP 2: A water-soluble protein produced by asexual stages and young gametocytes of P. falciparum, abundantly expressed on the surface of infected red cells.
• pLDH and Aldolase: Metabolic enzymes found in the glycolytic pathway of malaria parasites; produced by both asexual stages and gametocytes of all four human malaria species.

Gene Amplification and Serological Methods

Gene amplification methods, such as PCR, have been developed for detecting malaria parasites. These techniques are valuable in cases of low parasitaemia, mixed infections, ambiguous parasite speciation, for reference purposes, microepidemiology, and research.

Serological assays—IFA, IHA, and ELISA using cultured P. falciparum infected erythrocytes as antigen—are well characterized for antibody detection. However, because they reflect past exposure rather than acute infection, they are not suited for diagnosing acute malaria in endemic areas, though they can be useful for retrospective diagnosis in non-endemic regions.

Other Tests

Additional diagnostic tests and measurements include:

• Haemoglobin or packed cell volume (especially in cases of heavy parasitaemia in young children and pregnant women).
• Blood glucose measurement to detect hypoglycaemia, particularly in young children and pregnant women with severe falciparum malaria.
• Total white cell count and platelet count.
• Coagulation tests (plasma fibrinogen, FDP’s, etc.) if abnormal bleeding is suspected.
• Urine tests for free haemoglobin (in cases of malaria haemoglobinuria) and protein (when nephrotic syndrome is a concern).
• Blood urea or serum creatinine to monitor renal failure.
• Screening for G6PD deficiency before treatment with oxidant drugs such as primaquine.

Key Points in Diagnosis

• Blood smears should be prepared upon patient admission; a clear fever pattern may not be apparent early in infection.
• Both thick and thin blood smears should be examined, with at least 200–300 oil-immersion fields reviewed before a smear is declared negative.
• Wright’s, Wright-Giemsa, or (preferably) Giemsa stain is used for identifying blood parasites.
• Automated differential instruments may miss malarial parasites; even with careful review, light parasitaemia is likely to be overlooked.
• If the patient has been on prophylactic medication during the past 48 hours, more oil-immersion fields may need to be examined as the number of infected cells may be decreased.
• One negative set of blood smears does not rule out malaria.
• Serial thick film parasite counts remain a simple, cost-effective, rapid, and reliable method for identifying patients at high risk of recrudescence due to drug resistance and treatment failure.