Malarial Parasite And The Vector

Published Date: 02 Nov 2017

The first documentation about the pattern of malaria and the splenic changes that occur in this disease is attributed to The Father of Medicine Hippocrates around the time of 400 B.C who can probably be considered as the first malariologist. The description of malaria is found in the Greek writings around 500 BC who believed the evil spirits or malaria gods live within the marshes this led to the origin of the belief of Hercules and Hydra in the Greek fable.

The Babylonian God of destruction and pestilence who is pictured as a double-winged, mosquito-like insect in the in cuneiform script on clay tablets written several thousand years ago in cuneiform script on clay tablets attributes to malaria.

In 800 B.C. the sage Dhanvantari wrote, "Their bite is as painful as that of the serpents, and causes diseases as if burnt with caustic or fire, is red, yellow, white, and pink color, accompanied by fever, pain of limbs, hair standing on end, pains, vomiting, diarrhea, thirst, heat, giddiness, yawning, shivering, hiccups, burning sensation, intense cold..."

The description of features suggestive malaria and its consequences like fever and spleenomegaly is found in ancient texts all over the world like the Sumerian and Egyptian texts which were written 3,500 to 4,000 years ago, the Vedic and Brahmanic scriptures of Indus valley, texts of Charaka Samhita and the Susruta Samhita, written about 100 BC. The Charaka Samhita had also classified the fevers into five different categories, namely continuous fevers (samatah), remittent fevers (satatah), quotidian fevers (anyedyuskah), tertian fevers (trtiyakah) and quartan fevers (caturthakah).

Initially when the description of the disease was not well understood physicians used the term 'typhomalaria' to describe acute undifferentiated fevers. Osler by his careful observation in clinical features and fever patterns clearly differentiated malaria from typhoid fever.

EPIDEMIOLOGY OF MALARIA 8, 9

Malaria is considered as a tropical disease and is endemic throughout most of the tropics .About half of the world's population are at risk of malaria. Plasmodium falciparum species is responsible for most of the malaria followed Plasmodium.vivax.

Figure-Malaria is a major public health problem in India and one which contributes significantly to the overall malaria burden in Southeast Asia. A study on the global malaria mortality between 1980 and 2010 by Murray et al, published in, estimated the malaria mortality in India in 2010 at 46,800. 11

MALARIAL PARASITE AND THE VECTOR

Introduction 12

The disease malaria is transmitted by the bite of a female Anopheles species mosquito. This occurs mainly between dusk and dawn. The other known rare routes of the disease transmission include: congenitally-acquired disease, blood transfusion, sharing of contaminated needles, and organ transplantation.

Historical aspects of the malaria parasite 13 ,14

Alphonse Laveran in 1880 is credited with the discovery of the plasmodium parasites in the blood of malaria patients. Ronald Ross in 1897 described the whole of the transmission cycle in culicine mosquitoes and birds infected with Plasmodium relictum. In 1898 a group of the Italian malariologists, Giovanni Battista Grassi, Amico Bignami, Giuseppe Bastianelli, Angelo Celli, Camillo Golgi and Ettore Marchiafava demonstrated that mosquitoes are responsible for transmission of malaria in humans. Wojciech Krotoski in 1982 demonstrated the pre-hepatic phase of malaria parasite in the blood and the presence of dormant stages in the liver.

The life cycle of the malaria parasites, Plasmodium species 15

Malaria infection is caused by protozoan parasites of the Plasmodium genus. Malaria is transmitted to humans by the bite of female anopheline mosquitoes. It is very strange to note that this parasite has widely adapted itself to a variety of vertebrate hosts ranging from reptiles to mammals but human malaria is still not a zoonosis.

Human malaria is caused by infection with five different species of Plasmodium. The life cycles of the species of Plasmodium causing human malaria are similar and described below.

Various genetic changes have been acquired by the malaria parasite for the survival and development of the parasite within the two hosts, in intracellular and extracellular environments.

Definitive and Intermediate Hosts 17

The malaria parasite completes its complex multistage life cycle in two different hosts namely the intermediate hosts where the asexual cycle of reproduction takes place the anopheles mosquitoes and the definitive host where the sexual cycle of reproduction takes place the humans.

The life cycle of Plasmodium vivax 18, 19

The life cycle of Plasmodium vivax is divided into the following stages.

The Asexual Cycle - occurs in humans

The Sexual Cycle - occurs in female Anopheles mosquito.

The Asexual Cycle 19

The asexual cycle phase of the life cycle is further divided as follows:-

The Asexual Cycle

Pre-Erythrocytic Cycle

Erythrocytic Cycle

Trophozoite Stagesignet Ring Stage

Amoeboid Stage

Schizont Stage

Merozoite Stage

Post Or Exo-Erythrocytic

Pre-Erythrocytic Cycle

This stage begins with the bite of the infected female Anopheles mosquito to a healthy person .after its feed on human blood it spits in the place of the bite when numerous sporozoites enter into the human blood stream mixed with the saliva of the mosquito.

SPOROZOITES ENTER INTO THE HUMAN BLOOD STREAM

MIGRATE TO THE LIVER SINUSOID

PASSING THROUGH THE KUPFER CELLS & FORM PARASITOPHOROUS VACUOLES IN HEPATOCYTES

DETACHMENT OF THE INFECTED HEPATOCYTE

REMAIN WITHIN

DORMANT AS A HYPNOZOITE

DEVELOP TO FORM MEROZOITES

Erythrocytic Cycle

MEROZOITES EXIT THE HEPATOCYTES

RE-ENTER THE BLOODSTREAM

INVASION OF RBC’S

MULTIPLICATION IN RBC’S

THOUSANDS OF PARASITE-INFECTED RBC’S IN THE HOST BLOODSTREAM,

CLINICAL FEATURES OF MALARIA

This cycle is divided into the following stages:

Trophozoite Stage:-

After entering the blood stream the merozoites start invading the erythrocyte’s this stage is called as the trophozoite stage.

Signet Ring Stage:-

In this stage a non-contractile vacuole appears in the parasites cytoplasm which grows by feeding the substance of the erythrocyte. The nucleus is pushed on one side and parasite assumes a ring-like appearance hence this stage is called as the signet ring stage.

Amoeboid Stage:-

In this stage the vacuole of the signet ring stage disappears and the parasite assumes an amoeboid.

Schizont Stage:-

By using its pseudopodia inside the cytoplasm of erythrocyte, the parasite feeds on the hemoglobin and increases in its size to form the schizont.

Merozoite Stage:-

Once a particular size is reached the parasite does not grow in size but undergoes asexual multiplication. During this stage the nucleus then divides to form eight to 24 daughter individuals and known as the merozoites.

Gametocytes:-

Some of the merozoite-infected blood cells leave the cycle of asexual multiplication to develop into sexual forms of the parasite, called male and female gametocytes which circulate in the bloodstream.

Post or Exo-Erythrocytic Cycle

In the Exo-Erythrocytic stage the merozoites live within the liver asymptomatically for some years. After this dormant stage the merocytes can again become infectious by invading either fresh parenchyma cells or erythrocyte’s.

Sexual Cycle

FEMALE ANOPHELES MOSQUITO WHEN FEEDING ON AN INFECTED PERSON

INGESTS BOTH SEXUAL AND ASEXUAL FORMS

SEXUAL FORMS SURVIVE

ASEXUAL FORMS ARE DIGESTED

UNDERGO FURTHER DEVELOPMENT

The sexual cycle is divided in the following stages:

Gametogony

In this stage after entering the erythrocyte some of the schizonts specialize themselves as gamonts or gametocytes.

There are two forms of fully-grown gametocytes:-

Female or macrogametocyte, and

Male or microgametocyte.

Fertilization:

The actively moving male gamete is attracted by the female gamete and unites to form a zygote.

Sporogony:

The actively moving zygote moves vibrantly inside the mid-gut of the mosquito

bores through the wall of the gut wall

comes to rest under epithelium and the outer wall of the stomach.

The zygotes that fail to get shelter in the stomach wall of the mosquito

develop into Ookinete.

Ookinete secrete a thin membranous cyst wall at this stage it is known as the Oocyst.

The nucleus divides repeatedly and ultimately a large number of sickle-shaped sporozoites are formed from a single Oocyst.

The Oocyst ruptures about 10 days from its formation

sporozoites are liberated in the hemocoele of mosquito;

later they migrate to the salivary gland

the female Anopheles becomes infected,

The cycle of human infection re-starts when the mosquito takes a blood meal, injecting the sporozoites from its salivary glands into the human bloodstream.

The Malarial Vector 21

The female Anopheles mosquito is responsible for transmission of malaria in human beings. More than 60 species have been incriminated in the transmission of malarial infection but some species are more important as vectors because of variations in susceptibility to the parasite.

The life cycle of Anopheles mosquito is divided into the following stages 22:-

Laying of Eggs

The larval stage

The pupal stage

Adult form

Laying of Eggs:-

The female lays the eggs in batches of 70-100 on the surface of water like the irrigation channels, a pool of water in a tree trunk, and sewage effluent at night.

The larval stage:-

In the tropical temperatures the eggs hatch after two to three days and the larvae lie just below the surface of the water and feed on algae

The pupal stage :-

After 7-14 days the larvae turn into pupae during a five-minute process. The pupa is the least active stage and is comma-shaped of the Anopheles lifecycle.

Adult form:-

After two to four days the pupa metamorphoses into an adult mosquito.

The Lifestyle of the Anopheles Mosquito 23

The Anopheles mosquitoes usually mate during the flight. The male is attracted to the female mosquitoe by the tone of her wing beat which has an antennae that acts as a sound receptors.

Once mated, the female searches out a blood meal, following sensory clues such as host odour, carbon dioxide and convection currents and then she seeks out a resting place,.

When the blood meal has been digested, the ovaries develop and the mature eggs are laid at night.

Pathophysiology

The pathophysiology of malaria results from

1. Destruction of erythrocytes,

2. Liberation of parasites and invasion of erythrocytes by the merozoites

3. The binding of parasitized red blood cells (PRBC) to uninfected red cells

(Rosetting)

4. Binding of parasitized blood cells to endothelial cells in critical organs

(cytoadherence)

5. The induction of pro-inflammatory response cytokines most notably tumour

necrosis factor (TNF-a)

6. The host reaction to these events

7. Sequestration of parasites in microcirculation of vital organs interfering with

microcirculatory flow and host metabolism

Clinical Features of Malaria 25-28

The time from the initial malaria infection until symptoms appear differs in different species, is longer if chemoprophylaxis is taken or because of immunity due to previous infections but this period generally ranges from:

9 to 14 days in Plasmodium (P.) falciparum.

12 to 18 days in P. vivax and P. ovale.

18 to 40 days in P. malariae.

11 to 12 days in P. knowlesi.

Symptoms of malaria

•Fever.

Fever is the most common presentation in malaria.Three types of malarial fever may be classified, depending on the symptoms or the parasite that causes the fever. They are:

Tertian fever

Quartarn fever

Malignant fever

Tertian fever

In this type of fever, the fever attack occurs on alternate days.

Quartarn fever

In this type of fever, the fever attack occurs after an interval of two days.

Malignant fever

The other symptoms of malaria are

Chills and rigors

Headache

Sweats

Fatigue

Nausea and vomiting

Dry non-productive cough

Muscle and/or back pain.

Enlarged spleen

Impaired function of the brain or spinal cord,

Seizures, or

Loss of consciousness.

Severe malaria is characterized by one or more of the following features:

• Impaired consciousness/coma

• Repeated generalized convulsions

• Renal failure (Serum Creatinine >3 mg/dl)

• Jaundice (Serum Bilirubin >3 mg/dl)

• Severe anaemia (Hb <5 g/dl)

• Pulmonary oedema/acute respiratory distress syndrome

• Hypoglycaemia (Plasma Glucose <40 mg/dl)

• Metabolic acidosis

• Circulatory collapse/shock (Systolic BP <80 mm Hg, <70 mm Hg in children)

• Abnormal bleeding and DIC

• Haemoglobinuria

• Hyperthermia (Temperature >104 F)

• Hyperparasitaemia (>5% parasitized erythrocytes in low endemic and >10% in hyperendemic areas) Haematological Changes of Malaria

Anaemia

Anaemia due to Plasmodium infection is a major health problem in endemic areas for young children and pregnant women. The anaemia is caused by excess removal of nonparasitized erythrocytes in addition to immune destruction of parasitized red cells, and impaired compensation for this loss by bone marrow dysfunction. The pathogenesis is complex. In acute childhood malaria, there is suppression of the marrow response, sometimes accompanied by reduced erythropoietic activity. In chronic malaria, the marrow is hyperactive, but erythropoiesis is ineffective with morphologic dyserythropoiesis and absence of reticulocyte release.

Red cell destruction

Fever and the inflammatory reaction may shorten red blood cell (RBC) survival, but anaemia in patients with malaria is more pronounced than in other systemic infections. Only a limited number of erythrocytes undergo intravascular haemolysis, and most RBC’s are eliminated extravascularly, most likely by phagocytosis .The decrease in Haemoglobin is often greater than could be accounted for by the destruction of parasitized cells only, and that circulating monocytes contain phagocytosed nonparasitized RBC’s. Recent data in adults with acute uncomplicated malaria suggest that nonparasitized RBC’s account for more than 90% of RBC loss.

RBC deformability

RBC deformability is reduced in patients with malaria, which parallels disease severity and correlates with the degree of anaemia. Reduced RBC deformability has also been incriminated in the removal of normal senescent RBC’s from the circulation as they are trapped in the splenic cords and removed by immunologic effector cells.

Persistent anaemia

Recovery of Hb is often slower in patients with malaria than in those with other conditions with a comparable loss of blood volume, and anaemia may persist after complete clearance of parasitaemia. Possibly, there is continued destruction of nonparasitized RBC’s by antibody- mediated mechanisms.

Thrombocytopenia

Thrombocytopenia is a common finding in vivax malarias .In immune mediated thrombocytopenia, IgG forms a complex with the malarial antigen and the complex binds to and damages circulating platelets which are then removed from the circulation. Another mechanism is that platelets engulf malarial parasites and in the process become damaged and are thus removed from the circulation. Platelet count can fall as low as 10,000 to 20,000/ul. This is due to platelet sequestration in venules during malarial infection. The adherence of platelets to endothelium appears to be mediated through a specific receptor ligand interaction involving leucocyte function-associated antigen (LFA-1). Thrombocytopenia can also be due to excess removal of platelets by reticuloendothelial organs.

Coagulation changes

Coagulation studies show normal PT, aPTT and TT in vivax while prolonged PT, aPTT were seen in P.falciparum. Coagulation factors V, VIII and IX were most sensitive parameters in the expression of coagulation defects; most coagulation abnormalities were due to liver involvement.

Leucopenia and Leucocytosis

Mild leucopenia has been described in uncomplicated malarias .The other common changes seen are monocytosis found in population living in endemic areas.

Phagocytosis of malaria pigment by monocytes, macrophages and less frequently by neutrophil’s has been observed in peripheral blood and bone marrow of patients with malaria.

P. vivax clinical syndromes

The major severe P. vivax clinical syndromes documented include the following:

Thrombocytopaenia

Cerebral malaria

Acute renal dysfunctions

The spectrum of renal involvement in malaria includes:

Acute nephritic syndrome

Acute interstitial nephritis

ARF/ATN/pigment-induced/ischaemic nephropathy

Chronic and progressive glomerulopathy

Hepatic dysfunctions and

Pulmonary dysfunctions.

Investigations of Malaria

Complete Blood Count 30

Red blood cell analysis 31,32

White blood cell counts 30

White blood cell (WBC) counts during malaria are generally characterized as being low to normal due to the localization of leukocytes away from the peripheral circulation and to the spleen and other marginal pools, rather than actual depletion or stasis. Leukocytosis is typically reported in a fraction of cases and may be associated with concurrent infections and/or poor prognosis. Band cells were seen in P. vivax and P. falciparum malaria, although in higher numbers in P. falciparum malaria. 33

Platelet count 34,35

Severe thrombocytopenia is quite rare in P.Vivax malaria

Decreased thrombopoiesis, but bone marrow examination usually shows normal or increased megakaryocytes .Peripheral destruction, induced by the parasite, in which immune complexes are generated by malarial antigens lead to sequestration of the injured platelets by macrophages in the spleen, although this mechanism has not been properly evaluated in P. vivax malaria.The spleen has been implicated as a site of excess sequestration.In acute malaria infection platelets are found to be hypersensitive and there is increased concentrations of platelet-specific proteins such as beta thromboglobulin (ßTG), platelet factor 4 (PF4). Production of thromboxane A2 and prostacyclin also increased

Peripheral Blood smear 37,38

Light microscopy of giemsa-stained blood smears is the accepted standard for malaria diagnosis.

Thick and thin diagnostic blood smears are used

Giemsa-stained thin smears are prepared from a much smaller amount of blood than thick smears and are used to determine the Plasmodium species

The features to look for in a peripheral smear are as follows:-

• The size of the infected red cell (P. vivax and P. ovale prefer reticulocytes and

are found in larger red cells).

• The presence of stippling on the red cell (i.e. Schuffner's dots are characteristic

of P. vivax and P. ovale).

• The morphology of parasites (parasites forming a band across the red cell are a

feature of P. malariae) and the different parasite stages present (late

trophozoites and schizonts of P. falciparum are normally absent from the peripheral

blood).

• The shape of gametocytes (crescent-shaped gametocytes are characteristic

of P. falciparum, other species have round gametocytes).

• The quality of pigment (coarse pigment is suggestive of P. malariae).

Bone Marrow Biopsy 36

Plasmodium vivax malaria was not associated with any significant pathology in the bone marrow, except iron deficiency anaemia.