Lecture 23- Malaria

Question 1

Malaria definition

Answer 1

• obligate intracellular protozoa of the genus Plasmodium
• natural hosts are human
• produces acute or chronic infection, most commonly characterised by paroxysms of fever, anaemia and splenomegaly
• transmitted by Anopheles mosquitoes

Question 2

• Classification

Answer 2

• P. Falciparum
• P. Vivax
• P. Ovale
• P. Malaria
• P. Knowlesi

Question 3

Epidemiology

Answer 3

• 100 countries world wide with 2 billion people at risk
• 300-500 million incidence with an annual mortality of 1 million
• P malaria exists in Africa, haiti and the dominican republic
• P vivax and P falciparum are both present in mexico, central and south america, the indian subcontinent, southeast asia and oceania
• P vivax is rare in africa while falciparum is common in africa
• P ovale is found almost exclusively in africa

Question 4

Pathogenesis

Answer 4

• genetic factors involving surface proteins of erythrocytes have evolved in human societies by selective pressures
• heterozygous sickle cell trait is protective of P falciparum infection
• the absence of Duffy A and B blood group determinants is protective against P vivax
• cytoadherance and sequestration are the critical parasite-host interactions responsible for severe disease
• P falciparum sequesters in the microcirculation
• disruption of the microcirculation
• probable RBC receptors are ICAM1, CD36, VCAM1

Question 5

PApathophysiology

Answer 5

• gametophytes are the infected RBC - infect mosquitoes but do not cause the disease
• manifestations are a consequence asexual forms and their interactions in the blood compartment

Question 6

Disease may be a consequence of:

Answer 6

• haemolysis of parasites and normal cells
• haemoglobinuria and ARF
• hypoglycaemia secondary to parasite consumption of glucose and/or inadequate hepatic gluconeogenesis
• Acidosis, SIADH, hyponatremia

Question 7

Pathophysiology

Answer 7

• density of parasitemia is dependent on the proportion of susceptible erythrocytes
• reticulocutes are rpeferentially infected with Ovale and Vivax
• older erythrocytes are infected in Malariae infestations
• all erythrocytes are susceptible in falciparum infections

Question 8

Clinical features

Answer 8

• fever in the returned traveller
• prodrome 2-3 days
• paroxysm with rigors
• frequently have headache, myalgia, back pain, fatigue
• GI symptoms or resp symptoms may confuse the picture
• fever, pallor, jaundice, hepatosplenomegaly
• anemia, thrombocytopenia, leukopenia
• hyperbilirubinemia and non-specifically elevated transaminases are frequent
• hyponatremia and hypoglycemia may occur

Question 9

Clinical features of vivax/ovale

Answer 9

• incubation usually 12-18 days
• terrain malariae: febrile paroxysms are separated by intervals of 48 hr
• replapses usually occur within 6 months
• splenomgaly usually occurs within 2 weeks
• after 5-7 days may develop classing benign tertian periodicity
• good prognosis

Question 10

Clinical features: malariae malaria

Answer 10

• mildest form of malaria, produces chronic low level parasitemia
• incubation is usually >18 days
• diagnosis may be difficult, requiring multiple blood films
• recrudescence may occur decades later
• Quartan malaria: febrile paroxysms are separated by intervals of 72 hours
• good prognosis

Question 11

• Clinical features falciparum malaria

Answer 11

• should be considered a medical emergency in non-immune individuals
• incubation shortest: 10-14 days
• paroxysms often irregular
• clinical findings are similar to other forms of malaria byt mroe severe and acute
• more often more severe anemia and multisystem involvement
• prognosis: mortality up to 25% in non-immune individuals

Question 12

Clinica features of severe/complicated malaria

Answer 12

• parasitemia
• cerebral malaria
• shock
• Pulmonary oedema
• DIC
• macroscopic hemoglobulinuria
• renal failure: serum creatinine >265 umol/L
• severe anemia
• hypoglycemia
• academia

Question 13

Clinical features of cerebral malaria

Answer 13

• unrousable coma not attributable to other cause in a patient with falciparum malaria
• mortality 15-25%, neurological sequelea uncommon
• most common severe complication of falciparum malaria in children
• neurological findings are variable: raised ICP, focal gaze abnormalities, seizures
• rapid recovery in survivors

Question 14

Diagnosis: microscopy

Answer 14

• gold standard
• capillary rich blood, fresh EDTA/heparinised sample
• uses classing stains
• takes an hour

Question 15

Thick vs thin film

Answer 15

• thick film improves sensitivity by superimposing 202-30 layers. Measures parasite density +/- speciation, essential for low level parasitemia
• thin film: a fixed monolayer to permit speciation by preservation of parasite and RBC morphology

Question 16

Microscopy advantages

Answer 16

• good sensitivity: up to 50 parasites/up, but often only 500 parasites/up
• speciation possible, detection mixed infection
• quantification routine, essential for assessment of response, prognosis

Question 17

Microscopy disadvantages

Answer 17

• labour intensive and slow
• requires highly trained staff, performance is observer dependent and maintenance of skulls requires constant exposure
• difficulties in morphological diagnosis with prophylaxis and treatment
• sensitivity moderate requiring repeat films
• microscope expensive and limited use in field

Question 18

P falciparum microscopy characteristic

Answer 18

• normal size RBC
• multiple parasites per RBC
• rings
• banana shaped gametophytes, black pigment in RBC, schizont rare

Question 19

P Vivax microscopy characteristic

Answer 19

• enlarged RBC
• one parasite per RBS
• trophozoite is usually amoeboid form, often fragmented
• Schuffner granules, often see gametophyte and schizont

Question 20

P ovale microscopy characteristic

Answer 20

• enlarged oval shape RBC
• one parasite per RBC
• compact and regular trophozoite
• schuffner’s granules, often see gametophyte and schizont

Question 21

• P malariae microscopy characteristic

Answer 21

• normal size RBC
• usually one parasite per RBC
• compact trophozoite
• often see gametocytes and schizont

Question 22

Diagnosis: Immunochromatographic capture technique (ICT)

Answer 22

• point of care test to use in field
• uses labelled malarial monoclonal Ab in a mobile phase that binds to serum antigens and becomes immobilised by a second antibody capture system to produce a visible line
• Histidine rich protein 2
• parasite specific LDH
• plasmodium aldolase
  Present in sexual and asexual phases

Question 23

ICT stats

Answer 23

• sensitivity and specific you around 80-90% with expert microscopy but discrepancy results when parasite density

Question 24

Diagnosis: molecular techniques

Answer 24

• targets include conserved regions: 18S rRNA, circumsporozoite gene
• no commercial test, offered as in-house test in reference lab

Question 25

Advantages of molecular techniques

Answer 25

- sensitivity: 5 parasites/ul - ability to speciate in difficult morphological cases including mixed infection - allows subtyping for epidemiological studies and detection of resistance and virulence genes

Question 26

Molecular techniques disadvantages

Answer 26

- variation between laboratories because of lack of standardisation - not available for smaller centers, no commercial test - high cost and expertise required - not particularly rapid - not useful for following up as a test of cure because of persistence of organism

Question 27

MALARIA IMMUNOLOGY

Answer 27

- malaria has a highly effective immune avoidance strategy with the property of antigen switching - this involves the expression of the var gene which produces variants in the PfEMP-1 - PfEMP1 is responsible for cytoadherance and antigenic presentation on erythrocytes - 2-18% of each malarial proliferation cycle is associated with a switching event - exposure causes the development of strain specific non-neutralising immunity, with recrudescence corresponding to antigen switching events - the spleen has a critical role in clearing parasites erythrocyte from the circulation

Question 28

Malaria vaccination

Answer 28

- acquired immunity in natural population is non-neutralising and protects against disease rather than infection - immunity is induced after ongoing exposure, and is short lived in the absence of ongoing exposure - disease burden occurs disproportionately in children - passive immunity from infused immunoglobulin has been demonstrated - vaccines targets include recombinant proteins, synthetic peptides, DNA vaccines, inactivated whole parasites

Question 29

Vaccine

Answer 29

- Pf66: synthetic hybrid peptide polymer with antigens derived from P falciparum linked to CS protein - CS protein is a dominant pre-erythtrocytic and hepatic antigen expressed prior to the erythrocytic phase and has the potential to prevent disease - RCT revelead efficacy of 28% new infectin but overall no clinically significant benefit found

Question 30

Mosquirix: RTS,S vaccine

Answer 30

- based upon CS protein coupled with HBsAg - Effective vs P falciparum only - results 5 RCT, revealed modest efficacy

Question 31

Appropriate therapy requires

Answer 31

- identifying the species of malaria parasite - estimating the level of parasitemia - determining the immune status of the patient - ascertaining the likely geographic origin of the infecting parasite

Question 32

TREATMENT

Answer 32

- chloroquine and amodiquine - quinine, quinidine - artemisinin, artemether, artersunate - mefloquine - halofantrine - Dihydrofolate reductase inhibitors - dihydropteroate syntheses innhibitors - primaquine - tetracycline, doxycycline - atovaquone

Question 33

Chemoprophylaxis

Answer 33

- indicated for non immune persons entering malaria-endemic areas - chemoprophylaxis may also be appropriate for certain high-risk groups living in endemic areas - most prophylaxis do not prevent infection but aborts or modifies clinical attack - most suitable for drugs of longer T1/2 - chloroquine phosphare is effective for infection caused by Vivas, ovale, malaria and chloroquine-sensitive strains of falciparum - chloroquine resistant falciparum: artermether primary choice, doxcycline or the combination of atovaquone, proguanil are alternatives - administration should be initiated before entering a malarous area and should be continued for 4 weeks after leaving