Protozoa 3: Malaria

Question 1

What are the different species of malaria?

Answer 1

• P. falciparum
  
  • fever every 48 hours, found throughout tropics, causes most death
  • Schizonts of P. falciparum are seldom seen in peripheral blood. Parasitized cells avoid splenic capture by being sequestered deep in the tissues and this, combined with a rapid replication, renders such infections more dangerous than those caused by the other types of plasmodia.
  • This type of malaria can cause jaundice.
• P. ovale
  
  • fever every 48 hours, West Africa
• P. vivax
  
  • fever every 48 hours, Half of all malaria infections outside of Africa
• P. malariae
  
  • fever every 72 hours, Patchy distribution throughout tropics
• P. knowlsi
  
  • Knowlesi is a new form that is a simian zoonoses
• P. falciparum and ovale responsible for most pathology

P. ovale is virtually confined to West Africa. P. vivax has a wide distribution in China, India, Pakistan, Afghanistan, SE Asia, the Philippines, and Mexico and in South and Central America. P. malariae is scattered throughout all these areas but is not common. P. knowlesi is currently confined to areas of SE Asia (Indonesia, Malaysia, southern forests of Thailand). P. falciparum is present in most tropical regions but is concentrated in sub-Saharan Africa where it causes >75% of infections.

Question 2

What is the life-cycle of malaria?

Answer 2

• Infected mosquito bites and injects the parasite into the bloodstream (sporozoite)
• Within 30mins, it travels to the liver and infects a hepatocyte
• Sits quietly in the liver and keeps dividing (for one parasite that goes into the liver, 10,000 come out) -> takes 5 to 7 days
• Asymptomatic during that period
• After 5 days, the hepatocyte bursts and the merozoites are released into the bloodstream with the goal of infecting an RBC
• This is the stage that is pathogenic
• The merozoites divide within the RBC and goes on to infect more cells once burst
• The process from invasion to release takes around 48 - 72 hours (24hrs in knowlesi)
• This process will continue until immunity kicks in, drugs are given or the patient dies
• Also there will be production of gametocyte, which are non pathological but when a mosquito bites -> if it takes up blood that contains the gametocyte, then the mosquito has it
• The following process is quite complex but depends on temperature at 26 degrees it takes 14 days from bite to becoming an infectious mosquito -> takes 30 days at 20 degrees and a mosquito lives for around 30 days so the mosquito dies before transmitting

The malaria parasite life cycle involves two hosts. During a blood meal, a malaria-infected female Anopheles mosquito inoculates sporozoites into the human host . Sporozoites infect liver cells and mature into schizonts , which rupture and release merozoites . (Of note, in P. vivax and P. ovale a dormant stage [hypnozoites] can persist in the liver and cause relapses by invading the bloodstream weeks, or even years later.) After this initial replication in the liver (exo-erythrocytic schizogony ), the parasites undergo asexual multiplication in the erythrocytes (erythrocytic schizogony ). Merozoites infect red blood cells . The ring stage trophozoites mature into schizonts, which rupture releasing merozoites . Some parasites differentiate into sexual erythrocytic stages (gametocytes) . Blood stage parasites are responsible for the clinical manifestations of the disease.

The gametocytes, male (microgametocytes) and female (macrogametocytes), are ingested by an Anopheles mosquito during a blood meal . The parasites’ multiplication in the mosquito is known as the sporogonic cycle . While in the mosquito’s stomach, the microgametes penetrate the macrogametes generating zygotes . The zygotes in turn become motile and elongated (ookinetes) which invade the midgut wall of the mosquito where they develop into oocysts . The oocysts grow, rupture, and release sporozoites , which make their way to the mosquito’s salivary glands. Inoculation of the sporozoites into a new human host perpetuates the malaria life cycle .

Question 3

Which mosquito transmits malaria?

Answer 3

Only the female Anopheles mosquitoes can transmit malaria. Male mosquitoes do not bite mammals, but feed on nectar and plant juices

Question 4

What are the main differences between anopheles, aedes and culex msoquitos and which diseases do they transmit?

Answer 4

Female anopheles: malaria

Aedes: dengue fever, chikungunya, Zika fever, Mayaro and yellow fever viruses

Culex: West Nile virus, Japanese encephalitis, or St. Louis encephalitis, but also filariasis

Question 5

Where is malaria prevalent and how many cases and deaths are there annually?

Answer 5

• 60% of all malaria cases, and 80% of malaria deaths, occur in tropical Africa
• 86% of malaria death worldwide were in children <5
• 20 - 45% of all hospital admission in Africa and 18% of deaths of children <5
• Malaria risk exists in 91 countries but mostly confined to Africa
• 78% of P. vivax in 4 countries: ethiopia, india, indonesia, pakistan

Question 6

How infectious is malaria?

Answer 6

It takes 10-15 days for the symptoms of malaria to appear after infection. This is the length of time it takes for the parasite to invade the liver and then the blood.

In most cases, sporozoites disappear from peripheral blood within the first hour. A liver schizont produces thousands of merozoites whereas an intra-erythrocytic schizont only produces 12–24 schizonts. It is the release of the latter that coincides with the clinical disease.

Question 7

How do the different malaria species compare in their clinical presentation?

Answer 7

• P. vivax and ovale: some sporozoites do not develop further into a schizont but become latent, forming an infected cell known as a hypnozoite -> can be reactivated weeks, months or years later

Question 8

What are the clinical features of malaria?

Answer 8

• Generally categorised as uncomplicated or severe
• Uncomplicated:
  
  • ​nonspecific symptoms - headache, lassitude, fatigue, abdo discomfort, and muscle and joint aches, followed by fever chills, perspiration, anorexia, vomiting, and worsening malaise
  • Classical malaria attack lasts 6 - 10 hours
    
    • Cold stage (sensation of cold, shivering)
    • Hot stage (fever, headaches, vomiting, seizures in children)
    • Sweating stage (sweats, return to normal temp, tiredness)
• Severe:
  
  • If tx delayed, parasite burden increases and severe malaria may ensue
  • Cerebral malaria:
    
    • Acidosis, hyperosmolar states and hypoglycaemia occur in severe malaria and can cause seizures. However, even if you correct all these, the patient may still have true cerebral malaria. Direct ophthalmoscopy is a simple and reliable way of confirming this. Typical changes consist of white/grey patches, especially around the fovea, due to focal areas of ischaemia.
    • Neck rigidity and photophobia are not usually seen
  • Hypoglycaemia in malaria is most common in pregnancy and can also result from treatment with quinine

Question 9

How does the pattern of fever vary between the species?

Answer 9

Question 10

How does malaria influence pregnancies?

Answer 10

Question 11

How do we diagnose malaria?

Answer 11

• Gold Standard: Examination of thick and think smeak (Giemsa stain) -> simple and cheap but requires miscroscopist and equipment
• Rapid diagnostic test: simple to use but false positives
  
  • False positives are an issue becuase we do not want to waste money on drugs and they also increased the likelihood that later infections with malaria parasites will be exposed to sub-therapeutic doses of drug
• PCR: greater sensitivity and can identify species but expensive
• IFAT (immune fluorescence antibody test) can be used for retrospective diagnosis
  
  • The IFAT test, using blood-stage antigens, is extremely sensitive in detecting malaria antibodies against P. falciparum and P. vivax. While IFAT is not of use in diagnosing acute malaria, it does become positive from 8 –180 days after an attack

Question 12

How do we treat malaria?

Answer 12

• Falciparum
  
  • 1. Riamet (see below)
  • 2. Malarone: 4 Tabl. 1x/d für 3 - 5 Tage
  • 3. Parenteral bei schwerer Erkrankung, mind. 24h: 2.4mg/kg zum Zeitpunkt 0, 12 und 12h und dann einmal taeglich
  • WHO: Artemisinin-combination therapy (ACT)
    
    • Artemether + lumefantrine (co-artem/riamet)
  • You pay more for ACT in developed countries in order to subsidise drugs for developing world
  • Resistance to ACT in 4 countries currently: SE asia
• Non-falciparum
• Hydorxychloroquine 4 Tabl. bei 0 und 6h, danach einmal taeglich
• Im Anschluss Primaquine 30mg 1x/d fuer 14 Tage
  
  • ​Chloroquine
  • ACT for P. vivax if CQ resistance has been reported
  • Primaquine to prevent relapse (P. vivax)
• ​Prophylaxis
  
  • ​Atovaquone-proguanil (malarone) - start 1 day before and then continue for 7 days after travel
  • Mefloquine (Lariam)
  • Doxycyline
    *

1

Question 13

What are ways to prevent the spread of malaria?

Answer 13

• Two methods
  
  • Parasite control
    
    • ​Drugs
    • Vaccines
  • Vector control
    
    • ​Larvicides
    • Adulticides (spraying)
      
      • Effective in reducing mosquito numbers but -> ?resistance ?safety ?cost ?willingness
    • Bednets
      
      • Effective in reducing infection rates but -> ?cost ?insecticide resistance ?willingness
      • Nets are particularly effective in Africa because there most people are bitten between 10pm and 2am, while this is not the case in other parts of the world

Question 14

Are there any vaccines in development for malaria?

Answer 14

• No commercially available vaccine
• 27 candidates in clinical trials
• One vaccine recently passed phase III trials
• Seems achievable:
  
  • Natural immunity to malaria develops with increased frequency of exposure
• RTS,S - about 47% efficacy, more so in older children and adults, trialled as a complementray strategy in multiple african countries but will not replace current prevention methods

Question 15

How are some individuals immune to malaria then and how does HIV influence malaria infection?

Answer 15

• Common in adults and older children who grew up in areas of high malaria transmission
  
  • Frequent re-exposure is needed to maintain such clinical tolerance. This is an immune tolerance and is called premonition. Such individuals act as a reservoir for infection and often suffer from chronic anaemia, not eosinophilia. Asymptomatic infection = parasitaemia + no symptoms.
• Malaria-specific immunity following repeated falciparum infection
• Several host genetic factors at play
• Sickle-cell trais (AS) prevents high parasitaemia, probably because of red cells being removed by spleen before development in schizonts
  
  • However, malaria detrimental to sickle cell anaemia SS
• Alpha and beta thalassaemia traits similar protection
• P. vivax unable to infect red cells lacking the duffy blood group antigen
• HIV results in higher incidence of episodes of sypmtomatic malaria and increased likelihood of developing severe disease

Question 16

Which condition makes a person more prone to symptomatic malaria?

Answer 16

The Duffy Factor is a glycosylated protein antigen (Fya and Fyb) on the surface of red blood cells. This factor facilitates the entry of plasmodium into red cells, especially of P. vivax. Up to 85% of Caucasians are Duffy positive, only 10–20% of Blacks are Duffy positive. Asians occupy an intermediate position.

Question 17

What are the hallmarks of P. knowlesi?

Question 18

Which species create a latent form of malaria?

Answer 18

• P vivax and ovale cause some of their cells become hypnozoites
• Can cause relapse many weeks/months/years later
• Little known about hypnozoite stage
• Cannot be treated with anti-malarials

Question 19

What are the hallmarks of pregnancy associated malaria?

Answer 19

Question 20

Important image

Answer 20

Multiple rings stages within red blood cell of P. falciparum

Question 21

In short, what happens in the gut of the anopheles mosquito?

Answer 21

Mature oocysts rupture into the mosquito’s body cavity and release sporozoites, which travel to the salivary glands, to be injected into a recipient mammal during the next blood meal.

Gametes from different strains of plasmodium can form zygotes and this genetic mixing plays a part in subsequent drug resistance. The zygote in all cases is formed in the mosquito’s gut. This develops into a motile ookinete which pierces the gut wall before becoming an oocyst. The whole process from start to finish takes about 10 days.

Question 22

What is recrudescence?

Answer 22

A recurrence of falciparum malaria is called a recrudescence when a small number of the parasites persist in the blood and local body factors become favourable for the development of overt disease, even if on malrone prophylaxis. Generally such recurrences peter out in 12–18 months with the attacks getting less severe each time.

Reinfection with the same or another strain is possible, but falciparum resistance to malarone is extremely rare, and in any case, this patient took special personal precautions (impregnated nets and clothes, sprays, and mosquito repellent creams). Relapse up to several years occurs in vivax and ovale infections and will occur after chemoprophylaxis is stopped unless primaquine is used to eradicate liver hypnozoites. Primaquine is the only drug currently licensed for the radical cure of vivax and ovale malaria. This is despite some limited resistance to primaquine by some strains of P. vivax having been reported in SE Asia.

Question 23

What is best for the personal prevention of malaria?

Answer 23

Insecticide treated nets (ITNs), using Impregnation with permethrin, has reduced malaria worldwide, probably by a third. Permethrin is derived from the crushed flower of Chrysanthemum cinerarifolium and is effective against mosquitoes, mites, ticks, chiggers, lice, fleas and sandflies. It has the unique advantage of being a contact insecticide as well as of being an insect repellent.

Knockdown insect sprays are supposed to eliminate insects as they fly through the room. At least 2 studies have shown that they are ineffective in preventing malaria.

Coils burn slowly and release allerthin (a pyrethroid), DDT or lindane into the atmosphere. They are effective in reducing the number of biting mosquitoes but it has not been proved that they reduce the incidence of malaria. Recall that one bite from one infective mosquito can transmit malaria.

Neem oil, when vaporized, appears to be harmless to humans and certainly inhibits mosquitoes from landing on the skin. Although neem and many commercial bath oils provide a physical barrier no bath oil can be consistently relied on to protect against malaria.

Electric vaporizers have not been fully evaluated to date.

Note: DEET is better against Culex spp. (culicine mosquitoes) than against Anopheles spp. DEET only provides poor protection against ticks.

Question 24

What is the most realistic way of eradicating malaria?

Answer 24

Use ACT.

This is the best of the choices provided and is strongly recommended by the WHO. It goes hand in hand with the widespread use of insecticide treated bed nets (ITNs) and the strengthening of local health resources including education, elimination of poverty and early diagnosis of the disease.

Efforts at eliminating mosquito breeding sites, the introduction of sterile male mosquitoes, the use of personal protective measures (repellents, sprays) and regular disinfection of villages and towns are too expensive and always seem to break down somewhere along the line.

The ultimate success of ACT or of newer drugs or even of vaccines depends on the mosquito’s adaptability - which is considerable.

An added factor is the plethora of counterfeit drugs available in markets throughout developing countries, where one may buy a brand ‘medicine’ for a few pence.

The impetus to rid the world of malaria has been greatly stimulated in recent times by the generosity of many philanthropic groups.

Question 25

How effective is malarone?

Answer 25

Malarone can be used both for prophylaxis and treatment of malaria. So far, only occasional cases of resistance have been reported and these are mostly linked to the genetic configuration of codon 268 in the anopheles.

It is important to know that while malarone, if correctly taken, has been show to protect against 96% of falciparum malaria it only protects against 84% of vivax infections in Indonesia*

Question 26

How often should a long-lasting insecticide treated bed net be replaced?

Answer 26

Every three years

Question 27

What is the best single drug treatment of moderate non-falciparum malaria in a district hospital?

Answer 27

Chloroquine

The treatment of malaria should follow the guidelines given in each country but chloroquine, a synthetic 4-aminoquinolone, is the preferred option when the diagnosis is absolutely clear and when resistance to chloroquine in a falciparum-free area has not been reported.

Chloroquine is a powerful schizonticide which acts during the asexual blood phase of the parasite. It also has useful anti-inflammatory powers.

Chloroquine can be given in a hospital or in a health centre to all age groups, including pregnant women, either orally or parenterally. Most importantly, it is a cheap, widely available, well-tolerated drug.