Malaria

Malaria: mala aria

About the Disease

Medieval Italian:  mala aria  or “bad air”               
Thought to have been a link to the smell of swamps, where there were many mosquitoes
Malaria is an infectious disease common in tropical and subtropical regions.  It is responsible for 1 to 3 million deaths annually and is caused by a protistan parasite –  Plasmodium.

 

by Meredith Maxey [Honors Tutorial college/BIOS]

References

  Picture:  Painting of a swamp - malaria's namesake  Source

Brief Recap of Life Cycle
  • Anopheles  injects  Plasmodium  into human host when feeding…

  • Plasmodium  infects cells, replicates, and wreaks further havoc…

  • Some RBCs carrying  Plasmodium  can be transmitted back to a mosquito seeking a blood meal…

Symptoms            

                                                 
Picture:  Man with malaria being examined  Source

Generally, the infected person will be asymptomatic for 10-28 days.  Then the infected person may experience…

  • Characteristic waves of fever and chills (lasting 4-10 hours)

    • 2 day cycle for  P. vivax  and  P. ovale

    • 3 day cycle for  P. malariae

    • None for  P. falciparum (malignant tertian)

  • Anemia - hemolysis

  • Hemoglobinuria

  • Convulsions

  • Vomiting

  • Tingling of skin ( P. falciparum )

  • Splenomegaly

Image:  Note shape of enlarged spleen (splenomegaly)

  • Coma (“sticky” RBCs affect integrity of blood brain barrier)

  • Death

 Consequences of infection   

Young children with cerebral malaria may have severe neurological deficits.  Pregnant women may experience increased infant mortality, stillbirths, and babies with low birth weights.

 

Preferred Diagnostic Method

Microscopic examination of blood films allows for identification of the species of  Plasmodium.

P. falciparum

  • visible gametocytes

  • visible trophozoites

P. vivax

  • parasitized RBCs = 2x normal

  • schizonts have up to 20 merozoites

P. ovale

  • schizonts never have > 12 merozoites

P. malariae

  • parasitized RBCs appear smaller than normal

  • visible band forms across infected RBCs

What causes malaria?    

               Picture:   Plasmodium  Source

Answer:

A protistan parasite (Genus:  Plasmodium )

 

What is the vector for human parasitization?

Picture:   Anopheles  Source

Answer:

The female  Anopheles  mosquito

Plasmodium

Four species cause malaria in humans

  • P. falciparum (80% of infections; 90% of deaths)
  • P. vivax
  • P. ovale
  • P. malariae
Plasmodium  Life Cycle                 

When a mosquito wants a blood meal it penetrates the host’s skin, injects saliva, anticoagulant and 10-100 sporozoites.  Within 30 minutes, the sporozoites travel to the liver and enter the liver cells.  There, the sporozoites undergo schizogony, asexual division (lasts 2-10 days), and generate merozoites (10s of 1000s of these).  The merozoites invade other liver cells, enter bloodstream, and invade erythrocytes (RBCs).

In RBCs, merozoites enlarge into a uninucleate cell called a ring trophozoite.  The nucleus asexually divides to produce a schizont.  The schizont divides and produces 20-30 new multi-nucleated merozoites. 

The merozoites use an aspartic acid protease (plasmepsin) to degrade hemoglobin.  This causes the RBCs rupture and release toxins – resulting in the characteristic waves of fever/chills. Some merozoites develop into gametocytes which produce gametes; those RBCs do not rupture.  Finally, the gametocytes are extracted by mosquito.  Gametes are produced in the gut of the mosquito (they cannot be formed in humans).  The diploid zygotes develop in intestinal walls and differentiate into oocysts.  The subsequent mitotic divisions in oocysts produce many sporozoites.  The sporozoites migrate to salivary gland of mosquito, and the cycle continues...

 

Picture:  RBCs infected with  P. vivax    Source                                                          

The Impact of Malaria

From the WHO…  

Picture Source

 

Ancient History
  • Three of the parasites evolved over 30 million years ago in Africa
  • 2.5 million years ago for  P. falciparum
  • 323 BC: Alexander the Great’s death was attributed to malaria
M odern History

1880: Charles Louis Alphonse Laveran proposed that malaria was caused by a protozoan; this was the first time protozoa were identified as causing a disease

1881: Carlos Finlay suggested that mosquitoes transmitted malaria

1898: Ronald Ross showed that a mosquito species transmitted malaria to birds

1900: Findings of Carlos Finlay were confirmed by Walter Reed

1902: Ronald Ross received a Nobel Prize for describing the life cycle of the parasite

1907: Charles Louis Alphonse Lavern won a Nobel Prize for his discovery that malaria was caused by a protozoan (among other findings)

1955: WHO inaugurated its Global Malaria Eradication Campaign

Malaria as a cause of death
  • One death every 30 seconds
  • 350 to 500 million infected
  • 1.3 to 3 million deaths annually

85 to 90% of fatalities = sub-Saharan Africa

Children under 5 comprise the vast majority of cases

Death rate expected to double in the next 20 years

Picture:  Child being treated for malaria  Source

Why Children?
  • Malaria induces human immune response very slowly
  • Children only develop sufficient immunity after a few years of constant exposure
  • Immunity wanes if removed from exposure (specific parasite)                             

Picture:  Most of these children have already had malaria  Source

Distribution of Malaria

Complex

  • Malarial and malarial free zones can be found in close proximity
  • More common in rural areas
  • Even in West Africa, risk is lower in cities
  • Incomplete data because of location
Malaria’s Impacts on GDP
  • GDP between 1965 and 1990 rose only 0.4% annually in malarious countries vs. 2.4% in non-malarious countries
  • GDP per capita comparison in 1995 between malarious and non-malarious countries found a five-fold difference between the two
  • Malaria costs Africa $12 billion every year
Microscopic examination of blood films

Thin films are similar to usual blood films and allow for species identification.  Thick films, on the other hand, screen a larger volume of blood and are 11x more sensitive than thin (.0000001%).  However, the parasite appearance is distorted.

Problems with blood films                                       

Picture:  Common examples of "bad" blood films  Source

Films MUST be made shortly after sampling            

or sample can be altered by…

  • Warm temperatures (schizonts rupture à  P. falciparum )
  • EDTA ( P. vivax  and  P. ovale  à  P. malariae  size)
  • Cool temperatures (gametocyte divisions mistaken for other organisms)
If no microscopes…

Antigen detection tests can be used

  • OptiMAL-IT ®

    • P. falciparum  to 0.01%

    • Non- falciparum  to .1%

  • Para check-Pf ®

    • Detects parasites to 0.002%

    • Won’t distinguish between species

  • Rapid assays are being developed for the field

 Treatment           

Several families of drugs used for treatment of prophylaxis based on the dosing. Chloroquine is the standard, but  Plasmodium falciparum  is resistant.

Drugs for therapy                                                 

Picture:  Sample treatment instructions  Source

  • Artemether-lumefantrine (Coartem)
  • Artesunate-amodiaquine
  • Artesunate-mefloquine
  • Artesunate-sulfadoxine
  • Quinine – toxic to the malaria parasite by interfering with the bug’s ability to break down and digest hemoglobin.
  • Primaquine ( P. vivax  and  P. ovale  only!)
Drugs for therapy and prophylaxis
  • Atovaquone-proguanil (Malarone)
  • Chloroquine
  • Cotrifazid
  • Doxycycline                           
  • Mefloquine (Lariam)
  • Sulfadoxine-pyrimethamine
Chloroquine as a model drug

Mechanism

  • Caps hemozoin molecules

  • Leads to heme build up

  • Toxic to cell                                      

Picture:  Areas of chloroquine resistance  Source   

P. falciparum  Resistance
  • They can efflux chloroquine at 40x normal the normal rate. 

  • This gets rid of the toxic buildup of heme that chloroquine causes. 

  • Mutations in the  Plasmodium falciparum

  • Chloroquine Resistance Transporter (PfCRT) gene are thought to be responsible for this.

Standards for Treatment

The WHO (2001) recommends using artemisinin-based combination therapy (ACT) which costs up to 20X as much as older meds.  There are still problems with availability and cost -- even in the United States.  These problems are especially evident in developing nations.   It only costs between a quarter and $2.40 (USD) to treat these people; the problem is largely one of availability (vs expense).

How is the immune system evaded?

Trojan Horse Mechanism
  • merozoites hide in merosomes (dead liver cells) and release “cloaking chemicals” so that the body doesn’t clean up those dead cells like it should

 Adhesive surface proteins (PfEMP1)

  • P. falciparum  displays 60+ variations of PfEMP1 on surface of infected RBCs

  • PfEMP1 stick to walls of vessels and prevents travel to spleen (where they would have been destroyed)

 Anopheles  mosquito

  • 400 species

  • 30-40 transmit the four  Plasmodium  species

  • Anopheles gambiae  best known

    • transmits  P. falciparum

    • infects large proportion of RBCs (10x others) adheres to walls of blood vessels

 Anopheles  and malaria transmission

Anopheles  is innately susceptible to  Plasmodium.   Anopheles is also anthropophilic, which means they prefer to feed on humans.

 

A. gambiae  and  A. funestus

Anopheles  survive longer than required  Plasmodium  incubation period

 

Current Research

A. gambiae

In lab, strains are selected that have an immune response to parasites.  That mechanism involves the mosquito encapsulating and killing the parasites once in the mosquito’s intestinal wall.  Scientists are studying this mechanism with the thought of replacing wild mosquitoes.

Evolutionary pressure of malaria on human genes

High levels of mortality/morbidity especially associated with  P. falciparum

Sickle-cell anemia                           

Picture:  Sickle-cell on left, normal RBC on right  Source

  • mutation in a gene (HBB) that codes for a hemoglobin subunit (valine for glutamate)

  • normal allele = HbA; Sickle-cell = HbS

  • heterozygotes (HbA/HbS) have malaria protection

  • 10% sustained frequency of allele

  Duffy antigens
  • chemokine receptor on RBCs

  • carriers completely immune to  P. vivax

Picture:  Duffy antigen's resistance cycle  Source

  • carriers lack receptor on RBC membrane

  • rarely found in white populations

  • found in 68% of black people

Socioeconomic Impacts
  • Costs of health care                                     
  • Working days lost due to sickness
  • Days lost in education
  • Decreased productivity (especially with cases of cerebral malaria)
  • Loss of investment and tourism
Socioeconomic Impact Statistics

In some countries, the disease is estimated to account for…

  • 40% of public health expenditure
  • 30-50% of inpatient admissions
  • Up to 50% of outpatient visits
Vaccine Research                                                                   

No vaccine that will prevent malaria but much research is being conducted

-RTS,S/AS02A is in phase II trials

  • Funded by the Gates Foundation and GlaxoSmithKline
  • Reduces infection risk by 30% and severity of infection by 50%
  • Mozambican children in 2000
  • Commercial release by 2010? 

                                                             Picture:  Sample vaccine trial poster  Source

Other Advancements

2002: Genome of  P. falciparum  completed; this could provide targets for new drugs/vaccines

2002: University of Edinburgh (2002) announced the discovery of an antibody that protects against malaria

2002: Sterile insect techniques utilized by scientists at Case Western brought about a  Plasmodium -resistant strain of  Anopheles

Prevention and disease control: Prevention is cheaper than treatment

US and Southern European efforts were effective

  • Drain wetland breeding grounds/better sanitation
  • Monitoring/treating those infected

Some developing nations have also been successful

  • Conducive country conditions
  • Targeted technical approach
  • Active leadership at all levels of government
  • Decentralized implementation (control of finances)
  • Hands on technical/program support
Prophylactic drugs

Not practical for those living all the time in a malaria-endemic locale due to their side effects and expense.  However, they are effective for short-term visitors.  Their effects are not immediate, and so some must be taken as early as two week prior and four weeks after exposure.

                                                                  Picture Source

Indoor residual spraying

DDT developed around WWII; it was quite controversial because of its effects when used on a large-scale agricultural basis.  The WHO still advises the use of DDT in endemic areas, however.  It is quite effective, though still stigmatized, when used to spray interior walls with small quantities to control mosquitoes.

Mosquito Nets

Anopheles  feeds primarily at night, and thus, insecticide-treated nets (ITN) around sleeping people are effective.  In fact, they are about 2 times as effective as untreated nets.  However, only 1 out of 20 Africans own these nets.  Nets can be readily obtained for 2 to 3 euros from the UN, WHO, others… One downside to the nets is that they need to be re-impregnated every 6 months; this creates numerous logistical problems.

Picture:  Mosquito net  Source