This course introduces learners to a variety of infectious diseases using a patient-centered, story-based approach. Through illustrated, short videos, learners will follow the course of each patient’s illness, from initial presentation to resolution. Integrating the relevant microbiology, pathophysiology and immunology, this course aims to engage and entice the learner towards future studies in microbiology, immunology and infectious diseases. The patient-centered videos included in this course were created as part of the Re-imagining Medical Education initiative, led by Charles Prober MD, Senior Associate Dean of Medical Education at the Stanford School of Medicine. This initiative was the first of its kind to explore the collaborative creation of foundational medical education online content by inter-institutional teams of faculty. The content presented in this course was created by faculty from Stanford University School of Medicine, in collaboration with The University of Washington School of Medicine, Duke University School of Medicine, UCSF School of Medicine, and The University of Michigan Medical School. Support for this initiative was provided by the Robert Wood Johnson Foundation and the Burke Family Foundation. PLEASE NOTE: Information provided in this course is for educational purposes only and is not intended to be used for diagnostic and/or treatment purposes.
Protozoa: Malaria
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En provenance du cours de Stanford University
Stories of Infection
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À partir de la leçon
Other Infectious Pathogens
In Module Six, we will explore six cases of infectious disease caused by mites, protozoa, tapeworms, fungi, and tick-borne bacteria. We will also examine how arthropods like mosquitos and ticks can spread disease. While most of these diseases are caused by infectious pathogens other than viruses and bacteria, learners will be able to apply their knowledge of immunology and microbiology to understand and interpret the clinical presentation of these six infectious diseases.
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Maya Adam, MDLecturer
Stanford School of Medicine
Today, we're going to be talking about a parasitic infection
that, by World Health Organization estimates,
results in the death of a child every minute in sub-Saharan Africa.
The disease is malaria.
This three-year-old child, living in a small village in Uganda,
contracted a type of malarial infection caused by the parasite Plasmodium falciparum,
one of several species of the Plasmodium parasite that causes malarial disease in humans.
Falciparum malaria is the most common and one of the most deadly forms of the illness.
So, when three-year-old Farida was brought
to the small clinic in her village by her mother, the health workers were very worried.
Farida had been suffering from cycles of high fever,
shivering, and sweating, or diaphoresis, for the past 10 days.
Her mother had initially tried giving her some teas made with traditional medicines but,
every 36 to 48 hours, the paroxysms or cycles of fever, shivering, and sweating returned.
Farida had been listless and had started complaining of terrible pain in her head.
When the whites of her eyes started to turn yellow,
her mother realized she needed to seek medical treatment.
At the clinic, the health worker on duty examined Farida
and found her liver and spleen to be enlarged.
The health worker asked Farida's mother i
f the child had been sleeping under a mosquito net.
UNICEF outreach teams distributed one insecticide
treated net per household to the village homes about a year before,
as part of a coordinated malaria prevention effort.
But Farida's father thought
that because the women and girls weren't earning income for the family,
it was less important to protect their health by putting them under the net at night.
So Farida was left exposed, from sundown to sunrise,
to the bites of the Anopheles mosquito,
the primary host or carrier for Plasmodium falciparum.
Malaria is a mosquito-borne illness
that commonly occurs in tropical or subtropical parts of the world,
like Uganda, where rainfall and warm temperatures lead to standing water
that provides an ideal environment for incubating mosquito larvae.
Near Farida's home, there was a small dumpsite
where empty food cans and washing water were disposed of.
The cans were often full of stagnant water,
an ideal breeding ground for the Anopheles mosquito.
After sundown and all through the night,
the female Anopheles mosquitoes feed on human blood.
One of these insects drew blood from Farida's arm
and, in the process, transferred the Plasmodium parasite in its motile sporozoite
form into Farida's bloodstream.
Once inside the bloodstream of its human or secondary host,
the sporozoites travel to Farida's liver,
where they quickly colonized liver cells entering schizont phase,
a phase in which the parasites replicate inside cells, forming multiple merozoites.
Merozoites can then bud off of the hepatocyte
and enter the bloodstream undetected by the immune system
because they're wrapped in liver cell membrane.
Once these wrapped merozoites, also called merosomes, reach the lung vasculature,
the merosomes begin to break down over the next 48 to 72 hours.
Free merozoites are then released in pulmonary capillaries
where the red blood cells are densely packed and moving slowly
so that they can easily be invaded.
Once inside Farida's red blood cells,
the merozoites were again hidden from her immune system,
and they could start replicating again, forming characteristic ring shapes
and passing through the trophozoite phase,
before breaking out to infect more red blood cells.
In this way, the parasite presence was amplified in Farida's body in two phases:
one inside the liver cells, and that's also called the exoerythrocytic phase,
and one phase called the erythrocytic phase,
because the replication happens inside the red blood cells.
Each time the merozoites were wrapped in hepatocyte or red blood cell membranes,
Farida's immune system couldn't detect their presence.
But each time the parasite was in its exposed form in the bloodstream --
for example, when the merozoites would break out of the red blood cells --
the immune system could then detect their presence
and deploy defense cascades to try and eliminate the parasite.
This explains the cyclical nature of Farida's fevers.
In infected humans, some red blood cells also become infected with parasitic gametocytes
which are then ingested by other mosquitoes to continue the cycle of infection.
This is how the parasite exits its secondary host.
After examining Farida, the health workers at the rural clinic quickly decided
to send her to the regional hospital in the nearest town of Ibanda.
A neighbor offered to drive Farida and her mother to the hospital.
So, mother and child were loaded onto the back of a small moped,
Farida wrapped tightly to her mother's chest with a blanket.
During the 45-minute drive, Farida had a seizure and began to lose consciousness signs
that she was now suffering from cerebral malaria,
a serious complication of her falciparum infection.
Infected red blood cells
that circulate through the body are usually destroyed by the spleen.
This is why Frida's spleen was enlarged when she was first examined.
But the Plasmodium falciparum parasite tries to avoid this fate
by expressing adhesive proteins on the surface of infected red blood cells.
This gives the parasite a better opportunity to replicate
because infected red blood cells will stick to the walls of small blood vessels
and thereby avoid destruction in the spleen.
Unfortunately for Farida, this led to the blockage of many small vessels in her brain,
causing her seizure and other neurological symptoms.
Upon arrival at the hospital in Ibanda,
Farida's blood was immediately drawn for several lab tests,
and she was started on an IV anti-malarial medication called artesunate,
part of the artemisinin group of drugs that treat malaria.
This drug had recently replaced IV quinine,
in accordance with World Health Organization recommendations.
Farida's blood smear showed ring forms and gametocytes,
confirming the diagnosis of falciparum malaria.
Her blood tests also revealed a low platelet count,
elevated levels of bilirubin, consistent with the finding of jaundice,
hypoglycemia, and a normal white blood cell count --
all confirming the diagnosis of falciparum malaria.
Shortly after her arrival at the hospital, Farida fell into a coma
and, despite anti-malarial and supportive therapy, she died 48 hours later.
At present, there is no vaccine for malaria, but there is an urgent need for one.
And efforts to generate one are ongoing.
Malarial infections can be prevented with mosquito nets, insecticide spraying,
and even by raising public awareness about the risks of standing water in endemic areas.
But in high transmission areas, these measures may not be enough
because parasites can evolve to become resistant
to the insecticides commonly used on bed nets.
The cost of these preventative measures is also an issue and often means
that many people in the world today still don't have access to them.
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