Malaria and Antibiotic Resistance Flashcards
1
Q
Malaria Overview
A
- Malaria is a febrile illness
- Malaria is caused by protozoan parasites that infect red blood cells. It’s specifically the Plasmodium species that causes the illness
- Malaria is spread by Anopheles mosquitos
- Much more rarely can be transmitted from blood transfusion, organ transplantation, or sharing of needles
- Diagnosis is made by microscopic examination of the blood smear or antigen tests
- 200 million cases worldwide in 2017, 435,000 deaths
- 90% of deaths are in Sub-Saharan Africa
- Most deaths are in children < 5yo
- Treatment is w/ antimicrobials or (in severe cases) exchange transfusion
- Antimalarial resistance is an ongoing problem worldwide
2
Q
Parasite
A
Parasite is a broad, non-specific term for eukaryotic infectious organisms; some overlap with so-called “tropical” infections
“Parasite” can be used in different ways:
- Parasitic organisms (as opposed to commensals or symbionts) are any organisms that live on or within a host from which they derive benefits without making any useful contribution in return. So, bacterial pathogens could also be considered parasites.
- In microbiology and infectious disease we also use the term parasite to refer to invertebrate animals (as opposed to bacteria, fungi, or viruses) that infect humans. Of course, further complicating the matter is that some things we group with the parasitic infections are actually fungi.
3
Q
Medical Parasitology
A
- Medical parasitology encompasses a huge range of organisms across three different kingdoms: protozoa, fungi, and animals
- We focus on the Protozoa
- The two main phyla of importance in the animal kingdom are round worms and flat worms
4
Q
Protozoa
A
- Relatively simple, single-cell, eukaryotic organisms
- 2-100 microns in size
- Further classified by how they move
- Flagellates – flagella
- Amoebas – pseudopods
- Sporozoa – don’t move much; penetrate the host cell
- Ciliates – cilia
- Of all the parasites, these are most similar to bacteria (in terms of pathogenesis and immune response)
- We focus on Sporozoa, and specifically Plasmodium and Babesia
5
Q
Malaria Vector
A
The vector for Malaria is the Anopheles mosquite
- Exists worldwide
- Only females transmit Malaria
- Feed at dawn and dusk. It bites primarily at night
- Transmission does not occur
- Elevations > 2000 meters
- Temperatrues < 60oF
6
Q
Malaria Parasites
A
- Five different species cause disease in humans
- Plasmodium falciparum - most common and most deadly
- Plasmodium vivax - most common
- Plasmodium ovale - much less common, found in Africa
- Plasmodium malariae - much less common, found in Africa
- Plasmodium knowlesi - discovered only recently. Mostly in Southeast Asia
- Other Plasmodium species cause malaria-like illness in other species, including birds and reptiles
- Malaria has been around a long, long time; it evolved with us.
- That’s why we have sickle cell disease and thalassemia traits.
- People heterozygous for sickle cell are less susceptible to Malaria
7
Q
P. falciparum
A
- Causes the most severe infection; responsible for the most deaths
- This is because more than one parasite can infect a single red blood cell
- Very high parasitemia (levels of parasite within the blood)
- P. falciparum can also infect erythrocytes at ANY stage of development
- Occurs worldwide in tropical regions, but particularly in sub-Saharan Africa
- Also has the highest levels of drug resistance
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8
Q
P. vivax
A
- The second most common cause of malaria in humans
- Occurs primarily in South America and Southeast Asia
-
Requires the Duffy antigen for entry into host cells, and most people of African descent are Duffy negative. They evolved to not have the Duffy antigen
- So almost no vivax malaria in Africa
- Can stay dormant in the liver and then cause relapsing disease months or years later. These dormant forms are called hypnozoites
- P. vivac has a preference for young erythrocytes, and in general only one is found in a given red blood cell
- Prefers to infect young red blood cells, called reticulocytes
9
Q
Malaria & Human Evolution
A
- Malaria infects red blood cells
- Red blood cells contain hemoglobin to carry oxygen to tissues
- Some mutations in hemoglobin can protect against severe malaria
- Sickle cell trait – prevalent in people of African descent
- Thalassemia – prevalent in many areas with endemic malaria
- Also common in people of Mediterranean descent, even though there is no more malaria there. As there used to be.
10
Q
Worldwide distribution of Hemoglobin S
A
- Heterozygous for Hemoglobin S are protected from severe malaria
- People homozygous for Hemoglobin S are at a disadvantage - leads to sickle cell disease
11
Q
Worldwide distribution of 𝜶 and 𝜷 thalassemia
A
- Hemoglobin mutations that can result in partial immunity to malaria
12
Q
Malaria Life Cycle
A
The malaria life cycle:
- A mosquito bites you and injects sporozoites (haploid, infectious).
- The sporozoites travel to your liver where they mature into schizonts, which rupture and release merozoites into the bloodstream, where they go on to invade red blood cells. In some species (P. vivax, P. ovale) the parasite also establishes dormant forms, called hypnozoites, which can cause relapse of infection months or even years later.
- The parasite reproduces asexually within the RBC, going through phases named ring, trophozoite, and schizont, until eventually it bursts open, releasing more merozoites into the bloodstream, which then go on to infect more RBCs.
- Sometimes, the parasite develops into male or female gametocytes within the RBC. If a mosquito ingests one of each type, then they combine to form a zygote (diploid) in the gut of the mosquito (sexual reproduction).
- The zygote develops into an ookinete, and then an oocyst, which migrates out of the gut. It divides by meiosis to produce many (haploid) sporozoites that migrate to the mosquito salivary glands, where they are ready to infect the next person.
13
Q
Hypnozoites
A
- Only P. vivax and P. ovale can form hypnozoites, which are dormant stages of sporozoites
14
Q
Malaria - Clinical Presentation
A
- Onset 8-25 days after the bite (P. falciparum)
- Delay is due to parasite replicating in the liver (asymptomatic)
- Cycling fever and chills (non- P. falciparum)
- Corresponds to the cycle of RBC rupture
- Can be 24, 48, 72 hour cycles
- Anemia (killing red blood cells), splenomegaly (enlarged spleen)
- In severe cases
- Cerebral malaria (infects brain, presents as abnormal behavior, impaired consciousness, seizures, coma)
- Renal failure
- Caused by all of the toxic metabolites from the ruptured RBCs that overwhelm the kidney’s filtration system
- Can get dark urine without renal failure
- Hypoglycemia (b/c parasite eating up sugar in blood)
- Lactic acidosis (due to severe anemia and inability to carry O2 to tissues)
15
Q
Cerebral Malaria
A
- Usually caused by P. falciparum
- Children in sub-Saharan Africa most affected
- 575,000 cases per year
- Impaired cognition and progresses to coma, seizures
- Fatal if not treated
- Pathogenesis not completely understood
- But we believe it could be due to sequestration of RBCs, whereby the infected RBCs adhere to vascular endothelial cells, causing tiny clots
18
Q
How to distinguish forms of malaria
A
- Key thing to remember: P. falciparum causes high levels of parasitemia, high levels of resistance, infects all RBCs, and does not have hypnozoites. In contrast to P. vivax which tends to have lower levels of parasitemia, resistance is unusual, prefers young RBC (i.e., reticulocytes), and has hypnozoites
20
Q
Malaria - Prevention
A
- DON’T GET BITTEN BY THE MOSQUITO!
- Insecticide-treated bed-nets
- Clothing: Permethrin
- Skin: DEET or Picaridin
- DDT
- Eliminate standing water
- Long sleeves and pants (treated)
- Air conditioning, window screens
Malaria prophylaxis for travelers
-
Atovaquone-Proguanil
- Limited resistance
- Minimal side effects
- Daily dosing
-
Doxycycline
- GI side effects
- Sun sensitivity
- Twice daily dosing + 4 weeks
-
Primaquine/Tefanoquine
- Used for areas with Vivax b/c it kills hypnozoites
- Cannot be used in patients with G6PD deficiency
-
Mefloquine
- Can cause serious psychiatric side effects
- Weekly dosing
- Drug of choice for prophylaxis in pregnancy
Malaria Vaccine
- RTS, S vaccine recently approved in 2015
- Test in infants (0-4 months old) and babies (5-17 months old)
- Requires 4 doses
- 27% efficacy in infants
- 39% efficacy in babies
- Not amazing, but better than nothing
- As of June 2021, recommend by WHO for children in areas with moderate to high P. falciparum infections
Genetically engineered mosquitos
- CRISPR/Cas9 gene drives could spread an anti-malaria mutation through wild mosquito populations
27
Q
Antibiotic Resistance Testing - DIsk Diffusion
A
- Disks with a pre-determined mass of dried antibiotic are placed on an agar dish that has been newly plated with a ‘lawn’ of the organism.
- Antibiotic diffuses into agar
- Where antibiotic concentration in agar is sufficiently high, bacterial growth is inhibited
- Resistance is correlated to the zone of clearance around the disc
- Allows you to test multiple types of antibiotics all on one agar plate
29
Q
Antibiotic Resistance Testing - E test
A
- A gradient of antibiotic concentrations is immobilized along a rectangular plastic test strip and placed on an agar dish that has been newly plated with a ‘lawn’ of the organism
- After 48 hours incubation a drop-shaped inhibition zone intersects the graded test strip at the MIC. In contrast to the circular shaped zone of inhibition you get from disk diffusion
31
Q
Antibiotic Resistance Testing - Sequencing
A
- Works when there are known genetic determinants of resistance
- Research and discovery of resistance mechanisms
- Not generally used in clinical practice
- Look for patterns for something that may be a resistance gene (e.g., a gene on a plasmid, duplicates of a gene throughout genome, new promoter sequence that may change expression of the target of antibiotic, point mutation, or insertion or deletion mutation
32
Q
Mechanism of antibiotic resistance
A
Many resistance mechanims
- Antibiotic efflux. Can pump antibiotic out of cell. An efflux pump wouldn’t always want it on to conserve energy. So they can turn it on and off. Pseudomonas is known for their efflux pumps
- Reduce permeability (e.g., vancomycins cannot get into gram-negatives b/c of their outer membrane)
- Expression changes
- Antibiotic modification/degradation (e.g., Beta-lactamase in S. aureas can breakdown the B-lactam in the peniciillin antibiotic family)
- Target protection (e.g., new protein that prevents antibiotic from getting to target)
- Target modification. Includes acquisition of insensitive functional targets. (e.g., mutations in topoisomerase confer resistance to fluoroquinolone)
