Distinguished future physicians welcome to Stomp on Step 1 the only free videos series
that helps you study more efficiently by focusing on the highest yield material.
I’m Brian McDaniel and I will be your guide on this journey through Parasites.
This is the 2nd video in my playlist covering all of microbiology for the USMLE Step 1 medical
board exam.
We are going to review the most important parasites such as malaria, various GI protozoa,
Toxoplasmosis and Pinworm as well as some high yield treatment options for these diseases.
Parasites are organisms that live in or on a host.
These organisms gain some sort of survival advantage (such as gaining nutrients) while
their presence is often detrimental to the host.
Parasites usually don’t kill their host, but can cause disease if the parasite burden
is high enough.
For the exam, the most important group of parasites is Protozoa.
These are microscopic unicellular eukaryotes that are usually motile.
They move using a tail or foot like processes.
Different species have a predilection for living in different parts of their human host.
The most important protozoa for the USMLE Step 1 medical board exam are Malaria, Babesiosis,
Toxoplasmosis, Cryptosporidium, and Giardia Lamblia.
We will cover each of them individually in this video.
The other main group of parasites that cause disease in humans is the Helminths or worms.
However, these are largely low yield material so we will just briefly cover this group towards
the end of the video.
We will start with Malaria, which I give a high yield rating of 4 on a scale from 1 to
10 based on a number of factors including how frequently this topic appears in retired
step 1 questions.
Malaria is a disease caused by the Plasmodium Protozoa that is transmitted by Mosquitos.
The most common species are Plasmodium Falciparum, Plasmodium Vivax, Plasmodium Ovale, and Plasmodium
Malariae.
Each of these has slightly different characteristics, but for the most part these differences are
beyond the scope of Step There is a very complex life cycle, but learning
all of those details isn’t necessary for the exam.
When inside a human host these parasites mainly reside inside red blood cells.
Clinically, malaria presents with reoccurring cycles of spiking fevers and chills with other
non-specific symptoms like headache and sweating.
These “attacks” are interspersed with periods of complete remission.
The paroxysmal symptomatic periods of different species of malaria occur at different frequencies,
but in general the attacks occur every couple days or so.
Symptoms occur when mature schizonts rupture erythrocytes releasing immature merozoites.
Anemia may be present due to this rupture of red blood cells.
The question stem almost always mentions recent travel to a place like Africa or Latin America
as Malaria is not endemic to the United States.
One interesting correlation is that Sickle Cell Trait offers some resistance to certain
malaria species.
This is why sickle cell trait and disease is much more common in area where malaria
is endemic.
Sickle Cell trait actually gives a survival advantage due to its antimalarial property.
A peripheral blood smear will show enlarged RBCs with numerous small parasite “dots”
on Giemsa stain
Here is a picture of the histology.
You don’t need to be able to identify specific stages or species, but on the left we have
mature schizonts containing merozoites and on the right you can see ring shaped trophozoites
Antimalarials are a class of medication that can be used prophylactically to prevent malaria,
used to treat identified/suspected malaria, or used to periodically treat populations
in endemic areas.
The most commonly used antimalarials are Chloroquine, Hydroxychloroquine, Mefloquine, & Primaquine.
Quinine is primarily used for severe cases of malaria.
Doxycyline also has some action against malaria and is most often used for prophylaxis.
Most of the antimalarials have “Quine” in the name which makes them easy to pick
out of a list.
The mechanisms of these drugs is beyond the scope of the USMLE Step 1 exam.
Resistance to antimalarial medications is common and a huge public health concern.
Different areas of the world have different resistance patterns which helps dictate which
drugs should be used as first line treatments.
Combination therapy is often used now to prevent resistance and recurrence.
For example, combining chloroquine or hydroxychloroquine with primaquine is a common combination treatment.
Monotherapy is usually sufficient for chemoprophylaxis (or prevention).
There are also non-pharmacologic preventative measures.
Spraying insecticides, using personal protective clothing/mosquito repellent, draining areas
of standing water (to destroy mosquito habitats), and using bed nets can decrease patients exposure
to mosquitos.
Babesia Microti is a protozoa infection of the red blood cells similar to malaria.
It is usually asymptomatic, but in some patients babesiosis can present with similar symptoms
to malaria.
It is transmitted by the Ixodes Tick that is also the vector for Lyme disease.
Like Lyme disease, Babesia is largely found in the northeast US.
The gastrointestinal protozoa include Cryptosporidium, Giardia Lamblia & Entamoeba Histolytica.
All 3 are primarily spread via ingestion of cysts in contaminated water (AKA fecal-oral
transmission) and all 3 present with diarrhea.
Cryptosporidium presents as watery diarrhea primarily in AIDs patients.
Only symptomatic treatment is needed.
Oocysts may be found in the stool on acid fast stain
Here is a picture of cryptosporidium oocysts in stool on acid fast staining
Giardia Lamblia presents with bloating, cramping, flatulence and fatty floating stools.
Recent camping or hiking is often mentioned in the question stem.
Metronidazole is the treatment of choice.
On stool histology giardia can be seen as a symmetric pear shaped trophozoite.
Here is a picture of the giardia lamblia trophozoite
Entamoeba histolytica presents as bloody diarrhea and intestinal ulcerations.
Metronidazole is the treatment of choice.
Histologic examination of the stool shows trophozoites which have ingested red blood
cells.
Here is a picture of that histology.
Metronidazole is an antimicrobial medication that has antiprotazoal activity against Trichomonas,
Giardia Lamblia & Entamoeba Histolytica.
It is also frequently used for C. Dif, Bacterial Vaginosis, and H. Pylori.
Despite the “Azole” ending to the name metronidazole is not used as an antifungal
medication like many other Azoles.
A common side effect is inhibition of alcohol metabolism.
This “Disulfiram-like” reaction leads to an exaggerated effect even when a small
amount of alcohol is consumed.
You will remember from our earlier video on alcohol metabolism that Disulfiram is a medication
used to treat alcoholism as it gives them a really bad hang over if they drink any alcohol.
Toxoplasmosis (AKA Toxoplasma Gondii) is a parasite that primarily causes brain abscesses/cysts
in HIV patients with a CD4 T Cell count <200.
In the united states this protozoa usually infects a human host after they eat undercooked
pork or are exposed to cat feces.
Infections are usually asymptomatic and then can become reactivated when the patient becomes
immunocompromised.
The symptoms depends on where the lesion is.
Toxoplasmosis can present as a headache with a wide variety of neurological deficits, but
seizures are most common.
On MRI or CT scan the lesion(s) presents as a “ring enhancing” meaning it is a low
density circular area surrounded by a white high density rim
Given my copyright limitations I was not able to find an actual picture of Toxo, but I have
a couple examples of other diseases that cause ring enhancing lesions here.
These look relatively close to toxo, but you should consult your textbooks or a google
image search for better pictures.
Toxoplasmosis can cross the placenta and affect the fetus, but I will discuss this topic in
a separate video cover the TORCH infections.
I’m also not going to cover Trichomonas in this video even though it is a protozoa.
I am going to make an entire video about vulvovaginal infections so that information will end up
there
Most helminths or worms are not going to be covered in my videos since they are very low
yield.
However, I am going to quickly touch on a couple key topics.
Pinworm (AKA Enterobius Vermiculus) is a type of roundworm/nematode.
Infection occurs through the ingestion of eggs in food.
It is the most common helminth infection in the US and is primarily seen in children.
Patients are asymptomatic except for severe perianal itching.
This itching is caused when the worms, that live in the intestine, migrate during the
night and deposit their eggs outside of the anus.
Diagnosis is made by “Cellophane Tape Test” (AKA scotch tape test).
Here a piece of clear adhesive tape is applied to the perianal area and then examined microscopically
to look for eggs or adult worms.
Mebendazole or Albendazole are the first line treatment for most worm infections.
They are both broad spectrum anti-helminth that inhibits microtubule formation and decrease
nutrient uptake by the worm.
Dispite the “Azole” ending to the word, these medications are not in the antifungal
azole class.
A mnemonic to remember these medications is that WORMs are BENDy.
That brings us to the end of the video.
Please do comment below if you have any feedback, comments or suggestions.
Getting the number of comments up on our videos really helps us out.
If you are watching on a computer and would like to be taken directly to my next video
you can click on this black box here.
The next video in the microbio section is going to start our cover vulvovaginal infections
like BV, trichomonas and candida.
Thank you so much for watching and good luck with the rest of your studying