Chapter 19: Rickettsia, Ehrlichia, Anaplasma & Coxiella

Welcome to Last Minute Lecture.

This free chapter overview is designed to help students review and understand key concepts.

These summaries supplement not replaced the original textbook and may not be redistributed or resold.

For complete coverage, always consult the official text.

Hello and welcome back to The Deep Dive.

We are back with another stack of source material and today we are cracking open Lippincott Illustrated Reviews, Microbiology.

Specifically chapter 19.

Yes, chapter 19 and honestly I am excited about this one because we are tackling a group of bacteria that are veritable escape artists.

We're talking about Rickettsia, Ehrlichia, Anaplasma and Coxiella.

It is a fascinating chapter.

I mean these organisms are what we call obligate intracellular parasites.

Right.

They're microscopic invaders that have evolved a very specific and you know frankly a somewhat terrifying lifestyle.

Exactly and our mission today is to master these obligate intracellular parasites.

We need to understand how they get in, what they do to our bodies, specifically those nasty rashes and how we stop them because, and correct me if I'm wrong here, these aren't just obscure textbook bugs.

These are life -threatening infections.

They absolutely are.

If you look at the big picture, the reason understanding this chapter is so crucial for you know clinical medicine is that these bacteria cause generalized infections.

We are talking high fevers, distinctive rashes and if missed, they can be lethal.

Wow.

Yeah.

They are diagnostic puzzles usually linked to things we encounter, outdoors ticks, lice, fleas.

The creepy crawlies.

Precisely and the source material today is rich.

We have the text of course but also classification charts, electron micrographs and clinical photos of rashes that are absolutely essential for visualization.

We're going to paint a picture of those for you as we go.

Okay let's unpack this.

We'll start with the general nature of the beast, rickettsia.

The text calls them obligate intracellular parasites.

Now I know what a parasite is but what does obligate intracellular actually imply in this context?

That's a great question.

I mean people hear that and they think of viruses right?

Yeah I thought viruses were the ones that had to live inside cells.

And they do but viruses must be inside a cell because they don't have the machinery to replicate at all.

They need the host's factory.

Rickettsia and its cousins, they're true bacteria.

They have DNA, RNA, ribosomes.

They have the machinery but they've evolved to be totally dependent on the host cell for something else.

What's that?

Energy.

So they have the car but no gas.

That's a great way to put it.

The text explains that their plasma membranes are leaky.

Leaky.

That sounds like a design flaw.

You'd think so but it's actually a brilliant strategy because their membranes are so permeable they can steal nutrients, amino acids,

and vital energy, specifically ATP,

directly from the host cell cytoplasm.

So they are energy vampires.

Essentially.

They don't have to spend resources making it themselves to just take it.

And here is where it gets really interesting from an evolutionary perspective.

The genus rickettsia is actually closely related to the ancestor of mitochondria.

Wait the powerhouse of the cell?

The very same.

The theory is that mitochondria started out as a rickettsia -like bacterium that got swallowed by a primitive cell eons ago and just decided to stay and help out in exchange for a home.

And these guys decided to stay and steal.

Exactly.

They even have a similar electron transport chain.

It's a fascinating evolutionary connection found right there in the text.

That puts a whole new spin on symbiosis.

So if we were looking at these things under a microscope, what are we source the electron micrograph?

Visually they look like pretty typical prokaryotes.

They are small.

We call them cocobacilli, sort of short stubby rods.

Okay.

They have a gram negative cell wall structure.

But, and this is a key takeaway for the lab, they stain correlate with a standard gram stain.

So you can't just do a gram stain and call it a day.

No.

If you suspect these infections and run a standard gram stain, you're going to see nothing.

Nothing.

Because they live inside the cells and have minimal peptidoglycan, they are best visualized with polychrome stains like Jumsa or Machiavello.

You have to know what you're looking for to even order the right test.

Okay.

So we have these invisible energy stealing little rods.

How do they actually get into the cell in the first place?

They don't exactly knock on the door.

They use a process similar to phagocytosis.

They essentially trick the cell into eating them.

But typically when a cell eats a bacterium, it traps it in a bubble called a phagosome to destroy it.

Right.

With acid and enzyme.

Exactly.

But rickettsia has a trick.

Of course it does.

Once it's inside that bubble, it produces an enzyme called phospholipase C.

Phospholipase C.

Sounds like a chemical weapon.

It basically is.

It degrades the phagosome membrane.

This allows the bacteria to escape that trap and spill out into the cytoplasm before the cell can kill it.

And now they are free.

Now they are free in the

cytoplasm.

Now this is the part of the chapter that I found really cinematic.

The text divides rickettsia into two main groups based on how they move.

The spotted fever group and the typhus group.

Yes.

And visualizing this difference is critical for understanding the pathology.

Let's look at the spotted fever group first.

Like rickettsia rickettsia.

Okay.

These bacteria mobilize the host cell's actin fibral.

Actin fibral.

Those are like the structural cables inside a cell, right?

Correct.

And our rickettsia essentially hacks that system.

It polymerizes the host's actin behind it to create a tail.

It uses this actin tail to propel itself like a rocket right through the cytoplasm and punch into adjacent cells.

So it's like they're surfing on an actin wave, crashing from one cell into the next without ever going outside.

That's a perfect analogy.

It allows them to spread rapidly through tissue without being exposed to antibodies or immune cells in the bloodstream.

So that's the spotted fever group.

What about typhus group?

Well, you contrast that with the pyphus group like rickettsia prowoziki.

They lack that actin motility.

They can't surf.

So they're stuck.

They are trapped.

They just replicate and replicate until the host cell becomes so full that it bursts or dies, releasing all the bacteria at once.

So one is a surfer.

The other is a ticking time bomb.

Exactly.

But regardless of the method surfing or bursting, the target tissue is the same.

And I really need to emphasize this.

Rickettsia love endothelial cells.

The cells lining the blood vessels.

Yes.

And when you infect and destroy the lining of blood vessels, you get vasculitis inflammation of the vessels.

This leads to the formation of thrombi little clots and small hemorrhages where blood leaks out.

And that explains the symptoms, right?

The headache, the mental changes, and the rash.

The rash is a direct result of that vascular damage.

It's essentially blood leaking into the OK, let's get into the specific diseases because the source material goes into great detail here.

We'll start with the spotted fever group, the big one, rocky mountain spotted fever or RMSF caused by rickettsia rickettsia.

Now, I grew up thinking this was a mountain disease.

I mean, rocky mountains in the name.

But the text corrects that assumption pretty harshly.

It does.

While it was first identified in the Rockies, the text is clear that today it is much more prevalent in the southeast and south central United States.

We're talking North Carolina, Oklahoma, Arkansas, Tennessee, Missouri.

Wow.

OK, so if you're hiking in the Appalachians or the Ozarks, not just the Rockies, you need to watch out.

And the vector here is what a tick?

The wood tick or the dog tick.

And there's a mechanism here called trans ovarial transmission.

Trans ovarial.

That sounds impactful.

It means the infected female tick passes the bacteria directly to her eggs.

So the baby ticks, the larvae are born infected.

Yeah,

this allows the disease to persist in the tick population even without mammal hosts.

That is disturbingly efficient.

So you get bit.

What happens next?

About a week later, an average of seven days, you get high fever and malaise.

Then comes the rash.

We need to visualize figures 19 .3 and 19 .4 here.

Yeah, describe figure 19 .3 for the listener.

It's a photo of a child's hand.

It shows a spotted rash with what we call palpable purpura.

These are raised bumps caused by bleeding under the skin.

But the key feature is the pattern of spread.

It is centripetal.

Centripetal.

What does that mean in this context?

It means the rash starts on the extremities, the wrists, ankles, palms, and soles, and spreads inward toward the trunk.

OK, that is a high yield fact right there.

Palms and soles.

Most rashes spare the palms and soles, don't they?

Many do, yes.

So seeing a rash on the palms and soles is a major red flag for our MSF.

But then the text drops a really scary statistic.

Spotless RMSF.

Yes.

About 10 % of patients never develop the rash.

This is a nightmare for diagnosis, because if you're just waiting for the spots, you might miss it.

And RMSF is potentially lethal.

It can lead to kidney failure, heart failure, and death if untreated.

And the text notes specifically that two -thirds of cases are in children under 15.

Absolutely.

The peak incidence is ages five to nine.

So a child with fever and headache in summer in a tick -prone area needs to be evaluated for this, rash or no rash.

Before we move on, there's a brief mention of another spotted fever, rickettsial pox.

Right.

Caused by rickettsia cari.

The vector is a mite, and the reservoir is the mouse.

The unique feature here to remember is the eschar.

An eschar.

That's like a scab, right?

A dark, crusty scab at the site of the bite.

It's a helpful diagnostic clue that you don't typically see with RMSF.

Got it.

Okay, let's pivot to the typhus group.

We've got epidemic typhus and endemic typhus.

Let's start with epidemic rickettsia proezeki.

The vector here is the human body louse, and the transmission mechanism is,

well, it's memorable.

Oh, I read this.

It's gross.

It's not the bite itself, is it?

No.

The louse feeds, and while it feeds, it defecates.

The louse feces contain the bacteria.

When the human scratches the inch you bite, they inadvertently scratch the infected feces into the wound.

Talk about adding insult to injury.

It really is.

Now, typically we associate typhus with war and poor sanitation because lice thrive in those conditions, but the text adds a surprising fact about the United States.

The flying squirrels.

Yes.

In the eastern U .S., flying squirrels can serve as a reservoir for r proezeki.

Sporadic cases occur when humans come into contact with the ectoparasites, the fleas or lice of these squirrels.

Note to self, do not pet the flying squirrels.

Now, how does the rash compare to Rocky Mountain spotted fever?

It's the opposite.

The typhus rash is centrifugal.

It starts on the trunk, the chest, and back, and spreads outward to the extremities.

So RMSF is in, typhus is out.

Correct.

And unlike RMSF, the typhus rash usually spares the face, palms, and soles.

That is a crucial distinction.

The text also mentions something called Brill -Zinsser disease.

What is that?

It's also known as recrudescent typhus.

Basically, a person recovers from epidemic typhus, but the bacteria go into hiding.

They go latent in the reticuloendothelial system.

Decades later, maybe 10 to 40 years later, the infection can reactivate.

It's usually milder, but it shows you how persistent these organisms are.

Like a sleeper cell waiting for decades.

That is wild.

Quickly, what about endemic typhus?

That's rickettsia typhi.

It's called marine typhus because the reservoir is urban rodent's rats.

The vector is the rat flea.

It's generally a milder disease than epidemic typhus.

Okay, so that covers rickettsia.

Let's shift gears a bit to the leukocyte invaders.

Ehrlichia and anaplasma.

Right.

So structurally and behaviorally, they're a lot like rickettsia, but they have a different target.

They don't go for the blood vessel lining, they go for the white blood cells.

They attack the body's defenders.

Exactly.

And we distinguish them by which type of white blood cell they prefer.

Okay, let's break it down.

Ehrlichia.

Ehrlichia chuffinsis causes human monocytic ehrlichiosis, or HME.

As the name suggests, it parasitizes monocytes.

And anaplasma.

Anaplasma phygocytophyllum causes human granulocytic anaplasmosis, or HGA.

It targets granulocytes, specifically neutrophils.

There is a very specific visual associated with these two in the text, morally.

Yes.

When these bacteria grow inside the cytoplasmic vacuoles of the white blood cells, they create these berry -like inclusions.

Morala is Latin for mulberry.

Morally, like little berries.

Exactly.

If you see a monocyte or a neutrophil on a blood smear with a mulberry -shaped cluster inside, that is a morala.

It's diagnostic.

What about vectors?

Are we still talking ticks?

We are.

For Ehrlichia, HME, the primary vector is the lone star tick.

This is common in the southeastern and south central US.

It's the one with the white dot on its back.

That's the one.

And for anaplasma, it's transmitted by the deer tick and the dog tick.

So the same ticks that transmit lying disease can transmit anaplasma.

Double trouble.

So you could technically get both at the same time.

You absolutely can.

Co -infection is a real clinical concern.

Now, symptoms.

We talked about how rickettsia is all about the rash.

What about these guys?

And that's the important negative.

Rash is actually rare in ehrlichiosis and anaplasmosis.

You get the fever, the chills, the headache, the muscle pain, but usually no rash.

So fever plus a tick bite minus a rash equals suspect Ehrlichia or anaplasma.

That's a good rule of thumb.

Also, because they attack white blood cells and platelets, lab findings will often show leukocytopenia, a low white count, and thrombocytopenia, a low platelet count.

Okay.

Moving on to the final member of our quartet, the rebel of the group, Coxiella bernetti.

Ah, yes.

Coxiella.

The causative agent of Q fever.

What Q?

It stands for query.

For years, the cause of the fever was unknown in the abattoir workers who were getting sick, so they just called it query fever.

I love that.

We don't know what this is.

Let's just call it Q.

Why do you call it the rebel?

Because it breaks almost all the rules we just discussed for rickettsia.

First off, transmission.

It does not require an arthropod vector for humans.

No ticks?

No ticks.

Human infection usually occurs via inhalation.

You breathe in infected dust.

Dust from where?

Barnyards, slaughterhouses.

The reservoir is livestock, cattle, sheep, goats.

The bacteria are in high concentrations in the placenta and amniotic fluid.

Oh, wow.

So when an infected animal gives birth, those fluids contaminate the soil.

It dries out, turns to dust, and people inhale it.

So it's airborne.

Effectively, yes.

And that leads to the second difference, stability.

Rickettsia are fragile outside the host.

Coxiella is extremely resistant to heat and drying.

It forms a spore -like structure that can survive in that dust for months, maybe even years.

And inside the cell?

It has a unique niche.

It actually grows inside the phagalysosome.

Most bacteria are killed by the acidic environment there, but coxiella seems to be stimulated by the low pH.

It likes the acid.

That is tough.

So what does Q fever look like, clinically?

Since you inhale it, it often presents as interstitial pneumonitis, a type of pneumonia.

It can also cause hepatitis.

And there is a very serious complication mentioned, culture -negative endocarditis.

Endocarditis infection of the heart valves.

Why culture -negative?

Because remember, it's an obligate intracellular parasite.

If you try to grow it on standard blood agar in the lab to see what's infecting the heart, nothing will grow.

So you have a patient with signs of heart valve infection, but the cultures are blank.

You have to think of coxiella.

Okay, we've covered the bugs.

Now we need to know how to catch them and kill them.

Diagnosis and treatment.

For diagnosis, serology is the main tool looking for antibodies.

PCR is also used.

For rickettsia, you can do immunofluorescence on a skin biopsy of the rash.

The text is very specific about the timing of treatment.

It almost sounds like a warning.

It is a warning.

The golden rule is, do not wait for lab results.

If you have clinical suspicion fever, headache, tick bite history, you treat immediately.

Immediately.

Especially with Rocky Mountain spotted fever, delaying treatment beyond the fifth day significantly increases mortality.

Treat first, ask questions later.

And what is the magic bullet?

Doxycycline.

It is the drug of choice for almost everything we discussed today.

RMSF, ehrlichiosis, anaplasma, Q fever.

Doxycycline for the win.

Are there exceptions?

The text notes one major exception.

Pregnant women.

Tetracyclines like doxycycline can affect fetal bone and teeth.

So for pregnant women with RMSF, the alternative is chloramphenicol.

Chloramphenicol.

Got it.

And prevention.

There is no vaccine currently available in the U .S.

So it comes down to vector control.

Tick avoidance, wearing long sleeves, checking for ticks.

And interesting, note ticks usually need to feed for several hours to transmit the infection.

Really?

Yes.

Often 6 to 24 hours.

So finding them early really matters.

So check your socks and check your hair.

Okay, expert, we have covered a lot of ground.

I need the last minute lecture recap.

Give me the bullet points.

All right, let's condense this.

1.

Rickettsia.

They are obligate intracellular parasites that steal ATP.

They target endothelial cells causing vasculitis and rashes.

Okay.

2.

Rocky Mountain spotted fever caused by R rickettsia.

Vector is the tick.

Rash is centripetal, starts on palms and soles and moves in.

It can be lethal.

Got it.

3.

Typhus.

Caused by R prozeki.

Vector is the louse.

Rash is centripetal, starts on the trump and moved out.

And watch out for flying squirrels.

Don't pet the squirrels.

4.

Ehrlichia and anaplasma.

They target white blood cells.

They form more allay inclusions.

No rash usually.

Lone star tick for Ehrlichia.

Deer or dog tick for anaplasma.

5.

Coxiella.

Causes Q fever.

No bug bite.

It's inhalation from livestock.

Causes pneumonia and culture negative endocarditis.

And 6.

Treatment.

Doxycycline is your go -to.

Don't wait for the lab confirmation.

Boom.

This chapter 19 in a nutshell.

And here's a provocative final thought for you to chew on.

We mentioned spotless Rocky Mountain spotted fever.

About 10 % of cases.

Yeah, the invisible killer.

Think about how many people get a summer flu fever, headache, muscle aches and just write it out.

How many of those are actually missed cases of rickettsia or Ehrlichia that the immune system managed to fight off or perhaps didn't quite kill?

It makes you wonder how prevalent these infections really are.

That is a terrifying thought to leave us with.

Next time I get a summer fever, I'm going to be checking for ticks very carefully.

As you should.

Thank you so much for breaking this down with us.

To the learner, thanks for diving deep with the last minute lecture team.

Stay curious, stay safe from ticks, and we'll catch you on the next deep dive.

Goodbye, everyone.