Understanding Haemophilus influenzae: Key Characteristics and Infections

* Haemophilus influenzae* is a gram-negative coccobacillus that causes infections like epiglottitis and requires specific growth factors.

### Understanding Haemophilus influenzae: Key Characteristics, Infections, and Treatments
* 🦠 *Haemophilus influenzae* is a gram-negative coccobacillus that requires hemolytic factors, specifically **V factor (NAD)** and **X factor (hematin)**, for growth on chocolate agar or in the presence of *Staphylococcus aureus*.
* 🧬 This bacterium colonizes the nasopharynx and is transmitted via aerosolized droplets, with **unencapsulated strains being the most common cause of infection**, particularly following the introduction of the vaccine that **reduced capsulated strains**.
* 💡 *H. influenzae* is a facultative anaerobe and non-motile organism, named during the late 1800s due to a **misconception that it caused influenza** during a pandemic.
* 🔬 The virulence factors include IgA protease, which allows it to **evade mucosal defenses**, and a polysaccharide capsule that poses a risk to asplenic patients, leading to **severe infections**.
* 🤒 Clinical features of infections include mucosal surface infections such as epiglottitis, otitis media, sinusitis, and pneumonia, with **epiglottitis being a particularly high-yield topic for exams**.
* 📈 Symptoms of epiglottitis include fever, drooling, dysphagia, stridor, and the **tripod position**, which is a classic sign where the patient **leans forward to ease breathing**.
* 🩻 The thumbprint sign on a lateral x-ray is a **key imaging finding for epiglottitis**, distinguishing it from croup, which shows a **steeple sign**.
* 💊 Treatment varies: mucosal infections are treated with **amoxicillin plus clavulanate**, meningitis requires **ceftriaxone**, and close contacts should receive **prophylactic rifampin**.

In this video, we'll talk briefly about *Haemophilus influenzae*. This will probably be a quick video, and there's just a few high-yield things that you need to know, which I will really harp on. First, *H. influenzae*'s defining characteristics are

that it is a gram-negative coccobacillus. It grows on chocolate agar, which is sort of high-yield because examiners like you to know that in order for *H. influenzae* to grow, it requires hemolytic factors, and specifically it requires V factor or NAD and X factor or

hematin. So, you can either grow it on chocolate agar, or you can grow it in the presence of *Staphylococcus aureus*, and the *Staphylococcus aureus* itself being hemolytic will create those factors that the *H. influenzae* needs to grow. So, first high

yield point is that you should know that it grows on chocolate agar because chocolate agar contains those hemolytic factors. But, you should also know that if you can't grow it on chocolate agar, you can simply grow it in the presence of *Staphylococcus aureus* because *Staphylococcus aureus* is a

substitute that in and of itself will produce those hemolytic factors. That does show up on exams. So, please know that *H. influenzae* colonizes the nasopharynx. It is transmitted via aerosolized transmission. Unencapsulated strains are the most

common cause of infection. Now, to be clear, there are both unencapsulated strains and capsulated strains. If you've watched the previous video on *Neisseria*, then we already talked about opsonic and how asplenic patients are really at risk of infection with encapsulated

bacteria and *Haemophilus influenzae* is one of those encapsulated bacteria but since the introduction of the vaccine against *H. influenzae*, those capsulated strains have pretty much been eradicated and therefore the majority of infections are caused by the unencapsulated strains

which the vaccine does not prevent against. Lastly, *H. influenzae* is a facultative anaerobe and it is non-motile. A little cool aside here, this doesn't show up on exams of course, but the reason that this is called *H. influenzae* is because this bacteria was actually

discovered in the late 1800s during an influenza pandemic and the person that discovered it thought that this was the bacteria that caused the common flu and that's why this bacteria retained the name influenza in the name. So pretty

cool aside. You might impress somebody if you tell them that. Here's an image of *H. influenzae* again. The thing that I want to call your attention to is that it is a gram-negative coccobacillus, so keep that in mind. Virulence factors: if you've

watched the recent video that I posted on *Neisseria*, this really isn't a big surprise because it's similar. So first we have IgA protease and recall that IgA is very important for immunity and defense at mucosal surfaces. So bacteria

that have an IgA protease and can cleave that defense basically are going to cause infections at mucosal surfaces and as you'll see on the next slide, *H. influenzae* causes epiglottitis, otitis media, sinusitis, pneumonia, basically upper respiratory infections.

and it gets in at those mucous surfaces that's what IgA protease is doing the more high yield thing and just like we talked about in the Neisseria video is the presence of a polysaccharide capsule this causes concern for patients who don't have a

spleen or are functionally asplenic again we're concerned about overwhelming post splenectomy infection the polysaccharide capsule and I'm repeating myself so if you've watched the Neisseria video you've already heard

this but it's very very high yield and please see that video for a full pathophysiological explanation but the polysaccharide capsule allows the bacteria to evade the two-step process of opsonization and phagocytosis and so

in patients with no spleen who don't have those splenic macrophages this bacteria can cause an invasive and overwhelming infection now let's talk about the clinical features as I've already touched on there are really two types here one there are your mucosal surface

infections so the IgA protease in the bacteria is allowing it to infect the epiglottis the meninges the ear the upper respiratory system so this causes sinusitis pneumonia otitis media meningitis and epiglottitis okay so

that's what you want to keep in mind for mucosal infections and after somebody's infected with a mucosal infection the infection can become invasive so it can spread to other areas of the body this is really um less common so it's not something

that I would necessarily look for on your exam but just know that with any infection once there's a mucosal surface infection that obviously has the ability to spread now the big thing that I want to call your attention to and probably the most high-yield part of this video

is epiglottitis now epiglottitis is going to be caused by *Haemophilus influenzae* if it's happening on your exam and the way that they're going to give this to you is they're going to give you a presentation of epiglottitis and expect

you to then connect it to the causative bacteria so how I've seen this come up is that they're going to describe epiglottitis to you and then ask you what is the causative bacteria the answer obviously being *Haemophilus influenzae* but

in order to get that third order question correct you obviously need to know what epiglottitis looks like so let's talk about that now epiglottitis is extremely high yield on exams because it's got a lot of very distinct clinical

features and it also has a very famous imaging finding now let's start with symptoms so you're going to see fever you're you're going to see drooling because epiglottitis means inflammation in the epiglottis so with that being so

inflamed you're not able to swallow properly so we're going to see drooling we're going to see dysphagia and muffled voice because imagine that suddenly in the back of your throat your epiglottis is now hugely inflamed you're obviously

not going to be able to phonate like you normally do, we're going to see both stridor and respiratory retractions because now suddenly with an inflamed epiglottis we can't move air as efficiently as we previously could, and the highest yield clinical symptom that

I want to call your attention to because it shows up on exams all the time is what's referred to as the tripod position. So usually on your exam this is going to be in a child and they're going to be sitting up on their hands, usually

with their tongue out and their head forward. This is a position that the patient will automatically assume because they are trying to make it more comfortable for themselves to breathe and to be able to communicate, but mostly to breathe. And so this is called the tripod position, and so if you're taking

your exam and they describe the way that somebody is sitting, specifically in a child or adolescent patient, the bells need to be ringing in your head that they're trying to tell you this patient has epiglottitis. So again, this is referred to as the tripod position, maybe

they'll show you a picture, but more likely they're going to just describe it to you. So sitting up on their hands, tongue usually protracted or out a little bit, head bent forward, that's the tripod position. It's just a position that's more comfortable to be in if

you've got an inflamed epiglottis. Now they could describe these symptoms to you, and chances are they will, but they also could just show you imaging. If you see what's known as a thumbprint sign, you want to be thinking epiglottitis and

how this will look is you're going to get a lateral x-ray as you see here in this image and because of the inflammation in the epiglottis it looks like a thumb now please go ahead and Google thumbprint sign so you can see

various images of what this looks like because it's a very characteristic finding you want to know thumb print sign equals epiglottitis but something that's really important for test taking purposes is if you have a child patient in your vignette who has fever and some

respiratory symptoms and they show you Imaging if you have absolutely no idea what you're looking at and you're not sure what the diagnosis is or what the causative agent is you probably want to immediately narrow your differential to either epiglottitis or croup and the

reason that I say that is because epiglottitis has the characteristic thumbprint sign and croup has the characteristic steeple sign so in a child patient with fever respiratory symptoms and some type of Imaging in your question even if you have

absolutely no idea what you're looking at I would always be guessing either epiglottitis or croup because they wouldn't be giving you any Imaging if it wasn't either a thumbprint sign or a steeple sign so that's just a little

test taking strategy you have a 50% chance of getting the question correct if you approach it like how I just said lastly if you're taking step two or step three they could take this one step further and ask you what is the most

immediate next step or you know what's the appropriate which of the following is the most appropriate immediate action to take and if it's epiglottitis chances are the question is going to ask you this and the answer will be endotracheal

intubation because in somebody who has epiglottitis you want to protect their airway before you progress with further treatment now speaking of treatment let's conclude by talking about how you treat H influenza the treatment in the mucosal infections will be

amoxicillin plus clavulanate okay but if the patient has meningitis then you add in Ceftriaxone and lastly for close contacts you need to give them prophylactic Rifampin so there's three different answers here it depends on the

type of infection right either amoxicillin plus clavulanate in the mucosal infections or Ceftriaxone in your meningitis patients and then lastly if they ask you what to give to the close contacts of somebody who has H

Haemophilus Influenzae

Dirty Medicine

Defining characteristics and growth requirements of *Haemophilus influenzae*

Requires hemolytic factors (V factor/NAD and X factor/hematin) for growth.

Grows on chocolate agar due to presence of hemolytic factors.

Can grow in presence of *Staphylococcus aureus*, which produces necessary hemolytic factors.

Colonizes the nasopharynx and is transmitted via aerosolized transmission.

Types of *H. influenzae* strains and virulence factors

Unencapsulated strains are the most common cause of infection.

Encapsulated strains largely eradicated due to vaccination.

Produces IgA protease, allowing infection at mucosal surfaces.

Polysaccharide capsule increases risk for asplenic patients.

Clinical features and infections caused by *H. influenzae*

Mucosal infections: epiglottitis, otitis media, sinusitis, pneumonia.

Invasive infections can spread from mucosal sites, less common.

*H. influenzae* is the causative agent of epiglottitis.

Symptoms: fever, drooling, dysphagia, muffled voice, stridor, respiratory retractions.

Tripod position: characteristic posture of patient.

Differential diagnosis with croup (steeple sign).

Immediate action for epiglottitis: endotracheal intubation.

Mucosal infections: amoxicillin plus clavulanate.

Prophylaxis for close contacts: Rifampin.