Scratching the Surface: How We Miss Murine Typhus
- Heather McSharry, PhD
- Apr 15
- 15 min read
Updated: Apr 21
Summary
Murine typhus doesn’t announce itself. It begins with symptoms that feel familiar—fever, headache, fatigue—and unfolds in a way that doesn’t quite fit the story we expect.
In this episode, we follow that gap—between symptoms and recognition—to understand why infections like murine typhus are so often missed. From flea-borne transmission and urban ecology to the biology of endothelial infection, this is a closer look at a disease that never disappeared, just slipped out of focus.
Listen here or scroll down to read full episode.
Full Episode
[linked sources that are paywalled are provided as PDFs in the citation list after Heather's signature at the end of the post]
It doesn’t start with anything memorable.
No bite you noticed. No exposure you can point to. No moment where something clearly went wrong. Just a fever that feels…off. Not high enough to scare you. Not low enough to ignore. A headache that settles in and stays. Fatigue that feels heavier than it should. You tell yourself it’s viral. You’ve been around people. It’s that time of year.
But then a few days pass—and instead of resolving, it lingers. The headache sharpens. The fever doesn’t break. Maybe a rash appears—faint, easy to miss. Or maybe it doesn’t appear at all.
You go to urgent care.The labs are nonspecific. The viral panel is negative. You’re sent home. And this is the part that matters—Because in that window—between symptoms that don’t quite fit and a diagnosis no one is thinking about—something is already happening inside your body.
A bacterium most clinicians rarely see is moving through your bloodstream, infecting the cells that line your blood vessels, quietly turning your own vascular system into the site of infection.
This is Scratching the Surface: How We Miss Murine Typhus
SETTING THE FRAME
Before we go further, we need to clear up something that causes a surprising amount of confusion—even among clinicians. Typhus and typhoid are not the same disease. They’re not even closely related. They don’t share a transmission route. They don’t belong to the same category of bacteria. The similarity in names is historical—and misleading.
Both diseases were named in an era before microbiology was well developed, when physicians grouped illnesses based on how they looked clinically. High fever, confusion, systemic illness—those patterns overlapped. And the names stuck. But biologically, these are entirely different infections.
Typhoid fever is caused by Salmonella enterica serovar Typhi—often shortened to Salmonella Typhi. The word serovar is short for serological variant and refers to a distinct, unique strain within the Salmonella species classified by specific antigens on its surface (O, H, and Vi antigens). Salmonella enterica is transmitted through contaminated food and water and invades the intestinal tract, then disseminates systemically through the bloodstream. And serovar Typhi defines the specific variation that causes typhoid fever, rather than gastroenteritis.
So even though typhoid and typhus can present with fever, headache, and systemic symptoms—the underlying biology is completely different.
Typhoid is food- and water-borne—linked to sanitation, contaminated supplies, and ingestion.
Typhus, today's topics, is vector-borne—linked to fleas and the animals that carry them.
Same root word. Completely different diseases.
Now let’s get back to typhus. “Typhus” isn’t a single disease. It’s a category—caused by bacteria in the genus Rickettsia. Within that category, there are multiple distinct illnesses with different transmission routes and epidemiology and of those there are two major forms you've probably heard about. The murine or flea-borne typhus also called endemic typhus that we're talking about today. And epidemic typhus, which is transmitted by body lice and historically associated with war, crowding, and severe outbreaks. The one relevant here, murine, or flea-borne, typhus—caused by Rickettsia typhi. For the rest of this episode that's the one we're talking about. OK? OK. Here we go.
So Rickettsia typhi is an obligate intracellular bacterium. That means it doesn’t replicate freely in the bloodstream—it has to get inside host cells, specifically endothelial cells—my favorites—to survive. And that detail matters, because it explains nearly everything about the disease that follows.
Unlike most bacteria, which can live and divide on their own outside our cells, these organisms depend on the inside of a host cell for nutrients and a protected environment. And that makes them sound a little like viruses...but unlike viruses that need host cell machinery to replicate (because viruses are not cells and don't have that stuff) obligate intracellular bacteria are cellular organisms and carry their own bacterial machinery to replicate—they just can’t do it without the host cells, which provide food and a safe house.
Now, once these bacteria are inside, that changes how the body fights them—because they are partially shielded from antibodies and require strong cell-mediated immunity to clear the infection. That's why treatment is critical.
TRANSMISSION: WHAT ACTUALLY HAPPENS
So let's step back and look closer at how these bacteria are transmitted. Murine typhus is a vector-borne infectious disease, which means it's transmitted by living organisms—primarily insects like mosquitoes, ticks, and fleas—that carry pathogens from infected animals or humans to new hosts. It's not transmitted from person to person.
Now, there’s a tendency to think about vector-borne diseases in terms of bites...and for good reason. Mosquitoes bite. Ticks bite. Sand flies bite. And fleas of course, bite. I mean, plague anyone? but here’s the part most people miss... But flea bites are only part of the problem for typhus. Fleas feed on an infected host. The bacteria replicate within the flea. And then that flea moves on to a new tasty critter and feeds on that one then deposits infectious poop on the skin....and that ...umm... deposit is often really close to the bite site. Right? I mean fleas are tiny and always eating and pooping so even if they wanted to they couldn't follow the very wise old adage, don't crap where you eat. It just happens.
So the flea poop—microscopic and contaminated—ends up on your skin. And when the skin is scratched, or disrupted, or even exposed to mucous membranes, the bacteria are introduced. And with flea bites, scratching is even more likely. So unlike plague, which is transmitted through the flea saliva when it bites, typhus is transmitted when you scratch the flea poop into the skin, often near a bite. Most people never register the exposure at all. And that’s part of why this disease is so frequently missed.
Now I should also say that some experimental conditions have shown typhus can be transmitted via infectious saliva during bites, but at this point, it's not the primary mechanism by any means.
And I feel like I need to point out that this means, you could argue that typhoid and typhus are, technically, fecal transmission diseases. Typhoid is fecal–oral—human waste contaminating what you eat or drink. Typhus is flea feces…introduced through your skin when you scratch. Either way, getting infected with one of these is...well...shitty.
PATHOGENESIS: WHAT THE BACTERIA DO
So then what happens? How does it make you sick? Well, once Rickettsia typhi enters the body, it targets endothelial cells—the cells that line blood vessels. Once Rickettsia typhi enters the body, it targets endothelial cells—the cells that line blood vessels. That’s the central event in typhus. The bacteria invade these cells, replicate inside them, and trigger an inflammatory response that disrupts vascular integrity.
So, it turns out that endothelial cells are major players in our health. Under normal conditions, endothelial cells form a tightly regulated barrier between the bloodstream and surrounding tissue. They control what moves in and out—fluids, proteins, immune cells. It’s an active interface, not just a passive lining.
When Rickettsia infects these cells, two things happen:
First, the bacteria directly alter endothelial function as they replicate inside the cells, beginning to compromise the barrier.
Second—and more importantly—the immune system responds. Infected cells release cytokines and upregulate adhesion molecules, signaling that something is wrong. Immune cells adhere to the vessel wall and move into surrounding tissue, and inflammation builds. Because this is happening throughout the vascular system, the process becomes systemic.
As inflammation increases, the junctions between endothelial cells loosen. Fluid that should stay inside blood vessels leaks into surrounding tissue—what we call increased vascular permeability. That leads to microscopic edema, accumulation of inflammatory mediators, and less efficient circulation.
At the same time, the endothelial surface shifts slightly toward a pro-thrombotic state—though less so than in other rickettsial diseases. Large clots are uncommon, but microvascular dysfunction still affects tissue perfusion.
So when we say “vasculitis,” we’re not talking about a single event. We’re talking about a distributed process across thousands of small vessels at once—resulting in barrier dysfunction, inflammation, and altered blood flow. And that’s why the symptoms don’t localize neatly.
The headache reflects vascular inflammation in the brain. The rash reflects inflammation in skin vessels. In more severe cases, the same process affects the lungs, impairing gas exchange—or the brain, causing confusion—or the kidneys, where reduced perfusion begins to matter.
It’s one mechanism, playing out across multiple organ systems at once.
That’s what endothelial infection does. It turns the vascular system itself into the site of disease. It’s a vascular disease first. Everything else follows.
CLINICAL PRESENTATION: WHY IT’S MISSED— AND WHEN IT SHOULDN’T BE
So what does that actually mean clinically? Well, if you’re looking for a single defining feature of murine typhus—you won’t find one. And that’s the problem. The classic triad—fever, headache, rash—isn’t reliable in practice. Many patients never develop a rash, or develop it late. Others present with symptoms that are entirely nonspecific. Which means this disease often blends in with more common illnesses and gets missed.
But there is a pattern.
Most patients develop a persistent fever that doesn’t resolve on the timeline you’d expect for a routine viral illness. The headache is usually constant rather than episodic—often described as deep and not particularly responsive to typical medications. Fatigue is common, sometimes disproportionate. There may be muscle aches, nausea, or a faint rash that typically starts on the trunk—but again, not reliably. And lab tests don’t point to one clear diagnosis, but they often show subtle changes—like mild liver irritation, a drop in platelets, and sometimes low sodium levels, which together signal a systemic inflammatory process rather than a simple viral illness.
On their own, none of these symptoms are specific. The key is context.
That context includes:
exposure to fleas—on pets, in the home, or in environments where fleas are likely
living in or traveling through endemic areas like Southern California, Texas, or Hawaii
contact with animals that bridge that gap—cats, dogs, or wildlife like opossums
Most people won’t recall a flea bite. That’s not the signal.
The signal is the combination:
a persistent, unexplained fever
plus a headache that doesn’t fit the usual pattern
plus a plausible exposure window in the 1–2 weeks before symptoms began
That’s when this should enter the differential.
If symptoms are mild and improving, monitoring may be reasonable. But if fever persists, symptoms worsen, or recovery isn’t following the expected course, it’s time to seek care—especially if there are signs of progression, like:
Shortness of breath
Confusion or difficulty concentrating
Severe or persistent vomiting
Or simply a trajectory that feels like it’s escalating rather than stabilizing
At that point, you may be seeing the effects of systemic vascular inflammation.
And here’s the clinical reality: diagnosis is often presumptive. Early testing is frequently negative, so clinicians rely on pattern recognition—symptoms plus exposure—and start doxycycline empirically. That decision point matters.
When treated early, typhus is usually a short, self-limited illness. But if treatment is delayed, the infection continues spreading through the vascular system. What starts as a febrile illness becomes a more systemic process—affecting multiple organs and prolonging recovery. The mortality rate stays relatively low, but the disease itself becomes heavier, more complicated, and harder to ignore.
So the risk isn’t just the infection itself. It’s the gap between when the disease begins—and when it’s recognized.
SEVERITY: REFRAMING THE HEADLINE
In Los Angeles County, this dynamic isn’t theoretical—it’s playing out in real time. In early April, public health officials reported a record 220 cases of flea-borne typhus in 2025, continuing a steady rise from prior years. Among identified cases, nearly 90% required hospitalization.
That number is striking—but it needs context.
Murine typhus severity is often a function of recognition timing, not inherent virulence. Delayed or missed treatment doesn’t usually make the disease uniformly lethal, but it does increase the risk of complications, prolonged illness, and hospitalization.
At the same time, many milder cases likely go undiagnosed. So what shows up in official counts is skewed toward more severe presentations. That’s likely part of what’s driving the high hospitalization rate.
Cases have been identified across the county, including clusters in Central Los Angeles, Santa Monica, and the Willowbrook area—reflecting how widely established the transmission cycle has become. Exposure often occurs in and around the home, where infected fleas—commonly carried by rats, opossums, and free-roaming cats—go unnoticed.
The challenge is compounded by the disease’s early presentation. Symptoms are nonspecific, and initial lab testing is often negative. Confirmation may require delayed serology.
For that reason, public health guidance emphasizes starting treatment based on clinical suspicion alone, without waiting for confirmation—because by the time tests turn positive, the disease may already be well underway.
TREATMENT: STRAIGHTFORWARD, IF YOU THINK OF IT
And this is one of the more frustrating aspects of murine typhus. The treatment is simple. It's the antibiotic doxycycline. With it, patients often improve rapidly—sometimes within 48 hours. But that only happens if the disease is considered. There’s a pattern you see repeatedly in the literature: patients treated empirically with doxycycline early do well. Patients treated late—or not at all—are the ones who develop complications. So again, the limiting factor isn’t therapeutic capability. It’s diagnostic awareness.
ECOLOGY: WHY THIS IS HAPPENING NOW
Now this is where it gets interesting. If this were just a clinical story, it would be straightforward. It’s not. This is an ecological story. Historically, murine typhus followed a relatively well-defined ecological cycle: rats, fleas, humans. Urban environments. Ports. Dense housing. That model is no longer sufficient. What we’re seeing now—particularly in places like Southern California—is a more complex system. Cats. Opossums. Suburban interfaces. The pathogen didn’t change. The ecosystem around it did.
The traditional rat–flea cycle hasn’t disappeared—it’s broadened. In many regions, opossums have become important hosts for infected cat flea populations. And those fleas—Ctenocephalides felis—are particularly well-suited to this moment. They’re not host-specific. They feed on wildlife, pets, and humans, moving easily between species without breaking the transmission chain.
Domestic animals sit right in the middle of that system. Cats and dogs move between outdoor and indoor environments, often carrying fleas with them. They don’t have to be sick or even heavily infested to serve as bridges. It doesn’t take many fleas to create that connection.
Urban and suburban ecosystems have blurred.
Green spaces, dense housing, food waste, and shelter create ideal conditions for synanthropic wildlife. Synanthropic refers to organisms that have dapted to live near humans and evolved to benefit from human settlements and their environmental modifications. So animals like opossums, rodents, and stray cats that thrive alongside humans. At the same time, development pushes human activity deeper into previously wild spaces. The result isn’t separation. It’s overlap.
But from a One Health perspective, this isn’t just overlap—it’s coupling. Human population growth increases the number of pets, stray animals, and wildlife food sources. That, in turn, expands the host community available to fleas, allowing their populations to grow and persist. At the same time, environmental changes—especially warmer temperatures and urban heat island effects—accelerate flea life cycles, increase feeding and defecation rates, and even enhance bacterial replication within the flea itself . The biology of the vector, the behavior of animals, and the structure of human environments are all shifting in the same direction.
And with that, the interface between humans and vector populations has changed.
This isn’t about poor sanitation in the historical sense. It’s about proximity and continuity. The vectors don’t need dramatic conditions to persist—just stable access to hosts across multiple species. Fleas and their animal hosts together maintain the infection cycle, with transmission occurring both horizontally between hosts and vertically within flea populations, allowing the system to sustain itself even without constant new infections.
So this is wildlife adapting to human environments. Humans expanding into wildlife habitats. And vectors exploiting both.
That combination creates a self-reinforcing transmission network—one shaped by human behavior, animal ecology, and environmental change all at once. It doesn’t require large outbreaks to persist, just ongoing, low-level contact.
Which is exactly what makes it easy to miss.
PET OWNERS: WHAT ACTUALLY MATTERS
The messaging around pets tends to oversimplify. Part of the problem is language. Terms like host, reservoir, and vector get used interchangeably—but they don’t mean the same thing.
A vector is what actually transmits the pathogen. In this case, that’s the flea. Without the flea, the bacteria don’t move between species.
A reservoir is where the pathogen is maintained in nature over time. These are the animals that quietly sustain the infection cycle—classically rodents, and in many modern settings, opossums and other urban-adapted wildlife. They don’t need to look sick. They just need to keep the bacteria circulating.
A host is broader: any animal the flea feeds on. That includes wildlife, pets, and humans. Not all hosts are reservoirs—but they can still participate in the system.
Pets sit in an in-between role. They are not usually the primary reservoirs. They are part of the interface. Cats and dogs move between outdoor and indoor environments, picking up fleas in yards, alleys, and green spaces, and bringing them into close proximity with humans. They function less as sources of infection and more as transport platforms—bridging the gap between wildlife and people.
If you want an analogy: pets aren’t the factories producing the bacteria. They’re more like rideshare drivers for fleas—picking them up outside, dropping them off in your living room, and unintentionally completing the last leg of the journey.
So the intervention point is not avoiding pets. It’s controlling fleas. Consistent flea prevention. Limiting contact between pets and wildlife. Being aware of environments where flea exposure is more likely—shaded yards, areas with wildlife activity, places where fleas can persist off-host. This is vector control at the household level.
THE BROADER FRAME
And zooming out one step further, this isn’t just about individual risk or household exposure. It’s about how a disease persists over time. Flea-borne typhus isn’t emerging in the way we often think about emerging diseases. It’s re-emerging. Which is a different problem. It means the pathogen was always there. The biology didn’t change. What changed—twice—was the environment around it.
In the mid-20th century, typhus didn’t disappear on its own. It was actively suppressed. Large-scale public health campaigns targeted every part of the system at once: insecticides to kill fleas, rodenticides to reduce reservoirs, housing improvements to eliminate rodent harborage, and sanitation efforts to disrupt transmission pathways. Cases dropped dramatically—from thousands per year in the 1940s to fewer than a hundred within a decade .
But those interventions didn’t eliminate the underlying ecology. They controlled it.
Over time, those systems relaxed. Rodent control became less aggressive. Insecticide use changed. Urban environments evolved. And the transmission cycle adapted—shifting from a rat-centered system to one involving opossums, companion animals, and cat fleas, which are harder to control and more integrated into everyday human environments .
So this isn’t just re-emergence. It’s reconfiguration. The disease moved from something visible—crowded housing, obvious rodent infestation—to something quieter. More distributed. More embedded in normal life. Awareness declined. Surveillance became uneven. And the disease settled into a space where it doesn’t cause explosive outbreaks—but also doesn’t disappear.
It persists.
CLOSING
There’s a tendency to associate infectious disease risk with novelty. New pathogens. New variants. New threats. But a significant portion of infectious disease burden doesn’t come from the new. It comes from the familiar—misrecognized.
Flea-borne typhus is a case study in that dynamic. A pathogen with well-characterized biology. A transmission cycle that hasn’t disappeared—just adapted. An ecology that persists not through outbreaks, but through continuity. Quiet, stable, and easy to overlook.
And a clinical presentation that blends into the background. Nothing about it is particularly dramatic. That’s the problem. Because the outcome doesn’t depend on rare interventions or advanced technology. It depends on access to care—but not in the way we usually think about it. Not just whether care is available. Not just whether someone can get to a clinic or a hospital. But whether the need for care is recognized at all.
Whether the exposure is considered. Whether the pattern is seen. Whether the diagnosis makes it onto the list early enough to act on it.
Because in this case, the difference between a short illness and a complicated one isn’t just access to treatment.
It’s access to recognition.
And one more thing before we wrap—this episode marks the debut of my new free email newsletter, Field Notes.
Each week, I take one idea from the episode—something that feels like a hinge point—and sit with it a little longer. The episode gives you the science. Field Notes follows one thread and explores what it opens up.
Each issue comes out alongside the episode, and you can sign up at the link in the show notes or on the Field Notes page on my website, infectiousdose.com.
And next week, we’ll shift gears a bit—we’re going to talk about Lassa fever, which has had renewed interest recently, and what’s actually worth paying attention to there.
Thank you for being here! If there’s something you’ve been wondering about—something that doesn’t quite make sense or feels like it should connect to something else—you can always send it my way. My email is infectiousdose@gmail.com I read everything even if it takes me a minute to respond.
Until next time, stay healthy, stay informed, and spread knowledge not diseases.
Sources
George Cowan. 2000. Rickettsial diseases: the typhus group. Postgraduate Medical Journal. https://academic.oup.com/pmj/article-abstract/76/895/269/7059458?redirectedFrom=fulltext
→ Good general framework for explaining typhus classification.
Paywalled so here is the PDF:
Kaur and Jain. 2012. Role of antigens and virulence factors of Salmonella enterica serovar Typhi in its pathogenesis. Microbiological Research. https://www.sciencedirect.com/science/article/pii/S0944501311000784
Cerutti and Ridley. 2017. Endothelial cell-cell adhesion and signaling. Experimental Cell Research. https://www.sciencedirect.com/science/article/pii/S0014482717303336
Szperlinski, et al. 2025. Obligate intracellular bacteria and host cell death pathways-the matter of life and death. Cell Tissue Res. https://pmc.ncbi.nlm.nih.gov/articles/PMC12727909/
Sahni, et al. 2019. Pathogenesis of Rickettsial Diseases: Pathogenic and Immune Mechanisms of an Endotheliotropic Infection. Annual Review Pathology: Mechanisms Disease. https://www.annualreviews.org/content/journals/10.1146/annurev-pathmechdis-012418-012800
Maria A Caravedo Martinez, Alejandro Ramírez-Hernández & Lucas S Blanton. 2021. Manifestations and Management of Flea-Borne Rickettsioses. Research and Reports in Tropical Medicine. https://www.tandfonline.com/doi/pdf/10.2147/RRTM.S274724
→ Strong clinical overview including treatment and complications.
Lucas S. Blanton, 2023. Murine Typhus: A Review of a Reemerging Flea-Borne Rickettsiosis with Potential for Neurologic Manifestations and Sequalae. Infectious Disease Reports. https://www.mdpi.com/2036-7449/15/6/63
→ Comprehensive modern review covering clinical features, resurgence, and complications. Especially strong on neurologic manifestations and underrecognition.
Tsioutis, et al. 2017. Clinical and laboratory characteristics, epidemiology, and outcomes of murine typhus: a systematic review. Acta Tropica.
https://www.sciencedirect.com/science/article/pii/S0001706X16307264 → One of the best systematic reviews summarizing symptoms, hospitalization, and outcomes across studies.
Paywalled do here's the PDF:
Civen & Ngo. 2008. Murine typhus: an unrecognized suburban vectorborne disease. Clinical Infectious Diseases.
→ Classic paper showing the shift from urban rat cycles to suburban ecology.
Paywalled so here's the PDF:
Pieracci, et al. 2017. Fatal flea-borne typhus in Texas: a retrospective case series. American Journal of Tropical Medicine and Hygiene. https://pmc.ncbi.nlm.nih.gov/articles/PMC5417200/
→ Important for discussing severe and fatal cases (rare but real).
Azad, et al. 1997. Flea-borne Rickettsioses: Ecologic Considerations. Emerging Infectious Diseases. https://wwwnc.cdc.gov/eid/article/3/3/97-0308_article#
→ Foundational ecology paper—explains rat–flea cycles and alternative hosts.
Anstead, G. M. 2020. History, Rats, Fleas, and Opossums: The Ascendency of Flea-Borne Typhus in the United States, 1910–1944. Tropical Medicine and Infectious Disease.
Anstead, G. M. 2021. History, Rats, Fleas, and Opossums. II. The Decline and Resurgence of Flea-Borne Typhus in the United States, 1945–2019. Tropical Medicine and Infectious Disease. https://www.mdpi.com/2414-6366/6/1/2
Anstead, G. M. 2025. A One Health Perspective on the Resurgence of Flea-Borne Typhus in Texas in the 21st Century: Part 1: The Bacteria, the Cat Flea, Urbanization, and Climate Change. Pathogens.
→ Connects urbanization, climate, and animal reservoirs.
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