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Outbreak Watch: September 18, 2026

Writer: Heather McSharry, PhD
Heather McSharry, PhD
11 hours ago
16 min read

Updated: 6 hours ago

Summary

Three weeks can change an outbreak without the biggest number telling you how. In this Outbreak Watch, we look beneath the cumulative counts: signs of local Ebola transmission interruption alongside continuing spread in the DRC, the thresholds that would signal New World screwworm establishment, Florida’s growing dengue outbreak, rising pressure on the U.S. rabies prevention system, four measles-associated deaths and a fight over federal surveillance data in Pennsylvania, a wildlife anthrax outbreak crossing into humans in Zambia, and a malaria mystery unfolding around Frankfurt Airport.

Listen here or scroll down to read full episode.




Full Episode

Welcome back to Outbreak Watch. It’s been about three weeks since our last full check-in, and quite a bit has changed. Some of the outbreaks we’ve been following have continued to grow, but the cumulative numbers don’t necessarily tell us what is happening underneath them.

So, let’s start, as we usually do, in the Democratic Republic of the Congo.


The cumulative numbers are considerably higher than they were three weeks ago. According to DRC SitRep 125, published September 17 with data through September 16, the DRC has now reported 7,475 confirmed cases and 3,605 deaths, with 1,798 people recovered. The outbreak has affected 62 health zones across seven provinces. But those national totals aren't the most important part of the story, because the epidemic underneath them looks very different depending on where you look.

There are encouraging signs in some places. South Kivu has not reported a new case since May, and SitRep 122 identified seven health zones that had gone approximately 42 days without a new confirmed case. That's roughly two maximum Ebola incubation periods without another confirmed infection and good evidence that transmission may have been locally interrupted in those areas.

But that progress isn't happening everywhere. The latest report describes transmission as sustained in Ituri, North Kivu and Haut-Uélé, with more sporadic reporting in Tshopo and Bas-Uélé. And the concentration is striking: 68 of the 71 new cases reported on September 16 came from Ituri and North Kivu alone. So we're seeing local epidemics moving in different directions at the same time.

That's why the cumulative national count can be misleading. More than 7,400 infections sounds like nothing is improving, but cumulative counts tell us how much disease has occurred since the outbreak began—not what transmission is doing today. And the response remains uneven. Overall contact follow-up is 87.6 percent, but in North Kivu, standardized treatment facilities are now 96.9 percent occupied, 42.1 percent of hospitalized patients are being treated outside those facilities, and Butembo and Katwa have no available treatment beds. Those gaps make interrupting transmission more difficult.

Vaccination of frontline workers is also continuing. The latest report records 2,810 people vaccinated in Tshopo and 600 in Bas-Uélé, while preparations are underway in Ituri. Ervebo is licensed against Zaire ebolavirus but is being used here against Bundibugyo virus disease under a research protocol, so this outbreak may also generate important evidence for controlling future Bundibugyo outbreaks.

So now I'm less interested in whether the national cumulative total continues to climb—it will—and more interested in where transmission is continuing and where it is stopping. Do those health zones that have gone roughly 42 days without a case remain quiet? Do more join them? And can responders interrupt sustained transmission in Ituri, North Kivu and Haut-Uélé?

NEW WORLD SCREWWORM: MORE DETECTIONS, BUT NOT ESTABLISHMENT

Now let's move to New World screwworm in the US. The cumulative number of U.S. detections has continued to increase, but the signal I care about most still hasn’t appeared.

Texas now reports 39 affected premises across 17 counties, an increase in both since we last checked in. Across the U.S., 50 domestic-animal infestations have now been reported during the current outbreak.

Two of the newest detections are horses. A working ranch horse in Presidio County became the first confirmed equine case in the U.S. during this outbreak. And in a very late update as I'm recording, USDA confirmed the 50th U.S. case today, September 17—another horse, this time in Grant County, New Mexico. That's a new affected county and New Mexico's second case of the outbreak.

The timing is particularly interesting because USDA is preparing to reopen the Santa Teresa livestock port on September 24 as part of the phased reopening of livestock trade with Mexico that began with Douglas, Arizona, last month. It's not yet clear whether this new detection will change that plan.

But what would fundamentally change the U.S. risk picture is evidence that New World screwworm is becoming established beyond these individual domestic-animal infestations, especially confirmation in wildlife or detection of wild screwworm flies in the surveillance network. We still don't have that signal. USDA continues to report no wildlife or feral cases and no wild-fly trap detections.

And there are signs that control efforts can clear affected areas. USDA has already released infested zones in both Texas and New Mexico after response requirements were met, although today's New Mexico detection shows how quickly that can change.

So this is becoming an even more interesting test of the control system: can responders contain these individual infestations while livestock movement gradually resumes?

For now, we have more detections, a larger geographic footprint and a second equine case—but we still haven't crossed the threshold that would tell us New World screwworm has established sustained transmission in U.S. wildlife or wild fly populations.

DENGUE NEXT DOOR

Now to Florida, where the locally acquired dengue outbreak we've been following has grown substantially. The latest Florida Department of Health weekly report I can verify, covering through September 5, counted 111 locally acquired infections across six counties, including 95 in Hillsborough County alone. Florida is also still investigating the death of a Hillsborough County resident to determine whether dengue contributed to it.

So this isn't the usual imported dengue case occasionally producing a local case. We have continuing locally acquired human infections alongside dengue-positive local mosquitoes, and Hillsborough County is still conducting intensive mosquito-control operations.

I'm not going to spend time here explaining how we got here, because that's exactly what this week's Infectious Dose is about. In Dengue Next Door: How an Imported Virus Becomes a Local Outbreak, I walk through how an imported infection can seed local mosquito transmission, why Florida can support that process, and what would have to change before I would start thinking about this as something more persistent than a substantial local outbreak.

So my watch point is this: does transmission in Hillsborough begin to decline, or does locally acquired dengue continue expanding geographically? Right now, Hillsborough is the clear center of this outbreak. I’m watching to see whether it stays that way.

RABIES: THE SIGNAL ISN'T HUMAN RABIES

Next is rabies, but I want to be clear that there is currently no outbreak of human rabies. What I’m watching is happening farther upstream: and that’s the increase in potential human exposures and the public-health response required to keep those exposures from becoming human cases.

On September 10, CDC issued a national health advisory after seeing unusual activity this summer. During July and August, CDC received 17 percent more rabies-related inquiries than during the same period last year, including reports of mass-exposure events. At least eight state health departments reported increased use of rabies post-exposure prophylaxis called PEP, increased errors in administration of said PEP, or both. And pharmacy data through September 2 showed use of rabies vaccine up about 33 percent from the same period last year, while use of human rabies immune globulin was up 76 percent. CDC says there is currently no shortage of rabies vaccine or immune globulin in the US, although rabies biologics are under supply pressure elsewhere in the world—including a current vaccine shortage in Ireland—so appropriate use matters even when U.S. supplies remain adequate. But honestly, I’m curious how the CDC can actually impact anything positively here considering the cuts and anti-vax leadership. So for details on correct PEP administration, I recommend going to the WHO website linked in the blog post for trustworthy info right now.

Now, the petting-zoo investigation we've been following in North Carolina shows what increased exposure and PEP burden can look like. Three baby goats associated with the Farm Adventures mobile petting zoo tested positive for rabies, and a fourth untested goat is presumed to have been infected. Before anyone knew the animals had rabies, they'd been handled by visitors at the farm and at events across multiple counties.

And the exposure investigation has continued growing even though I haven't found evidence of additional infected animals. On September 2, 284 people had been assessed and 213 were recommended to receive PEP. By September 10, NCDHHS reported 346 people identified as exposed, with 263 recommended for PEP. So just three laboratory-confirmed rabid goats have generated hundreds of human exposure investigations and more than 250 recommendations for preventive treatment. To be fair, though, these weren't three adult goats standing behind a fence. They were baby goats at a petting zoo. People were holding them, cuddling them, feeding them and getting their faces and hands close to animals that, as anyone who has spent time with goats knows, can be pretty slobbery and even more so if they have rabies. Many of the visitors were children, and some were older adults. That creates far more opportunities for saliva to reach eyes, mouths, broken skin or small scratches than you would have in a typical encounter with a rabid wild animal. So this was almost a perfect setup for generating a very large number of exposures from a very small number of infected animals.

So that's why I'm adding rabies to the surveillance board in a slightly different way. Human rabies remains extremely rare in the US. But that rarity partly reflects everything happening upstream: animal surveillance and testing, exposure investigations, risk assessment and PEP before symptoms develop.

So I'm watching the exposure burden: whether mass-exposure events continue, whether demand for vaccine and immune globulin remains elevated, and whether PEP administration errors continue. North Carolina's petting-zoo investigation will eventually end, but the broader national signal may not. And by the time an increase in human rabies becomes the signal, prevention has already failed.

Next is measles, and unfortunately this signal has changed substantially since our last Outbreak Watch. Three weeks ago, Pennsylvania had just reported its first two measles-associated deaths in 35 years. On September 15, the state confirmed two more, bringing the total to four in a matter of weeks. Both of the newly reported individuals were unvaccinated.

One of those deaths involved an unvaccinated 18-year-old in Mifflin County who developed acute disseminated encephalomyelitis, or ADEM, a rare and severe neurological complication of measles involving inflammation of the brain and spinal cord. The other was an unvaccinated adult in Jefferson County. I'm deliberately using Pennsylvania's term measles-associated deaths because the state uses a specific epidemiological case definition to determine whether a death is associated with measles, and the circumstances of these four deaths haven't all been identical.

And there's another important development here, Pennsylvania is now asking CDC for an Epi-Aid, which would bring federal epidemiologists into the state for on-the-ground outbreak support.

But Pennsylvania says it will withdraw the request if CDC does not publicly recognize its four measles-associated deaths. And there's important context for why. CDC staff had already reviewed and accepted Pennsylvania's first two deaths through the agency's normal process, and they briefly appeared in federal measles data. They were subsequently removed after the antivax rfkjr directed CDC leadership to take them out and publicly questioned whether they should be attributed to measles.

Pennsylvania has continued to stand behind its classification, and the state is now essentially making this a question of federal data transparency as well as outbreak response. Public-health surveillance depends on knowing that data are being added—or removed—according to transparent epidemiological standards rather than the preferences of political leadership.

So there are really two surveillance signals here now: the outbreak itself—which has grown to 731 cases, 141 hospitalizations and four state-confirmed measles-associated deaths—and what happens to the public-health infrastructure responding to it.

And to be clear…this is an entirely preventable outbreak in which nearly one in five identified patients has been hospitalized and we’ve had four deaths.

So that's my watch point here. Four measles-associated deaths in one state within a matter of weeks, after Pennsylvania had gone 35 years without one, is a significant change in the severity signal. Most people with measles recover, but as outbreaks grow, rare complications increase. It’s a numbers game. I'm watching not only the case count now, but hospitalizations, severe neurological and respiratory complications, and any additional deaths.

Please get vaccinated. If you’re old like me get your antibody titer checked and get boosted if needed.

Now to Zambia, where authorities are responding to an anthrax outbreak in the Munyamadzi Game Management Area in Muchinga Province, where the first obvious signal, again, was dead wildlife before sick people. So far, authorities have reported 46 dead hippos, eight elephants, five buffalo and one crocodile. And the outbreak has crossed into humans: 12 people reportedly became infected after consuming meat from affected wildlife carcasses. All are receiving treatment, and no human deaths have been reported.

Anthrax is well-suited to this kind of outbreak because Bacillus anthracis forms durable spores that can persist in the environment for years, decades and even longer. Wildlife or livestock can encounter those spores while grazing, and infected carcasses can then contaminate the environment further. That's why authorities are warning people not to handle, butcher, sell or eat animals found dead, and hunting has been temporarily suspended while investigators determine the extent of the outbreak. Zambia has also intensified surveillance, carcass disposal and community outreach in the affected area.

And to clarify, when I say this outbreak has crossed into humans, I don't mean anthrax is now spreading from person to person. That's exceedingly rare. This is the same ecological event producing infections across species: infected wildlife created opportunities for human exposure.

So that's my watch point: does the wildlife die-off continue, do livestock become involved, do additional human infections appear, and does the affected geographic area expand? Because this is another example of dead wildlife giving us a warning signal.

And finally tonight, I want to return briefly to the strange malaria investigation in Germany—partly because the story has changed, and partly because I'm going to spend much more time on it in next week's Infectious Dose.

Frankfurt health officials are now investigating eight cases of Plasmodium falciparum malaria. Six earlier cases were associated with Frankfurt Airport, raising the possibility of what's called airport malaria, where an infected Anopheles mosquito hopped on a flight and went somewhere they never expected. Of those original six, two had died.

But now there are two additional cases in residents of Schwanheim, near the airport, and Frankfurt officials reported today that one of those patients has died. Neither person worked at the airport, neither had traveled to a malaria-endemic region, and officials are still investigating how they became infected. Specialized laboratory testing is underway to determine whether the infections might share a source, while mosquito surveillance and breeding-site control have been expanded into the surrounding residential area.

And that's where I'm going to leave this one, because we don't yet know how these people became infected. An imported infected mosquito remains one possibility under investigation, but proximity to an airport isn't proof of airport malaria. Do I think it’s likely? Sure, but we need to wait for some data on this one.

Next week on Infectious Dose, we'll dig into how airport malaria actually happens, how investigators solve cases in people with no travel history, and what this unusual Frankfurt cluster can—and cannot—tell us about malaria transmission in Europe.

WHAT WE'RE ACTUALLY WATCHING

And that's where I'm going to leave the board tonight.

If there's a thread connecting these very different stories, it's that the biggest number isn't always the most important signal.

And that's what I want Outbreak Watch to do: to not just keep a running tally of cases, but ask what would actually tell us that the situation has changed? Sometimes that's a declining epidemic curve, a positive mosquito trap or unusual pressure on a prevention system. And sometimes, as in Frankfurt, it's a case that doesn't quite make sense yet.

So I'll keep watching those thresholds, and when the evidence changes, we'll move the signal with it.

If you want to go deeper into the Florida dengue outbreak, this week's Infectious Dose, Dengue Next Door, is available now. I also have episodes on ebola, anthrax, measles, rabies, and screwworm if you want to learn more about those as well. And next week, we'll follow that strange trail to Frankfurt and look at what happens when mosquitoes fly the friendly skies.

Until next time, stay healthy, stay informed, and spread knowledge, not diseases.








ANNOTATED CITATIONS

Accessing the literature: Whenever possible, I've linked directly to free full-text articles and books that are legally available online. For subscription-only journal articles, many researchers are happy to share a personal copy of their work if you contact the corresponding author. For books, I've included links to free online lending copies or library catalogs when available. If a title isn't freely available online, I've linked WorldCat to locate it at a nearby library or you can ask your local library about interlibrary loans.

Each citation is annotated to explain how the source connects to the episode and to help listeners who want to explore the science in greater depth.

DRC Ebola

Democratic Republic of the Congo Ministry of Health. 2026. Ebola Situation Report No. 124. 

➡️ Official outbreak situation report providing the most current case counts, deaths, affected health zones, and operational updates from the DRC Ministry of Health.

World Health Organization. 2026. Ebola disease caused by Bundibugyo virus – Democratic Republic of the Congo. Disease Outbreak News, 10 September 2026.

➡️ WHO’s detailed outbreak assessment summarizing the continuing Bundibugyo virus disease epidemic in the DRC, including transmission patterns, geographic expansion, mortality, response challenges, and the use of Ervebo under a research protocol. Useful alongside the more frequently updated DRC Ministry of Health situation reports for broader epidemiological context and WHO’s risk assessment.

New World Screwworm

USDA Animal and Plant Health Inspection Service (APHIS). 2026. Current Status of Confirmed U.S. Cases of New World Screwworm.

➡️Official USDA dashboard tracking confirmed New World screwworm detections in the United States and summarizing the federal response.

Centers for Disease Control and Prevention. 2026. New World Screwworm Outbreak. Updated 11 August 2026.

➡️ CDC’s public-health overview of the current New World screwworm outbreak, including U.S. human-health risk, regional spread, prevention guidance, and the federal One Health response. Complements USDA/APHIS animal surveillance, which remains the primary source for current U.S. animal detections.

Florida Dengue

Florida Department of Health. 2026. Florida Arbovirus Surveillance: Week 35, August 30–September 5, 2026. 

🌐 Free online resource (Link downloads the week 35 report) — Florida DOH Week 35 Arbovirus Surveillance Report

➡️ Primary state surveillance report documenting 111 locally acquired dengue cases across six Florida counties, including 95 in Hillsborough County, as well as serotype data and 16 DENV-2-positive mosquito pools in Hillsborough. This is the principal source for the Florida numbers used in this episode.

Centers for Disease Control and Prevention. 2026. Current Year Data (2026): Dengue. 

➡️ CDC’s national dengue surveillance resource for current U.S. case data and trends. CDC notes that its figures are provisional and may lag state and local reporting, making the Florida Department of Health report the preferred source for the most current Florida-specific outbreak numbers.

Mayer, R., & Sheridan, K. 2026. Florida investigates possible Hillsborough death related to dengue as cases hit 95. WUSF, 10 September 2026.

➡️ Reports confirmation from the Florida Department of Health that it is investigating whether dengue contributed to the death of a Hillsborough County resident. Also provides local context for the rapidly growing Hillsborough outbreak and mosquito-control response.

Rabies

Centers for Disease Control and Prevention. 2026. Nationwide Increase in Reported Human Rabies Exposures: Rabies Post-exposure Prophylaxis Administration. Health Alert Network (HAN) No. 00533, 10 September 2026.

➡️ CDC national advisory documenting increased reports of potential human rabies exposures and problems with PEP administration. CDC reported 17% more rabies-related inquiries during July–August than during the same period in 2025, reports of increased PEP use or administration errors from at least eight states, and estimated year-over-year increases of 33% in rabies-vaccine use and 76% in human rabies immune globulin use.

WHO 2024. Rabies Post-Exposure Prophylaxis Decision Tree: Decide with Confidence.

➡️ WHO Rabies Post-Exposure Prophylaxis Decision Tree: Decide with Confidence. Clinical guidance for evaluating and managing potential rabies exposures, including wound care, human rabies immune globulin and vaccination schedules. Useful background for understanding why correct exposure assessment and PEP administration are central to preventing human rabies.

Pennsylvania Measles

Pennsylvania Department of Health. 2026. DOH Confirms Two Additional Measles-Associated Deaths. 15 September 2026.

➡️ Official state announcement confirming two additional measles-associated deaths, bringing Pennsylvania’s 2026 total to four. Both newly reported individuals were unvaccinated. This is the primary source for the state’s classification of the deaths as measles-associated.

Pennsylvania Department of Health. 2026. Measles. 

➡️ Pennsylvania’s primary surveillance resource for the ongoing outbreak, providing updated information on cases, hospitalizations, deaths, geographic spread, exposures and vaccination. The state reported 731 cases across 38 counties, 141 hospitalizations and four confirmed measles-associated deaths as of September 16.

Centers for Disease Control and Prevention. 2026. Measles Cases and Outbreaks. 

➡️ CDC’s national surveillance page for U.S. measles cases and outbreaks. Provides regularly updated national case counts, outbreak information, geographic distribution and historical comparisons. Particularly relevant to this episode because Pennsylvania and federal health officials are currently disputing whether the state’s four measles-associated deaths should appear in federal reporting.

Reuters. 2026. Pennsylvania asks for CDC help on measles outbreak, demands agency recognize measles deaths. 17 September 2026.

➡️ Reports Pennsylvania Health Secretary Debra Bogen’s September 16 request for a CDC Epi-Aid to provide on-the-ground epidemiologic assistance during the outbreak. The request is conditional on CDC publicly recognizing Pennsylvania’s four measles-associated deaths and documents the dispute over their removal from federal reporting after intervention by HHS Secretary rfkjr.

Zambia Anthrax

Rukanga, B. 2026. Zambians warned not to eat dead wildlife after anthrax kills dozens of animals. BBC News, September 2026.

🌐 Free online news report — https://www.bbc.com/news/articles/ce8en6v3601o

➡️ Current reporting on the anthrax outbreak in Zambia’s Munyamadzi Game Management Area, where authorities have reported 60 wildlife deaths—46 hippos, eight elephants, five buffalo and one crocodile—and 12 human infections associated with consumption of affected wildlife carcasses. No human deaths had been reported at the time of publication.

ReliefWeb Sitrep ZMB: Epidemic - 08-2026 - Anthrax (2026-09-14). Posted 15 Sep 2026

➡️ Current report on Zambia’s anthrax outbreak derived from IFRC and Zambia Red Cross reports.

World Health Organization. 2023. Anthrax – Zambia. Disease Outbreak News, 8 December 2023.

➡️ Background resource on anthrax epidemiology and previous outbreaks in Zambia. Explains the environmental persistence of Bacillus anthracis spores, transmission through infected animals and carcasses, Zambia’s history of wildlife- and livestock-associated outbreaks, and the importance of One Health surveillance and safe carcass management. This source describes the 2023 outbreak and should be used for background only, not for the current 2026 case counts. 

Frankfurt Malaria

City of Frankfurt am Main Health Department. 2026. Zwei neue Malaria-Fälle im Umfeld des Frankfurter Flughafens [Two new malaria cases in the vicinity of Frankfurt Airport]. 15 September 2026; updated 17 September 2026.

➡️ Official announcement of two additional Plasmodium falciparum infections in residents of Schwanheim near Frankfurt Airport who neither worked at the airport nor had traveled to a malaria-endemic area. The investigation is ongoing. The page was updated September 17 to report that one of these two individuals has died. It also documents six earlier airport-associated cases, two of which were fatal.

City of Frankfurt am Main Health Department. 2026. Flughafenmalaria FAQs [Airport Malaria FAQs].

➡️ Official public-health background on airport malaria and Frankfurt’s current investigation, including transmission, risk to the public, mosquito surveillance and control measures, and guidance for clinicians and residents near the airport.

Wieters, I., et al. 2019. Two cases of airport-associated falciparum malaria in Frankfurt am Main, Germany, October 2019. Euro Surveillance.

➡️ Published investigation of two severe P. falciparum infections believed to have been acquired at Frankfurt Airport in 2019. Neither patient had recently traveled to a malaria-endemic area, both worked at the airport, and parasite genotyping showed identical patterns. Provides an especially useful historical precedent for understanding how airport malaria is investigated and why malaria must sometimes be considered even in patients without relevant travel history.            

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