Outbreak Watch: Line of Sight
- Heather McSharry, PhD
- 12 hours ago
- 13 min read
Summary
Outbreaks are often described through case counts, maps, and headlines. But the most important question is often much simpler: How much of the event can we actually see?
In this edition of Outbreak Watch, we examine three ongoing biological events at very different stages. The Bundibugyo Ebola outbreak in the Democratic Republic of the Congo has become the largest outbreak of its kind ever recorded as contact tracing, sanitation, and response systems come under increasing strain. The Andes hantavirus outbreak linked to the expedition cruise ship MV Hondius appears to be nearing its conclusion, offering a real-world example of what successful outbreak containment looks like. And new detections of New World screwworm in the United States raise difficult questions about surveillance, preparedness, and what happens when officials are forced to respond while rebuilding critical response capacity.
Together, these stories reveal a common challenge: making public health decisions with incomplete information.
Listen here or scroll down to read full episode.
Full Episode
It's Thursday night, June 18th, and I've decided to do an Outbreak Watch episode for you. These episodes are intended to communicate meaningful change in ongoing biological events—or when outbreaks give us insight into a larger story that's worth paying attention to. This week, both things happened.
Aside from important updates for Ebola, Hantavirus, and Screwworm, all three events seem to be circling the same question: How much of the threat can we actually see?
And as it turns out, the answer to that question often determines what happens next.
OK, so, tonight we're looking at three very different biological events unfolding at three very different points on the same timeline.
Over the last few weeks we've been following two outbreaks in particular: the Bundibugyo Ebola epidemic in eastern Democratic Republic of the Congo and Uganda, and the Andes hantavirus cluster linked to passengers aboard the expedition cruise ship MV Hondius. Check out my episodes on both (Ebola; Hanta 1; Hanta 2; Outbreak Watch 1) if you want deeper dives in those.
Both have continued to evolve, but not in the same direction.
The Ebola outbreak continues expanding under extraordinarily challenging conditions.
The hantavirus cluster, meanwhile, is approaching the end of its monitoring period.
And then there's screwworm. Unlike Ebola or hantavirus, screwworm isn't a virus. It's a parasite. And while it isn't causing a human epidemic, new detections in the United States are raising difficult questions about surveillance, preparedness, and what happens when a biological threat emerges and the response system has to be rebuilt.
Taken together, these three stories illustrate something important. Outbreaks have life cycles. They emerge. They expand. They stabilize. Sometimes they're contained before most people ever hear about them. Sometimes they're not. But they also reveal something about the systems built to find and stop them. Tonight we're going to look at all three stages. One outbreak growing. One outbreak ending. And one emerging threat whose fate remains to be seen. Together, they tell a larger story about surveillance, preparedness, and what happens when public health officials lose sight of a threat—or struggle to see it clearly in the first place.
This is Outbreak Watch: Line of Sight.
Ebola: When Containment Starts Slipping
Let's start with the outbreak that's still growing. When we last talked about Bundibugyo Ebola, there were already signs that the official numbers were underestimating the true scale of transmission. Modeling groups were warning that confirmed cases represented only a fraction of what was actually happening on the ground.
Today, the outbreak has become the largest Bundibugyo Ebola outbreak ever recorded—a distinction nobody wanted it to earn.
As of tonight, June 18th, the Democratic Republic of the Congo has reported 875 confirmed cases and 202 confirmed deaths. Nearly 400 people remain hospitalized in isolation units, and the outbreak continues to be concentrated in Ituri Province, although cases have also been reported in North and South Kivu. Across the border, Uganda has reported 19 confirmed cases and two deaths, but importantly, no new cases have been reported there since June 5th.
At first glance, that might sound like two very different outbreaks. In reality, it's the same outbreak unfolding under two very different sets of conditions. The virus hasn't changed.
The biology hasn't changed. What differs is the environment the virus is moving through.
Part of that environment is the humanitarian crisis unfolding around the outbreak. WHO continues to describe eastern DRC as a complex conflict-affected setting characterized by population displacement, insecurity, and highly mobile communities. People are moving between mining sites, towns, displacement camps, and neighboring regions, often under difficult conditions. At the same time, security incidents affecting health facilities and response teams have made it harder to conduct surveillance, investigate cases, and maintain consistent contact tracing. Those conditions create opportunities for transmission that have very little to do with the virus itself and everything to do with the environment in which the outbreak is unfolding.
When epidemiologists talk about controlling Ebola, they tend to focus on a handful of key tools: identifying cases quickly, tracing contacts, isolating infected individuals, providing safe care, conducting safe burials, and maintaining enough community trust that people are willing to cooperate with the response.
The challenge in eastern DRC is that several of those pillars are under strain at the same time. One of the clearest warning signs is the decline in contact tracing. According to Oxfam, only about 43% of known contacts are currently being successfully monitored. To put that in perspective, during the major Ebola outbreak in this same region between 2018 and 2020, nearly 80% of contacts were being followed at a comparable point in the response.
That difference is important. Contact tracing is how you get ahead of Ebola. Every missed contact is a transmission chain you can't see. Every unseen transmission chain becomes an opportunity for the virus to move into a new family, a new workplace, a new village, or a new health zone before anyone realizes it's there.
At the same time, response teams are facing other logistical challenges. Oxfam reports that only about one in five health facilities in parts of Ituri Province have reliable access to sufficient clean water. In some communities, access to sanitation infrastructure is equally limited. That's a major problem because Bundibugyo Ebola is one of the Ebola viruses for which we have no licensed vaccine and no approved antibody therapies. Supportive care, infection prevention, hygiene, and rapid identification of cases remain some of the most important tools available.
And those systems are under pressure.
Over the last week we've also seen reports of attacks on response personnel. An attack on a safe and dignified burial team was reported in Mongbwalu Health Zone. There have also been reports that several outbreak response workers were taken hostage after being falsely accused of spreading the disease themselves.
If that sounds irrational, remember that outbreaks don't occur in a vacuum. When communities are frightened, when people see neighbors dying, when health systems are overwhelmed, and when trust in authorities is already fragile, rumors often spread faster than the virus itself.
And that's what makes this outbreak concerning. Not because Ebola is behaving unexpectedly. It's doing exactly what Ebola has always done. It's exploiting weaknesses in the systems trying to stop it. The virus isn't winning because it's changing.
It's winning because the response environment continues to deteriorate.
That doesn't mean containment is impossible. Uganda's experience over the last two weeks shows that transmission can be interrupted when cases are identified quickly and contacts are successfully followed.
But it does mean that the trajectory in eastern DRC remains worrying. Three weeks ago, we were talking about an outbreak that appeared larger than the official numbers suggested. Today, we're talking about the largest Bundibugyo outbreak ever recorded—and one where many of the tools needed to control it are becoming harder to deploy, not easier.
One last thing to mention about this outbreak, yesterday the WHO released its first comprehensive clinical management guidelines for filovirus diseases, including Ebola and Marburg viruses, emphasizing early supportive care as a critical component of improving survival. For Bundibugyo virus disease, supportive care remains one of the most important tools available to clinicians. So to have standardized care guidelines is helpful.
Hondius Hantavirus: What Success Actually Looks Like
Now let's move to a very different story.
When we last discussed the Andes hantavirus outbreak linked to passengers aboard the expedition cruise ship MV Hondius, the outbreak was still behaving exactly the way we expected it would.
If you've listened to my previous episodes on this event, you'll remember that one of the defining features of Andes virus is its unusually long incubation period. Unlike many respiratory viruses, where exposed individuals typically become sick within a few days, Andes virus can take weeks before symptoms appear. That means cases don't all emerge at once.
Instead, they appear gradually as the monitoring period progresses.
And that's exactly what's been happening here.
Since the outbreak was first identified in early May, public health officials have tracked exposed passengers and crew across multiple countries. Additional cases were identified, bringing the total outbreak count to 13, but there has been no sustained chains of person-to-person transmission spreading through communities after passengers returned home. Knowing the transmission dynamics of Andes virus, it's what we expected.
In the United States, eighteen potentially exposed individuals were transported to the National Quarantine Unit at the University of Nebraska Medical Center for a 42-day monitoring period. As of today, twelve have already completed their monitoring to return home, while six remain under observation. Importantly, none have developed hantavirus disease.
At the same time, a separate group of passengers who had already returned to the United States before the outbreak was recognized completed their own monitoring period on June 6th. No cases were detected among them, and no further follow-up was required. Also... almost all of the passengers and crew quarantined in the Netherlands are now allowed to return home, including non-nationals.
That's all good news.
And it's a useful reminder of what containment actually means. A successful outbreak response isn't the absence of new cases. When an outbreak is identified there are already exposures that haven't turned into cases yet so there will be new cases. A successful outbreak response identifies potential exposures, understands who is at risk, monitors them appropriately, and prevents further transmission from occurring.
And that's what we had with this hantavirus outbreak. From an epidemiologic standpoint, the story over the last several weeks has been consistent with what we know about Andes virus. A limited number of cases. Slow emergence over time. Careful monitoring of exposed individuals. And no evidence that the virus has escaped those expected transmission patterns.
That may not sound dramatic, but in public health, boring is often good. And the Hondius outbreak is nearing the end of its story, not the beginning.
Screwworm: The Threat We Can't Fully See
Our final update tonight isn't about an outbreak. At least not yet. And whether it becomes one may depend less on the biology of the parasite than on our ability to see where it is.
Recently, officials have confirmed additional detections of New World screwworm in animals in the United States. As of tonight, the USDA APHIS dashboard indicates the number of confirmed cases is 12 in livestock and 0 in wildlife, but the response has continued to expand, with increased surveillance, additional trapping, movement restrictions, and efforts to increase sterile fly releases.
The reason we don't talk about screwworm very often today is because one of the most successful eradication campaigns in modern agricultural history eliminated it from the United States in 1966. The cornerstone of that effort was the Sterile Insect Technique. Scientists mass-produced male screwworm flies, sterilized them, and released them into the environment. Female screwworm flies only mate once. If they mate with a sterile male, they produce no offspring. Release enough sterile males, and eventually the population collapses. It's one of the most elegant biological control strategies ever developed. But one thing worth paying attention to in the current response is the language officials are using.
You'll hear that surveillance is being increased, sterile fly releases are being expanded, and response capacity is being ramped up. Those statements are true. But they can also obscure an important reality: many of these efforts are starting from a diminished baseline. Over the past year, programs involved in screwworm surveillance, sterile fly production, and adult fly suppression experienced significant reductions in capacity. As a result, some of the systems now being expanded are the same systems that are simultaneously being rebuilt.
There's some other news that can be misleading too. Two days ago, the USDA announced "USDA Invests in Projects to Strengthen New World Screwworm Preparedness and Response". Sounds like they're ramping up response for the current situation but that's not what they mean. They are talking about grants for new screwworm technologies, including wildlife surveillance tools, improved traps, ecological modeling, and expanded sterile-fly capabilities. Those investments may prove important in the future. But they can't help the current response. And another important point here is that the federal government recently proposed giving all grant funding oversight to the executive branch and political appointees. If that goes through, it gives them not only the power to decide what science grants are funded in the first place, it also gives them carte blanche to terminate grants already funded, with no notice. So who knows if these "awarded" grants will actually get to do the work they proposed.
But it's worth noting again that surveillance works. Falling surveillance in Ebola clearly demonstrates the importance of surveillance, but we have an example here in the US, too. Earlier this year, officials detected screwworm larvae in a horse imported from Argentina while the animal was still moving through Florida's quarantine process. The infestation was identified before the animal entered the broader livestock population, and officials reported no additional spread beyond the quarantine facility. So here's where the system worked. Surveillance happened. Detection happened. Control happened. When public health infrastructure is funded and operational, it protects us.
The challenge facing Texas is very different from that Florida win. A quarantined horse moving through a controlled import process is controllable. A screwworm population moving through livestock, wildlife, and thousands of square miles of open landscape in some areas that don't even have a veterinarian, is very different.
Now, livestock surveillance gives us visibility into part of the problem. Ranchers notice wounds. Veterinarians investigate suspicious cases. Agricultural inspectors can examine animals moving through the system. But wildlife is a different ballgame. We don't routinely inspect every deer, feral hog, coyote, squirrel, or other potential hosts moving through these ecosystems. Wildlife surveillance has always been more challenging, and reduced capacity only increases that uncertainty. So when that USDA dashboard says zero wildlife detections, it means detections, not cases. Zero found is not the same as zero cases.
The response itself has also become a subject of debate. Texas Agriculture Commissioner Sid Miller has argued that USDA should deploy additional tools, including the Screwworm Adult Suppression System, or SWASS, which was used during previous eradication campaigns. USDA has instead emphasized surveillance, trapping, movement controls, and expansion of sterile fly production. As of this recording, I have not found evidence that SWASS has been deployed. I don't know why that is. Considering it's so effective, there's no reason not to deploy it unless maybe they don't have the capacity after all the cuts to US public health infrastructure.
Either way, one of the most important questions right now isn't how many screwworm cases have been detected. It's how many haven't. The confirmed detections tell us where screwworm has been found. They don't tell us everywhere screwworm is. And that uncertainty is what makes this story important.
Unlike the Ebola outbreak, where the challenge is maintaining visibility as contact tracing breaks down during an active epidemic, the screwworm response faces a different problem: trying to understand the true scope of a threat while rebuilding some of the systems needed to measure it. For now, officials are racing to increase surveillance, expand sterile fly releases, and prevent screwworm from becoming reestablished in the United States. Whether those efforts succeed may ultimately depend on something far less dramatic than the parasite itself. Not what we know.
But what we don't.
Closing
If there's a common thread running through all three of these stories, it isn't that they're infectious pathogens. It's visibility.
In eastern Congo, public-health teams are struggling to maintain visibility into an outbreak as contact tracing becomes more difficult, health infrastructure remains under strain, and community trust continues to erode.
In the Hondius hantavirus outbreak, visibility gradually improved over time. Exposed passengers were identified, monitored, and followed. Uncertainty narrowed. The outbreak behaved within the boundaries we expected.
And with screwworm, the challenge is different altogether. The question isn't simply where we've found it. It's where we haven't looked, what we haven't detected, and whether surveillance systems can be strengthened quickly enough to answer those questions.
Different pathogens. Different environments. The same fundamental problem. Trying to make decisions with incomplete information. That's true in outbreak response. It's true in medicine. It's true in emergency management. And it's true in nearly every biological event we face.
Pathogens rarely announce themselves. They emerge in fragments: a handful of unusual illnesses, an unexpected laboratory result, an animal with an unusual wound, a cluster of cases that doesn't quite fit. The work of public health is turning those fragments into a picture that is clear enough to act on. Sometimes that picture sharpens . Sometimes it becomes more uncertain. And sometimes the most important thing we learn is how much we still don't know.
Over the coming weeks, the Hondius outbreak will fade from the headlines as monitoring concludes. The Ebola outbreak will almost certainly continue to challenge response teams in eastern Congo. And screwworm remains a story worth watching closely—not because we know exactly what comes next, but because we don't.
We'll continue following these events, and when there are significant developments—or when they've evolved enough to offer new insights—I'll bring you another Outbreak Watch update.
If you'd like weekly outbreak updates in the meantime, along with behind-the-scenes notes from the show, you can subscribe to my free newsletter, Field Notes.
Next week, we're closing out June's Summer Rain series with an Outbreak After Dark episode focused on one of summer rain's most persistent companions: the mosquito.
We'll follow a blood meal from the mosquito's perspective, explore what happens beneath your skin after the bite, and uncover the surprisingly sophisticated biology behind one of summer's most familiar irritations.
Until then, stay healthy, stay informed, and spread knowledge not diseases.
Linked Resources
European Centre for Disease Prevention and Control (ECDC) – Ebola Outbreak in DRC and Uganda
→ Weekly situation reports and risk assessments tracking the ongoing Bundibugyo Ebola outbreak in the Democratic Republic of the Congo and Uganda, including case counts, geographic spread, operational challenges, and assessments of risk to Europe.
World Health Organization Regional Office for Africa – Ebola Disease Outbreak in DRC
→ WHO Africa's central information hub for the current Ebola outbreak, providing official updates, situation reports, technical guidance, response activities, and public health resources.
Oxfam – Scale of Ebola Virus Outbreak Likely Underestimated
→ Oxfam's field assessment of conditions in affected communities, highlighting challenges related to clean water, sanitation, contact tracing, community trust, and humanitarian conditions that may contribute to under-detection of cases.
Centers for Disease Control and Prevention (CDC) – MV Hondius Hantavirus Outbreak
→CDC's official summary of the Andes hantavirus outbreak linked to passengers aboard the MV Hondius expedition cruise ship, including public health monitoring activities, repatriation efforts, and updates on exposed travelers.
USDA APHIS – New World Screwworm Information Center
→ The U.S. Department of Agriculture's primary resource on New World screwworm, including information on the parasite's biology, surveillance efforts, eradication strategies, sterile insect programs, and current response activities.
USDA APHIS – Confirmed U.S. New World Screwworm Cases
→ A regularly updated summary of confirmed New World screwworm detections associated with the United States, including case locations, affected animals, and ongoing response measures.