Field Notes #11: Nightfall
- Heather McSharry, PhD
- 4 days ago
- 5 min read
In the Margins
We tend to think of dusk as an ending.
The workday ends. The heat begins to fade. Porch lights flicker on. Conversations linger a little longer outdoors. It's the hour of campfires, evening walks, and summer dinners that stretch past sunset. But dusk isn't really an ending. It's a transition.
And transitions create opportunities.
For a brief period each evening, two worlds overlap. The creatures of the day haven't completely disappeared. The creatures of the night haven't fully emerged. Light remains, but darkness is coming.
For humans, dusk often feels peaceful.
For many other organisms, it's opening time. Mosquitoes become active as the heat of the day fades. Bats leave their roosts. Moths rise from vegetation. Owls begin hunting. Entire ecological communities that remained largely invisible during daylight suddenly come to life.
This isn't accidental. The threshold itself is useful.
Temperatures are lower. Humidity often rises. Creatures that depend on vision lose advantage while those adapted to darkness gain some. The environment shifts and certain species are built specifically to exploit it.
What feels to us like atmosphere is to them a chance. Perhaps that's why dusk has always carried a certain tension. Even when we find it beautiful, part of us recognizes that shift.
Underlined
These are some of the organisms that specialize in the liminal space between day and night:
Mosquitoes
Many mosquito species become most active around dusk, when temperatures drop, humidity rises, and hosts remain abundant.
Bats
Emerging from daytime roosts, bats use the transition to begin feeding on insects that are becoming active at the same time.
Moths
As daylight fades, many moth species leave shelter to feed, pollinate plants, and reproduce under the cover of darkness.
Owls
Exceptional low-light vision and hearing allow owls to hunt during the hours when many prey species are moving between daytime and nighttime behavior.
Crepuscular mammals
Deer, rabbits, foxes, and many other species concentrate activity during dawn and dusk, taking advantage of cooler temperatures and reduced exposure.
Individually, these species are very different. Together, they reveal that dusk is not simply a time of day. It's an ecological niche
What It Points To
Some of the most important things in nature happen not at the extremes, but at their boundaries.
Outbreak Updates
Updates will only include information verified through credible reporting or official public health sources.
As of mid-morning, June 24, 2026:
The Bundibugyo Ebola outbreak in the Democratic Republic of the Congo continues to expand and remains the most concerning outbreak in this week's update.
According to WHO and multiple international reporting sources, the outbreak has now exceeded 1,000 confirmed cases. Current reporting places the total at approximately 1,094 confirmed cases and 277 deaths in the DRC. Uganda has reported 20 confirmed cases and two deaths, an increase of one case since last week's update. Response officials continue to emphasize that the outbreak is growing faster than containment efforts can currently keep pace with.
The same challenges remain: delayed case detection, incomplete contact tracing, insecurity, limited treatment capacity, and difficulties conducting safe burials in affected communities. WHO officials have warned that the outbreak is still outpacing the response despite substantial international support.
This week also saw the first Ebola case identified in France linked to the outbreak. The patient, a physician returning from humanitarian work in the DRC, was isolated immediately upon arrival and is reported to be in stable condition. French health authorities have begun contact tracing and monitoring of exposed individuals. Officials continue to assess the risk to the broader public as very low.
The outbreak remains especially concerning because Bundibugyo virus currently has no approved vaccine or specific treatment.
MV Hondius hantavirus investigation
The MV Hondius Andes hantavirus cluster remains unchanged at 13 cases and 3 deaths.
This week's major development is that the U.S. monitoring effort has officially ended. The final passengers housed at the National Quarantine Unit in Omaha completed their 42-day monitoring period and were released without developing hantavirus disease. All 18 U.S. passengers who underwent monitoring have now completed follow-up successfully.
CDC has announced that it is concluding its outbreak response related to the voyage. Health officials continue to emphasize that the risk to the general U.S. public remains extremely low.
While the outbreak resulted in fatalities and required an unusually large public health response, no evidence of sustained transmission occurred among monitored U.S. passengers. Operationally, this outbreak can now be considered largely closed unless new information emerges.
New World screwworm remains an active animal health and agricultural concern in the United States.
The number of confirmed U.S. animal cases has continued to increase. USDA reporting now places the total at 19 confirmed cases, all associated with Texas except for a previously reported dog case in New Mexico. Recent detections include additional livestock cases in west Texas.
Federal and state agencies continue surveillance, quarantine enforcement, sterile fly release operations, animal movement controls, and treatment guidance. The primary objective remains preventing reestablishment of New World screwworm within the United States.
Veterinarians are also increasingly emphasizing the risk to pets. While livestock remain the primary concern, recent public advisories have highlighted the importance of monitoring wounds in dogs and other companion animals, particularly in affected areas.
Current Impact: Southern livestock import restrictions remain in place, surveillance continues to expand, and sterile fly release operations remain active across affected regions.
Economic Impact: Most current costs continue to stem from containment efforts, surveillance, inspections, movement controls, and trade restrictions rather than direct livestock losses.
Potential Impact: If New World screwworm were to become reestablished in the US, the consequences could extend beyond livestock production to wildlife management, companion animal health, interstate animal movement, and international trade. The longer-term economic consequences would likely far exceed the costs currently being incurred for containment.
My July 1 episode will take a deeper look at the biology, history, and current response to New World screwworm.
Postscript
Thank you for subscribing. 🫶
Last week's episode explored one of my favorite things: summer rain. This week's issue is about another.
I've always loved nightfall. Not quite day. Not quite night. The hour when the first stars appear and the world transforms. It's peaceful right up until you remember that many of the creatures becoming active at that moment are looking for something to eat. But that's half the fun.
Speaking of which, I'm also beginning to ramp up preparations for October's Month of the Macabre episodes. This year will be the second Month of the Macabre and, somehow, October also marks the first anniversary of Outbreak After Dark. It will be a fun month here at Infectious Dose.
For now, enjoy the summer evenings—and don't forget the mosquito repellent.
— Heather
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