Above 10 New World screwworm infestation cases in the United States during 2026 [Subquestion: Above 10]

probability: 1%

rationale:

Summary The likelihood of the United States recording more than 10 human cases of New World screwworm (NWS) in 2026 is extremely low. As of mid-June 2026, the US has reported exactly zero human cases for the year cdc.gov. Historically, human NWS infections in the US are exceedingly rare; the country was declared free of the pest in 1966, and only one human case has been confirmed in recent decades—a 2025 travel-associated infection in a patient returning from El Salvador 3 sources. While an active animal outbreak in Texas and New Mexico began in early June 2026, it remains very small, with fewer than 10 confirmed animal infections nationwide 44 sources. To reach the threshold of 11 or more human cases by the end of 2026, the US would need to experience either an unprecedented failure of current containment protocols leading to tens of thousands of domestic animal infections, or an inexplicable, massive surge in travel-associated cases. Evidence from the ongoing outbreak in Central America and Mexico—which has recorded over 185,000 animal cases but roughly 2,100 human cases—demonstrates that human infections typically represent 1% or less of total cases 2 sources. Given the superior hygiene standards, rapid healthcare access, and proactive containment measures in the US (such as sterile fly releases and quarantines) 2 sources, translating the current US animal outbreak into a double-digit human caseload within six months is highly improbable. Consequently, the baseline risk to the public is correctly assessed as low by health agencies 3 sources.

Strongest Arguments for Yes • Recent domestic emergence: In early June 2026, NWS was detected in domestic animals in Texas and New Mexico, signaling that the flies are now circulating locally and presenting a novel domestic exposure risk 3 sources. • Massive regional outbreak: Central America and Mexico have documented over 2,100 human cases and 185,000 animal cases cdc.gov, significantly increasing the pool of potential travel-associated cases entering the US. • Enhanced surveillance and reporting: Public health reporting systems are heavily sensitized, with the CDC instructing providers to report all suspected cases immediately cdc.gov. This means mild or isolated human cases that might have historically gone unnoticed are highly likely to be counted.

Strongest Arguments for No • Zero cases midway through the year: Despite the regional surge and the new domestic animal outbreak, there have been exactly zero locally acquired or travel-associated human cases reported in the US over the first half of 2026 cdc.gov. • Highly unfavorable human-to-animal case ratio: In heavily endemic areas, human cases constitute around 1% of total infections 2 sources. The US currently has under 10 animal cases ksat.com, mathematically falling far short of the base rate required to organically generate 11 human cases. • Aggressive domestic containment: Federal and state authorities have rapidly deployed robust interventions, including quarantine zones across multiple Texas counties, activation of the National Veterinary Stockpile, and large-scale sterile fly releases designed to eradicate the local population 3 sources. • Protective structural factors: Broad access to modern wound care, indoor living conditions, and higher hygiene standards in the US naturally suppress the vulnerability of human populations to screwworm myiasis.

Key Uncertainties • Trajectory of domestic containment efforts: If sterile fly releases and quarantines fail to halt the spread, an explosion of thousands of animal cases could materially elevate the human risk. Conversely, swift eradication of the US animal outbreak would firmly cap the human risk near zero. • Volume of travel-associated imports: An unexpected, concentrated spike in infections imported by travelers returning from endemic zones in Mexico or Central America could bypass domestic containment and push the US total toward the threshold. • Potential spillover into wildlife: If the infestation establishes itself in wild animal populations, it would become significantly harder to monitor and contain, increasing ambient environmental fly circulation and the corresponding risk of human exposure over the remainder of the year.