A billion flies are now in the screwworm fight. Will there be enough?

A lot is riding on the shiny green backs of large red-eyed sterile flies that scientists from the United States, Mexico and Panama have been raising.

The US government just released its billionth fly into Texas from a facility that opened in June in Chiapas, Mexico, near the Guatemalan border, and its sister operation in Panama in order to fight a pest that could potentially cause $2.5 billion dollars in damage in the Southwest US alone.

The New World screwworm lays eggs on warm-blooded animals – which can include humans – and its larvae screw themselves into the living flesh, leading to infections and sometimes death. While not considered a food safety issue, it can be a food supply problem.

Screwworm had been considered eradicated from the US, Mexico and most of Central America decades ago . In 2023, to the frustration of cattlemen and officials alike, the flesh-eating fly broke through containment lines in the Darien Gap, a stretch of dense rainforest between Colombia and Panama, and spread north.

Governments in the region launched a massive response, including traps, sniffer dogs, drone surveillance and most importantly, sterile flies. Despite those efforts, New World screwworm infested thousands of animals in Panama and Mexico, and in June, the US Department of Agriculture confirmed the first US cases in years.

Decades ago, the creation and release of sterile flies to interrupt the pest’s breeding cycle was key to driving it out. But scientists said they’re worried the screwworm could threaten wildlife, pets and cattle — the country’s highest-value agricultural commodity — for years, largely because sterile flies can’t be produced fast enough.

Screwworm is squeezing ranchers on both sides of the border

USDA Secretary Brooke Rollins said she is not concerned about the pace of the administration’s efforts.

“The Trump Administration has moved at warp speed to ramp up sterile fly production across Texas, Mexico, and Panama to protect our Great American Cattle Herd,” Rollins said in an emailed statement last week. “Through this all of federal, state, local, and private rancher effort, we have been able to keep cases in America low with only 48 total - 30 in June, 14 in July, and just 4 cases in August. This is a far cry from the common narrative in early June that the herds would be decimated, wildlife would be destroyed and it would take a decade to recover. Right now, we only have three active cases, and not a single case in cattle in over a month.”

There have been fewer than 50 screwworm cases recorded in the US this year, according to the USDA . That may not sound like much, but entomologist Dr. Dave Taylor, who spent years developing methods to defeat New World screwworm with the USDA’s Agricultural Research Service, warned that the numbers probably won’t stay low.

Although the flies live for only about 21 days, an adult female may lay up to 3,000 eggs during her lifespan, and they will grow into adult flies in less than a week, according to the US Centers for Disease Control and Prevention .

There were only 473 confirmed cases in 1971 . The next year, the US recorded nearly 100,000 cases.

“This is the reproductive potential of this fly,” Taylor said. “I’m not saying that’s going to happen this time, but there could be many, many, many more cases at this time next year.”

That is particularly true, he added, if ranchers don’t report all the cases they see in their animals to the USDA, or if cases go undetected in Texas’ wildlife population — a $31 billion industry.

Taylor, who was also former editor-in-chief of the Journal of Medical Entomology, died August 6 at the age of 71. When he spoke with CNN in July, he said he was frustrated, after 40 years of successful work keeping screwworm out of the country, “to see all that progress wiped out in three to four years.”

The only way the US has succeeded in eradicating screwworm is by breeding millions of flies, sterilizing them using radiation and dropping them in areas with infestations. Wild female New World screwworms mate only once, so when sterile males mate with them and they don’t reproduce, the local population dies off.

Even before cases were identified in the United States, Rollins described the need for more flies.

“I went to the president, and I said, ‘This is going to be a massive problem because we need 400 million to 500 million flies per week to beat this back,’” Rollins said in June. “We only have one facility in our entire hemisphere, and it’s only producing 100 million flies per week.”

When the US initially beat back screwworm, it did so by releasing 500 million sterile flies per week in the 1960s and ’70s. That production capacity was lost when the immediate threat passed. The plants closed long ago.

“It’s totally my opinion, but I think it was somewhat overconfidence with the success of the program,” Taylor said.

Brendan Smialowski/AFP/Getty Images

One of the only women left in Trump’s Cabinet has mostly been doing damage control

Taylor had worked with the USDA’s screwworm program until the 1990s, when he was reassigned to work with other biting flies until his retirement about five years ago. His first assignment with the screwworm was with a plant that had been producing sterile flies in Mexican state of Chiapas since 1976.

The plant closed in 2012, eventually leaving only the Panama plant to produce sterile screwworm. “There was no contingency for an outbreak such as this,” he said.

Dr. Joe Annelli, a former senior adviser for the USDA’s Animal and Plant Health Inspection Service Veterinary Services, said the government has often been reactionary about threats like screwworm.

“Unless there’s a fire in front of us, we don’t want to put the effort in to maintaining preparedness,” said Annelli , who also managed USDA’s involvement in the White House Avian and Pandemic Preparedness implementation plan.

Mauricio Palos/Bloomberg/Getty Images

“We don’t value the preparedness aspect of a disease. How do you measure something that doesn’t happen? It becomes difficult to justify continuing to spend money on a situation that hasn’t been a problem.”

The Trump administration is working to rebuild capacity, but it will take time to reach that level again.

Rollins says the nation is moving at “Trump-speed” to fight the pest. At capacity, the facility in Panama can produce 100 million flies a week. The new plant in Mexico should also be able to generate 100 million per week but a senior USDA official told CNN that it takes about six months from the time to facility opened to reach that production level.

Tonelson/iStockphoto/Getty Images

New World screwworm is disgusting. How could it not be a food safety issue?

The USDA also broke ground on a plant in Texas in April. It’s set to open late next year and should be able to generate 100 million flies per week by the end of 2027. By late 2028, according to the USDA , the Texas plant could produce 300 million more flies per week.

In July , the USDA said it would also invest $25 million in a sterile fly dispersal facility in Arizona. The agency did not give a timeline for when that facility would open.

Even when the Texas facility reaches full production, the country would likely still be below the 500 million benchmark that worked before. The facilities in Panama and Mexico do not produce flies exclusively for the US; they are shared with those countries.

And half the flies they produce don’t help because they’re female.

The US is developing an initiative to create genetically altered male-only flies called Novofly, which the USDA says are currently in field trials in Panama.

Male-only flies would be a “tremendous improvement,” Taylor said, since they would double production. And, he said, sterile female flies can be a distraction to the released sterile males. “So if you eliminated those sterile females, the sterile males would more than likely be concentrated upon just the fertile wild females,” and they may not even need as many males to do the job. “In theory, it should be a major, major improvement.”

Fredy Rodriguez/Reuters

What to know about the New World screwworm

A senior USDA official told CNN that the ongoing results from field tests “are great” and that they were optimistic about the outcome.

The sterile fly program complements efforts to monitor wildlife, inspect animals and work with ranchers in areas where the fly has been spotted to encourage regular checks for infestations. The USDA says that the reception from landowners has been positive and that landowners want the flies.

But not everyone is confident the USDA’s effort will solve the problem any time soon.

Joel Angel Juarez/Getty Images

“I don’t think that USDA is equipped with the tools that it needs to do the job,” said Maximillian Seunik, executive director of the Screwworm Free Future, a nonprofit dedicated to advancing science and policy to eradicate the parasite. “There’s a whole round of new technologies that could be fast-tracked, and they need support to do that.”

In January, the USDA launched the Screwworm Grand Challenge, a $100 million funding opportunity to support “innovative projects that enhance sterile NWS fly production.”

Seunik said Novofly would accelerate the effort significantly.

“We just can’t produce enough flies with what we have right now,” he said. “If it was deployed and scaled, it would really ramp up the capacity of the US just to produce flies.”

来自美国、墨西哥和巴拿马的科学家们一直在饲养大型红眼不育苍蝇，而这些苍蝇闪亮的绿色背部承载着很多东西。

美国政府刚刚从位于墨西哥恰帕斯州（靠近危地马拉边境）的一个设施（该设施于 6 月份启用）及其位于巴拿马的姊妹设施向德克萨斯州释放了第十亿只苍蝇，以对抗一种可能仅在美国西南部就造成 25 亿美元损失的害虫。

新大陆螺旋蝇会在温血动物（包括人类）身上产卵，其幼虫会钻入活体组织中，导致感染，有时甚至死亡。虽然它本身并不被视为食品安全问题，但却可能对食品供应构成威胁。

几十年前，人们认为螺旋蝇已在美国、墨西哥和中美洲大部分地区根除。然而，2023年，令牧民和官员们都感到沮丧的是，这种食肉蝇突破了达连地峡（位于哥伦比亚和巴拿马之间的一片茂密雨林）的隔离带，并向北蔓延。

该地区各国政府迅速采取了大规模应对措施，包括设置陷阱、部署搜救犬、开展无人机监控，以及最重要的——投放不育蝇。尽管如此，新世界螺旋蝇仍然感染了巴拿马和墨西哥的数千头牲畜。今年6月，美国农业部确认了多年来美国境内的首例病例。

几十年前，培育并释放不育果蝇来阻断害虫的繁殖周期是消灭这种害虫的关键。但科学家们表示，他们担心螺旋蝇可能会在未来数年内威胁野生动物、宠物和牛——美国价值最高的农产品——这主要是因为不育果蝇的生产速度跟不上需求。

蛴螬正在挤压边境两侧的牧场主。

美国农业部长布鲁克·罗林斯表示，她并不担心政府工作的进展速度。

“特朗普政府已迅速采取行动，在德克萨斯州、墨西哥和巴拿马大力推广无菌蝇的生产，以保护我们伟大的美国牛群，”罗林斯上周在一份电子邮件声明中表示。“通过联邦、州、地方和私人牧场主的共同努力，我们成功地将美国的病例数控制在较低水平，总共只有48例——6月份30例，7月份14例，8月份仅有4例。这与6月初普遍的说法截然不同，当时人们认为牛群将遭受毁灭性打击，野生动物将面临灭顶之灾，需要十年才能恢复。目前，我们只有3例活跃病例，而且一个多月以来都没有牛群感染病例。”

据美国农业部统计，今年美国记录在案的螺旋蝇病例不足50例。这听起来似乎不多，但昆虫学家戴夫·泰勒博士警告说，这个数字可能不会一直保持低位。泰勒博士曾在美国农业部农业研究局工作多年，致力于研发防治新世界螺旋蝇的方法。

据美国疾病控制与预防中心称，虽然这种苍蝇的寿命只有大约 21 天，但一只成年雌蝇一生中可产下多达 3000 个卵，这些卵会在不到一周的时间内发育成成蝇。

1971年仅有473例确诊病例。第二年，美国记录了近10万例病例。

“这就是这种苍蝇的繁殖能力，”泰勒说。“我不是说这次一定会发生这种情况，但明年这个时候可能会出现更多、更多、更多的病例。”

他补充说，如果牧场主没有向美国农业部报告他们在牲畜身上看到的所有病例，或者如果德克萨斯州野生动物种群（一个价值 310 亿美元的产业）中的病例未被发现，情况尤其如此。

泰勒曾任《医学昆虫学杂志》主编，于 8 月 6 日去世，享年 71 岁。他在 7 月接受 CNN 采访时表示，经过 40 年成功阻止螺旋蝇入侵美国的努力，“看到所有这些成果在三四年内被抹杀”，他感到非常沮丧。

美国根除螺旋蝇的唯一方法是人工繁殖数百万只果蝇，用辐射使其绝育，然后投放到虫害肆虐的地区。野生新大陆螺旋蝇雌性一生只交配一次，因此，当不育的雄性与它们交配后无法繁殖，当地的螺旋蝇种群就会灭绝。

甚至在美国发现病例之前，罗林斯就描述了需要更多苍蝇的情况。

罗林斯在六月份表示：“我去找了总统，我说，‘这将是一个巨大的问题，因为我们每周需要4亿到5亿只苍蝇才能控制住疫情。’我们整个西半球只有一个养殖场，而且每周只能生产1亿只苍蝇。”

美国最初遏制螺旋蝇疫情时，曾在上世纪六七十年代每周投放5亿只不育果蝇。但随着疫情威胁的解除，这种生产能力也随之消失。相关工厂早已关闭。

“这完全是我个人的看法，但我认为这有点过于自信了，因为这个项目取得了成功，”泰勒说。

Brendan Smialowski/AFP/Getty Images

特朗普内阁中仅存的几位女性之一，主要负责危机公关。

泰勒曾在美国农业部的螺旋蝇项目组工作到上世纪90年代，之后被调去研究其他叮咬性蝇类，直到大约五年前退休。他最初负责螺旋蝇的研究工作是在墨西哥恰帕斯州的一家工厂，该工厂自1976年以来一直在生产不育蝇。

该工厂于2012年关闭，最终只剩下巴拿马工厂生产无菌螺旋蝇。“对于像这样的疫情爆发，我们没有任何应急预案，”他说。

美国农业部动植物卫生检验局兽医服务部前高级顾问乔·安内利博士表示，政府对螺旋蝇等威胁常常反应迟钝。

“除非眼前发生火灾，否则我们不想投入精力来保持准备工作，”安内利说道，他还负责管理美国农业部参与白宫禽类和流行病防范实施计划的工作。

Mauricio Palos/Bloomberg/Getty Images

“我们不重视疾病预防方面的准备工作。如何衡量一件从未发生过的事情呢？继续在从未造成问题的情况下投入资金，就很难令人信服。”

特朗普政府正在努力重建产能，但要恢复到之前的水平还需要时间。

罗林斯表示，美国正以“特朗普式的速度”应对这种害虫。巴拿马的工厂满负荷运转时，每周可生产1亿只苍蝇。墨西哥的新工厂也应该能够每周生产1亿只，但一位美国农业部高级官员告诉CNN，从工厂投产到达到这个产量水平大约需要6个月的时间。

Tonelson/iStockphoto/Getty Images

新大陆螺旋蝇令人作呕。这怎么可能不是食品安全问题呢？

美国农业部今年4月还在德克萨斯州破土动工兴建一座工厂。该工厂预计将于明年年底投产，到2027年底每周可生产1亿只果蝇。据美国农业部称，到2028年底，这座德克萨斯州工厂每周的果蝇产量有望增加3亿只。

今年7月，美国农业部表示，还将投资2500万美元在亚利桑那州建设一座无菌果蝇散播设施。该机构并未给出该设施的启用时间表。

即使德克萨斯州的工厂全面投产，美国的苍蝇产量可能仍会低于之前设定的5亿只的目标。巴拿马和墨西哥的工厂并非专门为美国生产苍蝇，而是与这两个国家共享。

而且它们产下的苍蝇有一半是雌性，所以对它们没有帮助。

美国正在制定一项计划，旨在培育名为 Novofly 的基因改造雄性果蝇。美国农业部表示，目前正在巴拿马进行田间试验。

泰勒说，只用雄性果蝇会是“巨大的进步”，因为这样产量就能翻倍。他还说，不育雌性果蝇会分散释放的不育雄性果蝇的注意力。“所以，如果消除这些不育雌性果蝇，不育雄性果蝇很可能会集中精力捕食野生雌性果蝇，”而且它们可能甚至不需要那么多雄性果蝇就能完成交配。“理论上来说，这应该是一项非常重大的改进。”

弗雷迪·罗德里格斯/路透社

关于新大陆螺旋蝇，你需要知道些什么？

美国农业部一位高级官员告诉 CNN，田间试验的持续结果“非常好”，他们对结果持乐观态度。

这项不育蝇类防治计划是对野生动物监测、动物检查以及与发现蝇类地区的牧场主合作的补充，旨在鼓励他们定期检查蝇类是否泛滥。美国农业部表示，土地所有者对此反应积极，他们希望引入这些蝇类。

但并非所有人都相信美国农业部的努力能够很快解决这个问题。

Joel Angel Juarez/Getty Images

“我认为美国农业部还没有配备完成这项工作所需的工具，”致力于推进科学和政策以根除螺旋蝇的非营利组织“无螺旋蝇未来”（Screwworm Free Future）的执行主任马克西米利安·塞尼克（Maximillian Seunik）表示，“有很多新技术可以加快推广应用，而这些技术需要支持才能实现这一目标。”

今年 1 月，美国农业部启动了螺旋蝇大挑战计划，提供 1 亿美元的资金支持，以促进“提高无菌 NWS 蝇生产能力的创新项目”。

Seunik表示，诺沃弗莱将大幅加快这项工作。

他说：“以我们目前的条件，根本无法生产足够的果蝇。如果这项技术能够部署并扩大规模，美国的果蝇生产能力将大大提升。”