The shaky science behind ivermectin as a COVID-19 cure

Studies are inconclusive and misinformation is rampant. But many Americans now see a deworming medicine as a go-to drug to prevent and fight the Delta variant.

Over the last month, Frank Wallmeyer and several other farm supply store owners in some parts of the United States noticed an antiparasitic medication called ivermectin flying off the shelves. At his own store in Jacksonville, Florida, ivermectin sales have nearly tripled, and the phone rings at least a dozen times each day with inquiries about the drug, Wallmeyer says.

But many of those inquiring weren’t looking to get rid of worms in cattle and horse intestines. Rather, they wanted to use the drug for themselves or their loved ones to prevent and treat COVID-19. Touted as a miracle COVID-19 cure by some doctors and campaigners, despite lacking scientific support, ivermectin seems to be in high demand among unvaccinated Americans. As the fast-spreading Delta variant ravages the country, the search for alternative medication has led vaccine sceptics to ivermectin. Although the Food and Drug Administration has approved ivermectin to treat certain parasites in humans and animals, its use against COVID-19 isn’t authorized.

Poison control centers in several states including Florida, Mississippi, and Texas reported a recent surge in calls and cases associated with ivermectin misuse and overdose. Also, the Centers for Disease Control and Prevention reported that in the week ending August 13, 2021, more than 88,000 prescriptions were written for ivermectin, representing a 24-fold increase from the pre-pandemic baseline of 3,600 prescriptions per week. That meant some physicians were prescribing the drug for COVID-19, despite the FDA’s stand.

“It vastly complicates the management of [COVID-19] patients because there are so many and there is so much misinformation,” says John Sinnott, an epidemiologist at University of South Florida’s Morsani College of Medicine who is also affiliated with Tampa General Hospital.

Also, the drug preparations and doses vary for animals and humans, and the FDA warned people of potential harm from consuming the concentrated animal version, which contains inactive ingredients not tested for use in humans.

In an August 21 tweet, the FDA issued a warning: “You are not a horse. You are not a cow. Seriously, y’all. Stop it.”

But even human-grade ivermectin, considered generally safe for approved purposes—worms, head lice, and skin conditions such as rosacea—can cause side effects including headaches, nausea, diarrhea, skin rashes, and spikes in blood pressure. And seizures can result from high doses, leading to hospitalization.

What science tells us

While some studies suggest reduced risk of death, and others hint at fewer COVID-19 patients progressing to severe disease after taking ivermectin at an early stage of infection, the evidence is shaky. “We don't know whether ivermectin is helpful or not in the fight against COVID-19,” says Stephanie Weibel, a biologist at the University of Wuerzburg in Germany. “Trustworthiness of the pool of available studies is limited.”

In a recent review of 14 ivermectin studies, Weibel and her colleagues found that often the trials enrolled few patients or weren’t designed well, sometimes leading researchers to overestimate ivermectin’s impacts. She encourages more robust clinical trials, like the one underway at the University of Oxford in the United Kingdom.

Even Merck, an ivermectin manufacturer, in a February 2021 statement said that its own analysis of the scientific literature didn’t support the drug’s use against COVID-19. However, the argument that supporters sometimes make is that even if ivermectin use may not provide obvious benefits, it can’t hurt.

“If there isn’t evidence that the product works, then any risk the product might convey is unacceptable,” says Peter Lurie, president of the Center for Science in the Public Interest and former associate commissioner at the FDA. “We have people who have gotten ill from ivermectin, who’ve wasted a bunch of money for no proven benefit, and the concern is that ivermectin is diverting people from things that actually work: vaccines, masks, and social distancing.”

Also, ivermectin supporters, who possibly weren’t able to acquire prescriptions for the drug from their physicians, may resort to the animal-version stocked in farm supply stores, not knowing the difference. The recommended dose for animals is much higher, and if people ingest ivermectin at that high dose, they’re likely to be poisoned, says Michael Teng, a health virologist at the University of South Florida.

Some vaccine sceptics are also resorting to ivermectin to prevent getting COVID-19, even though there is no strong scientific evidence to support such behavior and physicians warn against using ivermectin for extended periods. As of now, the FDA states that ivermectin should only be used or prescribed for COVID-19 in a clinical trial setting, which often involves follow-ups and health monitoring for the enrolled participants.

What triggered the ivermectin frenzy

Ivermectin was discovered and developed in the 1970s. In a hunt to find antiparasitic compounds, scientists identified a new species of bacteria—Streptomyces avermitilis—from soil near a golf course in Japan that eradicated worms in mice. The microbes produced therapeutic molecules called avermectins, which were responsible for deworming and which later led to the creation of a commercialized veterinary drug called ivermectin. In 1987, after human clinical trials showed its effectiveness against river blindness caused by the parasitic worm Onchocerca volvulusthe FDA approved the drug for humans in 1996 under the brand name Stromectol.

Since then, ivermectin has come to be recognized as a safe treatment for several tropical diseases caused by parasites ranging from mites to roundworms.

So, when scientists were looking to test safe generic drugs that could be repurposed for COVID-19 treatment, ivermectin made the list.

Among the earliest research included a study published online in April last year that showed how high ivermectin doses prevented the replication of SARS-COV-2, the virus responsible for COVID-19, in test tubes. While the researchers didn’t test the drug for treating or preventing COVID-19 in humans or animals, the study made headlines and piqued the general public’s interest in ivermectin. The FDA quickly issued a warning against its use to cure COVID-19. Two letters sent to the journal’s editor expressed concerns about the high ivermectin dose used in the experiments.

Around the same time, a controversial paper which had not yet been peer reviewed and was later retracted entirely, claimed large reduction in mortality among COVID-19 patients given ivermectin. Although the study didn’t make the cut for publication in a scientific journal, it helped popularize ivermectin in Latin America.

As the efficacy and safety of ivermectin continued to be tested in clinical trials across the world, the results of a November 2020 publication led by Egyptian researcher Ahmed Elgazzar renewed interest in the drug’s potential. The preprint study claimed substantial recovery among COVID-19 patients given ivermectin in the early stages of infection and a reduction in mortality exceeding 90 percent. But ethical concerns led to the paper’s withdrawal in July this year.

“It’s troubling that people are putting their faith in a drug not proven to be effective against COVID,” Teng says. “I wish people would just take the vaccine.”

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