President Trump has threatened that his administration will put tariffs on lettuce from Mexico, in an effort to control a parasite

During an Oval Office press conference, a journalist asked Trump when it would be “safe to eat lettuce again”. Trump responded: “I think we'll put a major tariff on Mexico because of the lettuce. And we'll put a big tariff on Canada because of the smoke.”

There is believed to be a parasite on lettuce imported from Mexico making Americans ill. It is potentially cyclosporiasis, which causes severe diarrhea and has been reported in all but four states.

The Centre for Disease Control says there has been 4,173 reported cases across the country since May 1, and an additional 7,400 cases that haven’t yet been confirmed.

The outbreak, which is transmitted through food contaminated with human faeces, has been linked to shredded lettuce from Taylor Farms, sent to Taco Bell chains. Taylor Farms has a voluntary recall in place and has temporarily closed its Mexico site.

A spokesman for Taylor Farms said on Thursday: “We’ve commissioned independent experts to conduct a top-to-bottom review of food safety processes and protocols in our central Mexico facility. We continue to work with health authorities, customers, and independent farming partners to address this issue.”

The health and human services secretary, Robert F Kennedy Jr, said this week that the outbreak was “under control”, but there has been a second outbreak of the parasite that is potentially unrelated to the first.

Health officials are recommending people wash their hands and fresh produce thoroughly before preparing food. They are also advising cooking food to at least 70C to kill the parasite.

Trump also joked about putting a tariff “on Canada because of the smoke”. This is in reference to wildfires causing air pollution. On July 15, the Canadian city of Toronto had the worst air pollution in the world.

Nearly 34,000 lightning strikes led to the series of wildfires in British Columbia this week, prompting 25 evacuation orders and 139 active fires. Smoke from these fires moved down into the US’ east coast and midwest cities.

The Canadian government mobilised more than 5,300 firefighters and about 300 water bombers, helicopters and evacuation aircraft to battle the fires. Mexico lended a hand by sending 200 firefighters.