The Centers for Disease Control and Prevention (CDC) said it had been notified of 1,031 cases – some people reported international travel while others did not.
At least 578 people did not report international travel so were likely infected in the US and became ill on or after May.
No specific vehicle has been identified and investigations are ongoing.
Search for infection vehicle
CDC said it was too early to say whether cases of Cyclospora infection in different states are related to each other or to the same food item(s).
Previous outbreaks have been linked to various types of imported fresh produce (e.g., basil, cilantro, mesclun lettuce, raspberries and snow peas).
People can become infected with Cyclospora by consuming food or water contaminated with the parasite; it is not transmitted directly from one person to another person.
Symptoms begin an average of seven days to 14 days after ingestion of the parasite. Most people have symptoms for six to seven weeks.
As of late July, 78 cases had been reported in the UK, of which 37 (47%) had travelled to Mexico.
It is the third year in a row that there has been an increase of infections with a link to Mexico.
Canadian cases and historic infections
The Public Health Agency of Canada (PHAC) is investigating locally acquired Cyclospora infections in three provinces with provincial public health partners, the Canadian Food Inspection Agency and Health Canada.
The source has not been identified. Previous outbreaks have been linked to imported fresh produce.
The parasite is most common in tropical and subtropical regions such as the Caribbean, South and Central America, South and South East Asia, the Middle East and Africa.
In Canada, non-travel-related illnesses occur more frequently in the spring and summer months.
A total of 157 cases have been reported in British Columbia (17), Ontario (136), Quebec (three) and Nova Scotia (one) between May and August of this year.
The majority of cases (52%) are female with an average age of 48 years.
Washing produce does not always get rid of the Cyclospora parasite that causes the illness.
Risk can be reduced by cooking produce imported from countries where Cyclospora is found and eating fresh produce grown in countries where the parasite is not common, such as Canada, the US and European countries.
In 2016, the UK had 440 cases of cyclosporiasis between June and October and 359 went to Mexico, mostly the Riviera Maya and Cancun regions.
This year cases in travellers returning from the same region were seen also by Belgium and France.
Public health agencies in the UK and Canada noted an outbreak in travellers coming back from Mexico in 2015; 79 cases in the UK and 97 in Canada.