When school superintendent David Hile saw a television report on the Brussels terrorist attacks last week, he knew he had to make an unpopular move: Cancel a high-school trip to Europe that was scheduled to leave two days later.

"I immediately knew we had to cancel," said the administrator of Licking Valley High School, based in a suburb outside Columbus, Ohio.

The March 22 bomb blasts in Brussels, which killed at least 35 people and wounded more than 300, came four months after the deadly terror attacks in Paris had already reduced travel to Europe, by far the most-visited region in the world.

The impact of the Brussels terror onslaught was immediate. Within days, occupancy rates at some hotels in the city plunged to just 25%, according to preliminary estimates from STR, a company that tracks hotel industry data. The rate had been 82% the night before the bombings.

Other European cities felt ripple effects. Hotel-occupancy rates in London slid to 58% by last Thursday, and those in Paris fell to 67%, according to STR. Those figures, based on a sample of hotels in each city, represent declines of 24.7 percentage points and 15.1 percentage points, respectively, compared with rates a year earlier, the company said.

For Europe, a thriving travel industry has been crucial in sustaining the region's fragile economic recovery. Hospitality and tourism make up about a 10th of the region's gross domestic product, economists estimate.

The industry was already bracing for slower growth after Paris: The United Nations' World Travel Organization in January forecast a 3.5% to 4.5% rise in international tourist arrivals in Europe this year, below the 5% increase in 2015. U.K. travel firm Thomas Cook said last week that its summer bookings were 5% lower than last year.

"The uncertain geopolitical environment is causing some customers to postpone booking their holidays," Chief Executive Peter Fankhauser said.

The State Department last week issued a strongly worded advisory, warning Americans of the "potential risks" of travel in Europe--its first continentwide alert since 2010. It cautioned U.S. citizens that "terrorist groups continue to plan near-term attacks throughout Europe, targeting sporting events, tourist sites, restaurants and transportation."

The U.K. lifted its warning against Britons traveling to Brussels and told its citizens to follow the advice of Belgian authorities. Belgium lowered its terror alert level to the second-highest level on Friday.

While less of a tourist destination than Paris, Brussels is an important transport hub. It is home to the North Atlantic Treaty Organization and European Union institutions, and a key overnight stop for business travelers.

Up to 1,500 people from the plastics-recycling industry were gathering in Brussels on March 22 for a conference that had already been pushed back after the Paris attacks, when the Belgian capital went into lockdown amid a terror alert.

The organizers decided to cancel the event a second time, despite having to incur a loss.

"We didn't think it was right to proceed," said Steve Crowhurst, managing director of Crain Communications, which runs the trade show. "The money side had to take a back seat."

The targets of the Brussels attack--the main airport and its subway system--could also instill greater fear among travelers about other potential targets, say industry specialists.

"It is a different story now," said Wolfgang Arlt, director of the China Outbound Tourism Research Institute in Hamburg. "People are seeing Europe as not being safe anymore, and for the Chinese, they don't differentiate between France, Italy, Germany or Spain. To them, all of Europe isn't secure."

The number of Chinese travelers staying in Paris hotels dropped 3.1% in December from a year earlier, according to the Paris Convention and Visitors Bureau, after having risen 46% over the first 10 months of the year.

Hotel consultancy MKG Hospitality said the Paris average revenue per available room--a measure for the hotel sector's health--was down 13% over the first two months of 2016 compared with a year earlier.

The impact from the Nov. 13 Paris attacks was felt around the world. Priceline Group, which owns sites including Booking.com, said its annual growth in global hotel-room sales was 10% lower in the weeks afterward. Airlines bookings declined and passengers booked flights closer to their departure dates. Air France-KLM SA, Europe's largest airline by traffic, said it lost out on EUR120 million ($134 million) in possible sales, although bookings were starting to recover at the end of the year.

Retailers, especially luxury shops that depend on international tourist shoppers, have also experienced slowdowns. Axel Dumas, chief executive at Hermes International SCA, said last week that traffic in its Paris stores this year remained below levels from a year earlier.

Mr. Hile said Licking Valley High School's trip had already been modified in response to the Paris attacks. France was dropped from an itinerary that was to include the U.K., and 11 out of 20 people--students, staff and accompanying adults--dropped out due to security concerns.

While he wouldn't rule out student trips to Europe in the future, he predicted it would take a long time before he would feel their security could be assured.

"We hear there are over 400 terrorists in cells in Europe," Mr. Hile said. "That's going to have to be rooted out before we can feel safe.

Write to Jason Chow at jason.chow@wsj.com, Nick Kostov at Nick.Kostov@wsj.com and Sam Schechner at sam.schechner@wsj.com