Last year at this time, Minnesota turkey growers were in the middle of a full-blown disaster, the likes of which they’d never seen.

Between March 4 and June 5, the highly pathogenic H5N2 bird flu claimed about 5 million turkeys and 4 million egg-laying chickens on more than 100 farms in the state. Minnesota, the nation’s No. 1 turkey producer, lost 10 percent of its production.

This year? Nothing.

All growers are back at full production, except for one egg farm. The trepidation has not yet gone away, though. Farmers live with the fear of it happening again, and also the expense of taking security precautions in hopes of keeping the flu at bay if it reappears.

The University of Minnesota Extension reported that lost turkey and egg production and processing because of bird flu cost the state’s economy nearly $650 million in 2015, including about $240 million in direct losses to poultry producers, $65 million in direct processing losses, and $170 million in lost wages, salaries and benefits.

Despite those costs, scientists don’t know for sure how the bird flu reached Minnesota and how it spread so quickly. The virus showed up in the Pacific Northwest in December of 2014 and jumped to the Midwest in spring. It killed nearly 50 million birds in 21 states, mostly commercial poultry. Only Iowa — where 29 million birds died, mostly chickens — was hit harder than Minnesota.

Viruses are always changing, said Minnesota Board of Animal Health Assistant Director Beth Thompson, and it’s clear that some part of the H5N2 virus originated in Europe and Asia, and likely spread through migrating birds.

“What type of birds, and whether it’s the fall migration or the spring migration, there are still a lot of questions about that,” said Thompson. “Across the country everyone is still looking at the virus and trying to understand what happened.”

That’s not comforting news to John Burkel, a turkey grower in northwestern Minnesota’s Roseau County who walked into one of his barns last April and sensed immediately that something was different.

Burkel lost 26,000 toms and hens. He estimates that his actual production loss was 50,000 turkeys because of the flu and the subsequent down time — about two-thirds of his normal production for a year.

“It’s surreal to think that it even happened,” Burkel said. “Other than the fact that when you go to your barn now and open your door, and you remember what it looked like, and you just kind of hold your breath and say ‘I hope they look good today.’ ”

Minnesota has about 1,000 poultry farms, said Steve Olson, executive director of the Minnesota Turkey Growers Association. The H5N2 virus was confirmed at 105 turkey farms, four egg-producing chicken farms and one backyard flock.

Most of the affected producers lost one-third of their normal production, Olson said. The U.S. Department of Agriculture indemnified farmers for the loss of their birds and cleanup costs, he said, but not for the weeks of down time when their barns needed to be cleaned, disinfected and inspected before restocking.

Animal health scientists generally believe that the bird flu is transmitted by migrating waterfowl, usually through their feces. Bird droppings can contaminate farm fields, and the virus can survive cold temperatures in soil.

A study by the University of Minnesota Center for Animal Health and Food Safety released in January concluded that tilling of fields near turkey farms was a risk factor during the early part of the outbreak, but that other factors may have helped its “lateral spread.”

Among other vectors, the flu could have spread via windblown particles of soil, or by poultry workers, trucks and farm equipment traveling from farm to farm.

That’s why many turkey producers have ramped up biosecurity since last year, including measures commonly used in the swine industry to prevent disease. A method called the “Danish entry” establishes a line of separation between everything outside a barn that is considered dirty, and everything inside that is clean.

There are many variations, but generally the strategy involves cleansing. Workers enter the work room of a poultry barn, step into a disinfectant, then wash their hands and change from outside boots and coveralls to inside footwear and clothing. Only then can they move farther inside the barn to care for the birds.

Many turkey producers previously posted warnings at their gates for visitors, said Sally Noll, professor and extension poultry specialist. But the highly pathogenic bird flu showed that a more rigorous layer of protection was needed for individual barns. The costs of implementing cleaner procedures, signage and other measures varies widely and can cost many thousands of dollars, depending on the size of operations and how many changes are made.

Robert Orsten lost 32,000 birds to the flu on two of four farms that he owns with his brother in the Willmar area. Since 1967 their family business has produced turkey eggs for sale to hatcheries. Orsten said the birds that were euthanized last year would have produced 3.2 million eggs, and that his losses exceeded $500,000.

Orsten remembers sitting down with his brother at a restaurant last May after learning that their second flock was infected.

“I said that if we have any thoughts about getting out of the turkey industry, right now is the time because we’ll need to spend a lot of money for biosecurity improvements,” Orsten said. “We just both looked at each other and said that this is what we do. We’re turkey farmers. So we need to pull through and make it. I was pretty emotional.”

Orsten has built an enclosed walkway connecting one of his barns and a separate building where eggs are taken to be sanitized and stored before shipment, so that workers never need to go outside during their shifts. He no longer brings straw into the barns twice a week to replace turkey bedding, but uses artificial material that can be cleaned and stays in the barn.

Orsten also purchased extra lawn mowers and skid loaders, so that no equipment moves between his farms anymore. And he’s posted warning signs and installed locks on every door of every barn. The total investment, he said, has been $1.4 million.

Olson said no one knows when or whether the bird flu may return to Minnesota, but he’ll feel a little less anxious when mid-June arrives, because the virus does not survive in warmer weather.

“Ultimately what we learned is that when flocks are affected, we need to depopulate within 24 hours,” he said. “We are now better prepared to put them down quicker so that the virus is less likely to spread to other farms.”

The state and USDA now have the equipment to do that, and agencies have updated their emergency notification procedures and response duties, Thompson said. “If something did hit tomorrow, we have people who have specific jobs, and we wouldn’t be waiting for people from USDA and other states to come and help us.”