West Nile Virus had never been seen in North America when it appeared in New York City in 1999.

By the end of 2002, the mosquito-borne virus had spread to the Pacific Coast and in that one season caused 4,156 cases of human disease. More than half the cases involved brain swelling and meningitis, and 284 people died.

West Nile’s sweep from the warm, wet sewer drains of New York across the arid West in three years surprised epidemiologists. But not Oregon State University plant pathologist Chris Mundt.

The disease had spread in the same pattern as a wheat fungus he studies on experimental plots near Madras, as well as a corn blight that hit the United States in 1970 and the potato blight of 1845. The more time passes, the greater the jumps between infection sites. “What happens there is that distance actually increases exponentially over time,” he said. “It’s more or less doubling every generation.”

The pattern boils down to the same formula that predicts the diffusion of light from a single source, Mundt said, and it’s much different than the one scientists looked to in the past. The classic model described disease marching from the initial infection site at equidistant intervals. It didn’t explain how wind-blown spores of wheat stripe rust, or avian influenza, carried by migrating birds, could spread across continents.

Just how far a disease ultimately travels depends on its infectiousness, plus other conditions, such as susceptibility of the hosts and major changes in climate or geography, he said.

A far-reaching epidemic arises when a highly contagious pathogen meets a susceptible host under favorable weather conditions.

“This is not at all meant to be something that describes every possible scenario,” Mundt said. “What it does is it sets up the worst-case scenario. And that’s what we have to worry about.”

Viruses for which no vaccine is available are emerging, mutating and reaching North America all of the time. The United States saw its worst outbreak of avian influenza yet in 2015. While there were no human cases here, the World Health Organization has counted 826 since 2003 in east and central Asia and Africa.

The Centers for Disease Control is monitoring chikungunya, a mosquito-borne virus that until 2013 was not considered local to the Americas. The first U.S. local case, meaning local mosquitoes carry the disease, was found in Florida in 2014.

The CDC is also monitoring Zika, another mosquito-borne virus that’s been linked to birth defects in the babies of Brazilian women who were infected while pregnant.

“What about the many, many other ones that are circulating in other parts of the world and haven’t moved into these parts of the world?” said Heidi Brown, a professor of epidemiology and biostatistics at the University of Arizona. “Are they going to?”

Brown was part of a team that won a contest, sponsored by the Defense Research Advanced Projects Agency, or DARPA, to develop the most accurate chikungunya prediction model. The government would like to see disease modeling refined to the point that it’s like weather forecasting, but the science has a long way to go, Brown said.

Mundt is not so much interested in predicting how many cases of a given disease will arise, or where exactly it will go, but instead hopes to find a rule of thumb for controlling the spread. He landed a $5 million grant from the U.S. Department of Agriculture through a joint program with the National Science Foundation and National Institutes of Health to study wheat stripe rust in collaboration with experts in three other diseases: sudden oak death, which spread from California to Oregon and has the potential to bring drastic change to coastal forests; foot-and-mouth disease, which is highly contagious among livestock; and a group of insect-borne viruses that includes West Nile virus.

The notion that a single rule of thumb could apply to a crop fungus as well as to a human disease seems far-fetched to Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the Univerity of Minnesota. Bird flu behaves differently from Middle East Respiratory Syndrome (MERS) because they’re caused by different viruses, he said. “That’s part of the challenge of the modern disease world,” he said. “Each of these are going to be very different.”

Osterholm, who is concerned about a flu pandemic, thinks the best approach is to concentrate on the diseases that could do the most harm, rather than those that are just exotic. “We clearly need a game-changing flu vaccine,” he said. “We have lots of candidates right now, but nothing close to going to phase-three trials.”

The fact that vaccines can’t keep up with emerging threats is why it’s important to look at other ways of controlling outbreaks, Mundt said. In agriculture, diseases are controlled through quarantine and by culling — that is, killing — plants and animals adjacent to an infected host.

While that’s obviously not an option for controlling human disease, the field of public health could benefit from the level of precision that’s possible in plant science, simply because plants can’t move, said Samuel Scheiner, director of the National Science Foundation’s ecology and evolution of infectious diseases program, which is funding Mundt’s project.

Scientists know, for example, that how closely individuals are quartered affects the spread of disease, Scheiner said, so when there’s a flu outbreak, kids are kept home from school. Beyond that, there aren’t more specific guidelines. “We know as a general principle that structure matters,” he said. “It’s been more difficult to figure out exactly how structure matters.”

One idea arising from Mundt’s previous work with wheat is that early intervention at the focus site is more important than extensive culling. Killing off entire crops or herds of livestock can have huge economic consequences. During the foot-and-mouth epidemic of 2001, for example, the British destroyed millions of livestock, but it might have been better to spend the time and human resources tracking down the source of the outbreak, he said.

During human disease epidemics, public health resources are split between isolating the source and responding to new cases, Mundt said. In the new five-year study, Mundt will have multiple collaborators looking at the same question of timing versus extent of intervention as it applies to different disease outbreaks. “I think what this project could do is it could do a better job of convincing people … it really is all in the timing,” he said. “It’s going to be really important that we really emphasize making that happen.”

Brown said she hopes the Oregon State project is successful because public health officials need more specific guidance, such as “Where do I stockpile my Tamiflu?” she said. “Where do I put those resources, and when do I move them? That would be phenomenal.”

Other researchers are using mathematical models to compare containment strategies.

Neil Ferguson at the Imperial College of London found that to head off a bird-flu pandemic, countries would need stockpiles of antiviral medication, plus vaccines, according to a 2005 study published in the journal Nature. Quarantining entire households of infected individuals and closing schools could buy time to acquire medication, but closing borders would do very little, Ferguson found.

Mundt, 58, has been studying wheat since he arrived at OSU 30 years ago. Most of his time is spent giving talks to farmers and working with the wheat breeding program to develop disease resistant varieties of one of Oregon’s most valuable crops. Grown primarily in Central and Eastern counties, Oregon wheat was valued at $368.2 million in 2013.

To study stripe rust, Mundt has to plant varieties of wheat that are no longer in commercial production, so his experiments won’t ruin surrounding farmers’ crops. This year he has two sets of plots on two different fields he rents from a farm in the Madras area.

Stripe rust spread by the airborne spores of a rust-colored fungus that causes yellow stripes on the leaves of the plant. Plant breeders kept it under control for close to 40 years with genetically resistant varieties of wheat, but in the past decade stripe rust has moved around the world, so it’s turning up in warmer wheat-growing areas like Kansas and Texas, Mundt said. “These are much more aggressive than the older races and also much more adaptable to extremes of temperature,” he said.

Oregon had a bad epidemic in 2010, and fortunately, Mundt said, ag scientists learned from that experience and refined the recommended timing of fungicidal sprays. “If we hadn’t had fungicide in 2011, we would’ve lost at least 60 percent of the crop in the entire state.”

Traditionally, ag scientists talk about eradicating disease, but Mundt is starting to wonder whether that’s possible for wheat rusts. Eradication efforts extend farther and farther, yet cases of rust persist. And then the spores are airborne again. “We’re kind of shocked at how far out you find cases,” Mundt said.

Again, the bridge to human health could boil down to math. Many of the models that are supposed to predict the U.S. invasion of some little-studied virus are based on assumptions, Mundt said. Historically, the epidemiology of human disease has not dealt with pathogens that disperse at long distances, whereas that’s the most common pattern of dispersal in the plant world, he said. “We’ve got a lot more data on the plant pathogens.”

— Reporter: 541-617-7860, kmclaughlin@bendbulletin.com