Last week, officials in Dallas County in Texas began spraying insecticide from airplanes to curb the worst outbreak of West Nile virus that Dallas has ever seen. With no options for treating or preventing the disease, officials hope to prevent more people from contracting the potentially fatal illness, which is transmitted through mosquito bites.

Almost half of all West Nile human infections are in Texas this year, according to the Centers for Disease Control and Prevention. Given that it is still early in the outbreak season, this year could be one of the worst on record in the United States. “Last year, there were 700 cases [total], and right now we are already pushing 700, and it’s only coming into end of August,” says John Roehrig, a microbiologist with the CDC’s Division of Vector-Borne Diseases.

Most human infections have no symptoms and go unnoticed and unreported. But about 20 percent of people infected develop flu-like symptoms that can last for days or weeks, according to the CDC. About one out of every 150 people infected will develop neurological effects such as confusion, convulsions, and paralysis, which may become permanent.

But the unpredictable nature of the outbreaks makes testing potential treatments difficult. Scientists can’t accurately predict where the most intense outbreaks will be each year, so drug developers have a hard time setting up trials to test their therapeutics. “It’s very difficult to get the clinical trials done in a way that is defined by federal regulation,” says John Morrey, director of the Institute for Antiviral Research at Utah State University. “If you have a disease that is sporadic or you can’t predict it, or there are not a lot of people admitted to the clinical center, you just can’t do the clinical trial.”

Such was the fate of a treatment that Morrey had been developing with a Rockland, Maryland, biotech company called Macrogenics, which develops antibody-based therapeutics for cancer, infectious diseases, and autoimmune disorders. With National Institutes of Health support, the company had launched a study of the safety and efficacy of an antibody that specifically recognizes the West Nile virus. The team set up trial sites in areas where cases had occurred frequently in past years, but was not able to enroll enough patients to properly test the drug.

With no treatment available, vaccine designers cannot set up controlled trials in which participants would receive either the vaccine or a placebo and then be subjected to the virus. “West Nile is too dangerous,” says Elliot Parks, CEO and president of Hawaii Biotech, a Honolulu-area company developing a vaccine for the disease.

The other way companies normally test a vaccine, says Parks, is to set up a large study in an area affected by the disease to see how natural rates of infection differ between those who receive the candidate vaccine and those who don’t. Such is the case for Sanofi Pasteur’s ongoing trial for a dengue fever vaccine, the efficacy of which is being tested in more than 10,000 children in Thailand, where the disease is prevalent and occurs year-round (see “Dengue Fever Vaccine in Sight”). But for West Nile, such a trial is “impractical, because we never know from year to year where any particular West Nile outbreak will occur,” says Parks.

Instead, his company plans to further demonstrate the safety of its West Nile vaccine in humans and show its efficacy in lab animals that exhibit similar courses of infection to humans.