YAKIMA, Wash. — Exterminator Dan Nissen always got the occasional call about a rat problem, but about four years ago things changed.

That’s when Yakima-based exterminators started getting calls nearly every week, mostly about black rats, or “roof rats,” a species that Nissen, owner of Valley Pest Control, hadn’t seen around here before.

There had always been big, brown rats, or “Norway rats,” and bushy-tailed pack rats out in the country. But the small, black roof rats were something new. Calls about those rats have increased every year since, he said.

Prior to that he had believed roof rats, so-called for their tendency to live in roofs and trees, were limited to parts of Washington west of the Cascades.

“Somehow they were brought here and introduced,” Nissen said. “Their favorite foods are fruit and nuts; they’re in heaven.”

Most of the roof rat calls recently have come from the generally middle or upscale Yakima residential neighborhoods between 16th and 66th avenues and between Englewood Avenue and Nob Hill Boulevard, he said. Bob York of York’s Exterminating believes some areas, such as the Barge-Chestnut neighborhood between 16th and 36th avenues and Tieton Drive and Summitview Avenue, are particularly susceptible because of their density and the number of shrubs and trees.

Indeed, the Barge Chestnut Neighborhood Association sent residents a notice last month saying rats are a growing problem, and urging them to address the issue without leaving rat poison outdoors where neighborhood pets might get into it.

“Look how close the houses are together in the Barge Chestnut neighborhood,” York said. “And there’s so much (plant) growth. They can get an infestation around their house and not even realize it. ... I have one (client) here in town, he pulled all of his shrubs out because of this.”

Like Nissen, York pegged the increase in rat-related calls as starting about four years ago. Prior to that, York, who has been in business for 42 years, rarely got rat calls.

Whether that means the population has actually increased — or species have been introduced as Nissen suspects — is difficult to know. Neither the state Department of Health nor the Yakima County Health District tracks incidents of rat-human interaction. But Gordon Kelly, director of environmental health for the county agency, said he is not aware of any increase in the number of rats in Yakima County. It’s possible that people are seeing them more frequently as development or other environmental factors push them from established habitat into new neighborhoods, he said.

“There’s always a rat population,” Kelly said. “And they’re reclusive. They don’t like people and people don’t like them.”

From a public-health perspective, there certainly isn’t any panic over rats in Yakima County. The Chelan-Douglas Health District last month announced that rat-bite fever was found in several Chelan and Douglas county residents and possibly in a Grant County resident. But that disease, which causes flu-like symptoms and is treated with antibiotics, is exceedingly rare and has not been identified in Yakima County, Kelly said.

“I talked to a nurse that’s been here 25 years, and she has never seen it,” he said.

In fact, those cases in Chelan and Douglas counties are the only ones the Health Department is aware of statewide, said agency spokesman Donn Moyer. Still, there are other diseases that can be transmitted by rat bites or droppings. The department’s website, www.doh.wa.gov includes a page with instructions on keeping rodents away from homes. It’s essentially the same advice exterminators like Nissen and York give: don’t leave pet food or bird seed around, seal up holes, screen ducts and vents, and if you spot them repeatedly, call a professional.

The same goes for mice, Kelly said. They may not be as mean-looking as rats, but they’re more likely to be found in local homes and pose many of the same problems.

“Rats are more of a nuisance,” Kelly said. “I would say mice are more a concern to people from a public-health standpoint.”

• Pat Muir can be reached at 509-577-7693 or pmuir@yakimaherald.com.