Invasive 25-pound rodent spreads in Bay Area, posing health and infrastructure threat

SACRAMENTO, Calif. - A disturbingly large, invasive, and destructive rodent, double the weight of a cat, is causing widespread concern as the animal’s population spreads in the Bay Area, threatening infrastructure, the state’s water supply and posing a health risk to humans, pets, and livestock.   

The semi-aquatic mammals, known as nutria, are native to South America. They were first introduced to the U.S., in Southern California’s community of Elizabeth Lake, in the late 1800s as part of the fur trade. 

Often mistaken for a beaver or a muskrat, nutria are sometimes referred to as swamp rats. The rodents can weigh up to 25 pounds and are about 2 feet long excluding the tail.

One defining feature is their bright orange teeth. 

California eradicated the species from the state in the 1970s, but in 2017, a reproducing population of the rodents was discovered in the San Joaquin Valley, according to the California Department of Fish and Wildlife (CDFW).

Since then, the agency has been working to stop a potential infestation and tracking how many it has killed as part of that effort. 

Just this year, more than 1,100 nutrias have been trapped and killed in the state, as the animal’s presence has spread to the Bay Area and been detected in Contra Costa and Solano counties. 

Almost 5,200 have been killed since 2017. 

"In California, nutria are classified as a nongame mammal," said the state wildlife agency, which added that CDFW code allows property owners to kill nutria by legal means to address damage to crops or property. There are restrictions, though, on the types of traps that can be used. 

State wildlife officials said the process to eradicate the rodents involves humane euthanasia, using pellet pistols and following guidelines set by the American Veterinary Medical Association. 

Officials stressed that neither poisons nor pesticides are used to control nutria populations.

The animals are burrowers, making their habitats in embankments, dams, and dikes.

This leads to streambank erosion and can cause widespread damage to critical waterways, irrigation networks, levees and other infrastructure. The animals have even been blamed for roadbed collapses.

Beyond infrastructure damage, nutria’s feeding habits can harm the ecosystem as they threaten endangered species in wetland habitats.

"They can consume up to 25% of their body weight in above- and below-ground vegetation each day, but they waste and destroy up to 10 times as much, causing extensive damage to the native plant community and soil structure, as well as significant losses to nearby agricultural crops," state wildlife officials said on the CDFW website.

And the rodents pose a risk to both people and animals, as they can spread disease-causing bacteria and parasites, threatening our water supply.

"Nutria also serve as hosts for tuberculosis and septicemia, which are threats to humans, livestock, and pets," according to the CDFW. "Additionally, nutria carry tapeworms, a nematode that causes a rash known as ‘nutria itch,’ and blood and liver flukes, which can contaminate swimming areas and drinking water supplies," the agency explained.

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To prevent an infestation of the animal, the department has asked residents in areas where nutria populations have been detected, to provide written permission for nutria survey teams to access private properties to carry out trapping efforts.

CDFW said it has deployed survey teams from Suisun Marsh in Solano and Contra Costa counties, through the Delta, and down the San Joaquin Valley.

The agency said it has also established cameras in the Delta to identify the presence of the rodents, and wildlife agents are using traps to eradicate the infestation.

State figures showed that so far, Merced County has had the largest number of nutria discovered and killed, with 2,527 "taken" since 2017.

In the Bay Area, the CDFW has eradicated at least 486 nutria in Solano County. 

So far, there have been two killed in Contra Costa County as of Thursday.

The Contra Costa County Department of Agriculture has launched into action to try and get the word about the rodents and said it’s working with state wildlife officials to detect and stop the species from spreading. 

"The Nutria is an invasive species that can be destructive to wetland habitat, crops, and can weaken levees to the point of failure. With Nutria in the Delta, it is easier for it to disperse into other regions," the county’s agricultural commissioner Matthew Slattengren told KTVU in an email. 

Populations can explode rapidly, as female nutria can produce their first litter by the age of 8 months. The animals breed year-round, and can produce three litters per year, with litters typically ranging from two to 13 pups, according to wildlife experts. 

Officials urge anyone who believes they have spotted a nutria or notices potential signs of the animal's presence to take a photo and promptly report it to state wildlife officials by emailing them at Invasives@wildlife.ca.gov or by calling (866) 440-9530.

They also warn, "If this species is captured, do not release it." Instead, residents are advised to immediately contact their local CDFW office or alert their county agricultural commissioner.