Recent California headlines have put the spotlight on potentially fatal illnesses that can be transmitted to humans by common pests.
Here’s what to know about the infections and how to avoid them
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Plague
The news: A South Lake Tahoe resident contracted plague this summer, possibly from a flea bite while camping.
How common is it? The Centers for Disease Control says an average of seven human plague cases are reported each year in the United States. It’s most common in Western states, particularly northern New Mexico and northern Arizona.
Is this the same as bubonic plague? Bubonic is the most common of the three forms of plague. The name refers to one of the symptoms, painful lymph nodes.
Symptoms: Early signs include fever, headache, chills, weakness. It is treated with antibiotics, which are effective enough that the plague mortality rate in the U.S. is low — the CDC recorded 15 deaths from 2000 to 2023.
How to avoid it: Plague is caused by a bacterium most commonly transmitted to humans by the bite of an infected flea or through the handling of infected animals. There is no current plague vaccine. The Centers for Disease Control advises using insect repellent while outdoors — DEET on the skin, permethrin on clothes — and treating pets with flea control products.
West Nile disease
The news: This year’s first human case of the mosquito-borne infection in Northern California was confirmed last week. The state’s count is now 17 human cases and one death.
How common is it? West Nile counts vary widely from year to year because of periodic epidemics, the CDC says. The recent average for the U.S. is 2,345 human cases per year and 120 deaths. In California the highest risk is in the Central Valley.
Symptoms: The milder symptoms include headaches, body aches and rash. Some patients experience long-lasting fatigue and weakness, and the most severe cases involve encephalitis or meningitis. There is no specific treatment other than addressing the symptoms.
How to avoid it: Use insect repellent on skin and clothes when outdoors. Take steps to eradicate mosquitoes near your home, such as eliminating standing water.
Lyme disease
The news: Though the tick-borne Lyme disease is still rare in California, its geographic footprint is expanding. According to the latest data, the California county with the highest case count is Santa Cruz.
How common is it? In 2023, there were more than 89,000 Lyme cases nationwide. New York had 22,000, Pennsylvania 16,000 and California only 109.
Symptoms: Early stages present as flu-like symptoms and a rash. The disease is treated with antibiotics. Untreated, it can develop into a debilitating complex of symptoms including severe pain, fatigue and brain fog.
How to avoid it: It is spread by the bite of an infected blacklegged tick or Western blacklegged tick. In California, high season for Lyme disease is late fall to early summer. Use an insect repellent (DEET and permethrin) and check for attached ticks after time outdoors, the CDC says. A tick must be attached for at least 24 hours to transmit the bacteria.
2012: A researcher from UC Berkeley holds a deer mouse he trapped in the Glen Aulin area of Yosemite National Park. (BANG archives)
Hantavirus
The news: Three Mammoth Lake residents died this year from hantavirus, which is spread by mice. It is also believed that Betsy Arakawa, wife of actor Gene Hackman, was experiencing hantavirus symptoms before her death at their New Mexico home.
How common is it? California — which has one of the higher hantavirus counts in the United States — has averaged two or three cases a year since the 1980s, the state public health department says.
Symptoms: In the syndrome most common in the United States, early signs are fatigue, fever and muscle aches, followed by coughing and shortness of breath. There is no specific treatment. The mortality rate in the U.S. is about 35%, the CDC says.
How to avoid it: The virus is spread by contact with rodents (in the U.S., specifically deer mice) or their urine or droppings. People cleaning up rodent nests or droppings, inside or outside, should wear gloves and use a bleach solution or disinfectant.
Valley fever
This disease is spread not by a pest but by a fungus in soil in several Western states.
The news: Valley fever (coccidioidomycosis) has been on the rise in California for several years; cases this year are on track to set another record, the state’s Department of Public Health says.
How common is it? Last year, California had nearly 12,500 cases. Most were contracted in the Central Valley or the Central Coast area, but the fungus has been spreading to other parts of the state. In the first seven months of this year, more than 300 cases were reported in the Bay Area, the public health department said.
Symptoms: Flu-like symptoms related to lung infection. Not every patient needs treatment, and the mortality rate is low. People with weakened immune systems are candidates for antifungal medication.
How to avoid it: Outdoor workers and others who are exposed to dirt in areas where Valley fever is common should be aware of the symptoms and consider wearing an N95 respiratory mask.
