Black plague in California: Sierra Nevada case raises prevention alerts

A resident of El Dorado County, in California’s Sierra Nevada region, was diagnosed with the black plague after being bitten by an infected flea while camping in South Lake Tahoe in August 2025. The patient, whose identity was not disclosed, is recovering at home under medical care. The infection, caused by the Yersinia pestis bacterium, was confirmed by local health authorities, who have intensified rodent monitoring in the area. This is the first case in the region since 2020, prompting alerts about preventive measures for outdoor activities. The case underscores the importance of avoiding contact with wild rodents, using repellents, and protecting pets. Despite its severity, experts emphasize that the disease is rare in the United States, with effective treatment if diagnosed early.

The black plague, historically linked to devastating epidemics, is now managed with antibiotics but requires attention in rural areas. California’s Department of Public Health has identified the bacterium in 45 rodents since 2021, including four in 2024. The situation has spurred educational campaigns for campers and hikers.

• Preventive measures: Avoid rodent contact, use repellents, and keep pets protected.
• Risk areas: Sierra Nevada, Arizona, and New Mexico see most U.S. cases.
• Effective treatment: Antibiotics ensure recovery if administered promptly.

What is the black plague and how it manifests

The black plague is caused by the Yersinia pestis bacterium, primarily transmitted by fleas living on wild rodents like squirrels, chipmunks, and armadillos. Infection can occur through flea bites, contact with infected animal fluids, or, rarely, inhalation of droplets from someone with pneumonic plague. In the United States, human-to-human transmission hasn’t been recorded in over a century. The disease presents in three main forms, each with distinct traits. The bubonic form, most common, causes painful swelling of lymph nodes. The septicemic form affects the bloodstream, while the pneumonic form targets the lungs, being the most severe.

Symptoms appear between two and 14 days after exposure. High fever, weakness, nausea, and muscle aches are early signs, followed by lymph node swelling in the bubonic form. Without treatment, the disease can progress rapidly, risking death. The recent California case was caught in time, enabling recovery with antibiotics.

• Bubonic form: Swollen lymph nodes, fever, and localized pain.
• Septicemic form: Blood infection, risking septic shock.
• Pneumonic form: Affects lungs, transmissible via droplets.
• Early diagnosis: Critical to avoid severe complications.

Risk areas and monitoring in California

The Sierra Nevada region, particularly the Tahoe Basin, is a hotspot for the black plague in the United States. Since 2021, California’s Department of Public Health has monitored wild rodents like squirrels and chipmunks, which may carry infected fleas. In 2024, four rodents tested positive for the bacterium in the area, underscoring the need for ongoing vigilance. Beyond California, states like Arizona and New Mexico also report occasional cases, with an annual average of seven human infections nationwide.

The South Lake Tahoe case, linked to camping, highlights the risks of outdoor activities in forested areas. Local authorities have ramped up awareness campaigns, urging campers to adopt preventive measures. Rodent populations are regularly tested, with samples collected from parks and trails.

Prevention for campers and hikers

For those frequenting natural areas, prevention is the best defense against the black plague. Health authorities recommend simple yet effective measures to minimize infection risk. Avoiding direct contact with rodents like squirrels or chipmunks is crucial, as they may carry infected fleas. Additional personal and pet protection measures are widely promoted.

• Proper clothing: Wear long pants tucked into boots to prevent bites.
• Repellents: Apply DEET-based products to exposed skin.
• Pet protection: Use flea collars and keep pets on leashes.
• Camping hygiene: Avoid leaving food scraps that attract rodents.

These precautions are vital in regions like the Sierra Nevada, where infected rodents are documented. Campers should also inspect clothing and gear after outdoor activities.

History of the black plague in the United States

The black plague, though rare, has never fully disappeared from parts of the United States. Since the early 20th century, when outbreaks were controlled, the disease has been tied to rural areas in the western U.S. California, with its vast forested regions, is among the most affected states, though cases remain low. In 2020, another case was reported in El Dorado, also linked to camping.

The average of seven cases per year reflects the effectiveness of modern healthcare systems, which quickly identify and treat the disease. However, fatal cases, like one in Arizona in July 2025, show the plague can still be deadly if untreated. Ongoing rodent surveillance and public education have been key to keeping the disease in check.

Situation in Brazil and global comparison

In Brazil, the black plague is extremely rare. The last human case was reported in 2005 in Pedra Branca, Ceará, during an isolated outbreak. Since then, the Ministry of Health has reported no new cases, and Brazil maintains surveillance in rural areas. The absence of cases reflects both low bacterium circulation and less favorable climatic and geographic conditions compared to regions like the Sierra Nevada.

Globally, the black plague persists in countries like Madagascar, where seasonal outbreaks occur, and parts of Asia and Africa. In the United States, strict control and antibiotic access keep the disease manageable, but vigilance remains essential.

• Brazil: Last case in 2005, with ongoing rural surveillance.
• United States: Average of seven cases yearly, concentrated in the West.
• Madagascar: Seasonal outbreaks with higher incidence in rural areas.
• Global prevention: Focus on rodent control and treatment access.

Advances in treatment and diagnosis

Early diagnosis is the greatest ally against the black plague. Rapid laboratory tests, such as detecting the bacterium in blood or lymph node samples, allow antibiotic treatment, like doxycycline or gentamicin, to begin within hours. These drugs have a high success rate, with full recovery in most cases. However, delays in diagnosis can lead to severe complications, especially in the pneumonic form.

Medical research is advancing in vaccine development and more precise detection methods. While no widely available human vaccine exists, animal studies show promise. Genomic surveillance of Yersinia pestis helps track mutations and prevent outbreaks.