What is the primary reason for most cases of plague in California?

Plague in California: Transmission and Ecology

Most cases of plague in California occur due to recreational activities in areas with infected wild rodents and their fleas, particularly in coniferous forest habitats of the Sierra Nevada and Southern Cascade mountains. 1, 2

Epidemiology of Plague in California

California accounts for approximately 9% of all plague cases in the United States, with the majority (80%) occurring in the southwestern states of New Mexico, Arizona, and Colorado 1. The disease in California has distinctive characteristics:

• Plague in California is primarily maintained in wild rodent populations and their fleas in specific ecological niches 2, 3
• Unlike other states where cases are often acquired near residences, California cases are more frequently associated with recreational activities in endemic areas 1
• The disease follows a seasonal pattern, with most human cases occurring during summer months when flea activity is highest 1, 4

Key Ecological Factors in California

Host Species

• The primary rodent reservoirs in California's coniferous forests include:
  • Chipmunks (Tamias senex and T. quadrimaculatus)
  • Pine squirrels (Tamiasciurus douglasii)
  • California ground squirrels (Spermophilus beecheyi) 2, 3

Vector Species

• The most commonly infected flea species in California include:
  • Ceratophyllus ciliatus and Eumolpianus eumolpi (from chipmunks)
  • Oropsylla montana and O. idahoensis (from ground squirrels) 2

Geographic Distribution

• Plague is endemic in 15 of California's 58 counties 5
• High-risk areas include coniferous forests in the northern Sierra Nevada and Southern Cascade mountains 2, 3
• Climate modeling suggests plague risk may shift northward along the coast and Sierras by 2050 due to climate change 3

Transmission Dynamics

The persistence of plague in California depends on complex interactions between:

1. Host resistance: Some rodent species (particularly chipmunks) show moderate resistance to plague, allowing them to survive infection and serve as reservoirs 2

2. Multiple flea vectors: The presence of multiple flea species with varying competence levels helps maintain the disease cycle 6

3. Environmental conditions: California's diverse ecological zones provide ideal conditions for both rodents and fleas 2, 3

4. Epizootic cycles: Plague persists through cycles of intense activity (epizootics) followed by periods of quiescence 6, 5

Public Health Implications

The ecological characteristics of plague in California create specific challenges:

• Recreational exposure in wilderness areas makes prevention more difficult than in residential settings
• The wide geographic distribution of plague foci requires extensive surveillance
• The potential for large-scale epizootics, as demonstrated in Kern County in 1995, where plague was detected across 500-600 square miles 5

Prevention Strategies

To reduce plague risk in California's endemic areas:

• Avoid rodent-infested areas during recreational activities
• Apply insect repellents when in endemic regions
• Maintain surveillance of sentinel species (chipmunks, ground squirrels)
• Continue carnivore surveillance programs, which effectively track plague activity 4, 3

The unique ecological characteristics of plague in California, particularly its association with recreational activities in specific forest habitats, explain why this state maintains endemic plague despite having fewer cases than southwestern states.