Plague Causes

The bacteria that cause plague can be transmitted from a host such as a rat to a human through the bite of an animal or insect (such as a flea). These bites transport the disease. The animal or insect that spreads the disease is referred to as a vector. More than 200 different rodents and other species can serve as hosts. Hosts can include domestic cats and dogs, squirrels, chipmunks, marmots, deer mice, rabbits, hares, rock squirrels, camels, and sheep.

The vector is usually the rat flea. Thirty different flea species have been identified as carriers of the plague. Other carriers of plague include ticks and human lice. Transmission can also occur when someone inhales plague-infected organisms that have been released into the air. The inhalation form of the plague can be aerosolized, as in acts of terrorism. People infected by pneumonic plague can transmit air-borne plague in the form of coughed droplets. Close contact with plague-infected tissue or fluid can also transmit plague.

• Types of plague:
  
  
  • Bubonic plague: The bacteria that cause plague can thrive and grow in the flea’s esophagus. This crowding of bacterial growth prevents food from entering the flea’s stomach. To overcome starvation, the flea begins a blood-sucking rampage. Struggling to swallow, the flea vomits the plague-causing bacteria into the victim’s skin during a bite. The germs invade nearby lymph glands in the bitten animal and produce an inflamed lymph node called a bubo. The plague spreads along the lymph system to every organ. In rare cases, plague spreads to the covering of the brain. Severe illness follows. Bubonic plague has a 1-15% death rate in treated cases and a 40-60% death rate if left untreated.
    
    
  • Pneumonic plague: Direct inhalation of the plague-causing germs results in pneumonic plague. Severe illness follows. The death rate for pneumonic plague is 100% if not treated within the first 24 hours of infection. Plague bacteria may be released into the air as a weapon of biological warfare or terrorism causing this type of the disease, or plague may be contracted through the inhalation of droplets coughed from the lungs of a person with pneumonic plague.
    
    
  • Septicemic plague: This form causes severe blood infection throughout the body. It may occur quickly if a person is bitten in the mouth or throat area (primary). This type of plague can also develop from one of the other types of plague (secondary). Septicemic plague has a 40% death rate in treated cases and 100% in untreated cases. 
  
  
• Risk factors: The following conditions may increase the likelihood of a person contracting a plague infection.
  
  
  • Living in a rural area and especially in areas where plague is common
    
    
  • Having contact with sick animals, small rodents, or other possible hosts
    
    
  • Participating in wilderness activities (such as camping, hiking, sleeping on the ground, hunting)
    
    
  • Exposure to flea bites
    
    
  • Exposure to naturally occurring plague in the community
    
    
  • Employment as a veterinarian
    
    
  • Outdoor activity during the summer months
    
    
• Travel: Anyone who has traveled recently in the southwestern and Pacific Coast regions of the US, particularly in New Mexico, Arizona, California, and Utah, might have had a flea bite. Although contracting plague while visiting another country is rare, doctors may suspect that a flea might have bitten a patient with plaguelike symptoms who has recently traveled abroad to areas where plague is present. Fewer than 10% of flea-bitten people remember a flea having bitten them.
  
  
• Animal contact: Close contact with infected animals and travel through rural areas are risk factors for contracting plague. Historically, rats have been the principal hosts of the plague. Currently in the US, ground and rock squirrels are the most common hosts. In recent years, the domestic cat has emerged as a prominent host of fleas that transmit the plague to veterinarians.