Legionella pneumophila

Introduction

Legionnaires disease has a 25% mortality rate (Medscape)

Hosts

Human and animal.

Transmission / Exposure Route

Infection normally occurs after inhaling an aerosol (fine airborne particles) containing Legionella bacteria. ^[1] 
Anywhere from 8,000 to 18,000 people are hospitalized each year in the U.S. with Legionnaires' disease. (EH&E) Between 1980 and 1998, 35% of Legionnaires Disease cases met definition of nosocomial infection.^[2] 
Disease is usually associated with man-made environments, such as cooling towers, whirlpools, and building water systems, where warm water (25C–42C) and biofilms support growth and survival of Legionella species. ^[3]

Case Fatality Ratio

Legionnaires disease has a 25% mortality rate. (Medscape) 
Legionnaires' disease can be very serious and can cause death in up to 5% to 30% of cases. ^[5] 
Between 1980 and 1998, the case fatality ratio decreased from 34% to 12% (P<0.001) for all cases, 46% to 14% (P<0.0001) for nosocomial cases, and 26% to 10% (p=0.05) for community-acquired cases. ^[2]

Incubation Period

During an outbreak in the Netherlands the incubation period was reported to be 2-19 days (median 7 days, n=136), 16% of these individuals showed incubation times exceeding 10 days. ^[4]

Burden of Disease

U.S. legionellosis cases reported annually increased 217%, from 1,110 in 2000 to 3,522 in 2009, and the crude national incidence rate increased 192%, from 0.39 per 100,000 persons in 2000 to 1.15 in 2009. ^[6] 
Between 1980 and 1998, the median annual number of cases was 360 cases/year. ^[2] 
Elderly individuals, smokers, and people with underlying respiratory or immunocompromising conditions are at increased risk for Legionnaires disease. ^[7] 

 
Figure 1. No. of cases of legionnaires disease (LD) and rate per 100,000 US residents, by age group, 1980–1998.^[2] (Surveillance data collected by the Center for Disease Control as presented in Benin, Andrea L, Robert F Benson, and Richard E Besser. “Trends in Legionnaires Disease, 1980–1998: Declining Mortality and New Patterns of Diagnosis.”)

Duration of infectiousness and disease

Symptomology

Symptoms include a high fever, chills, cough, muscle or headaches. These symptoms are common to most forms of pneumonia so diagnosis relies on chest X-rays and tests performed on sputum, blood, and urine. ^[5]

Latency

Asymptomatic Rates

Excretion Rates (see Exposure)

Immunity

Microbiology

A thin, aerobic, pleomorphic, flagellated, non-spore forming, Gram-negative bacterium of the genus Legionella.^[8] [9] 
Legionella species are weakly gram-negative bacteria found primarily around fresh water environments, such as lakes and streams, where the bacteria use free-living amoeba as hosts for intracellular survival and multiplication.^[3]