The following information about Legionnaires' disease is abstracted from the publication Control of Communicable Diseases Manual, sixteenth edition. (In an actual epidemiologic investigation, you might consult a reference publication such as this to refresh your memory on pertinent details about a specific disease).

Legionnaires' disease, or legionellosis, is characterized by pneumonia caused by the bacterium Legionella pneumophila. The name "Legionnaires' disease" was given to this problem following a large outbreak among persons who attended a convention of American Legion military veterans in Philadelphia, Pennsylvania, in July 1976. The incubation period (the amount of time from initial exposure to the infectious bacteria to the actual onset of illness) for Legionnaires' disease ranges from 2 to 10 days. The disease often begins with anorexia (loss of appetite), malaise (fatigue and overall sense of poor well-being), myalgias (muscle aches and soreness), and headache, followed by rapidly rising fever and chills. Chest X-rays typically show patchy areas of inflammation and fluid accumulation in the lungs. The diagnosis is confirmed by:

1. isolation of the bacterium on special culture media;or
2. demonstration of the bacterium by immunofluorescent stain of involved tissue or respiratory secretions; or
3. fourfold or greater increase in titers (antibody levels) between acute (early illness) and convalescent (long-term [usually 3-4 weeks after the acute period]) phase serum samples; or
4. a single high titer in a patient with an illness characterized by the symptoms and/or signs of Legionnaires' disease.

Bogalusa was served by a 98-bed private hospital (hospital A) and a 60-bed public hospital (hospital B). Three additional hospitals were located in the surrounding parish. All of the reported cases of Legionnaires' disease were for patients at Hospital A.

The number of patients discharged with a diagnosis of pneumonia at Hospital A since January 1986 is shown in Table 1. Between January 1986 and September 1989, only one pneumonia patient had been diagnosed as having Legionnaires' disease.

Review of charts of pneumonia patients at Hospital A during October revealed that many patients were admitted with a febrile illness (i.e., an illness especially including fever) characterized by weakness, lethargy, and mental confusion. Some patients had a dry cough, and several reported having watery diarrhea. Chest X-rays were consistent with a pneumonia. Most cases were residents of Bogalusa or the surrounding areas of Washington Parish.

• QUESTION 4:
  What are possible interpretations for the data in the table above? What would you do next? (i.e., If you wanted to intensify the investigation, what steps would you take?)

• QUESTION 5:
  Develop a case definition for this outbreak. Would you want a relatively sensitive or a relatively specific case definition in this setting?

  (Tip: A "sensitive" case definition is one that will identify nearly all cases of the disease being investigated ["true positives"], but at the same time may include other similar illnesses resulting from different causes ["false positives"]. In contrast, a "specific" case definition is one that excludes similar illnesses resulting from causes different from the disease under investigation [false positives], but at the same time may exclude some true cases with slightly unusual symptoms and/or signs [true positives]. In other words, would you want a case definition that identifies the maximum number of possible cases [high sensitivity] or one that only identifies true cases [high specificity]?)

• QUESTION 6:
  How would you go about case-finding? Do you need to find every case?