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Outbreak Investigation Overview


Since 1970, CDC’s Vessel Sanitation Program (VSP) has maintained a surveillance and reporting system for cruise ships that have both a U.S. and foreign itinerary and carry 13 or more passengers. The system was designed to minimize the introduction, transmission, or spread of gastrointestinal (GI) illnesses into the United States. This reporting system, along with other VSP components, has reduced and controlled the number of passengers and crew members aboard cruise ships with GI illnesses.

The cruise industry is rapidly growing and now serves more travelers and has more extensive itineraries than ever before. Because of the increases in itineraries and the number of passengers on cruise ships, more people are at risk for GI illnesses while traveling. Cruise ship passengers and crews are also at risk for new patterns of old diseases or newly recognized causes of diseases. VSP protects passenger and crew health by minimizing the risk of GI illness aboard cruise ships.

What Are Cruise Ships Required To Do?

Cruise ships are required to log and report to VSP the number of passengers and crew members that say they have symptoms of GI illness. Ships are also required to keep a list of all passengers and crew members that have requested antidiarrheal medication.

What Does VSP Look for When Monitoring GI Illness Reports?

VSP provides assistance to cruise ships when the number of ill passengers or crew members reaches 2% or more of the total number of passengers or crew members or when an unusual GI illness pattern or characteristic is found.

VSP monitors and evaluates GI illnesses aboard ships to determine the cause of the outbreaks and to make recommendations for control. To accomplish this, VSP

  • Reviews the magnitude of GI illnesses among passengers and crew.
  • Monitors distribution of GI illnesses among both passengers and crew members.
  • Assists in assessment of changes in sanitation practices, infectious agents, and types of diseases.
  • Evaluates the effectiveness of prevention and control strategies.
  • Provides lab support to confirm the cause of the illness.
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What Is a Reportable Case of GI Illness?

VSP defines GI illness as three or more loose stools within a 24-hour time period or what is more than normal, OR vomiting along with one of the following symptoms:

  • diarrhea, or
  • muscle ache, or
  • headache, or
  • abdominal cramp, or
  • fever.

What Are Unusual GI Illness Patterns or Characteristics?

Examples of unusual GI illness patterns or characteristics are

  • Relatively high incidences of illnesses in successive cruises,
  • Unusual severity of illnesses or complications,
  • High numbers of persons reporting illnesses over a brief period, or
  • Suspicion of an uncommon causative agent.

When Does CDC Conduct an Investigation?

VSP may conduct an investigation

  • If 3.0% of the ship’s passengers or crew members have GI illness, OR
  • If an unusual GI illness pattern or characteristic is found (even if the illness rate is less than 3%).
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What Happens During an Onboard Investigation?

During an onboard investigation, VSP works with cruise ship staff and the cruise line to determine the cause of illness.

The objectives of an investigation are to

  • Determine the magnitude of illness aboard the ship.
  • Identify the pathogen causing the illness.
  • Identify risk factors associated with the illness.
  • Formulate control measures to prevent or stop the spread of illness.

By determining the cause of an outbreak, VSP can help devise effective intervention strategies to prevent a recurrence of the outbreak.

The investigation has three parts:

  1. Epidemiological investigation: The size and spread of the GI illnesses are evaluated on the basis of passenger and crew member activity questionnaires, general cruise ship layout, menus, medical records, and other pertinent data.
  2. Laboratory investigation: Medical staff may collect stool, vomit, or blood specimens to send to a land-based lab to confirm the cause of illness.
  3. Environmental health investigation: An environmental health investigation is based on the preliminary findings of the epidemiological investigation. For example, if water is suspected to be the source of the illness, more attention will be given to reviewing the water source.
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What Happens to Information Collected During an Investigation?

VSP reports findings and recommendations to the master of the ship and ship management before leaving the ship.

How Is This Information Made Available to the Passengers?

Passengers can find a summary report on the Outbreak Updates for International Cruise Ships webpage.

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