Vessel Sanitation Program: Outbreak Investigation Overview

Outbreak Investigation Overview

Background
Since 1970, CDC’s Vessel Sanitation Program 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 illnesses into the United States. This reporting system, along with other Vessel Sanitation Program components have reduced and controlled the number of passengers and crew members aboard cruise ships with gastrointestinal illnesses (GI).
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, 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 gastrointestinal illness aboard cruise ships.

What are cruise ships required to do?
Cruise ships are required to log and report the number of passengers and crew members that have reported having symptoms of gastrointesintal illness. Additionally, vessels are required to keep a list of all passengers and crew members that have requested antidiarrheal medication.

What does VSP look for when monitoring gastrointestinal illness reports?
VSP provides assistance to cruise ships when the number of ill passengers or crew members has reached 2% or more of the total number of passengers or crew members or when an unusual GI 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 officials do the following:
1. Review the magnitude of gastrointestinal illnesses among passengers and crew.
2. Monitor distribution of gastrointestinal illnesses among both passengers and crew members.
3. Assist in the assessment of changes in sanitation practices, in infectious agents, and in types of diseases.
4. Evaluate the effectiveness of prevention and control strategies.
5. Provide lab support to confirm the cause of the illness.

What is a reportable case of GI illness?
A reportable case of GI illness is three or more loose stools within a 24-hour time period, or vomiting along with one of the following symptoms: muscle ache, headache, abdominal cramp, or fever.

What are unusual GI illness patterns or characteristics?
Examples of unusual GI illness pattern or characteristic 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 and other CDC officials may conduct an investigation if at least 3.0% of the ship’s passengers or crew members have GI illness, or if an unusual gastrointestinal illness pattern or characteristic is found (even if the illness rate is less than 3%).

What happens during an onboard investigation?
During an onboard investigation, CDC officials work with cruise ship staff to determine the cause of illness.
The objectives of an investigation are to:
1. Determine the magnitude of illness aboard the ship.
2. Identify the agent causing the illness.
3. Identify risk factors associated with the illness.
4. Formulate control measures to prevent or stop the spread of illness.
By determining the cause of an outbreak, CDC officials can help devise effective intervention strategies to prevent a recurrence of the outbreak.
The investigation has three parts:
1. Epidemiological investigation: The incidence and distribution of the GI illness is 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, blood, or vomit 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 spent reviewing the water source.

What happens to the information collected during an investigation?
When possible, VSP officials present a preliminary report of findings and recommendations to the master of the ship before leaving the ship.
When the investigation is complete, a final report is sent to the cruise line results on of the epidemiological, laboratory, and environmental analyses. Summary recommendations for control and prevention are also included in the report.

How is this information made available to the passengers?
Passengers can find a copy of the report on this website, emailing CDCINFO@cdc.gov, or by writing:
Chief
Vessel Sanitation Program
Centers for Disease Control and Prevention
4770 Buford Highway, N.E., Mailstop F-23
Atlanta, Georgia 30341-3724