LEGIONELLA - World Health Organization

7.1.3 Ship-associated cases
Legionnaires’ disease was first associated with a ship in 1977 (Rowbotham, 1998). At least 55
incidents (outbreaks or cases) were associated with cruise ships, ferries, cargo ships, fishing
vessels or naval ships between 1977 and 2004. Outbreaks of two or more cases are summarized
in Table 7.1. Some of these incidents have been linked to ship’s water systems, air-conditioning
systems and recreational hot tubs. However, in the majority of cases, the source of the
infection and/or the mode of transmission were not established.
The risk of exposure to Legionella on ships is difficult to assess. Surveys carried out on general
cargo ships have shown drinking-water and air-conditioning systems to be contaminated with
L. pneumophila. Serologic surveys of some seafarers on cargo ships showed that a high proportion
had antibodies to L. pneumophila, suggesting that those on some ships could be at increased
risk of legionellosis compared with communities on shore (Temeshnikova et al., 1996).
The number of outbreaks and cases reported in the literature is probably an underestimate of
the true incidence of the disease. As for hotels, outbreaks and cases associated with ships,
especially ferries, are difficult to detect because the incubation period of 2–10 days or more
(see Chapter 1) means that passengers may have dispersed widely, including to different countries,
before developing symptoms. To detect such outbreaks, an international surveillance scheme,
such as the European Surveillance Scheme for Travel Associated Legionnaires’ Disease, is necessary
(see Section 7.1.1).
Even when an outbreak or cluster of cases is detected on a ship or ferry, it is often difficult to
implicate that vessel as the source of infection, if passengers disembarked at different locations
or stayed in hotels before or after the voyage. Tracking previous incidents associated with
ships can also be a problem if the vessel’s name has changed, particularly if ownership has also
changed. To implicate a particular source during an outbreak investigation, it is necessary to
isolate environmental strains of Legionella and match them with clinical isolates. However,
this is often more difficult to do for ships, because the suspect vessel will often have sailed to
another country before a case is recognized. Unless there is good cooperation between international
port health authorities and maritime authorities in different countries, ships may escape
adequate investigation.
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