It has long been recognized that the occurrence of infectious diseases in seafarers is related to work at sea. Accident insurance data and seafarers registers of maritime authorities have documented the occurrence of infectious diseases such as malaria and tuberculosis, which are commonly recognized as occupational diseases with a relevant travel history or when contact to a case (of tuberculosis) has occurred.

However, studies which quantify the risk of infectious diseases in seafarers as compared to the general population are rare. This is especially true for diseases that commonly occur in the general population in some countries, like chickenpox or hepatitis A. This review shows that a wide range of infectious diseases occurs in seafarers. Disease may either result from person-to-person transmission of infectious agents or through vectors (food, water, mosquitoes) on board ships or in ports, as well as from a pre-existing condition.

The magnitude of infectious disease occurrence in seafarers as an occupational group is ill defined due to several challenges for researchers. The global population of seafarers cannot,  by the nature of the profession and organization of the international and national shipping fleet, be described and studied as such. Seafaring includes many different working environments, such as fishing, navy ships, ferries, cargo ships and many others. Also employers give seafarers contracts with varying durations. Ships under the management of one country may be registered under a different flag for profit reasons and acquire personnel from a crewing agency in another country.

With a few exceptions no national or international surveillance systems exist on infectious disease occurrence on ships. Most national or international surveillance systems for infectious diseases do not collect data on professions in general and seafaring in particular. This study identified national registers for seafarers, navy registers, statutory accident insurances, medical log books as potential sources of information for incidence of disease specific to seafarers. However, most studies on infectious diseases in seafarers are prevalence studies in convenient samples, or case and outbreak reports. A further source of bias is the fact that only a few research groups on maritime health exist globally. Most of them are in European countries (e.g. UK, Denmark, Poland, Germany) which traditionally have been and are involved in the maritime industry but no longer supply the majority of personnel to the modern shipping industry.

Studies included in this review that investigate general treatment and referral patterns showed that up to one fourth of treatments on board were caused by a presumed acute communicable disease such as gastroenteritis, acute respiratory disease, skin or eye infections.

The majority of studies on diseases transmitted via skin and blood including sexually transmitted diseases are from convenient samples in ports. In most studies serological tests were performed in a limited number of persons during pre-employment exams. The studies from convenient samples showed a high variability of prevalence of Hepatitis B, C and HIV infection in seafarers from one study population to the other. Longitudinal studies from low endemicity countries suggest a higher prevalence of Hepatitis B and HIV infection in seafarers than the general population. The authors identified duration and place of travel, country of origin, casual sexual contact, medical care and intravenous drug use as risk factors. Modes of transmission and vaccination rates have not been studied systematically.

Overall, the occupational risk of serious infections is low ,with the country of origin of the seafarer being the main risk factor for the presence of infection. The published case reports and studies show that seafarers were at risk early in the HIV/AIDS epidemic due to travel to and sexual encounters in endemic areas. Recent studies do not allow the magnitude of risk of HIV infection in seafarers to be assessed, despite widespread HIV testing as part of pre-employment examinations. Only one study described the occurrence of needle-stick injuries in seafarers during medical care on board.  Studies which focus on behavioural aspects, knowledge and perception of blood-borne and sexually transmitted diseases are rare. No specific recommendation for post exposure prophylaxis for risk contact of seafarers (by unprotected sexual contact or needle-stick injury) to HIV infected source persons exists .

Global travel is a well described risk factor for infectious skin disease. While it is the author’s experience, from their seafarer’s ambulance in Hamburg,Germany, that skin problems are a common complaint of seafarers, no specific study to this now exists. The study of Roman et al 1997 on clinically suspect skin infections for methicillin-resistant staphylococcus aureus (MRSA) is a timely reminder that further research is needed to define causes and quantity of infectious skin diseases and publish adequate recommendations for the medical treatment on board.[47]

The occurrence of food and water borne diseases on board are well documented by numerous published outbreak reports. Consistently attack rates in crew members tend to be lower than in passengers. However, outbreak studies performed by Public Health Authorities commonly do not aim to quantify the occurrence of disease in seafarers but in the population of passengers, thus attack rates in crew as compared to passengers may be under- or overestimated depending on the method of data collection. Data do not allow the risk of most diseases in seafarers to be compared to the general population. Hepatitis A is an exception as it was shown that the risk of infection was elevated in seafarers.  Repair work of the ship´s sanitary installation is an established risk factor for infection with Hepatitis A on ships. With ships moving between high and low prevalence areas and mixing of crew members with different levels of immunity, offering Hepatitis A vaccination to seafarers seems to be a prudent decision and it should be recommended to reduce the risk of infection. While no evidence exists that seafarers do suffer long-term sequelae from food- and water-borne diseases, the avoidance of disease outbreaks is a concern. From the view of the ship-management the aim is to reduce loss of work-days and maintain the ship’s safety, while crew member will want to avoid loss of employment when disease occurs. Prevention of food and water borne disease starts from well constructed accommodation, galley, water system and food storage areas. Safe sources for catering materials, training of cooks and everyday ship hygiene and maintenance are essential.

Contact with infective aerosols and droplets are a major cause of morbidity on board connected to the particularities of the living conditions and the mixing of persons from areas of low and high endemicity. The occurrence of tuberculosis, legionellosis, chickenpox, influenza, rubella, measles, diphtheria, pertussis and bacterial meningitis has been documented in seafarers.  Vaccinations are the main tool for the prevention of these diseases.

 Outbreaks of influenza are characterized by attack rates up to 42% in crew members. Though comparability of attack rates between studies is limited by different methods of case finding, definition and control measures, rates in crew and passengers were within the same range. This points to the annual influenza vaccination as a worthwhile investment for the safe operation of the ship and the wellbeing of the crew.

Vector-borne Diseases on ship´s and the implication of spreading the vectors globally by travel and trade have long been a focus for international disease control programs overseen by the World Health Organization. Beside the known risks of disease to seafarers and control measures for yellow fever and Malaria, emerging infections causing Chikunguya, dengue fever or West Nile virus fever are recognized as travel related risks. Though no case reports are published from seafarers it is safe to assume that they are a population at risk.

Overall, the review identifies the current state of knowledge on transmission of disease connected to shipping, but does not allow quantification of disease occurrence compared to the general population. In most studies no appropriate comparison group from the general population or other occupational groups were available. Many, but not all, infectious diseases which have been described in travellers are seen in seafarers as well. This may be due to the fact that more studies are undertaken in travellers than in seafarers but also risk patterns may differ.

To estimate the burden of infectious diseases in seafarers and identify occupational infectious diseases, well designed, internationally coordinated sentinel studies and/or integration in the routine surveillance system of communicable diseases is needed. The data retrieved from these systems will not only be of importance to the workforce in shipping but also be able to detect emerging and reemerging infections with relevance to the public health in a timely manner.

Infection from drug resistant bacteria is a concern also in seafaring. Travel has long been associated with the international spread with infectious diseases and has been recently discussed more in the context of travelling. Importation of resistant strains of Neisseria gonorrhoea for example, has for many years been associated with travel to countries in the Far East. Much attention is on antibiotic-resistant species of the Enterobacteriaceae to extended spectrum beta-lactamases (ESBL´s). Other examples are Acinetobacter baumanii and MRSA.[125]

While there are no data available on the importance of drug resistance bacterial infections in seafarers, physicians must be aware that seafarers may be at risk for hospital-acquired infection and carriage of multidrug resistant bacteria. Systematic testing of specimens is certainly warranted if a seafarer was treated in a hospital abroad and does not respond to empiric antibiotic treatment. In this case prompt microbiological investigations of appropriate specimens is indicated.