Infections in this cluster are transmitted by animals (zoonotic) or from agents in water and soil (environmental). Shipping is associated with the potential threat of the dispersal of microbes and their vectors, such as rodents and arthropods including cockroaches, mites or mosquitoes (1,2). Vector-borne diseases on ships 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. The International Health Regulations 2005 regulate the surveillance of disease, disinfection, deratting and disinfection of ships and port areas as well as transported goods and health measures for travellers, including yellow fever vaccination (3).
Beside the known risks of 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.
G.6.2 Risk assessment
For centuries seafarers have been aware of the risk of being bitten by mosquitoes and acquiring disease when visiting areas where such disease is prevalent.
Seafarers and international travellers are at risk of developing disease once infected because they are non-immune and are frequently diagnosed late or misdiagnosed when returning home. Clinical manifestations of the disease may be evident on board when no medical doctor is available for diagnosis and treatment, or in ports where the reliability of a diagnosis may vary and is unknown to the seafarer. Seafarers may develop symptoms when they have ended their contract and returned to their home countries, sometimes to places where such diseases are not endemic and thus physicians may not consider the diagnosis when a seafarer presents with typical symptoms. Often, specialist tropical and diagnostic tools will not be easily available in the home areas of seafarers.
G.6.3 Risk management
Chemoprophylaxis, specific treatment or vaccination is available for some of these diseases. However no chemoprophylaxis can provide complete protection and specific therapies are not available for all. Hence measures to reduce the risk of mosquito bites are of paramount importance.
There are specific measures that must be observed in relevant areas:
- Avoid being outdoors as much as possible and particularly between dusk and dawn
- Wear long-sleeved shirts and trousers when outside
- Systematic availability of insect repellent on board
- Air conditioning should be functioning and used at all times
- Window and door screens should be used or, if these are not available, living quarter doors and windows should be kept closed
- Use of an insecticide treated bed net when ashore
- Availability of anti-mosquito sprays or insecticide dispensers that contain tablets impregnated with pyrethroids to all cabins at night.
Indoor residual spraying is recommended only after consultation with a public health/tropical medicine expert who can judge on the epidemiological and entomological data. It should be performed by a professional pest management company
The WHO also promotes the ABCD for Malaria protection that also applies to seafaring:
- Be Aware of the risk, the incubation period, the possibility of delayed onset and main symptoms.
- Avoid being Bitten by mosquitoes, especially between dusk and dawn.
- Chemoprophylaxis, when appropriate.
- Immediately seek Diagnosis and treatment if fever develops one week or more after entering an area where there is a malaria risk. Fever may be delayed for up to 3 months, or occasionally longer.
The appropriate availability and use of rapid tests, medication and vaccination should also be considered in conjunction with TMAS and other shore based medical expertise.
G.6.4 Malaria - Human-infective Plasmodium species
Most malaria is transmitted by the bite of an infected female Anopheles mosquito that live in many tropical and subtropical countries (see map at www.who.int/ith/en). They are most active between dusk and dawn. Up to date information about the risk of acquiring malaria in a given port, is available at www.cdc.gov/travelregionalmalaria/index.htm.
The period between the infective bite and the appearance of clinical symptoms is typically 9-14 days for Plasmodium falciparum, 12-18 days for Plasmodium vivax and Plasmodium ovale and 18-40 days for Plasmodium malariae. Some strains of Plasmodium vivax may have an incubation period of up to 12 months. Classic symptoms are fever, chills, diarrhoea, headache, fatigue or anorexia.
Malaria in seafarers
Numerous studies have investigated malaria among seafarers. Mouchtouri et al. concluded from these studies a varied incidence rate from 3 to 12/1000 seafarers (4).
Tomaszunas estimated the total number of malaria in international seafarers to be between 500 and 1000 each year. The disease was found to be contracted onboard or in ports (5).
Hansen et al showed in a longitudinal study from Denmark of 24.132 seafarers employed between 1986 and 1993 that the majority of the 25 malaria cases occurred in connection with trade to West Africa. Irregularity of malaria prophylaxis and chloroquin resistance was assessed to be of importance for risk of infection (6).
Port health authorities in Croatia reported 23 malaria cases in 19 sailors and 4 tourists on merchant ships arriving in Croatia between 1990 and 1993 (attack rate 0.3% (23/ 8379) (7).
Over 100 cases of malaria were reported to authorities in Japan between 1990 and 1997. It was estimated that approximately 5% of imported cases to Japan concerned seafarers (8).
In Klaipeda, Lithuania, malaria cases were registered from1973 to 1998. 99 of 113 cases occurred in seafarers and fishermen (9). In Italy, seafarers and airline personnel represented 21% all registered malaria cases (10).
Though, by the nature of their job, seafarers cannot avoid exposure to malaria there are well studied preventative measures that greatly reduces the risks of severe disease and death in sailors. The mainstay of prevention in ships sailing to malarious areas is chemoprevention (of note: there is by now no vaccination against Malaria).
The International Medical Guide for Ships requires ships to carry the appropriate drugs as recommended by the World Health Organization or national guidelines for that specific region.
Before travel updated recommendations on the appropriate prophylaxis must be reviewed:
It will be in the responsibility of the ship owner to supply the appropriate drugs to their ships in sufficient amounts. However sailors often fail to use chemoprevention even if available on board, the ship master needs to to promote and facilitate the use of chemoprevention (5). It is well known that many seafarers avoid chemoprophylaxis due to unjustified fears of side-effects, thus necessitating appropriate medical counselling and written information.
All ships deploying to malarious areas must carry effective treatment. Provision of methods to confirm diagnosis by microscope, where staff have the skills to use it, or by using a rapid diagnostic test, is recommended.
The International Medical Guide for Ships (IMGS) recommends that ships carry the appropriate drugs as recommended by the World Health Organization or national guidelines for that specific region. It is the responsibility of the ship owner to supply the appropriate drugs to their ships in sufficient amounts. However, sailors often fail to use chemoprophylaxis even if it is available on board and the ship’s master needs to promote and facilitate its use (5). Many seafarers avoid chemoprophylaxis due to unjustified fears of side effects and appropriate medical counselling and written information is useful.
All ships deploying to such areas must also carry effective treatment. The provision of methods to confirm the diagnosis by microscope, where staff have the skills to use it, or by use of a rapid diagnostic test, is recommended. However, regardless of a test result, if a seafarer has travelled to a malarious region and develops typical symptoms of Malaria, not earlier than 6 days and typically 10-14 days or later after entering the area of risk, they should begin empiric full dosage treatment immediately. A positive rapid test will confirm the diagnosis but a negative result should not delay treatment and should be repeated after 24 hours. Management of these cases should be done in conjunction with TMAS or other shore based medical personnel. If symptoms persist after 3 days or the patient deteriorates, further medical assessment is necessary.
To date there is no available vaccine.
Use of a Malaria Rapid Diagnostic Test in the Ship Environment
Rapid tests are not currently listed as a part of the ship´s medicine chest in the IMGS. However they are increasingly carried on board to facilitate decision making in situations when a medical consultation ashore is not available. The advantage of the Malaria Rapid Diagnostic Tests is that they provide a quick result and are easy to use. However, the quality and performance of the many commercially available tests vary substantially and procurement must take into account product specifications.
Limitations of the tests
- Variabilty in their ability to detect specific malaria parasite densities and species by their target antigens. Some tests will detect P. falciparum or non-P. falciparum only, some detect both antigens. In some areas of South America, P. falciparum parasites do not express a certain antigen. If tests are based on this antigen, their sensitivity to detect P. falciparum from this region will be low - only tests that test pLDH will be effective in detecting P. falciparum in these areas of South America.
- Different tests are designed to detect infections in populations with different epidemiological situations of malaria, that is, countries where malaria is endemic as opposed to countries moving towards the elimination of malaria. Therefore, some tests are designed to detect low density infections, that is > 100 to 200 parasites /µl whilst others are designed for endemic countries and will detect high density infections only, that is > 2000 or 5000 parasites /µl.
- Tests continue to be positive in patients who have been treated with effective ant-malarial medicines within the last 5 - 14 days, depending on the test. This occurs because the test detects the antigen and not the parasite itself. Hence if a seafarer has recurrent or persistent signs and symptoms of malaria within 3 to 14 days after completing a recommend first line treatment, the rapid test may still be positive even if the treatment was successful and another cause of symptoms must be considered. But, it may also be that the treatment was not successful and second line therapy for Malaria may be required. In a seafarer who is not improving with first line therapy for Malaria, a blood film for malaria parasites is necessary to confirm the diagnosis and decide if this is treatment failure. However, if a patient develops symptoms and has a positive rapid test or blood smear for malaria parasites 14 days or more after initiation of first line therapy, with prior resolution of symptoms, this should be considered a new infection and the patient be treated with the first line drug.
- Depending on the test used, the test must be read 15 to 60 min after the buffer was added. After this time discoloration of the system appears and may result in false positive or false negative readings. Therefore, the rapid diagnostic test is not suitable for documentation of claims of a work related disease and in this situation it is recommended to use a blood slide for documentation, as described in the IMGS. This is a rather simple procedure that can be performed by a seafarer with minimal training.
If purchasing a test for use on board, care must be taken to choose the appropriate test. Appropriate tests must:
- be heat resistant, if storage in a refrigerator in the ship’s medical centre is not possible at all times,
- detect parasites at a threshold of 100 to 200 parasites /µl with a high sensitivity and specificity,
- detect P. falciparum and non-falciparum and, if used for South-America travel detect pLDH Antigen,
- be easy to use and have an appropriate shelf-life.
The WHO has a product testing program for Malaria rapid detection tests that are commercially available and the results are published on the WHO website. This may be useful information to guide the procurement process.
(1) Tatem AJ, Hay SI, Rogers DJ. Global traffic and disease vector dispersal. Proc Natl Acad Sci USA 2006;103:6242-6247.
(2) Lounibos LP. Invasions by insect vectors of human disease. Annu Rev Entomol 2002; 47:233-266.
(3) World Health Organization. International Health Regulations (2005). Geneva: WHO, 2006. http://www.who.int/csr/ihr/en/ Last access August 2013.
(4) Mouchtouri VA, Nichols G, Rachiotis G, Kremastinou J, et al. For the SHIPSAN partnership: State of the art: public health and passenger ships. Int Marit Health 2010; 61, 2:49-98.
(5) Tomaszunas S. Malaria in seafarers. 1. The magnitude of the problem and the strategy of its control. 2.The status of malaria in large ports of the world. Protective measures against malaria in crews of ships. Bull Inst Marit Trop Med Gdynia 1998;49:53-61, 63-71.
(6) Hansen HL. Occupation-related morbidity and mortality among merchant seafarers with particular reference to infectious diseases. Esbjerg: South Jutland University Press, 1996.
(7) Nikolic N, Poljak I, Troselj-Vukic B.. Malaria, a travel health problem in the maritime community. J Travel Med 2000;7:309-313.
(8) Shoda M, Shimizu K, Nagano M, Ishii M. Malaria infections in crews of Japanese ships.Int Marit Health 2001;52:9-18.
(9) Scerbaviciene R, Pilipavicius R. Malaria among seamen in Klaipeda during 1973-1998. Int Marit Health 1999;50:7-13.
(10) Herrador Aguirre J. El paludismo en los trabajadores del mar de Costa de Marfil. Med Marit 1996;1:112-117.
G.6.5 Dengue Fever and Dengue Haemorrhagic Fever
Dengue Type -1, -2, -3 and -4 viruses are transmitted by infected Aedes mosquitoes in areas where the winter temperature is above 10°C and the incubation time is commonly 4-7 days. Asymptomatic infection can occur. No direct person-to-person transmission occurs. Aedes aegypti mosquitoes are day-biting species, with increased biting activity for 2 hours after sunrise. The past 20 years have seen the spread of epidemic dengue fever and dengue haemorrhagic fever from South East Asia to the South Pacific Islands, the Caribbean and the Americas. Recent epidemics have occurred in Asia - Cambodia, China, India, Indonesia, Laos, Malaysia, Maldives, Myanmar, New Caledonia, Pakistan, Philippines, Singapore, Sri Lanka, Tahiti, Thailand and Vietnam and in the Americas - Brazil, Colombia, Cuba, Ecuador, El Salvador, French Guyana, Guatamala, Honduras, Nicaragua, Puerto Rico, Surinam and Venezuela.64 It is rare in African countries. Recovery from infection from one serotype provides lifelong immunity to that serotype only, but further infection with another serotype increases the risk of dengue haemorrhagic fever.
Dengue fever is an acute febrile viral disease characterized by the sudden onset of fever, intense headache, myalgia, arthralgia, retro-orbital pain, anorexia, nausea, vomiting and a generalized rash, often not seen in dark-skinned patients. Symptoms tend to last for 2-7 days. Dengue haemorrhagic fever and Dengue Shock Syndrome are serious courses of the infection characterized by acute febrile illness and haemorrhagic diathesis with abnormal blood clotting and increased vascular permeability causing petechia, echymosis or purpura with a tendency to develop hypovolemic shock. Immediate intravenous fluid therapy and further medical treatment is needed.
Studies estimate that the incidence of returning travellers with Dengue has risen from 2% in 1990 to 16% in 2005 (1 . No studies or reports were identified on the occurrence of Dengue and Dengue Haemorrhagic fever in seafarers, but cases are seen in clinical practice. Medical doctors caring for seafarers must be aware that seafarers, along with other frequent travellers, are at risk to acquire dengue fever and are at higher risk for more severe disease in the form of dengue haemorrhagic fever and/or dengue shock syndrome. However, the prognosis may be difficult to predict at the outset of the disease in an individual seafarer. There is no specific chemoprophylaxis or treatment medication and whilst vaccines are under development, they are not currently available.
G.6.6 Chikungunya Virus
Chikungunya is a virus that is transmitted to humans by the bite of infected Aedes mosquitoes. It is endemic in many South East Asian countries. It causes a febrile disease with symptoms of chills, headache, nausea, vomiting, joint pain, low back pain and rash. Despite the disease being increasingly recognized as a cause of fever in ill travellers (2), no reports have been published in seafarers to date.
G.6.7 Yellow fever
Yellow fever is caused by a flavivirus, transmitted by the infected Aedes mosquitoe and predominantly seen in tropical Africa, Central and Southern America. It presents as a febrile disease with fever, headache, jaundice, muscle pain, nausea, vomiting and fatigue that usually lasts 3 – 4 days. The incubation period is 3 – 6 days. Letality in hospitalized cases is high. There is no specific treatment although the early and appropriate management of dehydration and fever improves the outcome.
A vaccine is available and Yellow Fever is currently the only disease expressly listed in the International Health Regulations (IHR) for which countries can require proof of vaccination from travellers as a condition of entry. Many countries do require proof of immunization to Yellow Fever in travellers to, or through, their territory. The vaccine provides immunity after 10 days and there is evidence that it produces lifelong immunity in most immunocompetent persons. This is recognised by the IHR in an amendment that came into force in July 2016.
No reports have been published on cases of yellow fever in seafarers, also no severe side effects of the vaccination are known in seafarers. Note: The Yellow -fever vaccination is contraindicated in pregnant women and severely immunocompromised persons-
G.6.8 Zika Virus
In 2014 the WHO declared a cluster of microcephaly and other neurologic disorders reported in Brazil to be a Public Health Emergency of International Concern. They were caused by the Zika virus, transmitted primarily by the Aedes mosquitoes, which bite during the day. Outbreaks and evidence of transmission soon appeared throughout the Americas, Africa, and other regions of the world. To date, a total of 86 countries and territories have reported evidence of mosquito-transmitted Zika infection. In addition, Zika virus can be transmitted through sexual intercourse. This is of concern due to an association between Zika virus infection and adverse pregnancy and fetal outcomes. Zika virus infection during pregnancy can cause infants to be born with microcephaly and other congenital malformations, known as congenital Zika syndrome.
Most people with Zika virus infection do not develop symptoms but those that do experience a fever, rash, conjunctivitis, muscle and joint pain, malaise or headache. Symptoms typically last for 2–7 days. An increased risk of neurologic complications is associated with Zika virus infection in adults and children, including Guillain-Barré syndrome, neuropathy and myelitis.
Advice for the shipping industry is available at http://www.imo.org/en/MediaCentre/HotTopics/Pages/Zika-virus.aspx and the EU ShipSan project has published an Interim guidance on maritime transport and Zika virus disease (Mochtouri, www.shipsan.eu/Home/Zikavirus.aspx). They conclude that the introduction of human cases of Zika virus disease (seafarers or passengers, symptomatic or asymptomatic) to the EU through ship travel is considered very low. However, specific types of imported goods including used tyres and ornamental plants do introduce invasive mosquito species to EU and recommend integrated Pest Management Plans.
There is no specific treatment and no vaccine available. Protection against mosquito bites during the day and early evening is a key measure to prevent Zika virus infection.
G.6.9 Other diseases
No reports were identified on other diseases from the zoonotic-environmental cluster known to occur in travellers including West Nile fever, Leptospirae, Schistosoma, Sand flies or Helminths.
(1) Schwartz E, Weld LH, Wilder-Smith A, von Sonnenburg F, Keystone JS, Kain KC, Torresi J, Freedman DO, for the GeoSentinel Surveillance Network. Seasonality, annual trends, and characteristics of dengue among ill returned travellers, 1997-2006. Emerg Infect Dis 2008;14:1081-1088. http://www.cdc.gov/eid/content/14/7/pdfs/1081.pdfLast access August 2013.
(2) Townson H, Nathan MB. Resurgence of chikunguya. Trans R Soc Trop Med Hyg. 2008;102:308-9.
 Drăgănescu N, Duca M, Gîrjabu E, Popescu-Pretor I, Răducanu S, Deleanu L, Totescu E. Epidemic outbreak caused by West Nile virus in the crew of a Romanian cargo ship passing the Suez Canal and the Red Sea on route to Yokohama. Virologie 1977;28:259-262.
 Woodford N. Unwanted Souvenirs: Travel and Multi-Resistant Bacteria. Journal of Travel Medicine 2011; Volume 18 (Issue 5): 297-298.