The prevention and control of diseases on ships is not a new concept. Even as far back as the Middle Ages, efforts to prevent the global transmission of infectious diseases by maritime vessels existed. During the mid-14th century, when Europe was gripped by a bubonic plague pandemic known as the “Black Death,” Venetian officials required all ships arriving from infected countries to remain quarantined at anchor for forty days; the term “quarantine” derives from the Italian “quaranta giorni” meaning forty days.
The Black Death, one of the most devastating pandemics in human history, resulted in the deaths of an estimated 25-50 million people. While the horrors of the 1300s are behind us, the lessons learned regarding how maritime transportation can facilitate disease transmission have not been forgotten. The International Health Regulations (IHR), first adopted by the World Health Assembly in 1969, are an instrument for global collaboration “to prevent, protect against, control, and provide a public health response to the international spread of disease.” Article 24 of the current International Health Regulations (2005)—a legally-binding framework of international law adopted by 196 countries—requires all vessel operators to ensure their ships are “free of sources of infection or contamination” through the application of appropriate public health measures.
The recent COVID-19 pandemic was our generation’s bubonic plague. The global debate on its management by governments and the World Health Organization (WHO) still rages on today. When things go wrong, we tend to look for the reasons why, and sometimes, we find explanation in proverbs. Murphy’s Law—a colloquial statement that draws on mathematical principles of probability for validation—is perhaps the most widely used of all such maxims to rationalize adverse events. The ‘law’ taps into human tendency to focus on the negative; however, it also provides encouragement on how to prevent disease outbreaks.
The Origins of Murphy’s Law
Murphy’s Law is an adage that states: “Anything that can go wrong will go wrong.” While the seemingly pessimistic view it portrays has been around for centuries, the axiom itself was reportedly coined in the late 1940s after remarks made by Edward A. Murphy, an American aeronautical engineer, during U.S. Air Force rocket sled experiments conducted to test the gravitational forces a human being could withstand. Rocket sleds, which are essentially platforms that glide on tracks powered by rockets, were frequently utilized during the early stages of the Cold War to test equipment deemed too risky for live trials in manned aircraft.
Murphy blamed a particular failure—the incorrect installation of sensors—that occurred during one experiment on his assistant, saying, “If that guy has any way of making a mistake, he will.” When a reporter later asked how no one had been severely injured during the tests, the rocket sled pilot, Colonel John Stapp, responded by pronouncing they had always applied “Murphy’s Law”: that the team’s good safety record was a result of considering and addressing all the things that could go wrong beforehand.
Although Murphy’s Law is more a philosophical observation as opposed to a scientific rule, it aptly illustrates how communicable disease outbreaks can occur on board maritime vessels. Diseases are unpredictable, with most triggering a multitude of symptoms and potential outcomes. While the concept of “if something can go wrong, it will, and likely at the worst possible time” may appeal to our underlying sense of fatalism, it likewise serves as a warning to be prepared for any plausible scenarios in respect of shipboard illnesses.
What is a Communicable Disease?
An infectious disease is any infection caused by pathogenic (disease-producing) microorganisms such as viruses, bacteria, parasites, or fungi. Humans develop infections because of the invading agent attacking the body and causing our immune system to produce white blood cells and antibodies in defense. Certain infectious diseases like malaria or Lyme disease can only be transmitted through insect bites, while others, such as norovirus or E. coli, can cause illness when individuals consume contaminated food or water.
Communicable diseases are infectious diseases that can be spread, directly or indirectly, from one person to another via exposure to bodily fluids, contaminated surfaces, insect bites, or through airborne transmission. Some communicable diseases are also considered contagious diseases, meaning they are easily spread through contact with other people.
For instance, COVID-19 and influenza (flu) are highly contagious respiratory diseases that can be transmitted when a sick person coughs or sneezes, and an uninfected person breathes in the infectious particles. Conversely, Legionnaires’ disease—usually caused by the inhalation of water droplets containing the bacterium Legionella pneumophila—is not an illness that can be passed from one individual to another.
So, while all communicable diseases are infectious, not all infectious diseases are communicable. Simply being exposed to an infectious disease does not automatically result in an infection. The outcome of exposure to a pathogenic microorganism hinges upon a dynamic interplay between several determinants including:
- Infectivity: The likelihood that the pathogen will infect a host.
- Pathogenicity: The pathogen’s ability to cause disease.
- Virulence: The likelihood of the pathogen causing severe illness.
- Susceptibility: The infected host’s ability to resist infection or limit disease.
The consequences of Murphy’s Law are similarly dependent on a number of variables, albeit an immeasurable quantity. Infinite units of time are required for all possible events to take place, although, as the bubonic plague and COVID-19 pandemics proved, things can happen if there exists a potential. Murphy’s Law speculates that given the opportunity to act incorrectly, we’ll do so about 50% of the time. However, this merely mirrors the decisions we make; in other words, we do have a choice on what transpires.
How The Epidemiologic Triangle Explains Disease
An infection requires three essential components to manifest in humans. Among the simplest models for explaining disease causation and transmission in epidemiology—the study of health and disease in populations—is the epidemiologic triangle (or triad). The epidemiologic triangle consists of:
- An Agent: The pathogen that causes the disease (the what).
- A Host: The person who is exposed to and harbors the disease (the who).
- An Environment: External factors representing favorable conditions, such as temperature, sanitation, or the close proximity of individuals, for an agent to cause illness (the where).
At the center of the triangle is another factor: Time. Time may be used to represent the incubation period of a disease (the interval between infection and the manifestation of symptoms), the recovery phase of a sick person, or even the duration between infection of “patient zero” to the threshold level where an outbreak is declared. Without a definitive clinical diagnosis, the various incubation times and recovery periods associated with each infectious agent can aid in identifying the likely pathogen.
The epidemiologic triangle represents a framework that reveals the causative agent of a disease and the environmental and host factors that permit its propagation. Understanding the interactions between the three vertices of the triangle is fundamental to any outbreak prevention and response strategy. By determining the equilibrium of the three vertices, the triangle can be broken to prevent the transmission of infectious diseases.
A clearer grasp of the epidemiologic triangle can be achieved by observing its application by epidemiologists in deciphering the transmission of existing diseases.
The Norovirus Epidemiologic Triad
Norovirus is the leading cause of gastroenteritis outbreaks worldwide, with an estimated annual burden on global economies—in direct healthcare system costs—of approximately US$4 billion. The virus can cause a range of symptoms, with the most common being diarrhea, vomiting, and stomach pains. Although many norovirus-induced gastroenteritis cases are typically mild and self-resolving, there is a risk of serious complications, especially in young children or the elderly, that may lead to hospitalization or even death.
Due to its very low infectious dose—a single particle has a 50% probability to cause infection—norovirus is a highly contagious disease. Individuals can shed billions of viral particles in their stool and vomit making it easy to infect others. Norovirus is usually transmitted via the fecal-oral route: when contaminated fecal particles from an infected person are ingested by another, typically through improper hand washing practices.
According to a 2018 U.S. study published in The Journal of Infectious Diseases, person-to-person transmission is the principal causal factor in approximately two-thirds of all outbreaks, with foodborne transmission accounting for almost a quarter. Foodborne outbreaks are typically the result of either contamination at source, or during unsafe food handling processes.
The predominant norovirus genotypes (the particular strain of the virus) causing illness in humans include GI.1 and GII.4, with GII.4 responsible for the more severe health outcomes. Historically, novel GII.4 variants emerge every 2-5 years, with the U.S. Centers for Disease Control and Prevention reporting the latest variant of concern as the newly evolved GII.4 Sydney[P16] variant, responsible for an abrupt increase in diagnosed norovirus cases in China during 2021-2022.
The epidemiologic triangle for norovirus can thus be illustrated as: norovirus (agent), individuals susceptible to infection (host), and the extrinsic conditions (environment) promoting its survival and transmission which include factors such as contaminated food or water, unsanitary conditions, and close living quarters.
To prevent norovirus cases from spreading on board, determining how to break the triangle’s vertices by interrupting the connection between any of its three components is key.
- Agent-Host: This connection refers to measures that stop an agent from infecting a host. While influenza can be targeted through seasonal vaccination programs, there is no vaccine at present that provides immunization against norovirus. In addition, as there are no antivirals to treat sick individuals, medical case management is currently limited to supportive care and hydration.
- Environment-Host: Factors that disrupt the environment-host link focus on routine public health controls to avert scenarios where hosts may become infected. The promotion of hand washing, disease surveillance, surface cleaning and disinfection, or the removal of frequently touched items during outbreaks are some of the more familiar norovirus countermeasures.
- Agent-Environment: To break this association, actions must be concentrated on preventing the virus from spreading in different environments. Tactics include isolation protocols, safe food handling practices, or the effective regulation of chlorine within the vessel’s potable water distribution system.
Norovirus is a resilient pathogen capable of withstanding extreme cold and heat, enduring temperatures below freezing and as high as 60°C (140°F). The virus is remarkably persistent on surfaces, lingering for days or even weeks, and is not easily eradicated by traditional disinfectants or alcohol-based cleaning products.
Such characteristics contribute to its propensity to contaminate food, water, or inanimate surfaces, often resulting in expeditious outbreaks. This is especially problematic in closed environments like ships, particularly aboard cruise vessels where thousands of people spend time in close proximity to each other.
Can Disease Outbreaks Be Prevented?
Chapter 8 of the WHO Guide to Ship Sanitation—a comprehensive set of standards that serve as a global resource for mitigating the health hazards associated with ship construction and operation—specifically concerns the management of infectious diseases on board:
“Ships present a particularly high risk for extensive outbreaks. The focus of the control strategy for persistent infectious agents should be on taking all reasonable precautions to prevent transmissions at all times. Reliance should not be placed on any single control strategy, and multiple barriers should be actively maintained.”
WHO Guide to Ship Sanitation (2011)
While the risk of certain diseases cannot be completely eliminated as interrupting the agent-host link is required to achieve that result, with proper contingency measures established, it can be substantially reduced.
Within the cruise industry, it’s standard practice to have detailed Outbreak Prevention & Response Plans (OPRPs) in place to address the risks posed by acute gastrointestinal and respiratory illnesses, including norovirus, influenza, and COVID-19. However, it’s important to recognize that the cornerstone of true disease prevention and management is the thorough application of all public health strategies, encompassing food and water safety, as well as pest control measures.
Murphy’s Law essentially counsels that although there is always a chance for diseases to propagate and for preventative measures to fail, it is possible to be ready for the most adverse scenarios. The law is not a prophecy, nor is it necessarily a defeatist outlook on life. It simply suggests that all events, whether favorable or unfavorable, will ultimately happen.
While the probability of events occurring may be unavoidable, the outcomes are not. Murphy’s Law does not possess any mystical powers of providence. In reality, it is we who give the law its validity. The management of diseases is comparable to the governance of other safety-related systems on a ship. The consequences of system failure reflect the choices made: when you make a choice, you also choose the consequences.
If sufficient fail-safes are considered in advance, the repercussions of unexpected incidents can be mitigated. By identifying the potential risks associated with communicable diseases and taking appropriate preventive measures, both the likelihood and severity of communicable disease outbreaks on board significantly decrease, protecting a company’s brand, reputation, and revenue in the process. Safeguarding the health and safety of crew and passengers, therefore, is not a matter of chance, but a matter of choice.