The significance of Yellow Fever
Yellow fever virus is estimated to cause 200,000 cases of disease and 30,000 deaths globally each year, with 90% of those cases occurring in Africa (Centers for Disease Control and Prevention, 2018). The ‘yellow’ in the name refers to the skin color that occurs when the infection involves the liver (SA Health, 2017). Yellow fever has shaped evolutionary history and distribution from humans’ susceptibility to hosting the virus, to human trade and movement, and environmental factors that led to an increase in the abundance of vectors across the tropics and subtropics. Despite the unsolved puzzle of discovering the antiviral for the disease that has plagued humanity for centuries, preventative measures are encouraged to be practiced as it is the best current means of combating this disease.
Epidemiology
Yellow Fever is a mosquito-borne disease caused by a virus found in tropical and subtropical areas of South America and Africa, which is caused by a virus that is spread to people by the bite of an infected mosquito (Centers for Disease Control and Prevention, 2018). The Aedes aegypti mosquito transmits the virus back and forth between monkeys and humans as both are commonly infected (Mayo Clinic, 2018). Yellow fever has two cycles: the sylvan cycle in which mosquitos primarily spread the disease among forest-dwelling primates, and the urban cycle in which the infection is spread from human to human (National Organization for Rare Disorders, 2009). This viral infection causes damage to the liver, kidney, heart, and gastrointestinal tract (National Organization for Rare Disorders, 2009). Travelers to and residents of high risk countries, including sub-Saharan Africa and tropical South America, may be at risk of contracting the disease (Mayo Clinic, 2018).
History and Yellow Fever
Yellow fever virus, and its urban mosquito vector, both are believed to have originated in Africa where a number of primate species were infected (Alan & Ralph, 2017). According to Alan & Ralph, the virus and its mosquito vector were transported on ships sailing from West Africa to the West Indies. The eastern regions of the Americas later got infected due to transport by coastal shipping of both the vector and infected humans. Through further human associations, the urban mosquito vector of the yellow fever virus became established in more eastern subtropical and tropical regions of the world, including India, Indonesia and the southern Pacific (Alan & Ralph, 2017). For more than 300 years, the coastal Americas and Caribbean islands had rampant outbreaks starting from the 1640s, with the largest recorded yellow fever epidemic with the highest case fatality rate occurring in 1802 in Saint-Domingue (present day Haiti) (Marr & Cathey, 2013). There have been several controversies to the claim of discovering the geographical origin and the transmission of the vector by France, Cuba, and the USA. The emergence and reemergence of vector-borne diseases encompasses many factors but often yellow fever outbreaks occur if at least three conditions exist: the introduction of the virus into a non-immune human community, presence of competent and anthropophilic vectors and insufficiency of prevention and/or adequate management of the growing outbreak (Chippaux & Chippaux, 2018).
Clinical Manifestation
The yellow fever virus incubates in the body for 3 to 6 days and many people often do not experience symptoms. However, the most common symptoms that are associated with the virus during the acute phase includes fever, muscle pain with prominent backache, headache, loss of appetite, and nausea or vomiting (World Health Organization, 2018). Most people improve after 3 to 4 days but 15% of people enter into a second more toxic phase with liver and kidney failure. Of these severe cases, around 50% are fatal (SA Health, 2017). The signs and symptoms of the toxic phase can include jaundice, bleeding from the nose, mouth, and eyes, bradycardia, brain dysfunction, and abdominal pain and vomiting of blood (Mayo Clinic, 2018).
Incidence and Prevalence of Yellow Fever
Vector-borne diseases are currently a major problem in tropical developing countries and is causing an up rise in mortality and high levels of disability-adjusted life-years (DALYs). When the incidence, mortality, and case fatality rates were calculated for Caucasians and non-Caucasians during the 19th century yellow fever epidemics in the United States, statistical significance identified that Caucasians were more likely to succumb to yellow fever than non-Caucasians (Blake & Garcia-Blanco, 2014). From the sixteen epidemics for which data was available, the case fatality rate for Caucasians varied from 25.0 to 72.5% versus non-Caucasians of 1.1 to 14.1% with the odds of Caucasian patients succumbing to yellow fever being 14.6 times greater (with a 95% confidence interval of 13.59 to 15.76) (Blake & Garcia-Blanco, 2014). The World Health Organization (WHO) estimates that South America is now at greater risk of urban epidemics than at any time in the past 50 years (Butler, 2016). Figure 1. illustrates the reported cases in South America and Africa from 1980 to 2017.
Fig. 1. Confirmed case reports in Africa and South America between 1980 and 2017 (based on WHO, weekly epidemiological record: http://www.who.int/wer/en/ – accessed on October 17, 2018.
Standard Therapies and Yellow Fever
There is no antiviral to combat yellow fever but supportive care to treat dehydration and symptomatic relief of pain may be required. Preventative measures include vaccination (YF 17D), immunization by an approved yellow fever vaccination clinic, exclusion of people with the disease is recommended, and personal protection and environmental management of mosquitoes are very important (SA Health, 2017). Max Theiler, Professor of Epidemiology and Public Health at Yale from 1964-67, and whose work on virus attenuation created the modern vaccine and earned him the Nobel Prize (Frierson, 2010). Immunity from a vaccination usually develops after 10 days and lasts for more than 10 years. In some cases, side effects associated with vaccination may occur 5 to 10 days later. These side effects affect approximately 5 percent of individuals receiving immunization and may include headache, fever, and/or aching muscles (National Organization for Rare Disorders, 2009). The world’s vaccine production is just over 40 million doses annually, which have proven to be sufficient to replenish emergency stockpiles and contain small outbreaks (Butler, 2016). In 2006, an initiative led by the WHO upped mass vaccinations and introduced routine immunization of children in many high risk countries in Africa. However, with only four suppliers worldwide, supply is falling short of demand and diluting the vaccine to boost stocks requires a specific kind of single-use, small volume syringe, which is not commercially available (Butler, 2016). Due to insufficient research on the virus, recent growth of large cities and mosquito-infested slums, and unvaccinated people in high risk areas, many scientists warn that the re-emergence of the deadly infection could overwhelm vaccine stocks.
Diagnostic Testing and Yellow Fever
Yellow fever diagnosis is generally accomplished by testing serum to detect virus-specific IgM and neutralizing antibodies. In fatal cases, nucleic acid amplification, histopathology with immunochemistry, and virus culture of biopsy or autopsy tissues can also be positive. CDC, along with a few state laboratories are capable of doing specialized testing. A presumptive diagnosis of the disease if often based on the patient’s clinical features, places and dates of travel (if the patient is from a non-endemic country or area), activities, and epidemiologic history of the location where presumed infection occurred.
Conclusion
The yellow fever epidemic has played its part in changing the course of Old and New World history and altered the New World geopolitics (Blake & Garcia-Blanco, 2014). Since then, knowledge of the basic preventative and control measures were identified and established in order to combat the disease and mitigate outbreaks. Despite fear of an outbreak of greater extent, everyone can play their part in reducing risk factors and preventing future outbreaks. Through combined efforts of getting vaccinated, wearing protective clothing, avoiding contact with infected people, and practicing preventative measures while traveling, the yellow fever statistics can be a part of the past and fatalities reduced.
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