Yellow Fever

 

By Bopha Yok

 

Yellow fever also known as yellow jack, black vomit, or “vomito negro” in Spanish is a viral disease that causes hemorrhagic illnesses in Africa and South America.  (1)  Its means of transmission can be categorized into three types: sylvatic, intermediate, and urban.  Sylvatic, also known as jungle yellow fever occurs in tropical rainforests where monkeys are infected by wild mosquitoes.  The monkey is infected therefore passing the virus onto other mosquitoes that feed on them, and then these infected mosquitoes will bite humans that enter the rainforest leading to sporadic cases of yellow fever.  Intermediate yellow fever is caused by the infection of monkeys and humans by a semi-domestic mosquito.  In this type many different villages in an area suffer simultaneously from yellow fever.  There is also an area known as the “zone of emergence” where increased contact with infected mosquitoes and man leads to disease.  Lastly, urban yellow fever is the cause of large epidemics where migrants who are infected introduce the virus into a heavily populated area.  Domestic mosquitoes carry the virus from human to human and no monkeys are involved in transmission.  The usual reservoir for yellow fever is the mosquito, more specifically from several different species of the Aedes and Haemogogus (South America only).  (2) 

 

The etiological agent for Yellow Fever is the yellow fever virus, which is an arbovirus of the family Flaviviridae.  It is actually one of the smallest RNA viruses that have been isolated.  (1)  Key tests for identification of this virus include the following: a viral culture of blood or tissue specimens, identification of the yellow fever virus antigen or nucleic acid in tissues using immunohistochemistry, enzyme linked immunosorbent assay (ELISA) antigen capture, polymerase chain reaction tests, and sometimes the detection of the yellow fever specific IgM antibody by capture ELISA with conformation of greater than a four-fold rise in neutralizing antibody titer.  (3)

 

The Yellow Fever Virus was discovered in the early part of the twentieth century in Havana City, Cuba where a U.S. Army yellow fever commission led by Walter Reed proved that yellow fever was a viral disease transmitted by a specific mosquito named Aedes aegypti.  (4)  In fact, one of his colleagues, a physician named James Carroll allowed an infected mosquito to feed on him and he developed a severe case of yellow fever, to help his friend Walter prove that mosquitoes transmitted this disease.  (5)

Yellow fever is so named because of the jaundice that is displayed by some people who are infected by the yellow fever virus.  (2)  Jaundice causes the skin color to turn yellow hence the name Yellow fever.  The reason Yellow fever is also known as black vomit is because another symptom of this disease can be hemorrhaging in the gastrointestinal tract which causes bloody vomit with a distinctive coffee-grounds appearance hence the name black vomit.  (1)

 

The Yellow Fever virus can lie dormant within the body during an incubation period of three to six days.  Then there are two disease phases: acute and toxic.  In the acute phase the following symptoms may occur: fever, muscle pain (mostly backache), headache, shivers, loss of appetite, nausea, and/or vomiting.  After three or four days most patients recover and their symptoms improve.  However, 15% of those infected experience the toxic phase within 24 hours and the following symptoms may occur: fever reappears, jaundice, vomiting with severe abdominal pain, bleeding from the mouth, nose, eyes, or stomach, then blood will appear in feces and vomit, or kidney deterioration.  (2)

 

Since Yellow fever is caused by a virus one virulence mechanism can be the fact that the virus grows intracellularly inside the host cell, where components of the immune system cannot reach them.  Another virulence mechanism that contributes to this disease process is that the yellow fever virus can gain access to cells because they have attachment sites for receptors located on their target cells.  In this case the virulence factor for yellow fever is the FC receptor binding spot where a “Non- neutralizing antibody has been shown to enhance the virulence of the alpha and flaviviruses by promoting their binding to FC receptors.”  (7)

 

Control of yellow fever can be obtained by eliminating mosquito breeding sites.  This is an effective way to prevent the transmission of this disease however “for prevention and control of yellow fever, priority is placed on vaccination programs.”  (2)  There are other preventive measures that you can do to decrease the risk of infection such as wearing clothes that cover your entire body, wearing insect repellant, and remaining in well-screened areas.  (6)  Unfortunately there is no specific treatment for yellow fever.  However, when dehydration and fever is experienced it can be treated with oral rehydration salts and paracetamol.  If there is an associated bacterial infection then it should be treated with the appropriate antibiotic.  (2)

 

Locally in the United States there has not been a major yellow fever outbreak since the early 1900’s.  (1)  In fact the majority of yellow fever cases and deaths take place in sub-Saharan Africa where it is at epidemic levels.  In ten South and Central American countries and in several Caribbean islands, yellow fever has reached endemic levels.  (8)About 90% of an estimated 200,000 annual cases of YF occur in Africa, where outbreaks are common and where both the urban and the jungle type of transmission operate. In South America, the jungle type of YF predominates, either in individual cases or localized outbreaks.” (9)  Below you can view the most recent incidence figures for yellow fever in Africa and South America that can be found on the World Health Organization website.  (10)

 

 

 

The main type of prevention for yellow fever is vaccination.  The yellow fever vaccine works by causing your body to produce antibodies against the virus.  (11)  This protective immunity occurs within one week in 95% of the people who are vaccinated.  A single dose can provide protection from yellow fever for 10 years and most likely for life.  (2)  It is also very important to bring awareness of the yellow fever vaccine to areas in dire need.   “To prevent an epidemic in a country, at least 80% of the population must have immunity to yellow fever.”  This can be achieved by incorporating the yellow fever vaccine into childhood immunization programs and of mass campaigns. (2)  If this can occur then it will help the future spreading of yellow fever outbreaks.  Another effective means of control is surveillance of the disease in areas where vaccination is not available.  Many cases of yellow fever are not reported which increases the spreading of this deadly disease.  The World Health Organization has recently recommended that all countries at risk for yellow fever have at least one national laboratory where basic yellow fever blood tests can be performed.  (2)  “Given the likelihood that other cases have occurred (but have not been detected), one confirmed case of yellow fever is considered to be an outbreak.”  (2)  By confirming one case of yellow fever an investigation team can explore and hopefully terminate the source, and start an immunization process to protect others from getting infected.  

 

 

References:

 

(1)  Wikipedia contributors, Yellow Fever, 12/8/2006, URL: http://en.wikipedia.org/wiki/Yellow_fever, accessed on 12/9/2006.

 

(2) World Health Organization, Yellow Fever, 12/2006, URL:  http://www.who.int/mediacentre/factsheets/fs100/en/, accessed 12/9/2006.

 

(3)  Martin S. Cetron, M.D., Anthony A. Marfin, M.D., Kathleen G. Julian, M.D., et. al., Yellow Fever Vaccine Recommendations of the Advisory Committee on Immunization Practices (ACIP), 2002, 11/8/2002, URL:  http://www.cdc.gov/MMWR/preview/mmwrhtml/rr5117a1.htm, accessed on 12/9/2006.

 

(4) Chastel, C., Centenary of the discovery of yellow fever virus and its transmission by a mosquito (Cuba 1900 -1901), 08/2003, URL: http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=14582304&dopt=Abstract accessed on 12/10/06.

 

(5)  ANON, U.S. Army Physicians Discovered the Cause of Yellow Fever August 27, 1900, 4/24/2000, URL:  http://www.americaslibrary.gov/cgi-bin/page.cgi/jb/progress/yellow_1, accessed on 12/9/2006.

 

(6)  Centers for Disease Control and Prevention, Yellow Fever Vaccine What You Need to Know, 11/9/2004, URL: http://www.cdc.gov/nip/publications/VIS/vis-yf.pdf, accessed on 12/11/06.

 

(7)  ANON, Yellow Fever, 4/6/1999, URL: http://medinfo.ufl.edu/year2/mmid/bms5300/bugs/yellowf.html#AA4, accessed on 12/11/06.

 

(8) World Health Organization, Yellow Fever: A Current Threat, 7/22/2005, URL: http://www.who.int/csr/disease/yellowfev/impact1/en/index.html, accessed on 12/10/06.

 

(9)  World Health Organization, Weekly Epidemiological Record, 10/3/2003, URL: http://www.who.int/wer/2003/en/wer7840.pdf, accessed on 12/10/06.

 

(10)  World Health Organization, Epidemiological Trends and current situation of Yellow Fever, 7/22/2005, URL:  http://www.who.int/csr/disease/yellowfev/surveillance/en/, accessed on 12/10/06.

 

(11)  Medline Plus contributors, Yellow Fever Vaccine (Systemic), 12/12/06, URL: http://www.nlm.nih.gov/medlineplus/druginfo/uspdi/202689.html, accessed on 12/12/06.