Yellow Fever

Yellow fever was first recognized with an epidemic in the Yucatan of Mexico, in 1648, probably introduced there from Africa. One

28.4 Viral Diseases of the Lymphoid and Blood Vascular Systems 729

Table 28.6 "Kissing Disease" (Infectious Mononucleosis, "Mono")

Table 28.6 "Kissing Disease" (Infectious Mononucleosis, "Mono")

Symptoms

Fatigue, fever, sore throat, and enlargement of lymph nodes

Incubation period

Usually 1 to 2 months

Causative agent

Epstein-Barr (EB) virus, a DNA virus of the herpesvirus family

Pathogenesis

Productive infection of epithelial cells of throat and salivary ducts; latent infection of B lymphocytes; activation of B and T lymphocytes; hemorrhage from enlarged spleen is a rare but serious complication

Epidemiology

Spread by saliva; lifelong recurrent shedding of virus into saliva of asymptomatic, latently infected individuals

Prevention and treatment

Avoid sharing of articles such as toothbrushes and drinking glasses, which may be contaminated with the virus from saliva.Treatment: usually none needed; acyclovir of benefit in rare cases

of the worst outbreaks of yellow fever in the twentieth century occurred in Ethiopia in the 1960s, producing 100,000 cases and 30,000 deaths. In 1989, an epidemic of yellow fever occurred in Bolivia among poor people who moved into the jungle to try to make a living growing coca. There have been no outbreaks in the United States since 1905, but the vector mosquito has reappeared in the Southeast, raising the possibility of outbreaks of the disease if the virus is again introduced.

Symptoms

The symptoms of yellow fever can range from very mild to severe. Symptoms of mild disease, the most common form, may be only fever and a slight headache lasting a day or two. Patients suffering severe disease, however, may experience a high fever, nausea, bleeding from the nose and into the skin, "black vomit" (from gastrointestinal bleeding), and jaundice (hence, the name yellow fever). The mortality rate of severe yellow fever cases can reach 50% or more. The reasons for the wide variation in severity of symptoms are unknown, but probably have more to do with the size of the infecting dose and the status of human host defenses than with differences among strains of the causative virus.

Causative Agent

Yellow fever is caused by an enveloped, single-stranded, positivesense RNA arbovirus of the flavivirus family. The virus multiplies in species of mosquitoes, apparently without harming them, and the mosquitoes transmit the infection to humans.

Pathogenesis

The yellow fever virus is introduced into humans by the bite of an Aedes mosquito, the biological vector. It multiplies, enters the bloodstream, and is carried to the liver and other parts of the

Causative Agents
Figure 28.10 Distribution of Yellow Fever Extensions may sometimes also occur into Central America.

body. Viral liver damage results in jaundice and decreased production of clotting proteins, and injury to small blood vessels produces petechiae, tiny hemorrhages, throughout the body. The virus affects the circulatory system by directly damaging the heart muscle, by causing bleeding from blood vessel injury in various tissues, and by causing disseminated intravascular coagulation (DIC). Kidney failure is a common consequence of loss of circulating blood and low blood pressure. ■ biological vector, p. 491 ■ disseminated intravascular coagulation, p. 720

Epidemiology

The reservoir of the disease is mainly infected mosquitoes and primates living in the tropical jungles of Central and South America and in Africa (figure 28.10). Periodically, the disease spreads from the jungle reservoir to urban areas, where it is transmitted to humans by Aedes mosquitoes.

Prevention and Treatment

In urban areas, control of yellow fever is achieved by spraying insecticides and eliminating the breeding sites of its principal vector, Aedes aegypti. In the jungle, control of yellow fever is almost impossible because the mosquito vectors live in the forest canopy and transmit the disease among canopy-dwelling monkeys. A highly effective attenuated vaccine is available to immunize people who might become exposed, including foreign travelers to the endemic areas. There is no proven antiviral treatment. ■ attenuated vaccine, p. 422

The main features of yellow fever are presented in table

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