infected actually show symptoms. Those who experience infection, whether clinically
apparent or not, develop a certain degree of immunity.
(3) The course of an epidemic. The course of an epidemic may be altered
considerably by the development of antibodies in a group of people representing a large
segment of the population. Thus, medical data concerning a particular country or area
may not indicate the presence of an arbovirus disease. It may not reveal its real
importance if it exists at a low level in the population because of a relatively immune
population. When susceptible troops are introduced into such an area, however, the
disease may wreak havoc among the newcomers.
5-21. YELLOW FEVER
Yellow fever is one of a group mosquito-borne, acute, hemorrhagic
fevers of viral etiology.
a. Signs/Symptoms and Diagnosis. Typically, there is sudden onset of
headache, fever, vomiting, and prostration, and later jaundice and changes in blood
composition. Internal hemorrhaging is evidenced by bloody spittle and passage of dark
colored, pitch-like feces. Laboratory confirmation of the diagnosis includes isolation of
the virus from the blood, demonstration of typical lesions of the liver, and other
b. Prognosis. While the fatality rate among natives of an endemic area is less
than 5 percent, it may be as high as 40 percent among unvaccinated non-natives
entering the area. The patient is infective for mosquito vectors shortly before onset of
fever and for 3 days thereafter. The vector mosquito is infective for man some 9 to 12
days after taking a blood meal from an infected person and remains so for life.
Typically, onset of fever in man occurs 3 to 6 days after inoculation by an infective
mosquito. Recovery confers lasting immunity.
c. Treatment. There is no specific therapy for yellow fever. For the first 3 days
of illness, patients should be cared for in areas that are screened against mosquitoes or
have been treated with an effective residual insecticide.
d. Method of Control. The "man loving" Aedes mosquito vector can be
controlled in urban areas by the methods outlined in paragraph 5-18. Attempts to
control the jungle dwelling Haemagogus vector thus far have met with failure. Active
immunization is very effective in the prevention of yellow fever.
5-22. DENGUE (BREAKBONE FEVER)
Dengue is a mosquito-borne virus that causes an acute, febrile
infection of sudden onset, in sometimes-explosive epidemic
proportions. Main areas of occurrence are shown in Table 5-1.