Reprinted from the Bulletin of the Pan American Sanitary Bureau 
Vol. XLI, No. 2, August, 1956 
Printed in U.S.A. 
RESULTS OF RECENT STUDIES OF YELLOW 
FEVER IN MIDDLE AMERICA 
Preliminary Note*! 
Dr. CARL M. JOHNSON 
Director , Gorgas Memorial Laboratory , Panama 
Dr. STANFORD F. FARNSWORTH 
Zone Representative , Pan American Sanitary Bureau , Guatemala 
The Gorgas Memorial Laboratory, Pan¬ 
ama, in cooperation with the Pan American 
Sanitary Bureau, has been engaged for the 
past seven years in studies relating to the 
epidemiology of sylvan yellow fever in the 
Middle American area. During the past year 
the Gorgas Memorial Laboratory has re¬ 
ceived financial support for the yellow fever 
work from the Office of the Surgeon General, 
Department of the Army. 
This disease appeared in Panama in the 
final months of 1948 after an absence of 43 
years and has progressed in wave-like fashion 
through Costa Rica, Nicaragua, Honduras 
and into Guatemala where it is presently 
active in forest primates. While there have 
been a number of human fatalities recognized 
(six in Panama, 37 in Costa Rica, 11 in 
Nicaragua and one in Honduras), serious 
threat to the human population has been 
largely circumvented by prompt preventive 
measures. During the last year and a half 
the known fatalities have been limited to 
monkeys. After reaching Honduras in 
1953-54 virus activity became inapparent 
for a period of more than a year and did not 
again become evident until December 1955. 
Recent results of the yellow fever studies, 
which are still in progress, are of such public 
health significance that it seems desirable 
to present a preliminary summary. 
1. In January 1956 the Gorgas Memorial 
Laboratory received from the P.A.S.B. office 
* Received for publication in July, 1956. 
1 The information presented herewith will ap¬ 
pear in amplified and final reports under the 
authorship of the staff members who actually 
carried out the field and laboratory studies. 
in Guatemala the liver of a dead howling 
monkey (. Alouatta ) found by Dr. Jorge 
Boshell in the Rio San Francisco del Mar 
area on the east coast of Guatemala. Post¬ 
mortem degenerative changes were so pro¬ 
nounced that a definite diagnosis could not 
be made, but it was the opinion of one of us 
(C. M. J.) that the animal had died of yel¬ 
low fever. The following month, February 
1956, five additional liver specimens from 
howling monkeys were obtained and sent 
to the Gorgas Memorial Laboratory by Dr. 
Boshell. Of these five, four showed the 
changes characteristic of yellow fever infec¬ 
tion (two from near Esparta, Honduras, one 
from Las Caobas, Guatemala, and one from 
Rio Blanco, Bananera, Guatemala). The 
fifth specimen (from La Barra, Guatemala) 
was negative. An additional howling monkey 
liver from Pedrera del Tipon, Guatemala, 
received 12 May 1956, was histologically 
positive. In addition yellow fever virus was 
recovered by Dr. Enid de Rodaniche from 
a glycerinated specimen of the same liver. 
2. Since the established South American 
vectors of yellow fever had been found to be 
rare in Honduras and completely absent in 
Guatemala, an attempt was made to deter¬ 
mine the species of mosquito responsible for 
transmission in these areas. Beginning in 
late March, 1956, mosquitoes were collected 
in Guatemala under the supervision of Dr. 
Jorge Boshell in zones in which there was 
evidence of epizootic activity. They were 
forwarded on ice to the Gorgas Memorial 
Laboratory in Panama where they were 
identified and sorted into batches by species 
by Drs. Harold Trapido and Pedro Galindo. 
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