[Emerging Infectious Diseases]
[Volume 4 No. 2 / April - June 1998]

Dispatches

Reemergence of Plasmodium vivax Malaria in the Republic of Korea

Brian H. Feighner, Son Il Pak, William L. Novakoski, Lori L. Kelsey, and
Daniel Strickman
U.S. Army, Republic of Korea

  --------------------------------------------------
      Plasmodium vivax malaria reemerged in the Republic of Korea in
      1993. The number of cases has tripled each year since, with
      more than 1,600 cases reported in 1997. All 27 cases in U.S.
      troops resolved uneventfully with chloroquine/primaquine
      therapy. Disease is localized along the western Demilitarized
      Zone and presents minimal risk to tourists.

Plasmodium vivax malaria has been endemic on the Korean peninsula for
centuries (1). The primary vector is believed to be Anopheles sinensis, a
zoophilic and facultatively anthropophilic mosquito, which breeds in fresh,
sun-exposed water such as that found in rice fields (2). During the Korean
War (19501953), U.S. troops were exposed to malaria (3) and received
weekly chloroquine chemoprophylaxis. This regimen generally succeeded in
preventing malarial attacks while U.S. troops were in the Republic of
Korea. As the troops returned home, however, U.S. military hospitals were
inundated with cases of malariaat one point, as many as 629 cases per week
(3). More than 3,000 cases of malaria were documented in U.S. troops that
served during the war. Subsequent U.S. military research led to the current
use of primaquine therapy (4). The outbreak of malaria continued on the
peninsula after the armistice. In the 1960s and 1970s, the Republic of
Korea government combined active and passive case detection with extensive
use of pesticides in an ambitious eradication project (1). Indigenous
transmission of malaria appeared to cease in the mid-1970s, and in 1979 the
Republic of Korea was declared malaria-free by the World Health
Organization (5).

In 1993, P. vivax malaria reemerged in the Republic of Korea along the
western edge of the Demilitarized Zone (DMZ) (6). Since the reemergence,
the annual number of cases has increased geometrically, tripling (1995,
1996) or quadrupling (1997) in each of the past 4 years (Table 1). In 1997,
the Republic of Korea Army administered antimalarial chemoprophylaxis to
approximately 8,000 of its troops at greatest risk (U.S. Army/Republic of
Korea Army/Republic of Korea Malaria Symposium, pers. comm.). U.S. troops
in Korea were not placed on antimalarial chemoprophylaxis in 1997 after
local and U.S. Army experts concluded that personal protective measures and
community mosquito control would adequately limit transmission (7).

Table 1. Annual number of Plasmodium vivax malaria cases
in U.S. Forces-Korea (USFK), Republic of Korea (ROK) Army,
and ROK Civilians, 19931997

  --------------------------------------------------
                   ROK        ROK
Year    USFK      Army     Civilians          Total
  --------------------------------------------------

1993	  0          1           1                2

1994	  1         18          20               39

1995	 19          8          19              118

1996	 11        285          71              367

1997	 27      1,154         461            1,642
  --------------------------------------------------

Cases in 1997 were distributed as expected from June to October, with a
peak of 518 cases in August (Table 2). As in past years, more than 80% of
the cases were associated with military service along the western DMZ.
During 1997, 34 cases of P. vivax malaria in U.S. troops were attributed to
exposure in the Republic of Korea. This number includes 27 cases in U.S.
Forces-Korea troops, one case in a soldier stationed in Okinawa, Japan, who
was temporarily assigned to the Republic of Korea in August 1997, and six
cases in U.S. troops exposed in the summer of 1996. The U.S. Forces-Korea
incidence of P. vivax in all malarious areas between July 1, 1997, and
September 30, 1997, was estimated to be 0.5 cases per 1,000 per week. The
incidence northwest of the Imjin River was approximately one case per 1,000
per week. Site-specific rates, as well as rates for the Republic of Korea
Army, cannot be released because of security concerns; however, the
Republic of Korea Army has 50 times more exposed soldiers than does U.S.
Forces-Korea.

Table 2. Monthly distribution of Plasmodium vivax
malaria cases in the Republic of Korea (ROK), by
onset of disease, 1997

  --------------------------------------------------
                     ROK       ROK
Month   USFK(sup a) Army     Civilian      Total
  --------------------------------------------------
Jan          0        0         0            0
Feb          0        0         0            0
Mar          0        5         2            7
Apr          1       10         3           14
May          0       31         4           35
Jun          0      145        38          183
Jul          2      260       102          364
Aug         11      317       190          518
Sep         10      320        75          405
Oct          3       55        38           96
Nov          0        9         9           18
Dec          0        2       NA(sup b)      2
  --------------------------------------------------
(sup a)USFK, U.S. Forces-Korea.
(sup b)NA, Not available.

The epidemic remains focused along the western DMZ. Before 1997, almost all
civilian cases were attributed to exposure in areas within 10 km of the
DMZ. In 1997, however, dozens of cases occurred among civilians farther
than 10 km from the DMZ. The area where U.S. troops are exposed is strictly
controlled; tourists are not allowed in this area after dark. Other areas
under the Republic of Korea Army control, e.g., Yonchon and Chorwon, are
accessible to tourists but are seldom visited. The risk to foreign
tourists, therefore, appears to be negligible at this time.

Of the 27 U.S. Forces-Korea cases, 18 were in U.S. soldiers and 9 were in
Korean soldier augmentees to the U.S. Army. All cases were in men 19 to 40
years of age (median 21.5 years). With one exception, the cases occurred in
junior enlisted soldiers. As in the past 3 years, no cases occurred in
African American soldiers, who account for almost 30% of the exposed U.S.
troops. This is consistent with the known natural immunity against P. vivax
in African Americans, most of whom do not have the Duffy antigen on their
red blood cells (8). This relative immunity of African Americans was also
seen during the Korean War (3). The time from onset of symptoms to
diagnosis was 1 to 25 days (median 5 days). Three patients with mild
illness were treated as outpatients. All patients quickly recovered after
standard chloroquine therapy. All patients were also treated with
primaquine.

The Korean strain of P. vivax malaria has been well characterized by both
Korean and U.S. medical personnel (1,3). This type of malaria is termed
"temperate climate" P. vivax malaria (3). Transmission is restricted to
only a few summer months, when the Korean peninsula is warm enough to
support the parasite's reproductive cycle in the mosquito (1,3). This
Korean P. vivax malaria is remarkable in that 40% to 50% of cases have no
symptoms until 6 to 9 months after infection (3,9). The long incubation
period interferes with determining the start of the transmission season.
Malaria cases begin to occur in the late spring or early summer, before the
weather is warm enough to support malaria transmission. These early season
cases, arising from infections acquired the previous fall, blend into and
fuel infections transmitted each summer. Another result of this long
incubation period, given the extraordinary (>90%) annual turnover in U.S.
troops, is the return of infected soldiers to the United States before any
symptoms appear.

Malaria in the Republic of Korea is also characterized as epidemic or
unstable because conditions usually do not support widespread disease (10).
Factors contributing to this instability include the extremely short
transmission season and the zoophily of the major vector. Areas with
unstable malaria usually have low to very low rates of disease but can
occasionally have high rates in response to climatic, livestock, or
sociopolitical aberrations (10). The current epidemic may fit this pattern.
Heavy rains from 1994 to 1996 led to widespread crop failures in the
Democratic Peoples Republic of Korea. As a result, domestic farm animal
populations, the preferred source of blood for A. sinensis, fell sharply.
These factors may have contributed to the geometric increase of disease,
but the lack of data makes this explanation speculative.

Most Korean officials perceive the current epidemic to be a Democratic
Peoples Republic of Korea public health problem spilling over the DMZ and
discount the importance of indigenous transmission in the Republic of
Korea. Broad-scale epidemiologic investigation is necessary to confirm
indigenous transmission. Although it is not known if the epidemic will
continue its geometric increase and geographic expansion in 1998, a
decrease in disease without extensive intervention seems unlikely.

Acknowledgments

The authors thank Colonel Daniel F. Perugini, U.S. Forces-Korea Surgeon,
and Colonel Kenneth G. Torrington, Deputy Commander, 18th Medical Command,
for their support and critical review of this manuscript.

All equipment, supplies, and author salaries were received from the U.S.
Army.

Address for correspondence: Brian H. Feighner, HHC, 18th MEDCOM, Unit
#15281, Box 763, APO AP 96205-0054 USA; 822-7916-3028; e-mail:
ltc_brian_feighner@smtplink.seoul. amedd.army.mil.

References

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     Experimental Medicine 1988;58:55-66.
  2. Ree HI, Hong HK, Paik YH. Study on natural infection of P. vivax in
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  3. Hankey DD, Jones R, Coatney GR, Alving AS, Coker WG, Garrison PL, et
     al. Korean vivax malaria. I. Natural history and response to
     chloroquine. Am J Trop Med Hyg 1953;2:958-69.
  4. Jones R, Jackson LS, DiLorenzo A, Marx RL, Levy BL, Kenny EC, et al.
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  8. Miller LH, Mason SJ, Clyde DF, McGinness MH. The resistance factor to
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     Engl J Med 1976;295:302-4.
  9. Shute PG, Lupasco G, Branzei P, Maryon M, Constantinescu P,
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Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

URL: http://www.cdc.gov/ncidod/EID/vol4no2/feighner.htm

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