Henan Province, whose population is 90 million, consists of 170 counties and covers over 167 000 [km.sup.2] around the Yellow River in the centre of northern China (1). The Chinese civilization began on the banks of the Yellow River and the presence of malaria in Henan has been reported for at least 2000 years. In the twentieth century Henan has been an area of nonstable endemic vivax, malariae, and falciparum malaria, with recent major outbreaks occurring in 1954-55, the mid-1960s, and 1970 (2).
The 1970 epidemic affected the whole province, with 10.2 million cases reported. This was the highest annual incidence of malaria ever recorded in China, but over the next two decades the overall number of malaria cases in Henan fell. In 1992 there were only 318 reported cases, a provincial incidence of 0.37 per 100 000 (3), almost all of which were in the south, with county incidences below i per 10 000. Falciparum malaria, primarily a problem in southern Henan, also decreased, with 2922 cases in 1984, 5 in 1987, and none subsequently.
The fall in malaria incidence after the 1970 epidemic resulted from a massive investment in malaria control by the province. The programme began with chloroquine-primaquine treatments for passively detected clinical cases during the transmission season, relapse prevention using chloroquine-primaquine between transmission seasons, and mass prophylaxis with pyrimethamine every 10 days during the transmission season if the area incidence exceeded 10%. From 1971 to 1984 the government dispensed 280 million treatment courses, 1.8 x [10.sup.9] anti-relapse courses, and about 2 x [10.sup.9] chemoprophylaxis courses (2).
From 1981 to 1984 the government began active case detection and examined more than 2 million blood smears for malaria parasites. Falciparum malaria persisted in some poor southern border counties in the early 1980s (Fig. 1). Over the period 1985-89 a total of 19.6 million more blood smears were performed, mostly in the south of the province. Also from 1985 to 1992 annual indoor spraying of houses and animal sheds with residual DDT was carried out in southern border counties, protecting over 1 million people at highest risk (2, 4). Over the period 1988-92 annual bednet impregnation with pythrethroids was added to the vector control activities because it was cheaper; 95% of the population of 2 million covered had bednets. Overall, southern areas of the province known to have Plasmodium falciparum or its vector Anopheles anthropophagus were exposed to a least 2 years of insecticide-based adulticidal mosquito control.
[Figure 1 ILLUSTRATION OMITTED]
The disappearance of falciparum malaria in southern Henan Province since 1987 was a result of the integrated vector control (4). In the past, P. vivax malaria had been predominant in these southern counties, but outbreaks of P. falciparum malaria occurred in the early 1980s (2, 4-6). Those southern counties (S of latitude 33 [degrees] N) harboured the endoanthropophilic vector A. anthropophagus, which was responsible for the P. falciparum transmission (2, 6). The density of A. anthropophagus was greatly reduced by the vector control over the period 1985-92 (4), but the exophilic A. sinensis vector persisted. A. anthropophagus has a vectorial capacity 20 times that of A. sinensis and is therefore a much more dangerous malaria vector (2, 4-6).
By 1994 the whole of Henan Province, including the southern area, was held to have fulfilled the Chinese Ministry of Health criteria for "basic elimination of malaria" (see below) (7), and provincial support for the indoor spraying and the impregnated bednet programme ceased (4). At this time it was thought that residual threats would come mainly from the inefficient but widespread exo-zoophilic vector of vivax malaria, A. sinensis, coupled with cross-border importation of exogenous malaria and humans who slept outdoors during the hot season (5, 8, 9). …