The control process of malaria in Changsha from 1950 to 2019 included three stages (Figure 4), which demonstrated the great changes of malaria from control to elimination.
The first stage, from 1950 to 1989, was a period of effective control of malaria. From 1950 to 1962, measures taken in this phase included actively treating the infectious sources, carrying out environmental improvement, training technical team and conducting malaria surveys; from 1963 to 1978, the leading group implemented the prevention and treatment strategy like prophylactic medication, radical cure malaria victim in resting phase, current illness and long-term recurrence, with the aim to narrow the epidemic areas and decrease incidences, which was the main phase of the whole process; from 1979 to 1989, the workgroup detected and eliminated the remaining infectious sources, strengthened the monitoring and management to consolidate previous achievements and prepared for the eradication of malaria.
The second stage, from 1990 to 2010, was a period of elimination of malaria. Malaria surveillance measures have been adopted to detect the remaining sources of infection and manage the floating population [16,17]. Firstly, hospitals at all levels should do malarial parasite examination for fever patients, register, classify, investigate and manage all malaria patients . Secondly, the professional staff of Changsha CDC are supposed to carry out active investigations of the missed cases from the municipal, county and township hospitals every year. Except for using drug to kill mosquitoes, epidemiological investigation and treatment were carried out in the epidemic regions. Thirely, blood tests were taken on fever patients with history of travel or migrant population, students from high epidemic countries or areas of malaria in colleges should be registered and given preventive medicine [19,20].
The third stage, from 2011 to 2019, was a period of prevention of reintroduction. Changsha launched the Action Plan for Malaria Eradication in 2011 , established the leading group, and appointed Changsha First Hospital as the designated hospital for malaria treatment. Changsha CDC has set up a parasitic disease prevention and control department with 5 full-time staffs, and appointed commissioners in each district to ensure the orderly progress of malaria elimination. Measures like investigation and treatment of malaria cases, improvement of microscopy network, standard drug administration, strengthen mosquito monitoring have been implementated, the ability of scientific control for Malaria have been strengthen. More actions through media, radio, television, microblog, WeChat, Website and so on have been taken to strengthen health education and raise awareness of disease prevention. Since 2011, relevant personnel have been organized to participate in the training of provincial malaria prevention and treatment techniques every year and the knowledge of malaria prevention and treatment has been promoted throughout Changsha.
The results showed that before the year 1978, Changsha was the tertian malaria epidemic area with anopheles mosquito as the main transmitting vector; the incidence rate of malaria was at a high level, while the fatality rate was at a low level in Changsha. From 1979 to 1989, the incidence of malaria has decreased dramatically. In 1980s, cases of malaria was just 1542 and the fatality rate was 0; the epidemic of malaria in Changsha has been well controlled under the relevant prevention and control measures in the first stage, which have made full preparations for malaria eradication. After 1990, the incidence rate of malaria in Changsha has been controlled below 1/100,000, and in the 2000s, the incidence has reached the lowest to 1.17/1,000,000 which almost reached the standard of elimination. The last local case of malaria in Changsha was reported in April 2010, which means the elimination of malaria in Changsha has achieved a phased victory.
However, in recent years, with the acceleration of scientific, technological, economic and cultural globalization, especially the implementation the spirit of “The Belt and Road”, labor, economic and trade exchanges between Changsha and high endemic areas of malaria has become increasingly frequent [21-25]. Since 2011, all malaria cases in Changsha were imported. As the sources of imported malaria were persistent existed, the vector of malaria was not completely eliminated and the risk of malaria reoccur in Changsha would still exist, which posed a great challenge to malaria elimination in the province. In addition, we found that, after 2010, the fatality rate of malaria in Changsha has reached the highest level in the past 70 years. Infection with different kinds of plasmodium usually results in different clinical outcomes in patients [26,27]. The most virulent and fatal type of malaria in the worldwide is caused by pernicious malaria , which is a major cause of death and neurological disease. In the past, the common malaria cases reported in Changsha were tertian malaria, while in recent years, with the increase of imported cases, a number of ovale, quartan and mixed infection malaria have been reported and the proportion of pernicious malaria has increased rapidly. After 2010, the proportion of pernicious malaria in Changsha was more than 50%, with a maximum of 86.67% in 2014. This may be the main reason for the high fatality rate of malaria after 2010, which made it more difficult for diagnosis and treatment .
In this study, an adjusted comprehensive assessment method was used to assess the risk of re-transmission of imported malaria in different districts of Changsha. The results showed that the risk level of re-transmission in Liuyang was the highest, which may be related to the large number of migrant workers to Africa or Southeast Asia. Most of them had low education level and did not fully understand the protection knowledge of malaria. In addition, the medical level of Africa and Southeast Asia were backward, the migrant workers generally did not have malaria related antibodies, and it took several weeks or even months to get sick after the first infection of Plasmodium, which led to widespread infection and transmission. The risk level of re-transmission in Yuelu district were relatively high, which may be related to the fact that this district is the political and cultural center of Changsha. Municipal government, Xiangjiang new area and many colleges and universities are here, which has attracted many experts, scholars and students from malaria endemic areas to visit, study and exchange here. Kaifu District and Changsha County had the lowest re-transmissi risk because of their strong prevention and control ability and few imported cases.
Great importance should be attached for prevention and control of imported malaria outbreaks, with the focus on monitoring people from high epidemic area of malaria and making accurate diagnosis and in-time treatment, once suspected cases were been found, they should be dealt with actively to cut off the source fundamentally [30,31]; It's better to enhance the professional training of clinicians, inspectors and information managers of infectious diseases to improve their skills in malaria prevention, microscopy, clinical diagnosis, treatment and prevention and control, especially for some basic health service organization or general hospitals with weak technology; Moreover, it is necessary to strengthen the health education of local residents and establish their awareness of malaria prevention. Finally, cooperation among the departments of public health, immigration, entry-exit inspection and quarantine services, police and the commercial sector would play an important role in the detection, prevention and management of imported malaria , it is necessary to strengthen the cooperation between various departments to ensure the implementation of prevention and control measures.
After 70 years efforts, Changsha has achieved the goal of malaria elimination. However, the risk of re-transmission still exists, especially in Liuyang and Yuelu district. In accordance with the provincial and municipal work arrangements and requirements, we should continue to take malaria control and prevention as an important preventive work and maintain the sustainability of malaria surveillance, effectively manage imported malaria cases, implement various prevention and control measures as required to prevent the secondary spread of scientifically and effectively [33,34].
Due to the capacity of malaria testing and the availability of medical facilities before the year 2000, some underreporting and misreporting of malaria cases may exist during the study period, in addition, our assessment approach did not include meteorological or socio-economic indicators, which is the limitation to this study.