The Plasmodium vivax in China: decreased in local cases but increased imported cases from Southeast Asia and Africa

Currently the local P. vivax was sharply decreased while the imported vivax malaria increased in China. Despite Southeast Asia was still the main import source of vivax malaria, the trend of Africa become serious, especially for west and central Africa. Herein we have clarified the trend of P. vivax in China from 2004–2012, and made some analysis for the differences of imported vivax back from different regions. There are significantly different of P. vivax between Southeast Asia and Africa, also the difference was observed for different regions in Africa. Additionally, we have explored the possibility for the difference of the P. vivax between migrant workers back from west and central Africa and the prevalence of local population. This reminds us that surveillance and training should be strengthened by medical staffs on the imported P. vivax cases reported especially from west and central Africa, in order to reduce the risk of malaria reintroduction and, specific tools should be developed, as well as the epidemiological study to avoid the misdiagnosis such as P. ovale and P. vivax.

and Henan provinces where Anopheles sinensis mosquitoes was prevalent ( Figure 1). Since 2007, vivax malaria declined and the proportion of P. falciparum increased significantly.
After NMEAP launched in 2010, each malaria cases was then required to be investigated and identified as indigenous or imported, so since then every P. vivax case could be distinguished 16 . Though a 57.7% decrease in total P. vivax cases from 2010 (n 5 4,901) to 2011 (n 5 2,075), the proportion of imported P. vivax was has increased from 31.5% to 57.0%.
In 2012, a total of 1,143 P. vivax malaria cases were reported, responsible for 41.9% of the total malaria cases. The imported and local P. vivax was 909 and 234, taking up 79.5% and 20.5% of total P. vivax malaria cases, respectively. The local P. vivax was mainly occurred in Yunnan province (n 5 171 [73.1%]), particularly in the counties around the China-Myanmar border (Figure 2A) 17 . The imported P. vivax malaria was spread to most regions of China (23 provinces), among them, Yunnan (n 5 506 [55.7%]), Hunan (n 5 64 [7.0%]) and Sichuan (n 5 56 [6.2%]) had the largest number of imported vivax malaria cases ( Figure 2B). The common characterize of these provinces was imported vivax back from Southeast Asia, account for the most source of total P. vivax. For example, vivax cases from Southeast Asia into top five provinces including Yunnan, Hunan, Sichuan, Hubei  Difference between P. vivax from different regions of Africa. The P. vivax imported from different regions of Africa also varied. Regarding to west Africa, it was differing from other regions of Africa (Table 1). In 2012, P. vivax account for 9.5% (n 5 61) by west Africa, while this figure was 16.2% (n 5 131, p 5 0.0007) in other regions of Africa. The difference was also observed among the migrant workers from west Africa and central Africa (x 2 5 29.7962, p 5 0.0063), east Africa (x 2 5 33.9816, p 5 0.0002) and north Africa (x 2 5 64.2363, p 5 0.0002) ( Table 1). Surprisingly, there is little difference between the migrant workers from west Africa and south Africa (p 5 0.2043), where the P. vivax constituent ratio in south Africa was 9.2% (n 5 17).
Difference between P. vivax in migrant workers and local population in west and central Africa. Surveillance of malaria cases imported into China in 2012, reveals that 122 cases of P. vivax originated from 21 west and central African countries. In the same time period, there were 988 cases of imported P. falciparum from the same countries. This gives a surprisingly high ratio of 100512 P. falciparum to P. vivax infections coming back from these areas ( Table 2). In addition, low P. vivax prevalence was observed in west and central Africa because of the high prevalence of Duffy negative phenotype in the local populations, so comparing with this, the imported P. vivax into China from these regions was extremely high, which has aroused our interesting of the difference.

Discussion
P. vivax was the major species in China for a long time and was responsible for more than 90% of the local malaria 18 . Vector control and drug treatment of febrile individuals had been highly successful in the endemic areas in China 19 . However, P. vivax had outbreaks frequently in many counties of central provinces and serve as the predominant parasite species around 2006 12 . To solve this, the government has launched the National Malaria Control Program (2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015) and National Malaria Elimination Action Plan (2010-2020) to strengthen malaria prevention and control at all levels, therefore the epidemics have well controlled 14,20 .
However, due to the increasing Chinese investment and numbers of laborers working abroad, it may bring out high risks from malariaendemic areas such as Africa and Southeast Asia on malaria morbidity in non-endemic regions of China 21 . For example, it was estimated to be 40.8 billion investment and 22,717 million laborers in Africa in 2012, respectively; and these were increased by 13.0% and 38.3%, respectively, from numbers in 2011. Though Southeast Asia, particular Myanmar was still the main imported source of P. vivax malaria, patients from Africa especially west and central Africa was increased significantly. For instance, the proportion of vivax malaria from west and central Africa was only account for 2.1% (n 5 32) of imported P. vivax cases in 2010, but in 2012, this increased to 13.4% (n 5 122) ( Table 3). This may bring out the retransmission risk to the endemic areas where the suitable Anopheles sinensis are prevalent. Since little was known about the difference of the imported P. vivax by acquisition, here we characterize the P. vivax trend in China from 2004 to 2012, and made some analysis for the differences of imported vivax back from Southeast Asia and Africa in 2012. In this text, not every case could be distinguished as being indigenous or imported before 2010, because the data were not available. Only since the NMCAP launched in 2010, each malaria cases was then required to be investigated and identified as indigenous or imported.
Data showed Southeast Asia was still the major imported source of P. vivax, while migrant workers coming back from Africa were increased recently. The geographical distribution of P. vivax within Africa is patchy, with sporadic areas of transmission scattered throughout the continent, particularly extremely rare in west and central Africa, with the exception of the horn of Africa and other areas such as island of Sao Tome, which is known to harbour all four human malaria parasites 22 . Though P. vivax malaria was rare  reported in west and central Africa, migrant workers consistently return to China with vivax malaria was detected after visiting these regions, this was more obviously since 2010. We have wondered why the significant difference occurred between the imported P. vivax data and the low prevalence local population.
Rare accounts detailing P. vivax infections in non-African travellers returning from these areas 23 . For example, imported malaria surveys from the USA showed the data of 2004 revealing that 65% of P. vivax imported into the USA from Africa in that year (n 5 67) originated in countries in west and central regions 24 . Furthermore, 49 P. vivax cases were detected from the travelers coming back from Nigeria, Cameroon, Liberia, Republic of Congo/ Democratic Republic of Congo and Ghana during 2006 to 2011 [25][26][27][28][29][30] .
It was found that the difference mainly associated with human populations in which the Duffy negative phenotype is present at a lower frequency than elsewhere 23 . Migrant workers or non-African expatriates may preferentially visit areas of west and central Africa where there is a relatively high frequency of Duffy positive individuals in the local population and where P. vivax is more likely to be transmitted.
However, recent report has showed that P. vivax is endemic in west and central Africa for it has been identified in both chimpanzees and gorillas from this region through mitochondrial genome sequencing [31][32][33][34] . This genetically based evidence may explain at least in part, for the presence of P. vivax among the largely Duffy negative human populations of west and central Africa. Another report indicated that P. vivax transmission can be expected in populations with high levels of RBC Duffy negativity and in which malaria transmission intensities are sufficiently high, as is the case in many areas of west and central Africa 35 . The capacity of the environments in these regions to transmit malaria is so high that even a very small proportion of the human population of the Duffy positive phenotype could sustain the presence of P. vivax 36 .
Another factor that may contribute to this difference is the higher transmissibility of P. vivax under adverse conditions 37 . Features of the transmission biology of P. vivax give this species greater resilience than the less robust P. falciparum in the face of conditions adverse to the transmission of the parasites. This may lead to the consequence that there should be higher proportion of P. vivax in the vector mosquitoes than there is in the corresponding human population. Then the travellers, a probe of the infection rates in the local mosquitoes, can be expected to have higher proportions of P. vivax than are found in the endemic human populations. Similarity, recent finding of Ryan indicated the presence of P. vivax in 0.65% of mosquitoes from an area of west Kenya in the local population 38 .
The misuse of prophylactic anti-malaria drugs among travellers may also contribute to this phenomenon. According to the antimalarial drug policy of China, chloroquine was recommended antimalarial chemoprophylaxis used in the P. vivax endemic area, and piperaquine phosphate was recommended antimalarial chemoprophylaxis used in the mixed P. falciparum and P. vivax endemic area. However, the Chinese travelers often take artemisinin combined therapies (ACTs) to treat with malaria in Africa, and even many travelers are not taking the drugs during the radical treatment, therefore it will not protect against relapses after cessation of drug use. This is also true of P. ovale, which is also capable of producing hypnozoites, and may explain the higher rate of this parasite in returning travellers compared to the local populations ( Table 2).
It is also probable that a small proportion of imported cases may be P. ovale infections rather than P. vivax, as is often difficult to distinguish the two species by microscopy 39 . Despite 117 P. vivax cases (95.9%) were laboratory-confirmed including microscopy, PCR and Rapid Diagnostic Tests (RDTs), the cases detected by microscopy took up a large proportion. Though these cases were all re-confirmed using PCR by reference labs in the provincial CDC, it should continued to develop more accurate and specific skills detecting these two species, currently some accurate molecular detection systems were developed for imported P. ovale with genetic variations P. ovale wallikeri and P. ovale curtisi 40 .
In a word, the present study indicates that frequent identification of P. vivax in travelers coming back from west and central Africa in China, despite the prevalence of this parasite in local populations is relatively low. The imported P. vivax could be introduced into malaria-free localities during the transmission season, especially when a large number of cases are clustered in areas where Anopheles sinensis are prevalent. Though there are no secondary transmission of imported P. vivax in China, surveillance systems should also be strengthened to ensure timely recognition and prompt response for effective management. More attention should be paid by Chinese medical staffs on the imported P. vivax cases reported especially from west and central Africa. First, they should raise awareness that P. vivax could be imported from these regions. Second, training should also be strengthened and more accurate, specific tools should be developed, as well as the epidemiological study to avoid the  641  61  554  6  10  5  5  Central Africa  537  61  434  8  25  6  3  South Africa  35  4  29  1  1  0  0  North Africa  70  19  51  0  0  0  0  East Africa  97  31  64  0  2  0  0  Southeast Asia  940  697  228  3  2  9 1 misdiagnosis between P. ovale and P. vivax. Only the epidemiology and molecular study have proved the P. vivax was imported from west and central Africa, and this may indicated that DARC is only a relative resistance factor and P. vivax must be circulating at low concentrations in west and central Africa. Data collection. All individual cases from the web-based reporting system (China Information System for Diseases Control and Prevention V2.0) were carefully reviewed and analysed. In China, malaria is a statutory notifiable infectious disease, and cases are identified according to the unified diagnostic criteria issued by the Chinese Ministry of Health, which includes the definitions of clinically diagnosed and laboratory-confirmed cases. Clinically diagnosed cases were defined as a patient with malaria-like symptoms, having lived in or recently travelled to areas with known malaria endemic areas. Laboratory confirmed cases were defined as clinically diagnosed cases with microscopy, rapid diagnostic tests (RDTs), or polymerase chain reaction test. Health workers in both the public and private medical sectors were required to report malaria cases. The data was selected by dates of onset, reporting area and final review. This collecting type could cover all the P. vivax cases reporting during January 1 st to December 31 st of 2012. For the local and imported data, we collected it through annual malaria statistics reporting system.

Methods
Statistical analysis. A descriptive analysis was processed using Microsoft Excel and SAS software (SAS INSTITUTE INC., Version 9.2). The chi-square test (x 2 ) or Fisher's exact test was used to investigate whether the constitute ratio of P. vivax have the statistically significant difference between two groups (Africa and Southeast Asia, and different regions of Africa). The p values was calculated, in order to reduce the probability of false positive errors and considered statistically significant when ,0.01.