**2. Epidemiological aspects of the** *plasmodium vivax* **CSP variants in endemic areas around the world**

*P. vivax* is responsible for approximately 7 million malaria cases worldwide [1]. In 2017, 49 countries reported locally acquired cases of *P. vivax*, where it is estimated that more than 2.5 billion people are at risk of infection [15, 16]. The control of *P. vivax* infections is more challenging compared with *P. falciparum* due to some unique biological features, such as the existence of dormant liver forms (hypnozoites) and early sexual parasite development. Thus, treatment of blood-stage infection is less effective to reduce *P. vivax* transmission, and probably the vaccine development against this species will be indispensable for elimination and eradication of disease. Most efforts to develop a vaccine against *P. vivax* have focused on the CSP. For this reason, it is important to evaluate polymorphisms in *pvcsp* gene and how its variants are widespread in endemic areas [17].

Seroepidemiological and genetic studies performed in malaria endemic areas could provide valuable information on parasite transmission and dispersion. The genetic diversity of the *CS* gene has been useful in molecular epidemiological studies, understanding the transmission, dynamics, and evolutionary relationships [14]. Furthermore, eco-epidemiological conditions, biological and genetic characteristics of the parasite, the host immunity, and local vectors may influence the different patterns of demographic expansion. On the other hand, the human intervention can become a factor to reduce the risk of malaria, without necessarily modifying the environment [18]. Serologic surveys in *P. vivax* isolates from Thailand [19, 20] and Myanmar [21] demonstrated the presence of antibodies that recognize recombinant and synthetic polypeptides that represent the VK210 and VK247 (alone or mixed). PvCSP antibodies in high-risk malaria areas in Korea [22] and sub-Saharan Africa [23] were detected. The Korean findings suggest that antibody levels for the CSP antigen could become deficient or reduced over the long-term incubation period. Mexican [24, 25] and Peruvian [24] sera reacted with either VK210 or VK247 repeat domains. It has been shown that the prevalent phenotype of the *P. vivax* parasite in the study sites of Colombia is VK247, whereas VK210 accounts for one-third of the cases, and few *P. vivax* malaria cases correspond to mixed infection [26]. Antibodies to these three variants have been circulating in the Brazilian Amazon population [27–30]. In addition, Oliveira-Ferreira et al. [31] have observed the profile of IgG responders to these PvCSP variants, where more responders to VK210 were found, followed by *P. vivax*-like and VK247 in Rondônia state. Cross-sectional cohort study included individuals from three different communities with malaria transmission in Acre state, demonstrating that VK210 presented the highest prevalence of responders to IgG antibodies, followed by *P. vivax*-like and VK247 [32]. Besides, the higher frequency of antibodies to VK210 is according to studies that described this variant as the most common in Amazon, while VK247 was rarely reported as a single infection [14, 27]. However, unlike previous reports from different areas of the Brazilian Amazon, the VK247 variant was found in higher frequencies than VK210 in Goianésia do Pará, Para State [29]. The higher frequencies of VK247 in current infections compared with higher antibody responses to VK210 may suggest that VK210 protein is more immunogenic than VK247 [33, 34], and also that the VK210 variant could have been more prevalent in endemic area from Amazon region in the recent past, as the majority of the patients had a previous history of malaria infections. Nevertheless, nucleic-acidbased diagnosis evaluates the genotypes of the current infection while antibody-based techniques are able to signalize positive results several months and even years after

the end of the infection. Furthermore, there is a possibility that hypnozoites/relapses do influence the antibody response to sporozoites, since the presence of a blood-stage infection may not necessarily indicate a new infection [29, 34].

Using DNA techniques, the VK210 and VK247 sequences were found in *P. vivax* isolates from Philippines [35] and Cambodia/Thailand [36]. The cosmopolitan variant VK210 also predominate in Sri Lanka [37], Myanmar [38], Iran [39], Vanuatu Island [40], and China [41]. VK247 seems to be more common in South-eastern Asia [42, 43]. In fact, VK210 and VK247 genotypes have a worldwide distribution; however, the *P. vivax*-like genotype has been only detected in Papua New Guinea, Brazil, Indonesia, and Madagascar [13, 42, 44]. Sequence analysis of the C-terminal nonrepeat region of Myanmar VK210 variants revealed 27 distinct haplotypes. The sequence of haplotype 22 was identical with Sal I (GU339059) and accounted for 9.8% of all the VK210 sequences. The C-terminal nonrepeat region of VK247 variants showed a lower level of genetic diversity than that of VK210 variants [45]. Moreover, *pvcsp* sequencing for samples collected in Oman displayed the VK210 type with a single haplotype (six repeats of GDRADGQPA and nine repeats of GDRAAGQPA), and the malaria vector in Oman is *Anopheles culicifacies* [46]. VK210 and VK247 ratio in China was extensively investigated in several provinces, and the coexistence was verified only in Yunnan, Hainan, and Liaoning provinces, whereas in Anhui, Hubei, Guangxi, Guangdong, Guizhou, Sichuan, Jiangsu, also endemic, only the VK210 type was detected. *P. viv*ax was transmitted by distinct *Anopheline* species in China, in Henan by *An. Sinensis,* and in Hainan by *Anopheles dirus* and *Anopheles minimus*, but these two provinces had the same genotypes [47].

Malaria transmission currently exists in 21 countries and territories of the American continent, where 132 million people are at risk of infection [48]. In southern Mexico, VK210 and VK247 CSP phenotypes were detected and associated to the parasite infectivity to the local mosquito vectors *Anopheles pseudopuntipennis* and *Anopheles albimanus* [49, 50]. In samples of *P. vivax* from Mexico, Nicaragua, and Peru, it was observed that the genetic diversity of the *CSP* gene is restricted mainly to the central repeat domain and 3′-terminal portion [18]. This variation occurs due to changes in the type of nucleotides and number of repeats of the repeat region, and it is possible that the repeat region of VK247 is more stable than VK210. The predominance of VK247 parasites was documented on the Pacific Ocean coast of Western Colombia and in Piura, Peru [51]. *An. nuneztovari* infected with the VK210 and VK247 genotypes in Colombia, showing its importance for malaria transmission in areas with anthropic intervention [52]. *P. vivax* samples in the North, North-West, and South of Guatemala [53] and in municipalities of Honduras [54] were all of the VK210 genotype.

In Brazil, the detection of three *P. viva*x CSP genotypes has been observed in different areas from Amazon [44, 55–58], pre-Amazon [59], as single and mixed infections, and extra-Amazon region [60]. The VK210 genotype remains the most prevalent, most likely because of the great susceptibility of the *An. darlingi* vector, which is the most abundant in the country, to this variant [61]. The *P. vivax*-like genotype had a low frequency of the genotyped samples; this frequency could be due to its recent introduction into the region or due to differences in the development of this genotype in the vectors present in the area [27, 55]. Furthermore, the VK210 and VK247 were detected in *An. aquasalis* and *An. darlingi* in endemic areas of Pará State [61]. Although the VK210 genotype remains the most prevalent in Brazil, new evidence reveals a strong adaptation of the VK247 variant in southeastern Pará state, as well as the association of this genotype with high parasitemia. *An. albitarsis* has

*Circumsporozoite Protein from* Plasmodium vivax *and Its Relationship to Human Malaria DOI: http://dx.doi.org/10.5772/intechopen.102529*

been found in a natural infection with the VK247 genotype in Goianésia do Pará [57], *Anopheles oswaldoi* in Acre State [62, 63]. Despite the association between high parasitemia and the VK247 genotype, the introduction of the VK247 and VK210 genotypes may have occurred at different times according to the endemic area [57].
