Epidemiological profile of imported malaria in Anhui Province, 2012–2019
In total, 941 cases of imported malaria were reported in Anhui province from 2012 to 2019. Cross-checks of the patient identifiers revealed that 6 patients had 2 clinical episodes owing to different malarial parasites after returning to China. These patients were reported twice owing to different dates of onset. PCR confirmed that they were co-infections (all were P. falciparum co-infected with P. ovale spp.). In this study, six cases were recorded as co-infections in accordance with the second clinical episode, and the number of imported malaria cases decreased to 935. P. falciparum was the dominant species, accounting for 733 cases (78.4%), followed by P. ovale spp. (115 cases, 12.30%), P. malariae (38 cases, 4.06%), P. vivax (35 cases, 3.74%), and co- infection (14 cases, 1.50%). Among 14 cases of co-infection, there were 12 cases of P. falciparum co-infected with P. ovale spp., one of P. falciparum with P. malariae, and one of P. malariae with P. ovale spp.. The proportion of imported cases due to P. ovale spp. peaked in 2015 (19.69%) and 2018 (19.35%), and increased from 2012 to 2018 (χ2 = 9.626, p = 0.002, excluded 2019) (Fig 1).
Origin of imported cases of P. ovale spp.
All 128 patients infected with P. ovale spp., including co-infections, were imported from 16 countries in Africa. In total, 127 (99.22%) P. ovale spp. cases were successfully confirmed via PCR; however, an inadequate blood sample was obtained from one patient for PCR and was only confirmed as P. ovale spp. through microscopy. The top four countries of origin for these infections were Equatorial Guinea (24, 18.75%), Angola (22, 17.19%), Nigeria (18, 14.06%), and Cameroon (12, 9.38%) (Table 1). One individual, infected in Angola, had a co-infection of P. o. curtisi and P. o. wallikeri. Therefore, 129 P. ovale spp. isolates were included in the analysis. Except for Ethiopia and Uganda (only one case each), P. o. curtisi and P. o. wallikeri were simultaneously detected in all countries. The proportion of P. o. curtisi ranged from 39.13% to 66.67% (Fig 2).
The incidence of P. ovale spp. in the primary countries of origin
In general, a high proportion of P. ovale spp. did not necessarily indicate a high incidence. In this study, we utilized the number of returnees (Anhui Statistical Yearbook, http://tjj.ah.gov.cn/ssah/qwfbjd/tjnj/index.html) from countries of infectious origin to estimate P. ovale spp. and P. falciparum incidence rates in the four main countries (Cameroon, Angola, Equatorial Guinea, and Nigeria). Using this method, the estimated average annual incidence rates of P. ovale spp. in Cameroon, Angola, Equatorial Guinea, and Nigeria were 2.48%, 0.30%, 1.87%, and 1.70%, respectively. The average annual incidence rates of P. falciparum in Cameroon, Angola, Equatorial Guinea, and Nigeria were 9.90%, 2.86%, 8.86%, and 8.11%, respectively (Table 2).
Epidemiological characteristics of P. o. curtisi and P. o. wallikeri
Of 128 patients infected with P. ovale spp., 113 were single-species infections, as determined through PCR. Sixty-two (62/113, 54.87%) cases of P. o. curtisi and the remaining 51 cases of P. o. wallikeri (51/113, 45.13%) were noted. Among the other 15 cases, there were 13 cases of co-infections, 1 case with co-infection of P. o. curtisi and P. o. wallikeri, and 1 case without species confirmation. The median latency period for P. o. curtisi (59.50 d, IQR: 23.0–192.75) was slightly but not significantly longer than that of P. o. wallikeri (34 d, IQR: 12–112.50) (P=0.070) (Fig 3, Table 3). In the study, the longest latency period of P. ovale spp. was 1299 d, which was noted in an individual co-infected with P. o. curtisi and P. falciparum. Furthermore, no significant differences in sex, age, occupation, or history of malaria were observed between the P. o. curtisi and P. o. wallikeri groups (Table 3). In this study, co-infections of P. ovale spp. and other species, predominantly P. falciparum (12 cases) were reported in 13 cases in total. One case presented a co-infection of P. ovale spp. and P. malariae. The co-infection rate was 10.16% (13/128). Of all 12 co-infection of P. ovale spp. and P. falciparum cases, six had only one clinical episode and six had two clinical attacks, whereby the first attack was due to P. falciparum and the second attack was due to P. ovale spp.. The intervals between the 2 clinical episodes were 33, 56, 127, 204, 295, and 1279 d, for the six patients, respectively. Among patients with one clinical episode, 4 were infected with P. o. wallikeri and two were infected with P. o. curtisi; among patients with two clinical episodes, 3 cases were of P. o. wallikeri and 3 were P. o. curtisi.
Diagnosis and clinical characterization of imported P. ovale spp. infections
To analyze the diagnostic data on imported P. ovale spp. infections, individuals with a P. falciparum infection constituted the control group in the study. The median latency period of P. ovale spp. was 49 (IQR: 16.5–169.5) d and that of P. falciparum was 6 (IQR: 2 –10) d; The median interval from onset to the first medical visit for P. ovale spp. was 1 (IQR: 0–3) d and that for P. falciparum was 1 (IQR: 0–2) d. The median interval from onset to the first medical visit to the diagnosis of P. ovale spp. infections was 1 (IQR: 0–3) d, while that for P. falciparum was 1 (IQR: 0–2) d. Based on the differences in the median intervals between the two species, P. ovale spp. had a significantly longer latency than P. falciparum (Z = -12.947, p < 0.001).
The parasite identification rates for P. ovale spp. and P. falciparum were comparable at 93.04% and 93.59%, respectively. However, the rates of species identification were significantly different between P. ovale spp. and P. falciparum (χ2 = 255.841, p < 0.001). Furthermore, among the 115 P. ovale spp. cases, only 20% of the cases (23/115) had an accurate species identification on microscopy (Table 4), while 32.17% (37/115) of cases were misdiagnosed as P. vivax, 6.96% (8/115) as P. falciparum. In the remaining 40.87% (47/115) of cases, species identification was not attempted, and only the parasite was identified (positive or negative finding). Among 733 P. falciparum cases, species identification revealed accurate results in 86.90% of cases (637/733); however, results were not obtained for 11.87% (87/733) of cases. Only 9 cases were misdiagnosed with other malarial parasite species or with co-infections.
Of the 115 individuals with P. ovale spp. infection, common clinical symptoms included fever (99.13%), chills (82.61%), sweating (59.13%), and headaches (55.65%). Furthermore, 53.91% cases presented with typical clinical manifestations of malaria (chills, fever, and sweating).