Plasmodium cynomolgi Co-infections among Symptomatic Malaria Patients, Thailand

Abstract

Among 1,180 symptomatic malaria patients, 9 (0.76%) infected with Plasmodium cynomolgi were co-infected with P. vivax (n = 7), P. falciparum (n = 1), or P. vivax and P. knowlesi (n = 1). Patients were from Tak, Chanthaburi, Ubon Ratchathani, Yala, and Narathiwat Provinces, suggesting P. cynomolgi is widespread in this country.

Plasmodium cynomolgi, a simian malaria parasite, possesses biological and genetic characteristics akin to those of the most widespread human malaria parasite, P. vivax. Although P. cynomolgi circulates among monkey species such as long-tailed macaques (Macaca fascicularis) and pig-tailed macaques (M. nemestrina), experimental and accidental transmissions have been implicated in symptomatic infections in humans (1). Several mosquito vectors for human malaria can also transmit P. cynomolgi, posing the risk of cross-species transmission in areas where its natural hosts coexist with people (1,2). Among pig-tailed and long-tailed macaques living in various countries in Southeast Asia, including Thailand, P. cynomolgi infections are not uncommon (3,4). A case of naturally transmitted P. cynomolgi malaria in a human was reported from eastern Malaysia (5). Subsequent surveillance in western Cambodia and northern Sabah state in Malaysia revealed asymptomatic human infection, albeit at low prevalence (6,7). Symptomatic P. cynomolgi infection was diagnosed in a traveler returning to Denmark from Southeast Asia (8). During testing of symptomatic malaria patients in Thailand, we identified 9 co-infected with cryptic P. cynomolgi and other Plasmodium species.

The Study

We examined 1,359 blood samples taken from febrile patients who sought treatment at malaria clinics or local hospitals in 5 Thailand provinces: Tak (n = 192, during 2007–2013), Ubon Ratchathani (n = 239, during 2014–2016), Chanthaburi (n = 144, during 2009), Yala (n = 592, during 2008–2018), and Narathiwat (n = 192, during 2008–2010). Using microscopy, we found 1,152 cases in which malaria was caused by P. vivax (869 patients, 75.43%), P. falciparum (272 patients, 23.61%), or co-infection with both species (11 patients, 0.96%). Using species-specific nested PCR, including for P. cynomolgi (Appendix), targeting the mitochondrial cytochrome b gene (mtCytb) of 5 human malaria species for molecular detection, as described elsewhere (9,10), we found malaria in 1,180 patients; P. vivax infections exceeded P. falciparum infections (Table 1). Submicroscopic parasitemia occurred in 28/1,180 (2.4%) patients: 19 infected with P. vivax, 7 with P. falciparum, 1 with P. vivax and P. falciparum, and 1 with P. malariae.

Table 1. Distribution of Plasmodium infections diagnosed by PCR of blood samples taken from febrile patients who sought treatment at malaria clinics or local hospitals in 5 provinces, Thailand*.

Species	No. cases by province					Total no. cases	% Total cases
	Tak	Ubon Ratchathani	Chanthaburi	Yala	Narathiwat
P. vivax	98	57	141	467	59	822	69.66
P. falciparum	72	41	0	87	73	273	23.14
P. knowlesi	0	4	0	0	0	4	0.34
P. malariae	0	2	0	1	0	3	0.25
P. ovale	0	0	0	1	0	1	0.09
P. vivax + P. falciparum	21	8	0	11	15	55	4.66
P. vivax + P. knowlesi	0	3	2	0	4	9	0.76
P. vivax + P. cynomolgi	1	1	1	3	1	7	0.59
P. vivax + P. knowlesi + P. cynomolgi	0	0	0	1	0	1	0.09
P. falciparum + P. knowlesi	0	3	0	1	0	4	0.34
P. falciparum + P. cynomolgi	0	0	0	1	0	1	0.09
PCR-positive	192	119	144	573	152	1,180	100.00
PCR-negative	0	120	0	19	40	179	NA
Total no. samples tested	192	239	144	592	192	1,359	NA
*NA, not applicable.

The mean age of all patients was 26.3 (range 7–85) years; 940/1,180 (79.7%) of patients were men. Febrile symptoms, lasting 1–7 days (mean 3.1, SD ±1.3 days) before blood sample collection, developed in all PCR-positive malaria patients. Monoinfection with P. knowlesi occurred in 4 patients, P. malariae in 3, and P. ovale in 1. We detected co-infections in 77 (0.93%) patients; of these co-infections, 55 were P. falciparum and P. vivax. In total (i.e., including both monoinfections and co-infections), P. knowlesi was detected in 18 patients, of which 10 cases were newly identified from Ubon Ratchathani Province, which borders Cambodia and Laos.

We detected P. cynomolgi in 9 patients, all of whom were co-infected with P. vivax (n = 7), P. falciparum (n = 1), or both P. vivax and P. knowlesi (n = 1). The overall prevalence of P. cynomolgi infections was 0.76%. Patients infected with P. cynomolgi were found in all provinces. Although 5 of these patients were from Yala Province, the proportion of P. cynomolgi infections among malaria cases in each malaria-endemic area (0.52%–0.87%) was comparable.

DNA from 10 P. knowlesi isolates from Ubon Ratchathani Province and the 9 P. cynomolgi isolates were subject to nested PCR amplification spanning a 1,318-bp region of mitochondrially encoded cytochrome c oxidase I (mtCOX1). Direct sequencing of the purified PCR-amplified template was successfully performed from all 10 P. knowlesi and from 6 P. cynomolgi isolates. The remaining 3 P. cynomolgi isolates could not be further amplified due to inadequate DNA in the samples. All mtCOX1 sequences of P. knowlesi from Ubon Ratchathani Province were different from one another and distinct from those from the previous case of natural human infection in Thailand (GenBank accession no. AY598141) (11). All 6 amplified P. cynomolgi isolates contained different sequences belonging to 2 clades. One was closely related to the Gombak strain (accession no. AB444129) and the remaining 5 isolates were clustered with the RO strain (accession no. AB444126) (Figure).

Figure.

Maximum-likelihood phylogenetic tree inferred from mitochondrially encoded cytochrome c oxidase I of Plasmodium cynomolgi and P. knowlesi from Thailand compared with other closely related species. Tree spans 1,318-bp region. Colors indicate province where human isolates were found: red circles, Yala; green triangles, Narathiwat; purple squares, Chanthaburi; and blue diamonds, Ubon Ratchathani. GenBank accession numbers of reference sequences are given in parentheses. Bootstrap values >50% based on 1,000 pseudoreplicates are shown on the branches. Scale bar indicates nucleotide substitution per site.

All but 1 P. cynomolgi infection occurred in male patients (age 15–53 years, median 32 years). Most P. cynomolgi malaria patients resided in areas where domesticated or wild macaques were living in proximity to humans. Infections with P. cynomolgi occurred in different annual periods; more cases were detected in rainy seasons than in dry seasons (Table 2). The parasite density of P. cynomolgi could not be determined from blood smears because of morphologic resemblance to P. vivax; an isolate co-infected with P. falciparum (YL3634) had very low parasitemia. Of 8 patients with P. cynomolgi co-infection, 6 had parasitemia <10,000 parasites/μL (<0.2% parasitemia). It remains unknown whether P. cynomolgi was co-responsible for symptomatic infections or merely coexisted asymptomatically with other human malaria parasites. However, self-reported defervescence among P. cynomolgi–co-infected patients occurred 1–3 days after antimalarial treatment with chloroquine plus primaquine after onsite microscopic diagnosis of P. vivax malaria or artesunate plus mefloquine for P. falciparum malaria. Unfortunately, data on long-term follow-up were not available.

Table 2. Demographic and parasitologic features of Plasmodium cynomolgi–co-infected patients among febrile patients who sought treatment at malaria clinics or local hospitals in 5 provinces, Thailand.

Patient*	Age, y/sex	Province	Month	Season	Monkey in proximity	Microscopy diagnosis	Parasites/μL‡	PCR diagnosis
TSY1522	38/M	Tak	2007 Nov	Dry	No	P. vivax	12,160	P. vivax, P. cynomolgi
CT606†	30/M	Chanthaburi	2009 Oct	Rainy	Yes	P. vivax	86,535	P. vivax, P. cynomolgi
UBY120	32/M	Ubon Ratchathani	2015 Aug	Rainy	Yes	P. vivax	570	P. vivax, P. cynomolgi
NR105	53/M	Narathiwat	2008 Jul	Rainy	Yes	P. vivax	4,620	P. vivax, P. cynomolgi
YL3179	15/M	Yala	2016 Apr	Dry	Yes	P. vivax	1,140	P. vivax, P. knowlesi P. cynomolgi
YL3634	40/F	Yala	2016 Dec	Rainy	Yes	P. falciparum	60	P. falciparum, P. cynomolgi
YL3680	49/M	Yala	2016 Dec	Rainy	Yes	P. vivax	3,720	P. vivax, P. cynomolgi
YL3685	18/M	Yala	2016 Dec	Rainy	Yes	P. vivax	4,680	P. vivax, P. cynomolgi
YL4278	21/M	Yala	2017 Oct	Rainy	Yes	P. vivax	7,440	P. vivax, P. cynomolgi
*Alphanumeric designations represent provinces and serial number of blood samples. †Patient from Cambodia, but had lived in Thailand for 1 year just prior to illness, with no history of travel outside of the country. ‡All species of malaria parasites (all stages) were determined from >200 leukocytes on Giemsa-stained thick blood films.

Conclusions

This report highlights the presence of P. cynomolgi in the human population of Thailand, where natural hosts, both pig-tailed and long-tailed macaques, are prevalent. All patients with P. cynomolgi infections harbored either P. falciparum or P. vivax in their blood, implying that this simian malaria species could share the same anopheline vectors or have different vectors with similar anthropophilic and zoophilic tendencies. The presence of P. cynomolgi in diverse malaria-endemic areas of Thailand suggests that cross-species transmission has occurred. Human infection with P. cynomolgi seems not to be newly emerging because it was detected among blood samples collected over a range of time periods since 2007. Undoubtedly, morphologic similarity between P. cynomolgi and P. vivax can hamper conventional microscopic diagnosis (1,5,8). Cryptic co-existence of simian and human malaria species could further preclude accurate molecular detection when inadequate diagnostic devices are used.

Previous surveys of Plasmodium infections in pig-tailed and long-tailed macaques have revealed the presence of P. cynomolgi and other simian malaria species in Thailand, mainly in the southern part of the country (4). Most patients infected with P. cynomolgi resided in areas where macaques were living in proximity to humans; therefore, the risk of acquiring malaria from this parasite could increase as people encroach into the habitats of infected macaques, as happened with malaria caused by P. knowlesi. Of note, co-infection with P. cynomolgi, P. knowlesi, and P. vivax occurred in a patient in Yala Province whose housing area was surrounded by several domesticated pig-tailed and long-tailed macaques.

Analysis of the mtCOX1 sequences of P. cynomolgi among 6 patients showed that all isolates possessed different genetic sequences, suggesting that several strains or clones of this simian parasite are capable of cross-transmission from macaques to humans. Meanwhile, P. cynomolgi seems to contain 2 divergent lineages (12), represented by RO and Gombak strains. The mtCOX1 sequences of both P. cynomolgi lineages were found in human-derived isolates in this study, further supporting that diverse strains of this parasite can infect people. Likewise, sequence diversity in the mtCOX1 of P. knowlesi from Ubon Ratchathani Province suggests that cross-transmission from macaques to humans may not be restricted to particular parasite strains.

Although human malaria from either parasite may be asymptomatic, infection with P. knowlesi can result in death, but patients infected with P. cynomolgi at worst had only benign symptoms (5–8). However, severe and complicated malaria has been observed in rhesus macaques experimentally infected with P. cynomolgi (13).

Whether severe cynomolgi malaria can occur in humans remains to be elucidated. However, if human infections with P. cynomolgi do become public health problems, diagnostic and control measures might be complicated by the morphological similarity between P. vivax and P. cynomolgi. This possibility makes further surveillance of this simian malaria in humans mandatory.

Appendix

Additional methodology for Plasmodium cynomolgi co-infections among symptomatic malaria patients, Thailand.

Biography

Dr. Putaporntip is a molecular parasitologist in the Molecular Biology of Malaria and Opportunistic Parasites Research Unit, Department of Parasitology, Faculty of Medicine, Chulalongkorn University, in Bangkok, Thailand. His work focuses on molecular diagnostics, population genetics, and the evolution of malarial parasites in human and nonhuman primates.