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Epidemiology of malaria in non-amazonian Colombian regions: importance of asymptomatic subjects as transmission foci Juan M. Vásquez-Jiménez1, Cristhian Morales-Plaza1, Sergio A. Ochoa-Orozco1, Nora Martínez1, John García1, Andrés F. Vallejo1, Yoldy Benavides1, Angélica Castellanos1, Juliana Henao-Giraldo1, Myriam Arévalo-Herrera1,2, Sócrates Herrera1 1Caucaseco Scientific Research Center, Cali, Colombia, 2 School of Health, Universidad del Valle, Cali, Colombia
RESULTS
ABSTRACT
Table 1. Number of sampled individual, cases by TBS and qPCR and species distribution by sentinel site and year .
Bv: Buenaventura, Ta: Tierralta, Tm: Tumaco, TBS: Thick blood smear microscopy, qPCR: Quantitative polimerase chain reaction, Pv: P. vivax, Pf: P. falciparum. + Buenaventura had 4 cases of mixed malaria in Tumaco had 1 case of mixed malaria in 201l. *Statistically significant (P<0.05).
Malaria control programs rely on the confirmation of parasite presence in blood of ill patients, mostly by the use of microscopy or antibody based rapid diagnostic tests (RDTs). Despite the significant contribution of these methods to malaria control/elimination they are not suitable for detecting the significant proportion of asymptomatic subjects harboring low levels of parasitemia, which endure untreated as potential reservoirs for transmission. The epidemiology of malaria in some of the most endemic regions of Colombia assessed throughout a four year follow-up, the importance of asymptomatic infections and their persistence for malaria elimination are discussed.
Methods:
We conducted a series of cross-sectional surveys between 2011 and 2014, in low to moderate malaria transmission sentinel sites (SS) of Tumaco, Buenaventura, and Tierralta municipalities of Colombia. A census was performed and a random sample of houses was selected from each SS prior to each survey. Inhabitants were asked to answer a questionnaire on clinical, epidemiological and demographic aspects, and to provide a blood sample for malaria diagnosis using microscopy and quantitative real time polymerase chain reaction (qPCR).
Results:
A total of 3,059 blood samples were obtained from all the SS, 58.5% of which were from women and displayed a malaria prevalence ranging from 3.6% to 9.7% in the fourth year study period. Almost all malaria cases (97.2%) were submicroscopic and only detectable by qPCR; 88.9% of the cases were asymptomatic at the time of blood collection.
While Buenaventura and Tierralta had a decreasing tendency through all the follow-up, Tumaco had a rise in 2013 and then a decrease in Plasmodium vivax accounted for the majority ( %) of cases in Tierralta and Buenaventura and for 25-50% of the cases in Tumaco.
Conclusions:
This study demonstrates an important prevalence of asymptomatic malaria cases not detectable by microscopy. This significant number of asymptomatic malaria carriers represents a parasite pool for malaria transmission that demands the introduction of alternative strategies for diagnosis and treatment in order to reduce transmission as appropriate levels to initiate malaria pre/elimination efforts.
Figure 1. Prevalence of malaria by year in each sentinel site
Figure 2. Proportion of species in each municipality through time.
TIERRALTA 
MATERIALS AND METHODS
BUENAVENTURA
Malaria prevalence by Plasmodium species per year in all study municipalities.
Of the infected cases in 2011, 83/113 (73.5%) were due to P. vivax which did not significantly change in 2013 with 56/72 (77.8%); however in 2014 the number decreased to 22/33 (66.7%). Nevertheless, the proportion of species varied depending on the municipality
From the total 3,059 samples collected from 2011 to 2014, only six were positive for P. vivax but none for P. falciparum when examined by microscopy. However, when the same samples were examined by qPCR, a total of 113 were found to be positive (83 Pv, 25 Pf, 5 Mixed) out of those collected in 2011, 72 (56 Pv and 16 Pf) in 2013 and 33 (22 Pv and 11 Pf) in 2014.
TUMACO
P. vivax
Malaria prevalence per Sentinel Site (SS) by species
The proportion of species varied depending on the municipality. P. vivax accounted for the majority
( %) of cases in Ta and Bv and for 25-50%
of the cases in Tm.
SUMMARY & CONCLUSIONS
There is a high prevalence of asymptomatic malaria in endemic regions in Colombia considered to be of low and moderate transmission. The fact that almost all (97.2%) of the cases were not detectable by TBS, which is the most widely used diagnostic method by NMCP, underscores the need to consider introducing molecular methods for malaria diagnosis. In addition, curative treatment should follow a positive diagnosis, even if asymptomatic, in order to effectively decrease malaria prevalence and further transmission, particularly in view of current plans to accelerate elimination activities
ACKNOWLEDGEMENTS
We thank the communities and health authorities of Bv (Freddy Córdoba), Ta (J. F.Quintero and. W. Hawasly) and Tm (H. Valencia).
This work was sponsored and conducted in sites corresponding to Centro Latino Americano de Investigación en Malaria (CLAIM) sponsored by NIAID/ICEMR (1U19AI ). The authors are very grateful with the participating volunteers, the representation of the ministry of health of Colombia through Malaria Project in INSALPA and Tierralta and Tumaco.
Caucaseco Research Center, Cali, Valle del Cacuca, Colombia /
Sócrates Herrera M.D. Km 6 via Cali-Puerto Tejada Colombia.
National Malaria Control Program