Infectivity of asymptomatic Plasmodium-infected human populations to Anopheles dirus mosquitoes in western Thailand

The infectivity of Plasmodium-infected humans in western Thailand was estimated by feeding laboratory-reared Anopheles dirus Peyton and Harrison mosquitoes on venous blood placed in a membrane-feeding apparatus. Between May 2000 and November 2001, a total of 6,494 blood films collected during an active malaria surveillance program were checked by microscopy for the presence of Plasmodium parasites: 3.3, 4.5, and 0.1% of slides were P. falciparum- (Pf), P. vivax- (Pv), and P. malariae (Pm)-positive. Venous blood was collected from 70, 52, 6, and 4 individuals infected with Pf, Pv, Pm, and mixed Pf/Pv, respectively, with 167 uninfected individuals serving as negative controls. Only 10% (7/70), 13% (7/52), and 0% (0/6) of membrane feeds conducted on Pf-, Pv-, and Pm-infected blood yielded infected mosquitoes. One percent (2/167) of microscope-negative samples infected mosquitoes; however, both samples were subsequently determined to be Pf-positive by polymerase chain reaction. Gametocytes were observed in only 29% (4/14) of the infectious samples. All infections resulted in low oocyst loads (average of 1.2 oocysts per positive mosquito). Only 4.5% (10/222) of mosquitoes fed on the seven infectious Pf samples developed oocysts, whereas 2.9% (9/311) of mosquitoes fed on the seven infectious Pv samples developed oocysts. The probability of a mosquito becoming infected with Pf or Pv after a blood meal on a member of the human population in Kong Mong Tha was estimated to be 1 in 6,700 and 1 in 5,700, respectively. The implications toward malaria transmission in western Thailand are discussed.     

Short report: Failure of the OptiMAL rapid malaria test as a tool for the detection of asymptomatic malaria in an area of Thailand endemic for Plasmodium falciparum and P. vivax

We evaluated the efficacy of the OptiMAL assay in a cross-sectional malaria survey in western Thailand from April to August 2001. Expert microscopy of Giemsa-stained thick and thin blood films was used as the gold standard. Positive control lines were evident in 99% (1,128 of 1,137) of the assays tested. However, 34% (384 of 1,128) of assays produced an aberrant result (a positive P. falciparum-specific line and a negative panmalarial line). False-positive panmalarial and Plasmodium falciparum-specific lines occurred in 25.9% (270 of 1,042) and 60.3% (628 of 1,042) of microscopy-negative samples, respectively. Due to the preponderance of false-positive test results, it was necessary to develop subjective criteria for test positivity based on line intensity. For determination of assay performance during this study, we therefore considered all test lines that were scored as intermediate or strong as positive and lines that were faint as negative. Using these criteria, we determined that the sensitivity of the OptiMAL assay for P. falciparum was 25% with > 500 parasites/microl and 10.5% with > 100 parasites/microl, while for P. vivax, the sensitivity at the same parasite rates was 100% and 41.7%, respectively. Further studies are required to determine whether the problems we identified are limited to this particular lot of OptiMAL assays.     

The retrospective identification and molecular epidemiology of porcine circovirus type 3 (PCV3) in swine in Thailand from 2006 to 2017

Porcine circovirus type 3 (PCV3) has recently been detected in pigs worldwide, with similar clinical manifestations to porcine circovirus‐associated disease (PCVAD) from porcine circovirus type 2 (PCV2) infection. Here, we report the identification and molecular epidemiology of PCV3 in swine in Thailand from clinical samples retrieved from 2006 to 2017. The epidemiological data revealed co‐infection with PCV2, PRRSV, and PCV2/PRRSV was common in our samples. Circulating PCV3 from this study shared a high similarity of nucleotide and deduced amino acid sequences of the partial capsid gene (96.7%–100% and 96.7%–100% respectively), indicated the genetic stability of PCV3 in Thailand. Phylogenetic analysis based on the capsid gene revealed scatter clustering with current PCV3 having no relation to the geographical origin of the virus strains. In this retrospective study, results have demonstrated that PCV3 has spread extensively within Thai swine from as early as 2006 and may also be involved in PRDC and PCVAD.     

Blood feeding patterns of Aedes aegypti and Aedes albopictus in Thailand

Aedes aegypti (L.) and Aedes albopictus (Skuse) were collected with aspirators from Mae Sot, Nakhon Sawan, Nakhon Ratchasima, Surat Thani, and Phatthalung study sites in Thailand from July 2003 though April 2004. The sandwich-B enzyme-linked immunosorbent assay was used to analyze 1,021 blood-fed specimens. Ae. aegypti almost exclusively fed on humans (99%, 658/664) in single host species, and 97% (86/88) of multiple-host bloodmeals included at least one human host. A low frequency of other hosts, including bovine, swine, cat, rat, and chicken were detected, but they represented <1% of bloodmeals. An even higher percentage of human feeding was detected in Ae. albopictus. Hosts of Ae. albopictus collected from sites in southern Thailand were entirely human (100%, n = 105) from both single and mixed meals. In the small number of double-host meals from Ae. albopictus, we detected 3.8% as swine-human and <1% from dog-human and cat-human. Forage ratios for Ae. aegypti indicated that human, dog, and swine were preferred hosts in order of preference. In contrast, bovine and chicken were avoided hosts for this species in Thailand.     

Chinese-like Strain of Porcine Epidemic Diarrhea Virus, Thailand

Since late 2007, several outbreaks of porcine epidemic diarrhea virus (PEDV) infection have emerged in Thailand. Phylogenetic analysis places all Thai PEDV isolates during the outbreaks in the same clade as the Chinese strain JS-2004-2. This new genotype PEDV is prevailing and currently causing sporadic outbreaks in Thailand.     

Improving Dengue Virus Capture Rates in Humans and Vectors in Kamphaeng Phet Province,Thailand, Using an Enhanced Spatiotemporal Surveillance Strategy

Dengue is of public health importance in tropical and sub-tropical regions. Dengue virus (DENV) transmission dynamics was studied in Kamphaeng Phet Province, Thailand, using an enhanced spatiotemporal surveillance of 93 hospitalized subjects with confirmed dengue (initiates) and associated cluster individuals (associates) with entomologic sampling. A total of 438 associates were enrolled from 208 houses with household members with a history of fever, located within a 200-m radius of an initiate case. Of 409 associates, 86 (21%) had laboratory-confirmed DENV infection. A total of 63 (1.8%) of the 3,565 mosquitoes collected were dengue polymerase chain reaction positive (PCR+). There was a significant relationship between spatial proximity to the initiate case and likelihood of detecting DENV from associate cases and Aedes mosquitoes. The viral detection rate from human hosts and mosquito vectors in this study was higher than previously observed by the study team in the same geographic area using different methodologies. We propose that the sampling strategy used in this study could support surveillance of DENV transmission and vector interactions.     

Age and body size influence male sperm capacity of the dengue vector Aedes aegypti (Diptera: Culicidae)

Understanding mosquito mating biology is essential for studies of mosquito behavior, gene flow, population structure, and genetic control. In the current study, we examine the effect of age and body size on spermatozoa number in two laboratory strains of the dengue vector, Aedes aegypti (L.), Thailand and Rockefeller (ROCK), and in wild-collected mosquitoes from Thailand. Body size was a major predictor of total spermatozoa number, with significantly greater sperm numbers in large (2.27-mm wing length) versus small males (1.85-mm wing length) within the same age group. Total sperm capacity also varied by male age. Spermatozoa numbers in virgin Ae. aegypti males increased significantly up to 10 d after emergence and then leveled off until 20 d. Significant variations in sperm number were detected among Ae. aegypti strains, with wild-collected mosquitoes having the greatest total number of sperm. Our study provides the first evidence of spermatogenesis in adult mosquitoes and indicates high rates of spermatogenesis in male mosquitoes up to 10 d of age (3.3 degree-days). Our results emphasize the potential role of body size and age on the mating capacity of this important vector of dengue and yellow fever viruses.     

Behavioral observations and sound recordings of free-flight mating swarms of Ae. Aegypti (Diptera: Culicidae) in Thailand

Sound plays an important role in the mating behavior of mosquitoes, including Aedes aegypti (L). Males orient to the fundamental wing beat frequency of females, and both sexes actively modulate their flight tone before mating to converge at harmonic frequencies. The majority of studies on mosquito mating acoustics have been conducted in the laboratory using tethered individuals. In this study, we present the first free-flight recording of naturally forming Ae. aegypti swarms in Thailand. We describe mating behaviors and present results on the flight tone frequency and dynamics of wild pairs in free flight. To assess the importance of these behaviors in vector control programs, especially those using genetically modified mosquitoes, it will be critical to use methods, such as those described in this work, to measure mosquito mating behaviors in the field.