Diversity and prevalence of Leucocytozoon in black flies (Diptera: Simuliidae) of Thailand.

backgroundLeucocytozoonosis, a parasitic disease of birds, is caused by haemosporidian protozoan parasites of the genus Leucocytozoon, which infect diverse avian species, including poultry. These parasites are transmitted by several black fly species, but knowledge of the factors determining the diversity and prevalence in these vectors, which is crucial for fully understanding disease epidemiology, is largely unexplored. In this study, we investigated factors associated with the prevalence and diversity of Leucocytozoon species in black flies from Thailand.

methodsAdults of two black fly taxa (Simulium asakoae Takaoka and Davies complex and S. khelangense Takaoka, Srisuka and Saeung) were collected using sweep nets at nine locations in northern and northeastern regions of Thailand. Specimens were identified morphologically and the results corroborated by DNA barcoding. Molecular methods using specific primers for amplification of the mitochondrial cytochrome b (cyt b) gene of Leucocytozoon were used to detect the parasite in black flies. Species and lineages of Leucocytozoon were determined using the MalAvi database of malaria parasites and related haemosporidians in avian hosts. Regression analysis was used to examine relationships between Leucocytozoon diversity and prevalence, black fly abundance and habitat characteristics.

resultsA total of 11,718 adult black flies were collected, of which 4367 were members of the S. asakoae complex and 7351 were S. khelangense. For molecular detection of Leucocytozoon, we randomly selected 300 individual female black flies of the S. asakoae complex and 850 females of S. khelangense pooled into groups of five individuals (= 170 pools). A total of 34 of the 300 specimens of the S. asakoae complex and 118 of the 170 pools of S. khelangense were positive for Leucocytozoon. Fifty-four lineages (haplotypes) were identified, all of which belonged to those reported in domestic chickens, Gallus gallus, with one exception that was identified in S. khelangense and found to be closely related to the Leucocytozoon lineages reported in owls; this is the first record of the latter lineage in Asian black flies. Among these haplotypes, nine and 45 were exclusively found in the S. asakoae complex and S. khelangense, respectively. No lineage was shared between these black fly taxa. Analysis of similarity (ANOSIM) revealed significant Leucocytozoon lineage composition between the two black flies. Phylogenetic analysis found that Leucocytozoon lineages in the S. asakoae complex and S. khelangense are largely isolated, agreeing with the ANOSIM result. The overall prevalence of Leucocytozoon in the S. asakoae complex was 11.3% and ranged from 9% to 13% in each collection. Leucocytozoon prevalence in S. khelangense was 21%, varying from 13% to 37% in each collection. The Shannon H' index indicated greater Leucocytozoon diversity in S. khelangense (H' = 3.044) than in the S. asakoae complex (H' = 1.920). Regression analysis revealed that Leucocytozoon diversity was positively related to black fly abundance and negatively related to maximum air temperature.

conclusionsThe results of this study show that the prevalence and diversity of Leucocytozoon lineages in the S. asakoae complex and S. khelangense from Thailand were associated with the abundance of these black flies and with air temperature. The Leucocytozoon lineages identified also showed some degree of black fly taxon specificity, possibly related to different abundance peaks of these vectors. The environmental conditions that favor the development of black flies are possibly a driver of Leucocytozoon prevalence, diversity and vector-parasite co-evolution.