Survey of infection and determination of the transmission vector of Onchocerca fasciata in camels (Camelus bactrianus) in Inner Mongolia, China

Abstract

Onchocerciasis in camels is caused by adult Onchocerca spp. and results in great economic losses to the camel industry. However, only a few studies on Onchocerca have been conducted, especially regarding the intermediate host and vector(s). In the present study, 192 camels were examined from December and January during 2013 and 2016, and the filarial larvae suspected to be Onchocerca spp. were further identified. Furthermore, aquatic dipteran insects in the living environment of camels were collected from May to September between 2013 and 2017 and dissected. Eventually, onchocercal lesions were observed in 95 of 192 (49%) camels and the captured insects were classified into 49 species from 42 genera in 21 families, among which 18 species were newly recorded in Inner Mongolia and 14 were haematophagous species. The filarial larvae were found in Culicoides puncticollis and identified as Onchocerca fasciata, indicating that C. puncticollis is the vector of O. fasciata in Inner Mongolia. These findings provide an estimate of onchocerciasis infection in camels and an alternative method of identifying insects and screening vectors using molecular methods. Important data are also provided for the diagnosis and control of onchocerciasis, thereby further filling the gap in knowledge regarding transmission vectors in China.

Scanning electron microscopy of Onchocerca fasciata (Filarioidea: Onchocercidae) adults, microfilariae and eggs with notes on histopathological findings in camels

“…As a close relationship between the biting habits of the vectors and the distribution of onchocercal nodules have been demonstrated [32], the difference in location of nodules in camels of various localities might be related to dipteran vectors in these regions. The vector of O. fasciata has been identified in Inner Mongolia, China, where out of 49 aquatic dipteran insects in the living environment of Bactrian camels, only Culicoides puncticollis was found infected with larvae of this nematode [16]. It is known that, generally, species of the genus Onchocerca are transmitted either by simuliids or by ceratopogonids [33].…”

“…For instance in a study where both Simulium ornatum and Culicoides nubeculosus were experimentally infected with L1 of Onchocerca lienalis and O. gutturosa, development of O. lienalis to L3 occurred only in S. ornatum, whereas O. gutturosa L3 developed only in Culicoides nubeculosus [34]. Regarding O. fasciata, finding of a Culicoides species infected with larvae [16], and the fact that The SEM observation of the male caudal region revealed an arrangement of 18 papillae which differs from previous descriptions. In their comprehensive redescription in 1981 Bain & Nasher [14] mentioned 18 or 19 papillae with no medioventral papillae posterior to the cloaca.…”

“…Since the original description of O. fasciata in 1910 the information about morphology of this parasite has been scant until Bain & Nasher re-described adult males and females in 1981 [14]. In recent years, morphological and molecular characteristics of this nematode have received attention [2,15] including studies about the identification of potential blood-sucking insect vectors in China [16]. However, there are still gaps in the knowledge about the morphological features of this filarial nematode.…”

Zoonotic Implications of Onchocerca Species on Human Health

“…Several Onchocerca species also have been found in the locations described above, but infecting camels and dromedaries [12]. Until now, what is known is that O. fasciata is a species specific in camels [50], being the most prevalent species in this host [48]. Camelidae are valuable animals in some areas of the world, as these animals are used as transport and their meat is used for human consumption; therefore, onchocerciasis have a detrimental impact associated with a loss of commercial value [48] and major issues in public health [54].…”

“…The identification of infective larvae in wild-caught blackflies is important in order to assess onchocerciasis transmission rate and putative natural Onchocerca vectors [38]. The screening of transmission vectors could be done by the morphological identification of microfilariae or via molecular methods, detecting their DNA by PCR amplification within vectors [48]. Although many species within the genus Simulium and Culicoides are likely to be putative vectors of Onchocerca spp.…”