FRUIT FLIES

The secretophagous non-biting flies of relevance for the transmission of the so-called ‘oriental eyeworm’ Thelazia callipaeda Railliet & Henry 1910 (Railliet & Henry, 1910, 1913; Faust, 1928) of carnivores and humans belong to the genus Phortica (Diptera, Drosophilidae). These drosophilids, commonly known as ‘fruit flies’, display a slightly ‘unusual’ behaviour as they feed on lachrymal secretions of vertebrates. During this feeding process they may act as vectors of parasites of the eyes of mammals (Otranto et al., 2005, 2006).

A second Thelazia species of importance for canines and humans, Thelazia californiensis Price 1930 (Price, 1930), has been reported in the western part of the United States (Doezie et al., 1996; Sobotyk et al., 2021; Taylor et al., 1996). Information about the vector of this eyeworm species is limited. Fannia canicularis and Fannia benjamini, the little house flies, have been implicated in its transmission (Burnett et al., 1957).

Due to the small number of papers on Th. californiensis and its vectors we concentrate mainly on drosophilids as vectors of Th. callipaeda on the website here.

Photograph of adult Phortica variegata
(by courtesy of D. Otranto, University of Bari, Valenzano (Bari), Italy)

GENERAL

Taxonomy

The family Drosophilidae comprises two recognised subfamilies, Drosophilinae and Steganinae, and a number of genera within an unconfirmed subfamily. It represents more than 4400 species in 100 genera commonly known as 'fruit flies' (Bánki et al., 2021). While members of the subfamily Drosophilinae feed and develop mostly on fruits and other vegetable matter (Bächli et al., 2004), flies belonging to the Steganinae subfamily display unusual feeding habits, and their ecology is much less known. Phortica variegata as transmitter of larvae of the 'oriental eyeworm' Thelazia callipaeda is belonging into the subfamily Steganinae, genus Phortica. Another species, Amiota okadai, was also considered to be a vector of this parasite in China (summarised in Otranto et al., 2006). Both, Phortica spp. and, to a lesser extent, Amiota spp. display a zoophilic behaviour, i.e. they feed on ocular secretions of animals and humans in addition to feeding on fruits and on fermenting tree sap (Bächli et al., 2004).

As potential vectors of Thelazia californiensis, a reported Thelazia species in carnivores and humans from the United States, the little house fly species Fannia canicularis and Fannia benjamini have been reported (Burnett et al., 1957), even though information about the vector of this eyeworm species is limited. The two species belong into the family Fanniidae, which comprises 8 genera and of which Fannia with 349 species is one (Bánki et al., 2021).

Due to the very limited information on Th. californiensis and its vectors and the considerably wider distribution and the superior data availability on Th. callipaeda, we mainly concentrate on the family Drosophilidae, in detail on the genus Phortica, on this website.

Classification of tabanids

Phylum:Arthropoda

Class:Insecta

Order:Diptera (i.e. two-winged insects)

Suborder:Brachycera

Family:Drosophilidae
Fanniidae
Subfamily:SteganinaeDrososphilinae
Genus:e.g., Phortica, Amiotae.g., Drosophilae.g., Fannia


Distribution

The distribution of Phortica variegata has to be considered here under the aspect of being a vector of the oriental eyeworm Thelazia callipaeda. This parasite was originally reported from Asian countries (Bhaibulaya et al., 1970; Shi et al., 1988). It has been found in humans and animals (dogs, cats, foxes, wolves and rabbits) in the former Soviet Union, China, South Korea, Japan, Indonesia, Thailand, Taiwan, and India (Anderson, 2000). Meanwhile there have been recorded autochthonous infections in carnivores also from several European countries (see under Thelaziosis for details and literature) and recently as well one from the north-eastern United States (Schwartz et al., 2021). Correspondingly, the main vector, the fruit fly Ph. variegata (Fallén, 1823), is widespread in Europe and Eastern Asia (Bächli et al., 2004). Phortica variegata fruit flies have also been found in areas in the eastern United States (see Werner et al., 2020 for more information), where they have also been experimentally proven to be competent vectors for Th. callipaeda worms (Otranto et al., 2018). Additionally, Ph. variegata has been reported from Canada meanwhile (Miller et al., 2019).

Ecological niche modelling demonstrated that the potential distribution of Ph. variegata ranges widely across southern and central Europe, extending as far north as Denmark, the southern tip of Sweden and south-eastern England (Otranto et al., 2006). Repeated modelling even extended the areas suitable for the development of Ph. variegata further east (e.g. over Ukraine, Russia and northern Turkey), implying a potential expansion also of the transmitted pathogen, Th. callipaeda.

Epidemiology

Phortica variegata is the intermediate host for the oriental eyeworm Thelazia callipaeda in Europe (Otranto et al., 2006a). It has as well just been confirmed as potential vector for the nematode in the United States (Otranto et al., 2018) and has been recorded in Canada (Miller et al., 2019). The presence of Ph. variegata is a critical determinant as the spread of the fly into new geographic regions may accompany an increased risk of Th. callipaeda infection to humans and other animals within those regions, with concomitant implications for both the human healthcare and veterinary sectors (Marino et al. 2018).

During studies in Italy, the flies were mostly concentrated in mountainous areas at 600-1200 m above sea level (a.s.l.) (Otranto et al., 2006b). Biological activity of Phortica flies was highest when temperature and relative humidity (RH) were 20-25°C and 50-75%, respectively. But flies were also collected at lower temperatures and higher RH suggesting that the latter parameter plays an important role in the maintenance of the biological activity of Phortica (Otranto et al., 2006b). Modelling showed that ecological niches of Ph. variegata in southern Europe seem to be limited to hilly areas with relatively high precipitation and continental temperatures (Otranto et al., 2006b). Generally, large areas of Europe are likely to represent a suitable habitat for Ph. variegata, mostly concentrated in central Europe (Otranto et al., 2006b). The fly collection sites in the reported studies were typically associated with oak (Quercus spp.) woodland habitats and fruit farms (Otranto et al., 2006b).

In southern Italy, the biological activity of Phortica flies occurs from May to October. An unpublished finding of one male Ph. variegata in Slovakia (at Zikmundova cave, 15 m from the cave entrance, 14/10/2002, collected by A. Mock, identification by J. Máca) suggests that Phortica flies may be able to use caves as winter shelters, a habitat they are presumably also able to exploit in southern Italy (Otranto et al., 2006b).

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