Musca domestica

Merodon equestris

Seasonal prevalence of  MdSGHV among populations of adult houseflies at four Florida dairy farms in 2005 and 2006. Adapted from Geden et al. (2008).

Virus Biology

Ecology

Distribution and Host Range

Field Studies

Blood-fed Glossina sp.

Distribution and Host Range

Viruses that cause salivary gland hypertrophy in the adult host have been reported from three different genera of Dipteran insects: the narcissus bulb fly, Merodon equestris (Syrphidae) (Amargier 1979); the house fly, Musca domestica (Muscidae) (Coler et al. 1993); and several species of tsetse flies, Glossina spp. (Glossinidae) (Jaenson 1978). The SGH viruses appear to be highly specific to their host insects. For instance, the MdSGHV, isolated from M. domestica, does not infect other dipteran species (such as Musca autumnalis, Sarcophaga crassipalpis, Drosophila melanogaster, and Glossina pallidipes); and the GpSGHV, isolated from the tsetse fly G. pallidipes, does not infect the house fly.  Attempts to infect various established insect cell lines have failed to provide any evidence that the SGHVs can replicate in vitro. 

Given the species-specificity of the SGHVs, their geographical distribution is tied to the distribution of the host insect.  To date, the MdSGHV has been isolated from its synanthropic host on several continents, and our lab currently maintains isolates from Florida, California, Kansas, the Virgin Islands, Denmark, Thailand, and New Zealand.  On the other hand, the tsetse SGHVs are restricted to the African continent (see Abd-Alla et al. 2009). The range of the MeSGHV outside France is unknown, as there has been only one report in the literature (Amargier 1979) linking a viral infection to symptomatic salivary gland hypertrophy in the narcissus bulb fly.

Field Studies

The best-studied SGH viruses are those associated with the tsetse fly complex. These viruses were detected over 60 years ago in feral flies (Whitnall 1934, Burtt 1945). Climatic changes and fluctuations in host density influence the abundance of SGHVs in wild populations of house flies or tsetse flies (Geden et al. 2008). Monitoring the incidence of MdSGHV at a field site in North Florida over the course of 17 months demonstrated seasonal fluctuations in abundance varying from no incidence of viral infection to as high as 34% incidence in sampled house flies (Geden et al. 2008).  Such seasonal fluctuations in virus prevalence may explain the failure of random sampling to detect this virus in wild populations; on several occasions, members of our lab have collected and examined hundreds of adult house flies without detecting any symptomatic individuals.

The natural incidence of the tsetse SGHV is lower, but also fluctuating, with 0.4-15.6% of field-collected flies showing symptoms of salivary gland hypertrophy (Jaenson 1978, Otieno et al. 1980, Odindo et al. 1981, Odindo 1982, Ellis and Maudlin 1987, Jura et al. 1988).  Significantly, among certain laboratory colonies of Glossina morsitans, incidence of symptomatic SGH has ranged from 1.1% (Jura et al. 1993) to 4% (Kokwaro et al. 1990).  In recent years, the GpSGHV has infected and decimated worldwide the tsetse colonies used for insect sterile male release programs (Abd-Alla et al. 2007, 2008).  The GpSGHV, unlike the MdSGHV, has the ability to spread through tsetse colonies in an asymptomatic form. Using a PCR based approach, Abd-Alla et al. (2007) recently concluded that many of the tsetse fly colonies infected by the GpSGHV were 100% asymptomatic. 

Locations are: MdSGHV (red), GpSGHV (blue), and MeSGHV (green).