House Flies and Blow Flies
The question of whether house flies can spread SARS-CoV-2 has been the topic of tweets and fact checks recently. At least two of them had definite answers in the negative. Our objective is to look at the question more closely, examine available evidence if any, and see whether we can be definite in answering with a No.
The common house fly, Musca domestica L., and various species of blow flies like Chrysomya megacephala (Fabricius) and allied species are also called filth flies. In the Philippines, there are about 15 species listed under the genus Musca while the blow flies (family Calliphoridae in the broad sense) includes at least 80 species. The larvae of these flies are more popularly known as maggots. [Note that we use here the two-word common names for these true flies (i.e. flies with two well-developed wings, insect order Diptera), and not the one word housefly and blowfly, to follow agreed conventions among dipterologists to differentiate them from other insects called flies like firefly (a beetle), dragonfly or a butterfly.].
These flies and their larvae and pupae are often encountered in decaying organic matter, and hence, are often associated with solid waste including human feces. They have been observed also feeding on sputum (or spit) but not reported on discarded mucus or phlegm. In particular, many blow fly species are also regarded as important evidentiary material for forensic investigations owing to the association of their maggots with cadavers. Many of these flies are also underrated pollinators of important crops especially fruit-bearing trees.
There are no studies on the relationship between COVID-19 and its cause SARS-CoV-2 and house flies or any species of true flies. Dehghani & Kassiri (2020) attempted a quick review of studies possibly related to the topic. For other coronaviruses, the one closest one to date is the study conducted by Calibeo-Hayes et al. (2003) on the mechanical transmission of the Turkey coronavirus by house flies. This virus is known to science as Avian coronavirus (with acronym IBV for avian infectious brochitis virus), a coronavirus species that belongs to the genus Gammacoronavirus, also under the virus family Coronaviridae. Properly replicated experiments generated conclusive evidence that detected virus infection in turkeys in contact with virus-exposed flies and more significantly, that “increased rates of infection were observed with higher fly densities.” The said study demonstrated the potential of the house fly as a mechanical vector of IBV.
In part 2 of this series, we cited a study by Wang et al. (2020) on coronavirus testing positivity rates. The high percentages of positivity on sputum (72%) and on feces (29%) may suggest higher possibility that they can be sources of inoculum if not properly disposed. The occurrence on feces was picked up by an Indian celebrity who understandably connected it with the problem of open defecation in certain areas of their country and the widespread occurrence of house flies. This was in turn retweeted by the head of their government. WHO in a tweet and infographic dismissed the connection between positive detection in feces and houseflies. This was retweeted and posted several times by different netizens.
The population of filth flies are not as uniformly distributed as before or as they we have thought them to be. Like all organisms, they concentrate in areas where their sources of food and/or breeding places abound. Under present conditions, house flies, blow flies, and other flies of significance to public health are more frequently encountered in areas where there are many poultry houses and piggeries, garbage dumps, and urban poor communities. It can be expected therefore that these filth flies are capable of contributing to local spread of the virus in places where there are no proper latrines or septic tanks and when people still have the habit of spitting in public places. These flies have sponging mouthparts, and the modifications of their labium enable them to feed on the liquified or fluid portions of solid wastes, including human wastes. The spaces between the pores of the labellum, or more technically, the diameters of what are referred to as pseudotracheal canals, are large enough at 30 micrometers to accommodate coronaviruses. The higher end of the size range of SARS-CoV-2 particle is 200 nanometers. [Note: 1 micrometer = 1000 nanometers]. Add to that the fact that the tarsi of their legs and external body surfaces can catch viruses and other microbial pathogens and smear them on surfaces where they may subsequently alight. The length of time that the virus remains viable or potent will have to be considered in determining the potential contributions of these insects to the spread of the coronavirus.
Going back to the tweets and posts, their definite negative answer was based on a WHO infographic which had the title: “FACT: COVID-19 IS NOT transmitted through houseflies” . . . In turn the WHO infographic was based on a supposed fact check. I checked the said fact check. The fact check was right in that to date, there are no studies on house flies and SARS-CoV-2. As we have mentioned in Part 1, this virus is new to us. However, the Fact Check cited a line of reasoning alluding to viruses spread by mosquitoes. We explained in Part 3 that mosquito transmission of flavivirid viruses are biological in nature. Let us not forget that some insects can also spread pathogens via mechanical transmission. The Fact Check’s actual conclusion was: “Therefore, there is no clear evidence to believe that Coronavirus can be spread through Houseflies, yet.” Yes, the last word was YET (my emphasis). The omission of one word, whether inadvertent or not, made a difference in meaning.
It is true that SARS-CoV-2, the virus that causes COVID-19 is mainly spread from one person to another through close contact, and aerosolized respiratory droplets when an infected person coughs or sneezes. Our primary preventive actions therefore should focus on this. However, there are no ecological impediments to a positive association between filth flies and other sources of contagion, especially when we take into consideration the consequences of high human population density including unsatisfactory sanitary conditions. The respective physical and social environments of more privileged human settlements and those of the urban poor communities are worlds apart. In the more affluent houses, screened windows and doors very easily keeps the pests outside and proper toilets solves the problem of open sources of contagion. However, that is not so in less privileged communities. In the Philippines, particularly in Southern Luzon, Laguna Lake would not gain the notorious image of being our country’s largest septic tank if the approximately 25% of households had proper toilet facilities. This coronavirus does not need to infect and multiply inside these flies. The flies just need to pick material where the virus particles are and then transfer them to another surface to contaminate it.
Yes, there is no evidence to connect these filth flies, fecal matter, sputum, and SARS-CoV-2. There is no evidence because no studies have been conducted. True, the more urgent matter is to follow recommended preventive measures. But yes, once more we are always reminded that “the absence of evidence is not an evidence of absence.” Hopefully, a collaborative work involving an entomologist, an environmental scientist, and a molecular virologist would provide the required evidence.
Like most of you, I do hope for the negative.
– Jun Lit (Ireneo L. Lit, Jr.)