Body dwelling mites are potential arthropod vectors responsible for viral pandemics.
by Deodoro Oliveira
This essay considers the hypothesis that microscopic mites (Arachnida: Acari) inhabiting the skin of vertebrates can function as vectors for the transmission of viruses. (i) Parasitic mites are prevalent in mammals, birds, and other vertebrates. (ii) Mites show certain host-species specificity, nevertheless, occasional cross infestations may provide a route for viruses to jump from one species to another. (iii) These diverse body dwelling critters – such as the skin burrowing itch mites causing scabies – can quickly spread in a host population. (iv) The presence of minuscule virus-infected mites in objects and dust resolves the current notion that some common viral diseases are airborne or propagated by fomites. Taken together, these considerations could help the understanding of some zoonotic viral diseases outbreaks, including the current 2019-20 coronavirus pandemic.
In the second half of the XVII century – a time when the theories of miasma and spontaneous generation were widely accepted –, G. C. Bonomo used a rudimentary microscope to observe the presence of mites in humans afflicted with scabies. The visualization of small mites, less than 0.5 mm, burrowing into the skin and their eggs hatching led G. C. Bonomo to suggest that mites themselves were the cause of scabies[1], an idea not readily accepted. Since then, the itch mite Sarcoptes scabiei (Acari: Acariformes) has been found to be a common ectoparasite on a wide variety of mammalian hosts, causing sarcoptic mange on them[2]. Varieties of S. scabiei present a certain degree of host specificity; however, both transient or complete cross infestations between different mammals seems to be possible[3,4]. Evidence for humans infested with S. scabiei varieties of other mammals is sparse. In spite of that, there are a few claims that the dog variety and the pig variety of S. scabiei were identified in humans[5,6]. Similarly, humans might be temporarily infested with Notoedres cati (Acari: Acariformes), the cause of feline scabies[7]. Proper species identification is a main issue since a large number of mites can infest mammals and birds[8]. The skin piercing red mite Dermanyssus gallinae (Acari: Parasitiformes) is normally associated with birds[9]. Dermanyssus gallinae is also known to infest humans (causing dermanyssosis) and infestations of D. gallinae in urban areas are a growing concern[10]. The mites in the genus Ornithonyssus (Acari: Parasitiformes) are other examples of avian mites that can parasitize humans[11]. A common characteristic of mites is that they can quickly spread in a population of humans, domestic animals, and wildlife[2].
In the late XIX century, the work of C. Finlay helped to determine that yellow fever was transmitted through mosquitoes[12]. Later, it was established that yellow fever was caused by a virus. The yellow fever virus is an enveloped virus with a single-stranded RNA genome, it is a flavivirus of the family Flaviviridae[13]. Multiple species of primates can serve as hosts for the yellow fever virus depending on geographical location[14]. Different mosquitoes can spread yellow fever within and between species of primates in somewhat complex cycles. The mosquito vector is also infected by the yellow fever virus, i.e. the virus is capable of replicating within mosquito cells – the mosquito vector species are also hosts for the virus. Furthermore, the virus finds its way to the eggs (transovarial transmission) and the next generation of mosquitoes is capable of spreading yellow fever. Many other RNA genome viruses can be transmitted by mosquitoes, for example: the dengue virus (Flaviviridae), the chikungunya virus (Togaviridae), the LaCrosse virus (Bunyaviridae)[15]. Mosquito control is a proven effective way to prevent viral diseases dissemination. Besides mosquitoes, other arthropods can serve as vectors for virus transmission. This is the case of ticks; for example, the tick Ixodes ricinus (Acari: Parasitiformes) can spread the tick-borne encephalitis virus (Flaviviridae) in a range of mammalian hosts. Furthermore, ticks are particularly fit to infect vertebrates with a quite heterogeneous number of RNA viruses[16]. The information that D. gallinae may transmit avian influenza A virus (Orthomyxoviridae) in chickens is an important finding that needs further scrutiny[17]. Less is known about viruses associated with the normally smaller acarines of the superorder Acariformes. Relatively recent data points that Leptotrombidium scutellar (Acari: Acariformes) and others trombiculid and gamasid mites – the larval stages of these mites feed on vertebrates – are naturally infected with Hantaan virus (Bunyaviridae), challenging the notion that the contagion of hantavirus by humans comes only from aerosols of rodent droppings[18]. Since all living things have their set of viruses[19], the likelihood of an obligate ectoparasite to share a set of viruses with its vertebrate host is considerable.
In the beginning of the XXI century (2003), a new contagious disease was recognized and named Severe Acute Respiratory Syndrome (SARS). The finding that the virus linked to SARS was a coronavirus[20] – severe acute respiratory syndrome coronavirus (Coronaviridae, relatively large single-stranded RNA genome viruses) – was surprising, since these group of viruses were not considered important causative agents of life-threatening illnesses in humans[21]. Prior to that, only two coronaviruses had been identified in humans and they were considered to induce mild symptoms (common cold). After the emergence of SARS, the number of known human coronaviruses grew to six[22]. In December 2019, a new SARS-inducing coronavirus, severe acute respiratory syndrome coronavirus 2, was isolated; it promotes influenza-like symptoms that can progress to pneumonia, multi-organ failure and death[23]. This coronavirus disease 2019 (COVID-19) is highly contagious and it has quickly spread globally resulting in the ongoing COVID-19 pandemic[24]. Much effort has been put in discovering the reservoir mammalian species of these SARS-related coronaviruses[25]. Bats are the usual suspects and many coronaviruses have been isolated from bats[26]. Coronaviruses are widespread in Mammalia, they have also been found in dogs, cats, rodents, caws, horses, pigs, camels, civets, pangolins, whales and probably most mammals have them[27]. However, the first coronavirus ever discovered was an avian coronavirus[28] and coronaviruses are also common in Aves[27,29]. Viral diversity is expected to be directly proportional to species number and/or population size for any given taxonomic group. Apparently, the possibility of an arthropod vector has been disregarded.
Transmission of viruses that cause influenza-like illnesses are historically considered to be airborne – transmitted by respiratory droplets produced when someone coughs or sneezes – or by contact with body fluids. The aerosol assumption still lacks scientific demonstration. The reasoning that a disease that affects the lungs and/or other respiratory tissues is airborne seems logical and the airborne model for COVID-19 transmission has driven attention to aerosols and fomites[30]. Although the airborne model can not be excluded at this point, this association between the route of viral contagion and the organs affected by the disease is unnecessary. Members of the Family Coronaviridae do not affect exclusively the respiratory tract, a number of distinct conditions, hepatitis, gastroenteritis, peritonitis, encephalomyelitis, can be linked to coronaviruses[27]. A modern definition of fomite would imply the presence of an active pathogen on an inanimate object. However, the feasibility of a virus present in a piece of clothing (or any other material) to reach the bloodstream of an individual causing the contagion of an illness has yet to be proved. The arthropod vector model is a well-understood model and should always be considered as an explanation for an RNA genome virus to reach the bloodstream of a vertebrate. A tiny arthropod could pass unnoticed on clothes or furniture and even be carried away with dust, and mites seem to be the best candidates. A small mite infestation, even a single individual, may be enough to pass a viral disease without promoting a noticeable skin condition. Obviously, mites can also be acquired through skin-to-skin contact and mite infestation is favored by crowded conditions. Among the concerns of COVID-19 pandemic is that it is eluding standard safety protocols exposing healthcare professionals to contagion. Proper understanding of the contagion mechanism is therefore paramount. This essay proposes taking into account a hidden arthropod vector: mites.
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