Epidemiologic Assessment of Scabies: Actuality of Airborne Transmission and Additional Standards to Reduce Spread of Contagion and Reinfestation

1.1. Innate Survival Skills of the Mite

The female burrows into the interface of the stratum lucidum and stratum granulosum to lay its eggs, while the male roams closer to the surface in the horny layer of skin. Mites do not feed off the lysed stratum corneum but ingest intercellular fluid for nourishment [9]. Host IgG antibody in the guts of mites is evidence that mites ingest host serum [11]. The procurement of host intercellular fluid is necessary for mites to obtain sufficient water to maintain their water balance [12].

The mite has an oval tortoise-like body that is ventrally flat and dorsally convex. All terminal segments of the legs of both sexes of scabies have claws for mobility. They move 1 inch per hour on human skin. Sarcoptes scabiei var. hominis is 0.35 by 0.3 mm in size. They are light with a fresh weight of 5.6 +/-1.25 ug and a dry weight of 2.8 ug+/-0.86 ug. Males are smaller, and their wet weight is 1.49 +/- 0.59ug, and their dry weight is 0.39 +/- 0.16 ug. One female produces 40-50 eggs over a life span of 26-40 days. Like lice, female scabies mites, following copulation, store semen internally in the spermatheca, a pouch that serves as a storage organ for sperm. Thus, one female organism can start a new colony of mites in a new host.

Mites are blessed with certain features, which aid in their viability when introduced into a location free from their previous host. They can ascertain proximity to a potential host by sensors detecting heat as well as human odor. Studies revealed that 100% of female mites moved directly to the host when placed 4.2 cm (1.62 in) away [13]. About 20% of mites migrated to the host when placed 11.2 cm (4.4 in) away. Moreover, if mites find themselves within 4 inches of the host, they stand a good chance of survival.

Mites also have been attracted to the fluorescent overhead light [14], accentuating their tendency to live on the outer edges of the epidermis.

The time duration that mites can survive off a host is a major factor in assessing their viability when landing on an inanimate object. Of note, they live longer if the relative humidity is high and temperatures are colder. Mites are prone to dehydration as they have limited ability to maintain their water balance. Females survived a week at 59 degrees with relative humidity above 75%. At a warmer temperature of 77 degrees, mite survival was 1-2 days [15]. Human mites survived 19 days at 10 degrees Celsius and 97% humidity. However, in dry climates, mites endure for less than 36 hrs.

Mites have the ability to penetrate the host skin fairly quickly. Female mites have been timed to take 20 minutes to burrow into the skin to lay eggs [15]. They secrete a clear fluid (saliva) that dissolves the stratum corneum, allowing the mite to descend into a skin depression in the outer epidermal tissues. They use their legs to crawl and swim into place. Furthermore, they can contort their body to navigate small openings. To merely find a firm footing on a new host, a mite would take even less time. Given their small size, they can penetrate most clothing materials easily.

1.2. Three Methods of Scabies Transmission

Mites are transmitted by three distinct methods:

(1) The most common mode is person-to-person physical contact, sexual or otherwise, which is referred to as horizontal direct contact transmission. Spread of the infestation among family members and other close contacts is common.

(2) Indirect contact transmission is when mites leave their safe haven and land on objects, such as clothes, furniture, clothes, toys, towels, linens, and bedding. Fomite transmission is when a new susceptible host touches these inanimate objects and transfers the parasite to a susceptible portal of entry, the skin. Human mites are positioned on the outermost skin layers, seemingly ready to take off their entire adult lives. They are programmed to leave their place for the hopeful colonization of a foreign host. If mites are rubbed off on sheeting or fall from their perch due to gravitational forces, that is referred to as normal fomite transmission.

(3) Vehicle transmission, in the case of scabies, refers to the transmission of pathogens through the air. Mites can be airborne due to kinetic, static electricity, or wind gusts. Kinetic energy is achieved from motion from the host, such as throwing a baseball, flicking rubber gloves off, or dancing. Of note, one of the body sites where mites reside is the hands and wrists [9], which are locations ideal for airborne transmission with the arm movement of the host. Another energy causing mites to go airborne would be static electricity. Static electricity in clothes is caused by the transfer of electrons between different fabrics when they rub together. It might occur with the removal of clothing and/or toweling. Additionally, gusts of wind, such as from a blow-dryer, fan, or forced air, could lead to airborne mites. If outside on a windy day, the distance of travel could potentially be further. Nature has made these organisms light and in the shape of balloons; they have minimal resistance anatomically, which might improve transport distance. For clarification, if a mite lands by gravity onto an inanimate object and then is resuspended into the air (for example, by a ventilating fan), this would then be defined as airborne transmission if it finds a new host or reinfests a previously treated individual.

The proof of airborne transmission would include that it is the first reasonable explanation for recovering mites on the floor, curtains, and chairs distant from the environment of affected individuals [16, 17]. An absence of knowledge for this form of transmission would also explain the lower-than-expected success rates with anti-scabietic treatments [18]. Differentiating airborne from fomite transmission has been pertinent in analyzing parameters for disease dissemination in other disease states [19]. Crusted scabies is presently felt to disperse mainly via fomite transmission, but airborne transmission, although never substantiated in the literature to date, would be operative in this condition as well [20, 21].

1.3. Present CDC Guidelines for the Control of Scabies Transmission

There is general agreement that for optimum preventive treatment, all family members and close contacts should be treated simultaneously with either oral or topical scabietic therapy. Simultaneously with treat- ment, individuals should wear clean clothing. All clothing, pillowcases, towels, and bedding used during the previous week should be washed in hot water and dried at high heat. Non-washables should be dry-cleaned, ironed, or stored in a sealed plastic bag in a warm area for 5 days. Floors, carpets, upholstery (in both homes and cars), play areas, and furniture should be carefully vacuumed. Fumigation of living spaces is not recommended.

The Centers for Disease Control also addresses the environmental control of mites in the institutional (hospital) setting. They have made three categories of scabies, namely, situations with a single case, multiple cases, and crusted scabies. For the first two forms, the CDC recommends avoiding direct contact as well as using gloves with hands-to-care for several days after the index case is treated. Only for the crusted variant, protective garments (gowns, disposable gloves, and shoe covers) are suggested.

As noted above, the CDC, as well as the bulk of the medical literature suggest that transmissions from inanimate objects may pose little concern in common scabies [1-6]. They suggest it is only important in a rare form of the disease called crusted scabies, in which the host has tens of thousands of mites, rather than less than a hundred in normal scabies. The CDC recommendations for crusted scabies include the use of gowns, disposable gloves, and shoe covers for three days after the treatment of scabies.

1.4. CDC Recommendations Might Need to be Expanded and Enhanced

Considering the transmission factors listed earlier, CDC recommendations for scabies need a few additions, including protecting index cases from reinfestation and infesting others. The enhancement relates to the fact that mites can navigate through cloth gowns, and one must protect against the mites with a more protective garb. Human mites are water-dependent and cannot navigate through waterproof materials [15]. Waterproof clothing should be recommended, which would include essential personal protective equipment (PPE) gowns that are made of polypropylene, as mites cannot penetrate such material. Other examples would include clothing often sold to painters and campers, rain gear for golfers, and parkas. Plastic bags, which are made of polyurethane, would be protective and could easily be worn over one's socks. Sofas, chairs, office furniture, and car seats need to be covered with plastic sheeting, which can be obtained at home improvement retailers. Blow dryers, fans, or major wind gusts in the house should be minimized, and if used, the air should be directed away from people.

1.5. The Use of Ivermectin

Ivermectin has been used in humans for oncho- cerciasis and strongyloidiasis since 1996. Although the FDA has not approved ivermectin for scabies, the CDC recommends it. With correct dosing, ivermectin kills the scabies mite on humans living in the skin layers entirely, as resistance is very rare. The only caveat is in patients with thickened nails and in patients with thick, crusted skin, topical agents would also be indicated for these areas. In such cases, a topical scabicide agent should be applied for 10 consecutive nights in addition to oral ivermectin.

Ivermectin reaches effective levels in the bloodstream within 4 hours and its half-life is 18 hours. The drug has an over a 2.5-fold increase in bioavailability when taken following a high-fat meal [22]. After one dose of ivermec-stin, it can be assured that for one day, one will not have any viable mites on the body. If reinfestation does not occur, the condition is resolved. Mites that have left prior to the oral treatment have three days to find a host before dehydration and death. Thus, control and protection measures only last for 96 hours with attention directed to one's home, car, and workplace.

Clinical efficacy for scabies has been achieved with the identical dosing for strongyloidiasis, namely 200 ug/kg [23]. Akin to the CDC, it is suggested to treat scabies twice, one week apart [24]. The reason for the second treatment is to improve one’s ability to avoid reinfestation, which is the cause of treatment failure with ivermectin. A lesser reason, that can now be ignored, is the concern as to whether the agent is 100% lethal to scabietic eggs, which might hatch days after the first treatment. In some countries like Italy, a single dose of ivermectin is recommended [25]. This means that a much more stringent adherence to fomite and airborne transmission is essential for cure.

Of note to this discussion, while taking oral ivermectin for three days, an index case of scabies would not have to worry about reinfestation from the environment. Trans- mission control would not be eliminated entirely as there would still be a risk of infesting another individual who is in close proximity during those three days. In severe cases, such as crusted scabies, and recalcitrant cases, a treatment of three days of oral ivermectin might be considered.