Demodecosis is a problem in both dermatology and ophthalmology due to the chronic nature of the infection. Most studies concern the occurrence of Demodex spp. infections in the skin of the face, while research on their incidence in the eyelids is scarce. The effect of Demodex spp. infection on the visual system is a contentious issue. Some researchers indicate that these mites participate in the etiopathogenesis of eye diseases, while others disagree [21,22]. However, there are studies which offer evidence that Demodex spp. infection may cause changes in the cornea and conjunctiva of the eye [23,24].
The incidence of Demodex spp. infection depends on climate, socioeconomic and sanitary conditions, as well as access to medical care and effectiveness of treatment . In human skin, the extensiveness of Demodex spp. infection may range from 20% to 80% patients . Studies on potentially healthy subjects showed that 10%–60% of respondents had Demodex spp. in eyelash follicles. A study by Kuźna-Grygiel et al.  showed that, in Szczecin (i.e. the capital of the region of West Pomerania, the area covered in our research), the frequency of infection with Demodex spp. in eyelashes was 61%. A similar prevalence of infection with Demodex spp. (59%) was reported by Czepita et al.  in their study also performed in Szczecin. Lower incidence of Demodex spp. infection was found by Sędzikowska et al.  in patients of hospitals in Warsaw (47%). Wesołowska et al.  determined the prevalence of infection of Demodex spp. in patients (n = 95) and staff (n = 75) of the Centre for Orthopaedics and Rehabilitation of the Regional Hospital in Wrocław, addicts from the MONAR Addiction Treatment Centre in Wrocław (n = 34), and students of the Medical University in Wrocław (n = 89). Those authors found Demodex spp. infection in 41% of respondents. The greatest prevalence of Demodex infection was reported by the staff of the Centre for Orthopaedics and Rehabilitation of the Regional Hospital (40%). Garbacewicz et al.  calculated the incidence of Demodex spp. infection at 36% people from central Poland.
All surveyed residents of the Social Welfare Home in Jaromin (n = 28) were infected with Demodex spp. In a study conducted at a similar institution, the Veteran’s House in Szczecin , infection with Demodex spp. was found in 80% of residents. The very high infection rate at the Social Welfare Home in our study may have been caused by the fact that adults with psychiatric illness can restore their mental health. In this group of patients, infection with Demodex spp. is generally higher than in control subjects. For example, in a study by Kokaçya et al. , patients with schizophrenia had a higher incidence of Demodex spp. (29.03%) than control group (6.7%). The same team of researchers found that in depressed patients, Demodex spp. infection was also higher than control group (23.8% vs. 9.5%) .
In our study, physiological flora was found in conjunctival sac swab in all patients with symptoms of blepharitis, all residents of the Welfare Center infected with Demodex spp., and in 9% of those not infected with D. folliculorum (control group). This may indicate that Demodex spp. promotes colonization of the conjunctival sac with physiological flora.
Spickett  showed that D. folliculorum may be a vector organism for leprosy mycobacteria (Mycobacterium leprae). Demodex mites may also transmit Staphylococcus spp. and Streptococcus spp. on its surface . In a study conducted on patients, staff, and visitors of the Optometry Clinic in Oklahoma , S. aureus and S. epidermidis were found in 16.8% and 75.8% of subjects. In the study, two or more mites (11.6% and 5.2%, respectively) were reported more frequently in patients infected with Staphylococcus aureus than in uninfected patients. Staphylococcus aureus was found in 21.9% of patients aged 1–29 years, 13.1% aged 30–59, and 15.1% aged 60–89. In another study, Türk et al.  found S. aureus in two patients with blepharitis and infected with D. folliculorum. In our study, S. aureus was isolated from 7% of patients with symptoms of blepharitis, including 14.3% of residents of the Social Welfare House infected with D. folliculorum. We did not find the bacteria in the control group. In contrast, Lee et al.  found no differences in the incidence of bacteria between people infected and not infected with Demodex spp. Coagulase-negative Staphylococcus spp., Corynebacterium diphtheriae, and S. aureus were found in patients of both groups. There were no differences in the occurrence of MRSA between those infected and not infected with Demodex spp. In our study, MRSA was found only in patients infected with D. folliculorum, including one resident of the Social Welfare House in Jaromin.
Acinetobacter baumannii is one of the most common etiological factors of hospital-acquired infections. It shows natural mechanisms of resistance to antibiotics and chemotherapy. In the present study, A. baumannii was isolated from the conjunctival sac of a patient infected with D. folliculorum with symptoms of blepharitis.
Lacey et al.  isolated Bacillus oleronius from inside D. folliculorum collected from patients with papulopustular rosacea, and found specific antigens against B. oleronius in the serum.
Li et al.  on sera from 59 patients with diagnosed rosacea showed a statistically significant correlation between a positive serum reaction with B. oleronius antigens and the presence of Demodex spp. on eyelashes and face skin lesions. Results of a study by O’Reilly et al.  showed that proteins derived from B. oleronius may be a neutrophil-activating factor. Such activation of neutrophils could take place if B. oleronius proteins released from mites entered the tissues surrounding the hair follicle. This, in turn, could result in the development of local inflammation in the perifollicular tissue . In our study, Bacillus spp. were isolated from the conjunctival sac of a patient with D. folliculorum infection. In a study by Szkaradkiewicz et al. , 23 strains of B. oleronius were isolated from eyelashes collected from 18 patients infected with Demodex spp. and with symptoms of blepharitis; the authors observed more severe symptoms of blepharitis in patients in whom B. oleronius was found. However, B. oleronius was also found in five healthy subjects, which may undermine its role in the development of blepharitis. The authors concluded that these bacteria, living inside the intestines of the Demodex mites as symbionts, can be excreted by these mites onto the surface of human skin. Due to the fact that B. oleronius plays a significant role in the process of digestion in termites, it seems that these bacteria may play a similar role in Demodex spp. [36,40]. Lacey et al.  stated that two specific antigenic proteins (62 and 83 kDa) produced by B. oleronius can stimulate and be responsible for inflammation of the hair follicle. Moreover, Li et al.  noted a correlation between Demodex spp. infection and serum immunoreactivity to B. oleronius proteins 62-kDa and 83-kDa.
Streptococcus pneumoniae can cause inflammation of the middle ear, paranasal sinuses, and conjunctiva and cornea of the eye, as well as pneumonia. S. pneumoniae infection can cause severe or chronic complications [41,42]. In the presented study S. pneumoniae was reported in a resident of a Social Welfare Home infected with D. folliculorum.