Found in tropical and subtropical regions, Ornithonyssus bursa (Berlese, 1888) (Acari: Macronyssidae) is a hematophagous mite that parasitizes domestic and wild birds (^{Arce et al., 2018}; ^{Mascarenhas et al., 2009}). It is generally found around the peak and eyes, where it feeds on the bird’s blood (^{Mascarenhas et al., 2009}), and it reaches high infestation rates, which causes anemia and might even lead to death (^{Guimarães et al., 2001}; ^{Lareschi et al., 2017}). It can also be found in nests of wild birds, both in urban and rural settings. This mite species has already been reported in bird species of the families Strigidae (^{Mascarenhas et al., 2009}), Tinamidae, Charadriidae, Columbidae, Furnariidae, Hirundinidae, Icteridae, Mimidae, Passerellidae, Tyrannidae, and Thraupidae (^{Oliveira et al., 2012}). However, it can be found in other wild birds. In the absence of its primary host, it migrates to residences, causing allergies in their inhabitants. Its bite causes pruritus and temporary skin lesions (^{Arce et al., 2018}; ^{Mentz et al., 2015}; ^{Oliveira et al., 2012}).

In Brazil, O. bursa was first reported parasitizing humans in the 1990's in southern Brazil (^{Mentz et al., 2015}; ^{Oliveira et al., 2012}; ^{Sarda-Ribeiro et al., 1992}). Humans are generally infested either through nests that are abandoned after nidification or by handling infested birds (^{Bassini-Silva et al., 2019}), regardless of age, humans become susceptible by acting as an alternative food source for the mites, which can occur in both urban and rural areas. The present study reports a new event of infestation by O. bursa in humans in an urban environment, in the South of Brazil.

The present case occurred in December 2020, in the urban area of the municipality of Cruzeiro do Sul, Taquari River Valley region, state of Rio Grande do Sul, Brazil. A three-year-old child presented some skin lesions characterized as erythematous papules (Figure 1) in their back and arms, followed by intense pruritus staying for approximately for about 10 days in the body. The presence of O. bursa was observed in several parts of the residence, e.g. bed, bedroom, walls, and wardrobe. A nest of Zenaida auriculata (Des Murs, 1847) (Columbiformes, Columbidae) was found in the air-conditioner beside the bedroom window.

Mites were collected with the help of paper towel, stored in plastic bags, and subsequently sent to Laboratório de Acarologia of Universidade do Vale do Taquari - Univates, Lajeado, state of Rio Grande do Sul. Samples were maintained under low refrigeration 8 ºC for subsequent mounting. A total of 20 specimens were mounted in Hoyer's medium and maintained in a furnace with temperature between 50-60 °C for approximately ten days for drying, fixating, distending, and clarification of the specimens (^{Silva et al., 2018}). Specimens were identified using a Leica S6E optical microscope with 2500 LED and phase contrast according to ^{Guimarães et al. (2001)}.

Figure 1 A. Skin lesion caused by the hematophagous O. bursa in the axillary region. B. Skin lesion caused by hematophagous O. bursa on the side of the abdomen. 

Morphologically O. bursa is very similar to O. sylviarum, however, the distinction between species is found mainly in the sternal shield, where it contains the third pair of arrows (Figure 2D). It presents a tapered genitoventral shield in the posterior region (Figure 2B), and a dorsal shield gradually decreases at the distal end (Figure 2A) with smaller arrows, it presents a chelicera in the shape of a chela (Figure 2C) (^{Guimarães et al., 2001}).

Figure 2: Ornithonyssus bursa. A. Dorsal view, B. Ventral view, C. Chelicera, D. Sternal plate. 

Ornithonyssus bursa parasitizes humans in the absence of their food source; this indicates that they temporarily become parasites since they cannot complete their cycle (reproduce) and that they live for long periods of time away from birds, considered as their main hosts (^{Orton et al., 2000}). In the present study, the infestation of this hematophagous mite towards the child was probably opportunistic, due to the recent abandonment of the chicks to the nest, which forced the transfer of the O. bursa to another food source. Therefore, infestations of this mite in residences only occur when conditions are favorable for its reproduction and propagation (^{Oliveira et al., 2012}).

The presence of O. bursa on humans in Brazilian territory has been reported in the southern and southeastern regions, associated to warmer seasons of the year (^{Mentz et al., 2015}; ^{Mesquita-Sousa et al., 2020}; ^{Sarda-Ribeiro et al., 1992}), which is corroborated by the present study. Avian-mite lesions caused on the child were located mostly in the arms, like previous reports (^{Oliveira et al., 2012}; ^{Suzuki et al., 2014}). Attacks on humans are usually more frequent in rural areas, especially in poultry farmers, as these mites are considered to attain pest status in the poultry chain where they act as a potential vector of the arbovirus (^{Santillán et al., 2015}). However, hematophagous mite infestations have occurred more frequently in urban areas due to the proliferation of pigeons due to the construction of their nests on roofs or air conditioning (^{Suzuki et al, 2014}), as birds are the main hosts of several species of hematophages (^{Bassini-Silva et al., 2019}).

Air-conditioners or eaves in residences might serve as shelters for birds to build their nests, thus increasing the frequency of cases. However, avian-mite dermatitis is generally misdiagnosed as bites from other arthropods (^{Waap et al., 2020}). Although there are no reports on the transmission of pathogens (^{Bassini-Silva et al., 2019}), the presence of bacteria of the genus Wolbachia associated to this mite species has already been reported (^{Lareschi et al., 2017}). Therefore, it is necessary that dermatologists - and especially general practitioners - become familiar with gamasoidosis, popularly known as avian-mite dermatitis. Dermoscopy criteria to identify dermatitis caused by hematophagous mites have yet to be described in medical literature (^{Suzuki et al., 2014}; ^{Wambier and Wambier, 2012}). These definitions might help establish an accurate diagnosis and hence find a suitable treatment.