Demodecidae are the most specialized parasitic mites of mammals; they typically inhabit the skin, but they have been found in other tissues and organs. They can cause demodecosis (a disease which is hazardous and difficult to cure) in humans, domestic animals and livestock. They are parasites with high host and topical specificity. They have been found for most orders of mammals, and they are common in the populations of numerous host species. Therefore, they not only constitute an important subject of veterinary and medical study, but also comprise an excellent model for faunistic and parasitological analyses concerning different aspects of functioning and evolution of the host–parasite relationship. The current level or knowledge of demodecid mites is irregular and fragmentary, and numerous questions require elaboration and ordering, from the taxonomic diversity to geographic distribution and relations with hosts. Such data may be of use i.a. for the development of more efficient and reliable diagnostic methods, as well as understanding the etiology and pathogenesis mechanisms of demodecosis, currently a contentious issue. The present paper lists all formally-described valid species of demodecid mites, together with other functioning specific names, verified and with comments on their status. This is significant for correct species identification and demodecosis diagnostics. The list has been drawn up on the basis of data acquired in the period 1842−2020. It contains 122 valid species of parasite, including their hosts and geographic distribution, data on parasitism, as well as only the second record of Demodex sciurinus in Eurasian red squirrel Sciurus vulgaris in over 100 years since its initial discovery.

Thelazia callipaeda is a zoonotic nematode transmitted by drosophilid flies. It causes ocular thelaziosis, a disease of carnivores, such as dogs, cats, and foxes, and also humans. The parasite has thus far been observed in various areas of Eurasia, including 20 countries within Europe. The present study documents its presence in the south-east region of Poland, near the Ukraine border. An adult nematode was removed from the conjunctival sac of a dog showing ocular inflammation and purulent discharge. The dog’s precise origin is unknown. Based on its localization and morphometrical features, the nematode was identified as a Thelazia callipaeda adult male. The present study is the first report of T. callipaeda in a dog in Poland.

The common harbor porpoise is a widely-distributed marine mammal with three known subspecies, including P. phocoena phocoena, with a clearly distinct and critically endangered (CR) subpopulation from the Baltic Sea (Baltic Proper). As part of an assessment of the condition and health threats of these mammals, it is important to conduct parasitological monitoring. The aim of the study was therefore to compare the data on porpoise parasitofauna from this subpopulation with those on porpoises from other areas. The study included 37 individuals from 1995 to 2019; eight species of parasites were found (prevalence 83.8%, mean intensity 724.2, range 2–3940), with a predominance of lung nematodes – Stenurus minor (94.7%), Torynurus convolutus (69.4%), Pseudalius inflexus (63.8%), Halocercus invaginatus (22.2%); the highest intensity was recorded for S. minor (989, 53–2928). Two species of Anisakidae (Anisakis simplex – 33.3%, Contracaecum sp. – 20.0%) were found in the digestive tracts, which were a new record for this population. The fluke Campula oblonga was found in the livers of 31.3% of porpoises. The tapeworm Diphylobothrium stemmacephalum was also recorded in the intestine of one individual; this is typical for these hosts, but previously undetected in the Baltic subpopulation. Parasites coexisted in numerous hosts, constituting a heavy burden for them. The obtained data were compared with those from the P. phocoena parasitofauna from other regions, based on a compiled checklist (1809–2021) including all species of porpoise parasites (55 taxa). Compared to the worldwide porpoise parasitofauna checklist, the number of parasites found in the nominative subspecies (Baltic Proper subpopulation) is small: including only 10 taxa (eight in the current study). These species are typical of porpoises and usually the most common; however, the level of infection of Baltic porpoises (intensity and total parasite load) is very high, which can undoubtedly have a negative impact on their condition and overall health.

The present paper lists all parasite species of the European eelAnguilla anguilla(Linnaeus, 1758), recorded in Poland, in both its saltwater and freshwater habitats. The list has been drawn up, based on data acquired since 1844. The majority of included parasite species are presented with fish infection parameters together with data on their developmental stages and occupied microhabitats, localities and dates of collection of the eels themselves. The database includes 62 parasite taxa (including 50 species, nine identified to the genus level and three to higher taxa), representing at least 47 genera and 39 families. The most frequently-noted parasites of the European eel are the cestodeBothriocephalus claviceps, the nematodesAnguillicoloides crassus,Camallanus lacustrisandRaphidascaris acusand the acanthocephalanAcanthocephalus lucii. Four alien species have been noted from this host:A. crassus, the monogeneansPseudodactylogyrus anguillaeandPseudodactylogyrus biniand the acanthocephalanParatenuisentis ambiguus. The present list includes both new host records and earlier records not included in previous lists of parasites of eels.

The bat skin mites from the closely-related Demodecidae and Psorergatidae families occur synhospitally, populating the same host species and perhaps neighboring microhabitats. However, data on their occurrence and parasitism are fragmentary and dispersed. Thus far, 27 Demodecidae and 18 Psorergatidae species have been described, but the coexistence of mites from both families was only demonstrated in six species of bats. This article presents a description of Demodex pusillus sp. nov. from Nyctalus noctula, including a new host record (first observation of demodecid mites in Nyctalus) and a new record concerning the occurrence of Psorergatoides kerivoluae in Plecotus auritus. It also includes an updated global checklist of the occurrence of Demodecidae and Psorergatidae in Chiroptera, including data on their records/distribution and location in their hosts. In both studied families, the mites exhibit preferences, and even topographic specificity, colonizing different microhabitats in the host, including the eye region (e.g., Meibomian glands of the eyes, corneal surface and eyelid vault), wing membranes and hairy skin on the body. Such colonization of separate microhabitats enables different species to co-occur within the same host, while the total number of parasites determines the level of parasite load, with higher levels being associated with the incidence of disease symptoms. It is worth mentioning that Demodex pusillus sp. nov. is the smallest known representative of the Demodecidae family and one of the smallest animals (70–80 micrometers in length).

The present study describes a finding of the demodecid mite Demodex phocidi in the seal Phoca vitulina in the Baltic Sea. This is the first identification in Europe and the second in the world. This is also the first observation of the Demodecidae family in the pinnipeds outside North America. A high density of demodecid mites was observed in the skin of the examined seal, but no symptoms of parasitosis were observed. Our findings also supplement the taxonomic description and morphometry of D. phocidi.