Medical and Veterinary Entomology (2011), doi: 10.1111/j.1365-2915.2011.00958.x Molecular investigations of cytochrome c oxidase subunit I (COI) and the internal transcribed spacer (ITS) in the poultry red mite, Dermanyssus gallinae, in northern Europe and implications for its transmission between laying poultry farms Ø. Ø I N E S 1 and S. B R ¨ ANNSTR ¨ OM 2,3 1 Norwegian Veterinary Institute, Oslo, Norway, 2 Department of Virology, Immunobiology and Parasitology, National Veterinary Institute, Uppsala, Sweden and 3 Department of Biomedical Sciences and Veterinary Public Health, Swedish University of Agricultural Sciences, Uppsala, Sweden Abstract. Samples of Dermanyssus gallinae (DeGeer) (Acari: Dermanyssidae) from more than 49 Norwegian and Swedish laying poultry farms, and additional samples collected from Scottish, Finnish, Danish and Dutch layer farms, were compared genetically. Analysis of partial mitochondrial gene cytochrome c oxidase subunit I (COI) sequences of mites from Norway and Sweden revealed 32 haplotypes. Only single haplotypes were found on most farms, which suggests that infections are recycled within farms and that transmission routes are few. Both Norwegian and Swedish isolates were found in the two major haplogroups, but no haplotypes were shared between Norway and Sweden, indicating little or no recent exchange of mites between these countries. There appears to be no link between haplotypes and geographical location as identical haplotypes were found in both the northern and southern Swedish locations, and haplotypes were scattered in locations between these extremes. The current data suggest that wild birds in Sweden are not a reservoir for D. gallinae infection of layer farms as their mites were genetically distinct from D. gallinae of farm layer birds. Transmission of the poultry red mite in Scandinavia is thus likely to depend on synantropic factors such as the exchange of contaminated material or infested birds between farms or facilities. Key words. Dermanyssus gallinae, cytochrome C oxidase subunit 1, poultry red mite, transmission. Introduction The poultry red mite, Dermanyssus gallinae, is a blood- feeding ectoparasite of poultry, argued to be the most important ectoparasite of laying hens (Chauve, 1998). It is of great economic significance and has the potential to cause anaemia and the death of its host if infections are severe enough (Kilpinen et al., 2005). The mite may also carry vector-borne diseases, including viruses and bacteria (De Luna et al., 2008). The mite visits the host only to feed and spends most of its Correspondence: Ø. Øines, Norwegian Veterinary Institute, PO Box 750, Sentrum, N-0106 Oslo, Norway. Tel.: +47 2321 6128; Fax: +47 2321 6001; E-mail: oivind.oines@vetinst.no lifetime hidden in the poultry house (Kirkwood, 1963). It is a common parasite of poultry; once established in a poultry house, its eradication is difficult and often involves costly downtime for the producer. Prevalence reports have asserted that D. gallinae is present in 80–90% of laying poultry farms in a number of countries (e.g. in Europe and Japan) (Sparagano et al., 2009). Parasite-infected hens have been shown to gain less weight than hens without D. gallinae infection (Kilpinen et al., 2005). Its effect on egg production is unknown, but mite infection in the layer house is believed to decrease © 2011 The Authors Medical and Veterinary Entomology © 2011 The Royal Entomological Society 1