Ticks are obligate haematophagous acarines that parasitise every class of vertebrate (including man) and have a worldwide distribution. An increasing awareness of tick-borne diseases among clinicians and scientific researchers has led to the recent description of a number of emerging tick-borne bacterial diseases. Since the identification of Borrelia burgdorferi as the agent of Lyme disease in 1982, 11 tick-borne human bacterial pathogens have been described in Europe. Aetiological diagnosis of tick-transmitted diseases is often difficult and relies on specialised laboratories using very specific tools. Interpretation of laboratory data is very important in order to establish the diagnosis. These guidelines aim to help clinicians and microbiologists in diagnosing infection transmitted by tick bites and to provide the scientific and medical community with a better understanding of these infectious diseases.

The clinical features and the titres to Ehrlichia equi, E canis, E risticii, Rickettsia rickettsii and Borrelia afzelii in 14 Swedish dogs, in which ehrlichiosis was diagnosed on the basis of the presence of inclusions in granulocytes, are reported. Most of the dogs were moderately ill but made a rapid recovery after treatment with doxycycline. The dogs with inclusions were thrombocytopenic. Analysis of the antibody titres indicates that serology to E equi will remain the most appropriate serological test for granulocytic ehrlichiosis in Swedish dogs, until a specific test is developed for detecting the recently identified subspecies of Ehrlichia.

A 14-month-old shorthaired cat was presented to the Animal Hospital in Skara, Sweden, with a two-day history of lethargy, anorexia and tachypnoea. Clinical examination and laboratory investigations revealed fever, dehydration, tick infestation, neutrophilia with left shift, lymphopenia, hyperglycaemia and intracytoplasmic neutrophilic Ehrlichia inclusions. After treatment with intravenous doxycycline and lactated Ringer's solution the temperature returned to normal. Oral treatment with doxycycline continued for 20 days. The ehrlichiosis diagnosis was confirmed by serology, polymerase chain reaction and DNA sequencing. No relapse was observed during the eight-month follow-up period. The granulocytotropic Ehrlichia strain found in the cat was later characterised by analysis of the 16S rRNA gene sequence which showed 100 per cent identity to DNA sequences found in Swedish canine and equine granulocytotropic Ehrlichia strains. This is, to the best of the authors' knowledge, the first reported case of granulocytic ehrlichiosis in a cat.

Seven beagles were inoculated experimentally with a Swedish canine Ehrlichia species isolate to study its pathogenicity. With respect to the 16S rRNA gene sequence, the isolate was identical to the human granulocytic ehrlichiosis (HGE) agent and closely related to both Ehrlichia equi and E phagocytophila. After an incubation period of four to 11 days, the most prominent clinical signs were high fever for two to five days and depression. All the dogs developed profound thrombocytopenia, moderate leukopenia and a strong serological antibody response. Ehrlichial inclusions were detected in blood neutrophils from four to 14 days after inoculation for four to eight days. Ehrlichial DNA could be detected by polymerase chain reaction during the parasitaemic stage and a few days before and after microscopic inclusions were visible. Postmortem, the dogs showed reactive splenic hyperplasia and non-specific mononuclear reactive hepatitis.