TRENDS in Parasitology Vol.17 No.2 February 2001 http://parasites.trends.com 1471-4922/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S0169-4758(00)01856-1 74 Review Susan E. Shaw Dept of Clinical Veterinary Sciences, University of Bristol, Langford House, Langford, Bristol, UK BS40 5DU e-mail: susan.e.shaw@bris.ac.uk M ichael J. Day Dept of Pathology and Microbiology, University of Bristol, Langford House, Langford, Bristol, UK BS40 5DU. Richard J. Birtles Dept of Pathology and Microbiology, School of Medical Sciences, University of Bristol, Medical Walk, Bristol, UK BS8 1TD. Edward B. Breitschwerdt College of Veterinary Medicine, North Carolina State University, 4700 Hillsborough St, Raleigh, NC 27606, USA. Being haematophagous, ticks are well designed to transmit disease agents such as viruses, bacteria and protozoa. Historically, they have been considered second only to mosquitoes in this ability 1 . Ticks attach securely to their hosts, facilitating not only effective transmission of infectious agents, but also the spread of both ticks and microorganisms to different geographical habitats via travelling pets. Pathogens ingested by ticks can be spread trans-stadially and/or trans-ovarially. As female ticks are extremely fecund, this allows effective dissemination of infectious agents in reservoir populations with which pets and their owners interact. Arthropods in general, and ticks in particular, have evolved as ectoparasites of wild animals 1 . Only a minority of tick species, generally those with a wide host range, transmit diseases to domestic animals and humans. The increasing prevalence of tick- transmitted diseases of pets and their owners has been associated with increased accessibility of traditional ‘wilderness’ environments and an increase in the reservoir of wild host species (deer, small mammals and foxes) that now have a closer association with human activity. In addition, there has been a rapid evolution of molecular-based techniques, which has allowed more sensitive and accurate detection of tick-borne pathogens. For example, granulocytic ehrlichiosis in dogs, cats, horses and humans is now considered to be a disease of major importance in Sweden 2 , the USA 3 , Scotland, Switzerland and Slovenia. Ticks and the diseases they transmit have a zoogeographical range restricted by host movement and, to some extent, climatic factors. However, the increased mobility of pets has resulted in rapid extension of the zoogeographical ranges for many species. In the UK, this will be exacerbated by recent alterations to the animal quarantine regulations (Box 1). Between 1995 and 1998, 36% of cases of monocytic ehrlichiosis reported in Germany occurred in dogs that had travelled for short periods to the Mediterranean area 4 . In addition, during the same period, both Ehrlichia canis infection and infestation with Rhipicephalus sanguineus, the ehrlichiosis vector traditionally found in southern Europe, were found in dogs that had never been outside Germany 4 . The zoogeographical range is also increasing because tick species are finding niches in different climatic conditions. Since the 1980s, Ixodes ricinus has extended its range in Sweden to include more northern and western areas 5 and Dermacentor variabilis, the major vector of Rocky Mountain spotted fever (RMSF), has extended its range to include the northeastern USA 6 . Dogs as sentinels for human infection The medical and veterinary importance of tick infestation in dogs lies in the transmission of a wide variety of infectious agents (Table 1). The most important tick-transmitted infectious diseases causing severe clinical illness in dogs are babesiosis, ehrlichiosis and, in the USA, RMSF and hepatozoonosis. However, although Borrelia burgdorferi and Rickettsia conorii infections commonly produce subclinical infection, as evidenced by seropositivity, their association with clinical disease in dogs is more difficult to evaluate 7 . Dogs also appear to be susceptible to infection with Coxiella burnetii (Q fever) and tick-borne viral encephalitides 8, but reports of clinical illness are uncommon. Other canine tick-transmitted infections include haemobartonellosis, bartonellosis, tularaemia (Francisella tularensis) and, rarely, louping ill (Flaviviridae). Several of the tick-borne infections that affect dogs can cause serious disease in humans, notably borreliosis, ehrlichiosis, RMSF, R. conorii infection and tick-borne encephalitis. However, the potential zoonotic threat posed by dogs is strongly influenced by the natural cycle of the specific agent with which the dog is infected. Three general epidemiological scenarios can be described. First, if transmission of an infectious agent involves ticks with a broad host range (such as I. ricinus), dogs can act directly as sentinels for infection of humans. Second, by acting as natural hosts for certain nidicolous ticks (such as R. sanguineus and Ixodes canisuga), dogs significantly increase contact between these species and humans, thereby increasing the risk of transmission 9 . Finally, there is a limited risk of transmission by exposure to infected-tick contents following damage to ticks during grooming of infested animals. This scenario has been reported for R. conorii 10 . Tick-transmitted infections are an emerging problem in dogs. In addition to causing serious disease in traditional tropical and semi-tropical regions, they are now increasingly recognized as a cause of disease in dogs in temperate climates and urban environments. Furthermore, subclinically infected companion animals could provide a reservoir for human tick-transmitted infectious agents, such as Ehrlichia chaffeensis, Ehrlichia ewingii, the Ehrlichia phagocytophila group and Rickettsia conorii. Here, we discuss the emergence of new canine tick- transmitted diseases, which results from several factors, including the expansion of the tick range into urban and semi-urban areas worldw ide, the movement of infected dogs into previously non-endemic areas, and the advent of novel molecular techniques for diagnosis and pathogen identification. Tick-borne infectious diseases of dogs Susan E. Shaw, M ichael J. Day, Richard J. Birtles and Edward B. Breitschwerdt