Ticks are major pests of dogs and to a lesser extent cats, that bite, feed on blood, cause local irritation and transmit several serious diseases. After piercing the skin with biting mouth parts, ticks cement themselves into the skin and secrete immunoactive chemicals to ensure an influx of inflammatory cells and prevent clotting. Tick bites produce local irritation and may predispose the area to secondary bacterial infection. Some infectious diseases transmitted by ticks include ehrlichiosis, anaplasmosis, babesiosis and Lyme disease. Some ticks can cause tick paralysis, an ascending flaccid paralysis of dogs and children, caused by a protein toxin elaborated most commonly by Dermacentor spp. females after a few days of feeding. The common ticks that bite dogs and cats in Europe are the sheep tick Ixodes ricinus, the hedgehog tick I. hexagonus, I. canisuga, Dermacentor reticulates, Rhipicephalus sanguineus and, more rarely, Haemaphysalis punctata and H. concinna. R. sanguineus and I. canisuga are 'peridomestic' ticks, meaning that they feed as larvae, nymphs and adults on dogs and cats and can thus complete their entire development near human environments. The other ticks require rodents or other wildlife to feed larvae and nymphs. Generally only nymphs and adults feed on dogs. Ticks feed on a dog or human for 2-8 days, most frequently in and around the ears and axillae.

Fully engorged ticks will drop off the dog and seek a suitable moist, dark and leaf-litter protected microenvironment to lay eggs. The brown dog tick R. sanguineus will lay eggs and moult in moist cracks of cement and other protected sites common in many shelters. If ticks are pulled off the dog before they are fully engorged, tissue trauma invariably accompanies the removal.

Dogs that have been heavily infested may require hospitalisation for observation and fluids and/or blood products if anaemic. Sites of attachment may become infected with secondary bacteria and may benefit from treatment. Acaricidal baths help kill ticks quickly and remove serum and crusting, which minimises secondary bacteria and provides comfort from pruritus. Spot-on acaricides (imidacloprid/permethrin or fipronil/methoprene) will kill ticks and provide protection from future infestations for weeks to a month. In areas where there is endemic tick-borne disease, it is reasonable to check serology 2-3 weeks after tick infestation for borreliosis, anaplasmosis and ehrlichiosis.

Anaplasma phagocytophilum

Granulocytic anaplasmosis is an emerging disease of humans and dogs in parts of Europe and eastern North America. In some geographical areas anaplasmosis in dogs is far more common than monocytic ehrlichiosis. A. phagocytophilum is a tick-transmitted rickettsial pathogen that targets the neutrophil. In Europe A. phagocytophilum is transmitted primarily by the sheep tick, and reservoir hosts are voles and field mice. The same pathogen can infect people, horses, dogs and wildlife. The clinical signs in dogs, horses and people include fever, muscle and joint pain and, in people, headache. Haematological and biochemical abnormalities may include thrombocytopenia, anaemia, leucopenia and elevated liver enzymes. Horses may develop icterus, head pressing and lower limb oedema and cattle can become depressed, thrombocytopenic, immunosuppressed, febrile or die. However, in all species, most infections do not manifest any abnormal signs and go unnoticed. A. phagocytophilum infection is associated with polyarthritis and thrombocytopenia in dogs.

Anaplasmosis is diagnosed by direct visualisation of the organism in neutrophils, serology and PCR (polymerase chain reaction). Within neutrophils A. phagocytophilum appears within small, cytoplasmic, membrane-bound vacuoles called morulae. PCR of whole blood is one of the most sensitive tests available for diagnosing active anaplasmosis because acute infections often are resolved before the dog seroconverts. Serology must be interpreted carefully, as many dogs in endemic areas are seropositive, documenting prior exposure although with little clinical relevance. The finding of a 4-fold increase in titre confirms recent infection. There is sometimes cross-reactivity between A. phagocytophilum and Ehrlichia canis; it is best to run both tests.

Granulocytic anaplasmosis responds well to treatment with tetracycline with resolution of the fever typically within 24 hours. Even without treatment, most cases of granulocytic anaplasmosis resolve on their own within about a week.

Anaplasma platys

Anaplasma platys (formerly Ehrlichia platys) is a rickettsial pathogen that infects canine platelets and causes canine cyclical thrombocytopenia. Little is known about the natural history, reservoirs or arthropod vectors of A. platys although a tick vector is suspected. Recent advances in molecular diagnosis have resulted in documentation of considerably more cases than were previously identified, suggesting that many dogs are subclinically infected. Diagnosis requires serology or PCR because affected dogs often have profound thrombocytopenia and thus few platelets are available in which to visualise the organism. Treatment includes tetracycline-class drugs, as well as steroids if the case is severe. Chronic thrombocytopenia may occur, although generally not severe, thus prolonged treatment may be necessary.

Ehrlichia canis

E. canis is a common ehrlichial agent found in sick dogs in mainland Europe but infrequently in Great Britain and Ireland. It is closely related to E. chaffeensis, the agent of human monocytic ehrlichiosis. The vector for E. canis infection is the tick Rhipicephalus sanguineus and the dog is the reservoir. E. canis targets the monocyte where it produces a morula similarly to A. phagocytophilum. Clinical signs and haematological abnormalities include fever, lymphadenopathy, anaemia and thrombocytopenia in the early stage. If the dog remains persistently infected, canine ehrlichiosis may enter a second subclinical stage where it may remain for months to years, although the serological titre may remain high or increase in this stage. Eventually in less than 10% of dogs, ehrlichiosis may enter a third stage of systemic disease with bone marrow suppression and pancytopenia. Dogs in this stage may have vague generalised signs of systemic disease associated with immune complexes and very high titres, with lethargy and weight loss, lymphadenomegaly and splenomegaly, uveitis and retinitis, and bleeding from eyes, nose, and in bowel movements. German Shepherd Dogs are predisposed to more severe disease.

As for granulocytic anaplasmosis, most acute cases of E. canis respond well to doxycycline. In contrast, chronic monocytic ehrlichiosis is not easily treated and has a poor prognosis. Dogs require anti-ehrlichial drugs (doxycycline or imidocarb), fluids and/or blood transfusion, possibly erythropoietin or granulocyte colony-stimulating factor and steroids.

Borrelia burgdorferi

Lyme disease or borreliosis is caused by the spirochaete B. burgdorferi. In Europe, the main species associated with infection in dogs and people are B. garinii and B. afzelii. Generally borreliosis is clinically silent or mild, but severe manifestations can include fever, polyarthritis, monoarticular arthritis, chronic arthritis and neurological and cardiac dysfunction. In dogs, particularly Labrador Retrievers, B. burgdorferi infection can cause severe immune complex-associated nephritis with protein-losing glomerulopathy and fatal renal failure. Vectors for B. burgdorferi are Ixodes ricinus, as for granulocytic anaplasmosis, and the main reservoir is the field mouse. After a tick bite, the spirochaete is inoculated into the skin and connective tissue, and eventually may disseminate to joints, heart or other areas of connective tissue. Infection in some dogs may last months to years.

Infection with B. burgdorferi should be considered in dogs with severe or chronic arthritis or nephritis. Blood samples are screened by immunofluorescent antibodies (IFA), enzyme-linked immunosorbent assay (ELISA) or an equivalent test and the diagnosis confirmed with western blotting, because IFA alone has a high rate of false positivity. Alternatively, a C6 protein ELISA is highly sensitive and specific. It is important when interpreting serology results to distinguish positive test results from vaccine reactions. Culture or PCR may be positive especially in joint fluid. Acutely infected animals may be treated with doxycycline, amoxicillin or azithromycin. Chronic disease, particularly nephritis, has a poor prognosis. In humans, neuroborreliosis is treated with intravenous ceftriaxone, a treatment that could be useful in chronic canine borreliosis. Nephritis treatment also requires immunomodulation (to minimize Arthus-type reactions) and possibly dialysis.

Tick-Borne Encephalitis and Louping Ill

These two diseases are caused by I. ricinus-transmitted flaviviruses that can cause severe disease in humans and sheep, respectively. While dogs can suffer from encephalitis with either virus, more typically dogs are asymptomatically infected and used primarily as sentinels for infection in humans and sheep.

Babesia canis

Babesiosis is a tick-transmitted disease most often caused by the protozoan B. canis in Europe. Rhipicephalus sanguineus and Dermacentor reticulates are the vectors, and disease is most prevalent in the Mediterranean and eastern Europe. Clinically, the presentation may be non-specific and variable, often manifesting mainly as a weak, depressed dog. Infected dogs may develop splenomegaly, haemolytic anaemia with bilirubinuria, and thrombocytopenia, eventually resulting in disseminated intravascular coagulopathy or immune-mediated glomerulonephritis. Coinfection with Borrelia burgdorferi, Anaplasma phagocytophilum or Ehrlichia canis may exacerbate clinical disease.

The diagnosis of babesiosis relies on identification of antibodies through serology, Babesia-specific DNA through PCR, or visualization of the parasite on thick blood smears. Treatment options include doxycycline, metronidazole, imidocarb dipropionate, clindamycin and prednisolone. Imidocarb appears particularly effective against Babesia canis. Supportive care, including blood transfusion, may be necessary. Important preventive measures include prevention of tick infestation and screening of blood products before administering transfusions. In shelters, the primary significance of babesiosis is in weak or anaemic dogs on a sporadic basis.