Ticks and Lyme Disease in Dogs

Ticks – Life cycle and vector potential

Ticks are arthropod ectoparasites that use a vast number of hosts to feed ontheir blood. There are about 800 species of ticks in the entire world and they are one of the most important groups of arthropod disease vectors in dogs. The two most important tick families are the Ixodidae and the Argasidae. In the Ixodidae family we can find the Rhipicephalus, Ixodesand Dermacentorspecies, which are really important vectors of diseases for dogs and cats, because they can transmit viral, bacterial and protozoan pathogens.The ticks in the Argasidae family are parasites that affect primarily birds,bats and reptiles.

The life cycle of ixodid ticks involves four stages: egg, larva, nymph and adult.During these stages the larvae,nymphsand adults have a number of blood meals. An adult tick can originate thousands of eggs which fall from the host into the ground where they grow until they can infect another host. Larval and nymphalIxodes are found in moist, protected areas such as under humus on the floor of hardwoodforests. They feed on small rodents, lizards and birds. Adult ticks climb to the tips ofvegetation, usually to a height appropriate for their host, because they can’t jump or fly, and then they transfer to the host and move over the skin surfaceto find their preferred attachment sites. In these sites they feed on the host’s blood for hours. That’s one of the aspects that makes ticks good vectors of diseases.

Infection of a host with tick-transmitted pathogens may be aided by salivaryanticoagulants and other active compounds thatmodulate host cutaneous immunity and inflammation,while enhancing vasodilation in order to bring moreblood to the feeding site. The salivaryfluid is the main vehicle of transmission of diseases in ticks.

Lyme disease (Borreliosis)

Etiology

Lyme disease is caused by a Spirochete from theBorreliagenus which is transmitted by adult and nymphalticks. The elevendescribed species in the Borreliaburgdorfericomplexare transmitted by hard ticks, primarily Ixodesspecies. Only three Borreliaspecies, B. burgdorferisensustricto, B. gariniiand B. afzelii, are clinicallyimportant in both humans and dogs.

Pathophysiology

Spirochete transmission requires at least 50 hours of tick attachment, during which the organism disseminates to the salivary glands and infects the host through tick saliva. There they replicate and migrate through tissues, beginning close to the tick bite. Later, they can spread and infect many tissues, including the joints.

Different mechanisms are used by Borrelia toavoid clearance by the immune system, resulting in apersistent infection and possibly chronic disease. Clinical syndromes are produced by the host’s inflammatory reactiontothe organism.

Clinical Signs

• Acute signs of fever (39.5°C to 40.5°C [103.1°F to 104.9°F]);
• Anorexia;
• Lethargy;
• Lymphadenomegaly;
• Episodic, shifting lameness (60 to 90 days after infection);
• Articular swelling;
• Polyarthritis – first limbs affected are usually the ones closest to the site oftick attachment. Lameness in aparticular limb often lasts for a few days and then may shift to another limb or even disappear;
• Renalinvolvement occurs frequently and may be an immunopathologicalsequela to the infection. This is more common in Labradors and Golden Retrievers.

Erythema migrans(EM) lesion seen in human infections does not occur indogs.

Diagnosis

Diagnosis is based on clinical signs, history of recent tickexposure, serological testing, and response to treatment.There are no specific hematological or serum biochemical changes in this pathology.

• Serology and western blotting – IFA or ELISA are insufficient to support a diagnosis of active borreliosis, as persistent antibody production occurs in asymptomatic dogs;
• PCR – PCR positivity in dogs in highly endemic areas may reflect the high risk of exposure to infected ticks, rather thanclinically significant infection;

 Treatment

Antibiotic treatment is usually begun before infection is confirmed.Doxycycline is the drug of choice. High-doseamoxycillin may be abetter choice in very young animals because of the effectsof tetracyclines on enamel formation. Improvement often occurs within 24 to 48 hours after antimicrobial therapyis initiated and most treatmentsare instituted for a minimum of 30 days. The greatest success is achieved with treatment in the initial phases of clinical illness.

Nonsteroidal anti-inflammatory drugs can relieve pain during episodes of painful arthritic complications.

 Monitoring and Prevention

Relapse of infection is possible even if treatment is continued for months.

There are vaccines available to prevent this disease, but they must be administered before exposure to ticks.These vaccines should be iniciated at 4 and 12weeks of age, depending on the product. Primary vaccination schedules consist of two inoculations 3 weeksapart. Vaccines given early in life before exposure offer the best mean of protection.

Dogs with naturally occurring borreliosis generally have ahistory of severe tick infestation. Consequently, prevention of heavytick infestations by regular use of long-acting topicalacaricides such as amitraz, fipronil or syntheticpyrethroidsapplied ascollars, topical powders, shampoos, dips or foams, sprays, spot applications and oral productsarethe key to prevent this disease. In addition,removing ticks within two days of attachment, will minimizetransmission and lower infectious load.

Application of acaricides to the environment is expensive and difficult. Ixodes species havea two-year life cycle and become redistributed by various hosts after feeding, making it difficult to treat all stageswith one application.

PUBLIC HEALTH SIGNIFICANCE

Borreliosis in humans is an important, debilitating diseasewith high morbidity in endemic areas. Clinical signs include skin, neurological and/or musculoskeletal problems,depending on the Borreliaspecies involved. Erythemamigransusuaally develops after an infected tick bite.

Dogs are not a direct threat to human beings.However, dogs and cats may carry infected ticks into thehouse, making it easier for humans to be bitten by infected ticks. However,it isunlikely that this representsa higher risk when compared with theexposure to infected nymphs derived from small rodentsor deer with access to the garden.

Precautions should be taken when removing ticks from your dog or cat in order to prevent the possibility ofexposure to Borreliae released from crushed tick salivaryglands, which might infect small wounds on the hand. In contrast todogs, humans can develop disease after a single bite, so precaution should be taken when entering areas of hightick density.