Causes of Lyme Disease (Borreliosis)
Lyme disease is caused by a single-celled, spiral-shaped bacterium known as Borrelia burgdorferi. The main reservoir for these bacteria in the United States is the white-footed mouse. The bacteria can also live and replicate inside of other small mammals, lizards and birds. The organisms are transmitted to larger animals by tiny, hard-shelled, slow-feeding ticks. The ticks become infected with the bacteria when they feed on the blood of infected animals – especially the white-footed mouse. Ticks are considered to be intermediate reservoirs for Borrelia burgdorferi. These ticks are also known as “deer ticks,” because their preferred host is the white-tailed deer. So, the “bad bugs” like to live inside of the white-footed mouse and other small mammals. The “bad ticks” become infected by feeding on one of these animals, and then eventually transfer the infection of Borrelia burgdorferi to deer, dogs or other larger mammals that they bite.
Dogs develop Lyme disease when they are bitten by an infected immature or adult female tick that feeds on them for a long period of time. There is some disagreement about how long an infected deer tick must be attached to a dog for the infection to be transferred. Some experts maintain that a dog can become infected in as little as 5 hours, while others suggest that the tick must remain attached to the dog for at least 20, 50 or even 70 hours to transmit the infectious microorganisms. In any event, it is clear that infected ticks need to feed on a dog’s blood for many hours before they can transfer Borrelia burgdorferi to the dog through the site of the bite. The actual transfer occurs when the tick regurgitates infectious microorganisms into the bite wound in its saliva, after it finishes feeding.
The infective bacteria multiple rapidly in and under the dog’s skin around the area of the tick bite, which causes a lot of local irritation and potential infection. People with Lyme disease typically develop a raised, red area at the inoculation site during this initial period of the disease. This is called an erythema migrans. Dogs do not develop these lesions. Within a matter of weeks to months after the bacteria are injected from a tick into a dog, they migrate throughout the dog’s body in its bloodstream. The bacteria tend to lodge in the connective tissues of joints, muscles, tendons and other areas that have high collagen content, such as skin, heart and lymph nodes, where they continue to replicate in large numbers.
The bacterial infection triggers an immune response in the dog’s body. This, together with the localized concentration of bacteria in and around joints, tendons and muscles, are responsible for the limping and lameness seen in most dogs with Lyme disease.
Preventing Lyme Disease
The only way to prevent a dog from developing Lyme disease is to prevent it from coming into prolonged contact with infected ticks. Because these ticks only regurgitate infective bacteria in their saliva at the end of a long blood meal, Lyme disease can be avoided by preventing ticks from attaching to dogs in the first place. This can be accomplished through appropriate use of repellent sprays, anti-tick collars, dips, powders, dusts and liquid spot-on treatments. Amitraz collars and various topical preparations (such as Frontline, Advantix and others) reportedly are quite effective at controlling ticks. Area treatment with insecticides usually is not necessary. Your veterinarian is the best one to advise you about appropriate tick control protocols for your pet in your geographical area.
Here are some other common-sense ways for owners to protect their dogs (and themselves) from tick bites:
- Avoid dense undergrowth. Walk dogs on closely mowed grass, dirt or paved walkways whenever possible.
- Wear long-sleeved shirts and long pants tucked into socks when out in endemic areas. Light-colored clothes make ticks more visible and easier to brush or pick off.
- Apply topical tick repellent to clothing and exposed areas of skin; apply topical veterinarian-approved tick repellant to all areas of the dog’s coat and skin, avoiding only the eyes and mouth.
- Inspect dogs thoroughly for ticks immediately after venturing outdoors in areas where ticks are known to be present in large numbers.
- Keep lawns mowed and weeds pulled to reduce the number of places for ticks to hide.
- Eliminate areas of dense, undisturbed vegetation from areas that the dog has regular access to.
- Discourage wild animals (raccoons, skunks, deer, mice, rats and other rodents) from coming into contact with domestic dogs. Wild animals can harbor the ticks that transmit the bacteria causing Lyme disease.
It’s a good practice for owners to go over their dog’s skin and coat regularly, especially after the dog has spent time outside and has been exposed to dense brush, grass or other thick vegetation. Dogs (and people) who spend a lot of time outdoors should be checked daily for ticks. Unfortunately, immature deer ticks are tiny and can be difficult to detect, especially on dogs with dark coats. Even adult deer ticks are small – about the size of the head of a pin. They can easily be mistaken for freckles, moles or specks of dirt.
It is best to remove ticks before they latch on. However, Lyme disease can also be prevented if a tick is removed after it has attached but before it has finished feeding. If a tick is found, it should be mechanically removed from the dog’s body as quickly as possible, using gentle traction. Owners should grasp the tick with tweezers at the point where its mouthparts enter the dog’s skin and pull straight outward with firm, constant pressure. Specialized tick-removal devices are also commercially available. Gloves should be worn to prevent contact with the tick’s blood. Once removed, the tick should be placed in rubbing alcohol in a sealed container and disposed of in an outdoor garbage receptacle. Tossing a tick into the toilet, or “squishing” it in a paper towel and throwing it into a waste basket, are not reliable disposal methods.
Vaccines against Lyme disease are available for domestic dogs. Currently, these vaccines usually are only recommended for dogs that live in or travel to high-risk areas. Lyme disease vaccines work by preventing migration of the infective bacteria from an attached tick’s gut to its salivary glands during the course of the tick’s blood meal. Regular booster shots have been shown to improve the protection afforded by Lyme disease vaccines.
Deer ticks have a specialized 2-to-3 year life cycle, depending upon climatic conditions and the availability of host animals for them to feed on. Adult females lay their eggs in warm weather, often in the middle of summer. On average, a single female lays approximately 2,000 eggs. She only does so once during her life. The larvae hatch a few weeks later and can live without feeding for up to 8 months. Larval-stage ticks become infected by Borrelia burgdorferi when they feed on birds or small mammals that carry the bacterial microorganisms. The most common of these is the white-footed mouse. The following spring, tick larvae molt into nymphs. If they were infected by Borrelia burgdorferi as larvae, they will remain infected as nymphs. If they were not infected at the larval stage, they still can become infected at the nymph stage by feeding on the blood of carrier mammals or birds. Nymphs can survive unfed for about 6 months. Infected nymphs molt into infected adult ticks during the following summer months.
Adult female ticks mate and then have a single, prolonged blood meal by attaching to a white-tailed deer or another large mammal. Adult males normally do not attach to or feed off of any animals. Once they come into contact with a host, adult female ticks tend to attach very quickly. Adult females do not feed intermittently; once they start, they almost always continue to feed on the same animal until they are satiated. After they have eaten as much as they can hold, engorged female ticks fall off of their hosts and land in surface vegetation. They live on or under leaves or brush until the following summer, when the cycle begins anew.
There is no reliable evidence that Lyme disease can be passed from an infected bitch to her unborn puppies. Theoretically, transmission of the infective organism through placental blood may be possible.