Lyme disease, or Lyme borreliosis, is an infectious, blood-borne bacterial disease that is transmitted by ticks. It’s caused by Borrelia burgdorferi species.
Now, the genus Borrelia contains several species. Lyme disease in people is caused primarily by Borrelia burgdorferi in North America and by B. afzelii, B. garinii, and B. burgdorferi in Europe and Asia. In domestic animals, only B burgdorferi is confirmed to cause Lyme disease.
Borrelia are spirochetes, which means spiral-shaped bacteria. They have outer surface proteins, abbreviated as Osp, which play a role in virulence; and sets of flagella that run between the cell wall and outer membrane, which they use to spin or twist to move in a wave-like motion.
Hard-shelled, Ixodes ticks, or deer ticks, are the vector for B. Burgdorferi, meaning they are the intermediate organism that spreads the bacteria. In the northeast and Midwest USA, I. scapularis, the black-legged deer tick is the main vector; while on the Pacific coast, it’s I. pacificus, the western black-legged tick. In Europe and Asia I ricinus and I. persulcatus are the primary vectors. Ticks like environments with moderate humidity and temperature so they’re often found in wooded areas, thick brush, marshes, and tall grass. The ticks are small, and even adults are only about 3 mm long, so they can be hard to notice.
Now Ixodes ticks feed on the blood from hosts throughout their life stages of larva, nymph, and adult. When they hatch as larvae, they are uninfected. When they feed on infected hosts as larvae or nymphs, they can pick up the B. burgdorferi bacteria. In the younger stages of their life, they often feed on smaller animals like rodents, birds and even lizards. When they grow into adults, they move on to larger mammals like dogs, cats, or horses.
A tick infected with B. Burgdorferi can transmit the bacteria to humans and animals through their saliva during feeding. In the first few hours after attachment, the bacteria in the tick’s midgut switch their outer surface protein from OspA to OspC, which helps with transmission and provides protection against the host’s immune system. After 24 hours of attachment, the bacteria passes from the ticks digestive system to the host’s skin, and then into the blood vessels that the tick is feeding on. So, the longer a tick is attached, the more likely transmission will occur. After the bacteria get into the skin, they cause a local infection that activates local immune cells, leading to an inflammatory response. After several days, the bacteria can disseminate through the bloodstream to distant tissues, like the heart, kidneys, and joints. Inflammatory responses in these tissues can cause swelling, damage and dysfunction, particularly in synovial joints and around nerves.
Clinical signs and symptoms don’t always develop in domestic animals. But if they do, they can be non-specific, like a fever from inflammation; or be related to the affected tissue, like limb and joint disease; and renal, neurologic, and cardiac dysfunction. Dogs also commonly have anorexia; lethargy; swollen lymph nodes, and intermittent or recurring lameness, with limping or pain when moving. They can also have renal failure, which can be fatal.
About 3 out of 4 infected humans develop a classic rash called erythema migrans. It usually occurs as a red bump at the site of the bite that expands, often with clearing between the center and periphery resembling a bull’s eye. If disseminated disease develops, other erythema migrans lesions may appear and people feel achy, tired and weak. Without treatment, about 20-25% develop neurologic complications like Bell’s palsy where there’s weakness on one side of the face, or cardiac manifestations like heart block or pericarditis.
Diagnosis is primarily based on clinical signs and symptoms; history of exposure in endemic areas; and ruling out other diagnoses. Antibody testing can be done and the standard is to use enzyme-linked immunosorbent assays, called ELISA. Seroconversion may take several weeks, so the decision to treat is usually made clinically. Additional tests to assess tissue dysfunction can be helpful: like autoimmune panels, CBC, blood chemistry, and radiographs.
Treatment with the antimicrobials tetracycline, like doxycycline; and penicillins, like amoxicillin, are used in suspected cases. In dogs, a 4-week standard course can be extended or restarted until clinical resolution. Additional treatments tailored to affected tissues may be needed. In humans, a variety of antibiotic regimens are used depending on the stage of the disease but they typically include amoxicillin, doxycycline, or ceftriaxone.
Prevention is often the best course of action and this includes tick avoidance and routine scanning for and quick removal of attached ticks; repellents; sprays; dips; and impregnated collars. Routine vaccination is available for animals and can keep antibody levels high - vaccines with recombinant OspA can prevent infection of the host; while lysate vaccines with OspA and OspC can offer protection to the host after infection.
So, to recap: Lyme disease in humans and domestic animals is caused by the bacteria Borrelia burgdorferi and is transmitted by Ixodes ticks in endemic areas, especially North America. If present, signs and symptoms are caused by the inflammatory response, which can lead to tissue damage and dysfunction. A clinical diagnosis guides treatment with antibiotics or other therapies tailored to tissue dysfunction.
Lyme Disease (https://www.youtube.com/watch?v=rOQvpcpxbCs) by Osmosis (https://open.osmosis.org/) is licensed under CC-BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/).