Tick Borne Diseases

Tick borne diseases are infections transmitted by the bite of infected ticks. They comprise a diverse range of causative organisms.

• Bacteria: Lyme disease, together with tick borne relapsing fever, ehrlichiosis and anaplasmosis
• Rickettsial organisms: various spotted fever group rickettsioses
• Less commonly encountered outside specific endemic regions: tick borne viral encephalitis

The shared epidemiological principle across this group is that risk correlates directly with outdoor activity in tick habitats.

Prompt, correct tick removal substantially reduces transmission risk, given that most tick borne pathogens require a prolonged period of tick attachment before effective transmission occurs.

Pathophysiology and classification

• Ehrlichiosis (caused predominantly by Ehrlichia chaffeensis) and anaplasmosis (caused by Anaplasma phagocytophilum) infect white blood cells, monocytes and granulocytes respectively.
• They produce a febrile illness with characteristic laboratory abnormalities including leucopenia, thrombocytopenia and deranged liver function tests.
• Both organisms are transmitted by Ixodes and related tick species. They share overlapping geographical distribution with Lyme disease in several regions, so coinfection is a recognised, clinically relevant consideration.
• Tick borne relapsing fever is caused by various Borrelia species distinct from those responsible for Lyme disease.
• It is transmitted by soft bodied Ornithodoros ticks, in contrast to the hard bodied Ixodes ticks responsible for Lyme disease and several other tick borne infections.
• It produces a relapsing fever pattern similar in principle to louse borne relapsing fever, though generally with a shorter interval between febrile episodes.
• Rocky Mountain spotted fever and related spotted fever group rickettsioses are caused by Rickettsia rickettsii and related species. They produce a systemic vasculitis.
• The characteristic rash, in contrast to typhus group rickettsioses, classically begins peripherally (wrists and ankles) before spreading centripetally toward the trunk. This is an important distinguishing clinical feature.

• Ehrlichiosis and anaplasmosis both present with fever, headache, myalgia and malaise, typically without a prominent rash (in contrast to several other tick borne infections). They develop one to two weeks after tick exposure.
• Characteristic laboratory findings of leucopenia, thrombocytopenia and elevated transaminases support clinical suspicion.
• Severe disease can produce significant multi organ involvement, particularly in older or immunocompromised patients.
• Tick borne relapsing fever presents with recurrent febrile episodes, similar to the louse borne form though generally with a shorter cycle length between episodes. Headache and myalgia accompany it and, in some cases, a rash.
• A history of exposure in a relevant endemic area is an important diagnostic clue. It is often associated with rodent infested structures such as cabins, where the soft tick vector resides.
• Rocky Mountain spotted fever presents with fever, headache and myalgia. Typically within a few days, the characteristic rash follows, beginning on the wrists and ankles and spreading centripetally. It notably includes the palms and soles, an important and relatively distinctive feature.
• Severe Rocky Mountain spotted fever progresses to significant vasculitic multi organ involvement, including renal failure, encephalopathy and, in severe cases, peripheral gangrene. It carries substantial mortality if treatment is delayed.
• Actively seek a specific history of tick exposure in any patient with a compatible febrile illness during the relevant transmission season in an endemic area. Ask about geographical location, outdoor activity, and, where recalled, the duration of tick attachment before removal.
• A substantial proportion of patients do not recall a specific tick bite despite genuine exposure.

• Diagnosis across this group frequently relies on a compatible clinical presentation in the appropriate epidemiological and seasonal context, with characteristic laboratory abnormalities. These are leucopenia, thrombocytopenia and deranged liver function for ehrlichiosis and anaplasmosis.
• Specific confirmatory testing supports the clinical diagnosis. Options are polymerase chain reaction and serology.
• For ehrlichiosis and anaplasmosis, examine a peripheral blood film for characteristic intracellular inclusions, termed morulae, within the relevant white cell lineage.
• Serological testing for the various rickettsial and Borrelia infections in this group frequently requires paired acute and convalescent samples for reliable confirmation, given a delayed antibody response.
• A single early negative result therefore does not exclude the diagnosis. It should not delay empirical treatment in a clinically compatible, potentially severe presentation.
• Vector distribution overlaps substantially between Lyme disease and several other tick borne infections, particularly ehrlichiosis and anaplasmosis.
• Consider coinfection in patients with an atypical or more severe than expected presentation of confirmed Lyme disease, or vice versa. Broader testing is warranted where the clinical picture does not fully align with a single diagnosis.

• Doxycycline 100 mg orally or intravenously twice daily is first line, empirical treatment across the great majority of bacterial and rickettsial tick borne infections in this group. These include ehrlichiosis, anaplasmosis, and Rocky Mountain spotted fever. It is typically continued for seven to fourteen days depending on the infection and clinical response.
• Start treatment promptly on clinical suspicion rather than delaying for confirmatory testing. Several conditions in this group, particularly Rocky Mountain spotted fever, are potentially severe if treatment is delayed.
• Doxycycline is generally recommended as first line treatment for suspected Rocky Mountain spotted fever and related severe rickettsioses regardless of patient age, including in young children. The risk of severe or fatal disease if treatment is delayed is substantial.
• This is an important and increasingly recognised departure from historical, more cautious approaches to tetracycline class antibiotic use in this age group, given the disproportionate risk benefit balance in this clinical context.
• Tick borne relapsing fever is treated with doxycycline or tetracycline. Anticipate and manage a Jarisch Herxheimer reaction after treatment starts, similar to the approach detailed under louse borne relapsing fever.
• Where coinfection with Lyme disease is confirmed or strongly suspected alongside another tick borne pathogen from this group, treatment should adequately cover both infections. In many cases doxycycline offers useful, broad activity across several of these organisms simultaneously.
• Prompt, correct tick removal at the earliest opportunity after any identified tick attachment is the single most effective preventive measure across this group. Transmission risk for the great majority of these pathogens increases substantially with duration of attachment.
• Remove the tick with fine tipped tweezers applied close to the skin with steady upward traction, avoiding crushing or twisting the tick body.
• Add protective clothing, tick repellent, and regular tick checks after outdoor activity in known endemic habitats.