Epidemiology

Untreated, tick-borne diseases (TBDs) can be associated with significant morbidity or mortality. Definitive treatments exist for the more serious diseases. TBDs usually can be confidently diagnosed clinically, based on pathognomonic or suggestive physical or laboratory findings that are available in the emergency department. Confirmatory laboratory testing is rarely available in real time and is often unnecessary for initial medical decision making. Therefore, physicians must understand both classic and atypical presentations and must be prepared to treat these illnesses based on clinical suspicion.

Because these illnesses frequently occur in the absence of a known tick bite, physicians must consider TBDs when patients present in the correct epidemiologic context and with a recognizable syndrome. This chapter considers these diseases in syndromic groups (patterns of presentation), rather than individually. Individual TBDs are listed in Table 180.1 (North America) and Table 180.2 (worldwide). Travelers may import the TBDs from one location to another.

Table 180.1 Tick-Borne Diseases of North America

Table 180.2 Other Important Tick-Borne Diseases Worldwide

Epidemiologic context is a key principle in diagnosing TBDs. As in a criminal investigation, the practitioner must assess the tick’s means, motive, and opportunity to transmit a TBD. The means has to do with tick anatomy—the ability to bite a mammal, usually without the mammal’s knowledge. This factor is crucial because most TBDs require many hours of tick attachment to transmit the infectious agent. The motive has to do with tick physiology; ticks require a blood meal from a host to live and transform to their next life stage. Opportunity relates to patient and geographic factors, that is, epidemiologic context. Taking a history that focuses on this epidemiologic context is essential to the diagnosis of all TBDs. Ask whether the patient has been bitten by a tick, but remember that most patients with TBDs do not recall a tick bite. For example, only approximately 25% of patients with Lyme disease (smaller ticks) and only two thirds of patients with Rocky Mountain spotted fever (RMSF) (larger ticks) recall the bite. Questions directed at whether a tick could have bitten the patient should be asked:

As for the season, most TBDs are acquired from April through September, but “tick season” depends on the stage of a disease, geography, and local weather patterns for that geographic area. For example, Lyme arthritis could manifest in January from a bite in July, and a warm spell in late fall in North Carolina could result in a case of RMSF that develops in November.

Pathophysiology and Tick Anatomy

Two families of ticks are important in human TBDs. Hard (Ixodidae) ticks tend to attach and feed for days, whereas soft ticks (Argasidae) feed in minutes to hours. Table 180.3 lists some characteristics of these different types of ticks. Because the hard ticks, which transmit most of the human TBDs, feed for so long, tick removal during the first 24 hours of attachment provides one strategy for disease prevention, a concept best studied in the context of Lyme disease. Figures 180.1 to 180.3 show various stages of Ixodes scapularis, Dermacentor variabilis, and Amblyomma americanum ticks.

Fig. 180.1 Ixodes scapularis ticks, nymphal stage.

Two nymphal I. scapularis ticks are shown next to a common household match. The tick on the left has been feeding on a mouse for 48 hours and is larger than the unfed tick on the right. Seen in three dimensions, the fed tick is more spherical, whereas the unfed tick is flatter.

(Photograph by Darlyne Murawski.)

Fig. 180.2 A Dermacentor variabilis tick next to an Ixodes scapularis tick.

This photograph shows an adult D. variabilis (left) tick next to an adult I. scapularis (right) tick, both next to a common match for scale. In cases of Rocky Mountain spotted fever, even with this larger tick vector, 30% to 40% of patients are not aware of a tick bite.

(Photograph by Darlyne Murawski.)

Fig. 180.3 Amblyomma americanum tick.

An adult female Lone Star tick next to a nymph of the same species. This tick is responsible for transmission of human monocytic anaplasmosis and Master disease.

(Photograph by Darlyne Murawski.)

Because multiple TBDs are covered in this chapter, specific pathophysiology for each of the TBDs is covered in the next section.

Presenting Signs and Symptoms

TBDs most commonly manifest as one of four syndromes (Table 180.4):

Table 180.4 Tick-Borne Disease Syndromes

Other possible presentations are possible, especially with respect to atypical presentations of any of these diseases and, most notably, Lyme disease. The other signs and symptoms of Lyme disease are discussed separately.

Before discussing the individual syndromes and diseases, the physician should know that as often as 20% of the time, a single tick bite results in multiple infections. Therefore, a patient may develop the rash of Lyme disease along with babesiosis or anaplasmosis. This possibility has implications with respect to the manifestations of the diseases and also the choice of antimicrobials.