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.

Localized Rash (With or Without Fever)

A localized rash from TBD occurs in early Lyme disease and ulceroglandular tularemia. Tularemia has multiple presentations, but the ulceroglandular form, which accounts for 80% of cases, usually starts with a papule that evolves into an ulcer on an extremity at the site of a tick bite (or animal exposure). The lesion evolves into a necrotic eschar and is often associated with regional lymphadenopathy, fever, and other systemic signs.

Mediterranean spotted fever and some of the African spotted fevers also manifest with a local eschar at the site of a tick bite, sometimes called a “tache noire.”

Erythema migrans (EM), the rash of early localized Lyme disease, is an important condition that emergency physicians should be familiar with because antibiotic treatment at this stage almost always leads to excellent outcomes. Lyme disease is by far the most common vector-borne illness in North America. EM develops at the site of the tick bite roughly 7 to 10 days later (range, 3 to 33 days), usually as flat erythema that is neither pruritic nor painful, although it can be either. The classic description of a target or bull’s-eye lesion with central clearing is not the most common. Most EM rashes are uniformly red. EM can be darker in the center, vesicular, or necrotic. Know the spectrum of morphology of EM to avoid misdiagnosis of this infection (Figs. 180.4 to 180.6).

Fig. 180.4 Classic erythema migrans.

Note the location (torso) and the bull’s-eye appearance in this patient with fever and localized rash, which was flat and neither painful nor pruritic. A suggestion of a central punctum, the location of the tick bite, is evident.

Fig. 180.5 Erythema migrans with a raised, vesicular center.

This patient presented in early July after having pulled a tick off himself 7 days earlier. Approximately 18 hours after starting doxycycline, the patient had a Jarisch-Herxheimer reaction (abrupt onset of chills, tachycardia, intensification of rash), which occurs with treatment of spirochetal diseases.

Fig. 180.6 Erythema migrans with a homogeneous color.

This young man had a rash that had been ascribed to a spider bite but was growing larger. The rash was painless, thus making a spider bite much less likely. The patient also had a tender, palpable lymph node in the right axilla.

The location of EM tends to be at the sites where ticks feed or experience an impediment to further movement (groin, popliteal fossa, axilla, an elastic underwear strap, or the hairline in children). Although the rash can be anywhere, it tends not to occur acrally, and the torso is another common location. Size is another important feature; EM becomes large, 16 cm on average. Cellulitis, a common diagnostic competitor, rarely attains this size without fever, significant tenderness, and other systemic findings.

Some patients with EM have fever, headache, myalgias, neck pain or stiffness, and other systemic symptoms. The fever is usually low grade, and high fevers suggest coinfecting babesiosis or anaplasmosis. Roughly 10% to 20% of patients with early Lyme disease have multiple cutaneous lesions (see later).

Although EM has always been thought to be pathognomonic of Lyme disease, EM-like lesions have been reported in the southeastern and central states; this has been called Master disease or southern tick-associated rash illness (STARI).¹ Although these patients’ rashes have some differences from classic EM in patients from Lyme-endemic states, significant overlap can occur in individual patients. Other novel Borrelia species may cause this lesion, and these patients should also be treated with antibiotics, similar to patients with Lyme disease.

Differential Diagnosis and Medical Decision Making

The differential diagnosis in these patients depends on the specific presenting syndrome and is largely covered in the previous section. A complete differential diagnosis of EM includes various other acute rashes.¹ Physicians should diagnose early localized Lyme disease and tick paralysis by history and physical examination. Fifty percent of patients with EM have a negative Lyme serologic test result; therefore, no testing is necessary because a negative result should not dissuade the physician from diagnosing EM and treating it. Serologic testing is not indicated in most patients with EM.

The differential diagnosis of undifferentiated fever is enormous and includes just about every cause of fever. In patients with fever and generalized rash, one must consider bacteremia and viremia, especially meningococcemia and streptococcal and staphylococcal sepsis. Physicians should also treat suspected RMSF empirically without waiting for diagnostic test confirmation. Table 180.6 gives clues to the diagnosis of TBDs.

Table 180.6 Clinical Clues (History, Physical Examination, or Laboratory) Suggesting the Diagnosis of a Tick-Borne Illness Manifesting as a Nonspecific Febrile Illness*

^* Apart from an epidemiologic context suggesting a tick-borne disease.

Physicians can often confidently diagnose babesiosis (Fig. 180.7), anaplasmosis (Fig. 180.8), and relapsing fever (Fig. 180.9) on the basis of a blood smear. Although numerous antigen and antibody tests are available for all these diseases, the results of these tests are hardly ever available in real time.

Fig. 180.7 Blood smear showing babesiosis.

This figure shows intraerythrocytic ring forms in multiple red blood cells from a patient with a high parasite count.

Fig. 180.8 Blood smear showing a morula in a patient with anaplasmosis.

This blood smear shows two morulae (intracellular clumps of anaplasma organisms in the white blood cell to the right) that can be diagnostic in a patient with anaplasmosis.

(Courtesy of Dr. J. S. Dumler.)

Fig. 180.9 Blood smear showing relapsing fever Borrelia.

This blood smear shows a spirochete in the blood from a patient with relapsing fever.

For later manifestation of Lyme disease, the Centers for Disease Control and Prevention currently recommend a two-step testing procedure (screening enzyme-linked immunosorbent assay followed by a confirmatory Western blot). Newer tests, especially the C-6 peptide, hold promise for a single-tier test. In these patients, the physician must carefully interpret the serologic results in the context of the patient’s symptoms, clinical course, and epidemiologic features.

In patients with acute weakness, tick paralysis can be confused with Guillain-Barré syndrome, myasthenia gravis, botulism, hypokalemic paralysis, organophosphate poisoning, transverse myelitis, and spinal cord compression.

Treatment

Tick removal is best accomplished by using fine forceps applied close to the skin and gradually pulling the tick upward and outward. Because ticks use a cement-like substance to embed, this removal procedure may take steady gentle pressure over a minute or two. Try to remove the entire tick. Retained mouth parts may cause a foreign body reaction or a staphylococcal or streptococcal skin infection, but they have no implications in terms of TBD transmission.

No specific antimicrobial therapy exists for Colorado tick fever or tick-borne encephalitis. The definitive treatment for tick paralysis is tick removal; improvement often begins within hours.

Table 180.5 shows the treatments for specific manifestations of Lyme disease. Treat patients with the remaining bacterial TBDs with specific antimicrobial therapy (Table 180.7). Data suggest a trend toward shorter courses of antibiotics in patients with early Lyme disease.^5,6

Table 180.7 Treatment of the Tick-Borne Diseases*

ICU, Intensive care unit; IV, intravenous.

^* The choices of antibiotics, route, dose, and duration should be made on a case-by-case basis, depending on the severity of illness, allergies, age of patient, pregnancy, and other individual factors.

The threshold to treat should be low, and frequently the decision to treat is based purely on presentation and epidemiologic context. Although tetracyclines are generally contraindicated in children less than 9 years of age, short courses of intravenous doxycycline are indicated in children with life-threatening manifestations of RMSF or anaplasmosis.

Follow-Up, Next Steps, and Patient Education

Most patients with TBDs can be treated as outpatients with primary care follow-up. Patients with syndromes requiring intravenous antibiotics, cardiac pacing, or intensive care for CNS, respiratory, renal, or circulatory failure should be admitted to an appropriate inpatient setting. Consultation with specialists is on a case-by-case basis.