Tick-Borne Diseases : An Overview

Introduction

A number of infectious diseases are transmitted by ticks. As the incidence of tick-borne illnesses increases and the geographic areas in which they are found expand, it becomes increasingly important that health professionals be able to distinguish the diverse, and often overlapping, clinical presentations of these diseases. This fact sheet describes the major tick-associated diseases seen in the United States.

Tick-borne illnesses are caused by infection with a variety of pathogens, including rickettsia and other types of bacteria, viruses, and protozoa. Because ticks can harbor more than one disease-causing agent, patients can be infected with more than one pathogen at the same time, compounding the difficulty in diagnosis and treatment.

In general, specific laboratory tests are not available to rapidly diagnose tick-borne diseases. Due to their seriousness, antibiotic treatment is often justified based on clinical presentation alone. A description of the major diseases transmitted by ticks follows.

Major Tick-Borne Diseases in the United States

Disease

Organism

Vector

Region

Lyme Disease Borrelia burgdorferi Ixodes scapularis
(= I. dammini)
I. pacificus
Northeast, Midwest,
Mid-Atlantic
West Coast
Rocky Mountain Spotted Fever Rickettsia rickettsii Dermacentor variabilis
D. andersoni
East, South
West
Babesiosis Babesia microti
B. equi
I. scapularis
I. pacificus
Northeast
West Coast
Ehrlichiosis Ehrlichia chafeensis
E. equi
D. variabilis Amblyomma
americanum
I. scapularis
South-Atlantic
South-Central
Relapsing Fever Borrelia species Ornithodoros species West
Colorado Tick Fever Coltivirus D. andersoni West
Tularemia Francisella tularensis A. americanum

D. andersoni
D. variabilis

Southeast
South-Central
West
Widespread
Tick Paralysis Toxin D. andersoni
D. variabilis
West
East

Lyme Disease

Etiology: Borrelia burgdorferi

Lyme disease has become the most common tick-borne disease in the U.S. More than 14,000 cases were reported to the Centers for Disease Control and Prevention (CDC) in 1994, primarily in the Northeast and Mid-Atlantic coastal states, and the north-central U.S. The Lyme disease bacterium is transmitted primarily by the tiny deer tick, after it has been attached to the host for more than 24 hours.

Signs and Symptoms:
More than half of the cases develop erythema migrans (EM) rash at the site of the tick bite within 7 to 10 days. The rash expands, with an area of central clearing. Accompanying constitutional symptoms may include low-grade fever, headache, myalgia, arthralgia, and regional adenopathy. The rash and early symptoms resolve within 3 to 4 weeks. Disseminated disease may produce a recurrence of original symptoms and multiple secondary rashes.

Musculoskeletal symptoms may include recurring bouts of asymmetric arthritis usually involving large joints, especially the knee. The course of arthritis may be prolonged, but usually resolves in 3 to 4 years with or without treatment.

Early neurologic involvement includes cranial neuritis (Bell's palsy), meningitis, and encephalitis. Chronic neurologic Lyme includes subacute encephalopathy, axonal polyneuropathy, and less frequently, leukoencephalopathy. Subacute encephalopathy is characterized by cognitive deficits, mood and sleep disturbances. These symptoms may persist for more than 10 years.

Cardiac symptoms such as an atrioventricular block, which sometimes requires a pacemaker, occur in a small minority of patients.

Diagnostic Tests:
Within 6 weeks, antibodies are usually detectable with an ELISA test. Because this test is prone to false positives, CDC recommends confirmation by a Western immunoblot. Nearly all patients with late Lyme disease are seropositive. Although not always positive, the most specific test is a culture of the organism from the EM lesion.

Treatment:
Early Lyme disease usually responds to several antibiotics, including oral doxycycline and amoxicillin, often prescribed for 2 to 3 weeks. When ehrlichiosis cannot be ruled out, treat with doxycycline, which is effective against both diseases. Persistent symptoms may require a second course of treatment. Neurologic involvement warrants 3 to 4 weeks of IV ceftriaxone or penicillin G, as may arthritis and other symptoms of late-stage disease. Chronic Lyme disease may not respond to antibiotic treatment. Prophylactic treatment after a tick bite is not routinely recommended.

Rocky Mountain Spotted Fever

Etiology: Rickettsia rickettsiae

Most infections are acquired in the southeast and west south-central regions of the country. A tick is most likely to transmit the rickettsial agent after it has been attached to its human host for at least 6-10 hours. More than 400 cases of Rocky Mountain Spotted Fever (RMSF) were reported in 1994.

Signs and Symptoms:
First symptoms appear 5-10 days after tick bite, including fever more than 102F, headache, and rash. Macular rash, present in 90 percent of patients, begins on the wrists and ankles and spreads inward to trunk, face, palms and soles. Skin lesions often become papular, petechial, or purpuric. Other symptoms that may develop include abdominal pain, diarrhea, conjunctivitis, confusion, meningismus, respiratory failure, renal dysfunction, and myocarditis. Common abnormalities, not specific for RMSF, include depressed leukocyte count, thrombocytopenia, elevated liver enzymes, and hyponatremia. In fulminant cases, widespread vasculitis causes intravascular coagulation and death.

Diagnostic Test:
Diagnosis must be made prior to laboratory confirmation, based on clinical presentation and patient history. Serologic tests available to confirm a diagnosis of RMSF include IHA, IFA, and latex agglutination. Antibodies appear within 7-10 days of onset of illness and decline within 2 months. The length of time required and the potential hazard to laboratory personnel make culturing the organism impractical for clinical management.

Treatment:
Prompt treatment has reduced mortality from 25 percent to 5 percent. Tetracyclines are recommended in adults with suspected RMSF. Chloramphenicol is the preferred treatment in children under 8 years of age. Intravenous chloramphenicol is recommended when central nervous system symptoms are present. Treatment should continue for 5-7 days or at least 48 hours after resolution of fever. Untreated patients can require weeks to months to recover. Survivors develop permanent immunity against future attacks.

Babesiosis

Etiology: Babesia microti in the Northeast
Babesia equi in West Coast states

This malaria-like illness is caused by a protozoan parasite that invades erythrocytes. While babesiosis most often occurs after a tick bite, the disease also has been transmitted through blood transfusion. The same ticks responsible for Lyme disease, I. scapularis and I. pacificus, are believed to transmit babesiosis. More than 450 cases have been reported since babesiosis first appeared in 1968, most occurring in the Northeast during the summer months. Because many infected persons have no symptoms, the incidence of babesiosis is unknown. Most illness occurs in persons of advanced age or who have underlying medical conditions (e.g., splenectomy). Since babesiosis is known to weaken the immune system, co-infection with Lyme disease can result in severe and prolonged illness. In spite of its ability to cause serious, and even life-threatening disease, babesiosis is only occasionally fatal.

Signs and Symptoms:
One to 3 weeks after tick bite, malaise, loss of appetite, fatigue, and dark urine are common. Initial symptoms are followed several days later by fever, myalgia, headache, and drenching sweats. Illness can range from a mild self-limited infection to severe hemolytic anemia, renal failure, and severe hypertension. Decreased levels of blood platelets may necessitate exchange blood transfusions.

Diagnostic Tests:
Laboratory tests may identify liver and blood abnormalities (anemia, decreased hemoglobin, hemoglobinuria). Most patients have thrombocytopenia. Wright's or Giemsa-stained blood smears may reveal babesia organisms. An IFA test confirms diagnosis, with titers for most acutely ill patients exceeding 1:1024. Elevated titers can persist for months after acute infection.

Treatment:
Most patients have a mild illness and recover without specific treatment. A 7-day regimen of oral quinine plus oral or intravenous clindamycin is recommended. Blood transfusions are given in severe cases. Fatigue and low-grade fever may persist for weeks or months after treatment.

Ehrlichiosis

Etiology: Ehrlichia chafeensis
Ehrlichia equi

The latest tick-borne disease to emerge in the U.S., ehrlichiosis was first described in 1987. Much less common than either Lyme disease or Rocky Mountain spotted fever, an average of 50 cases of ehrlichiosis are reported each year. Because of the often mild and self-limited symptoms, this rickettsial disease is likely to be underreported. Ehrlichiosis has been reported in many areas, but most cases have occurred in the Southeast and south-central U.S. Ehrlichia bacteria parasitize white blood cells, causing illness that may range from mild to severe and even fatal. Most patients, however, recover completely without treatment.

Signs and Symptoms:
Ehrlichiosis resembles Rocky Mountain spotted fever and cannot be reliably distinguished from it on the basis of its clinical presentation. A rash develops in only 20 percent of patients with ehrlichiosis and, unlike the Rocky Mountain spotted fever rash, is rarely seen on palms and soles. Most frequent symptoms are headache and high fever, which usually develop within 7-11 days after the tick bite. Other common symptoms are chills, nausea, vomiting, anorexia, and muscle ache. Patients may also develop cough, diarrhea, and swollen lymph glands. The symptoms last 1-2 weeks, and recovery generally occurs without long-lasting problems. However, complications have occurred in some people, including blood and kidney abnormalities, respiratory failure, and meningitis. A particularly severe form of disease, human granulocytic ehrlichiosis (HGE), can cause overwhelming infection, toxic shock, and death. Prompt recognition and proper treatment of ehrlichiosis is critical to avoid serious complications.

Diagnostic Tests:
Diagnosis is made on a clinical basis. Certain laboratory tests are most likely to detect abnormalities in the first 5 to 7 days of infection. These abnormalities are not specific for ehrlichiosis and may include leukopenia, thrombocytopenia, and elevated liver enzymes. Mulberry-like bodies called morulae can sometimes be seen in the cytoplasm of white blood cells by light or electron microscopy. Serologic tests, useful only to confirm past infection, include IFA and ELISA. Patients seroconvert in 4 weeks. A four-fold rise in antibody between the acute and convalescent stage verifies diagnosis.

Treatment:
Unlike Lyme disease, ehrlichiosis does not respond to amoxicillin, and misdiagnosis and incorrect treatment have led to overwhelming infection and fatalities. Prompt treatment with tetracycline or doxycycline for 10-14 days is effective. Like other rickettsial diseases, chloramphenicol can be used when tetracyclines are contraindicated. Convalescence can be prolonged.

Tick-Borne Relapsing Fever

Etiology: Borrelia hermsii in mountainous regions
Borrelia turicatae in semi-arid plains

An uncommon disease, relapsing fever is the only tick-borne illness transmitted by the soft tick, Ornithodoros, found mainly in remote mountainous settings. The true incidence of relapsing fever is unknown due to a lack of surveillance and frequent misdiagnosis. Epidemics have been traced to vacation cabins infested with rodents. The number of cases peaks in the summer months. Ornithodoros ticks feed at night for only 5-20 minutes and their bites usually go unnoticed.

Signs and Symptoms:
A 2-3 inch itchy black scab may develop at the site of the tick bite. Incubation is usually 7 days, at which time the patient abruptly develops high fever, chills, headache, tachycardia, muscle and joint pain, and abdominal pain. Neurologic involvement occurs in only 5-10 percent of cases. Rash is seen in less than half of cases, as fever wanes. Untreated, the fever breaks within 6 days and the patient experiences drenching sweats. Fever returns 8 days later, and subsequent episodes are milder. On average, a patient experiences 3-5 recurrences of the illness. Infants and the elderly may become severely ill and require hospitalization, but deaths are rare.

Diagnostic Tests:
Common abnormalities include leukocytosis, increased erythrocyte sedimentation rate, and thrombocytopenia. A rapid and specific diagnosis is made by observing the bacteria in a sample of blood under the microscope. Organisms are most likely to be seen in blood drawn during a febrile episode. Serologic tests are not standardized and cross-react with B. burgdorferi (the agent that causes Lyme disease). Western immunoblot confirms the diagnosis.

Treatment:
Either tetracycline or erythromycin for 5-10 days is effective. Rare treatment failures have occurred using doxycycline. Severely ill patients may require intravenous antibiotic treatment and hospitalization. A Jarisch-Herxheimer reaction occurs in one-third of patients treated with antibiotics. Severity of this reaction can be reduced by giving meptazinol or anti-pyretic or anti-inflammatory drugs.

Colorado Tick Fever

Etiology: Coltivirus

Colorado Tick Fever (CTF), also known as "mountain fever," is a viral illness transmitted by the Rocky Mountain wood tick, D. andersoni. CTF has also been transmitted by blood transfusions. Between 200-300 cases are reported annually in the U.S., but the actual incidence of the disease is likely to be much higher. Most cases occur in mountainous regions of the western states. Once infected, a person develops long-lasting immunity. A rare disease caused by another tick-borne virus, Powassan encephalitis, causes similar symptoms but more often leaves survivors with neurologic impairments.

Signs and Symptoms:
Three to 6 days after infection, symptoms begin abruptly with fever, chills, severe headache, photophobia, abdominal pain, nausea, and muscle aches. Symptoms last for up to 7-10 days and may recur several days later. The second bout of illness typically lasts only 2-4 days. While the convalescent period can be prolonged, especially in adults over age 30, prognosis is generally excellent. Rare complications include meningitis, encephalitis, and fatal hemorrhage. A transient macular or petechial rash is seen in only a small percentage of patients, but may appear anywhere on the body during the course of illness.

Diagnostic Tests:
Serologic tests, including complement fixation, neutralizing antibody determination, and IFA are available to confirm diagnosis. Less commonly, the virus itself may be cultured from blood. Because it may take several weeks to develop antibodies, blood samples should be taken during both the acute illness and post-convalescent periods.

Treatment:
There is no specific treatment. Therapy is limited to supportive care. Recovered patients should not donate blood for at least 6 months.

Tularemia

Etiology: Francisella tularensis

Tularemia, also called "Rabbit Fever" or "Deer-Fly Fever," is an infection that causes two forms of disease, a mild illness and a more severe one. While tularemia can be contracted by handling tissues of infected animals, especially by hunters who skin wild rabbits, more than half of all cases result from tick bites. Approximately 150-300 cases are reported each year, mostly from Arkansas, Missouri, and Oklahoma. Recovered patients usually develop long-lasting immunity, however cases of re-infection have been reported.

Signs and Symptoms:
A sore or ulcer develops within 24-48 hours at the site of tick bite, usually on lower extremities or trunk. Lymph glands become swollen and painful. The disease may be mild and self-limited, with fever abating within 4 weeks. A more fulminant disease leads to fever with chills, headache, abdominal pain, and severe prostration.

Diagnostic Tests:
There is no rapid diagnostic test for tularemia. A serum agglutination test can detect antibodies 10-14 days after onset of illness. Titer peaks in 4-6 weeks. Pre- and post-convalescent blood samples showing a four-fold rise in antibody confirms infection.

Treatment:
Streptomycin or gentamycin are effective. Tetracycline or chloramphenicol are alternatives, but relapses can occur if taken for less than 14 days. Considerable improvement is seen within 48 hours after treatment.

Tick Paralysis

Etiology: Neurotoxin

This rare disease is the only tick-borne illness that is not caused by an infectious organism. The illness is caused by a neurotoxin produced in the tick's salivary gland. After prolonged attachment, the engorged tick transmits the toxin to its human host. The incidence of tick paralysis is unknown.

Signs and Symptoms:
The toxin causes symptoms within 2-7 days, beginning with weakness in both legs that progresses to paralysis. The paralysis ascends upward to trunk, arms, and head within hours and may lead to respiratory failure and death. The disease can present as acute ataxia without muscle weakness.

Diagnostic Tests:
Diagnosis is based on symptoms and upon finding an embedded tick, usually on the scalp.

Treatment:
Removal of the embedded tick usually results in resolution of symptoms within several hours to days. If the tick is not removed, the toxin can be fatal, with reported mortality rates of 10-12 percent.

Prevention

No vaccine is currently available for any tick-borne disease. Individuals should therefore take precautions when entering tick-infested areas, particularly in the spring and summer months. Preventive measures include avoiding trails that are overgrown with bushy vegetation, wearing light-colored clothes that allow one to see the ticks more easily, and wearing long pants and closed-toe shoes. Tick repellents containing DEET (N,N, diethyl-m-toluamide) are effective and can be applied to skin or clothing. Although highly effective, severe reactions have occurred in some people who use DEET-containing products. Young children may be especially vulnerable to these adverse effects. Permethrin, which can only be applied to clothing, kills ticks on contact.

If an embedded tick is found, it should be removed promptly with tweezers, grasping the tick close to the skin and pulling with steady pressure upward, in a direction perpendicular to the skin. Engorged ticks should be handled only with gloves or other barrier and saved for identification by the physician.


Source: National Institute of Allergy and Infectious Diseases, National Institutes of Health, June 1996



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