D Adams, B Anderson, C Ammirati, K Helm
D Adams, B Anderson, C Ammirati, K Helm. Identification And Diseases Of Common U.S. Ticks. The Internet Journal of Dermatology. 2002 Volume 2 Number 1.
Background: Ticks are a major vector for cutaneous and systemic disease. In the United States, eight ticks are primarily responsible for ten different diseases. Physicians are asked by patients and other practitioners to remove and identify ticks. Accurate tick identification may be important to prevent, predict and treat tick-borne illness. We present a simple method of tick identification using six criteria. Specific tick-borne diseases are listed, as are their endemic areas and hosts.
Observation: We describe six major features from which most ticks affecting humans in the United States can be identified. The features are body shape, size, mouth parts (capitulum), color, dorsal shield (scutum) and festoons.
Conclusion: With the use of information presented, ticks affecting humans in the United States can often be identified by a physician and in difficult situations, by an entomologist, allowing the prediction and diagnosis of potential tick-borne illness and provision of appropriate patient care.
In the United States, ticks are responsible for the transmission of more vector-borne diseases than any other arthropod. 1 Tick-borne diseases, tick regions and hosts are listed in Table I. Lyme disease now accounts for more than 95% of all reported vector-borne illness in the United States and has been documented in 48 states (16,273 reported cases in 1999) 2. In addition to Lyme disease, tick bites in the United States have been responsible for the transmission of rocky mountain spotted fever (579 cases in 1999), Colorado tick fever (200-400 cases annually), tularemia (150-300 cases annually), ehrlichiosis (202 cases of human granulocyte ehrlichiosis and 99 cases of human monocyte ehrlichiosis in 1999), babesiosis (100+ cases reported since 1969), relapsing fever (sporadic, number unknown), Q fever (number unknown), tick paralysis (few sporadic cases), and painful tick bite reaction (number unknown). 1,2,3,4,5,6,7,8,9,10 Several tick-borne systemic infections have prominent skin findings and it is not uncommon for the physician to be consulted. If the tick remains attached to the skin or has been recovered, accurate identification, along with knowledge of endemic vector-borne diseases, can allow the physician to counsel and treat the patient appropriately. Available sources of tick identification are often difficult to use. 8,10,11 We propose a simple, rapid method of tick identification using six criteria based on published guides and personal observation. 7,8,9,10,11,12
Ticks known to cause human disease in the United States were obtained from the Department of Entomology at Penn State University and the Institute of Arthropodology and Parasitology, Georgia Southern University to provide a frame of reference for identification. In addition, ticks removed from or brought in by our patients, were included in this evaluation. To determine the six major identifying features, the ticks were compared and published data were reviewed. 7,8,9,10,11,12
In most cases, tick identification can be made based on six major features which are shape, size, mouth parts (capitulum), color, dorsal shield (scutum), and festoons (posterior abdominal markings) 8,10,11 (Fig 1).
In adult, non-engorged ticks, the three major body shapes are teardrop, full oval and rounded. One can see in the comparison photo (Fig 2) that these subtle differences are detectable. Both
Bottom row left to right:
Size is a less specific but commonly used feature for identification. Although male ticks are often somewhat smaller than female ticks, the comparison with a sesame or poppy seed is made because it provides a practical frame of reference (Fig 3). Adult
Mouth Parts (Capitulum)
Mouth parts also known as the capitulum (Fig 1). Mouth parts of soft ticks (
Color is affected by tick type, sex and engorgement. Color patterns are listed (Table II) and seen in photos (Fig 2). Season or developmental stage does not appear to affect color although engorgement causes a uniform grey color.
Dorsal Shield (Scutum)
The scutum is the dorsal sclerotized plate or shield covering the anterior part of the body in female and entire dorsal surface in male hard ticks (Fig 1). Soft ticks (
Festoons are delicate, usually rectangular, grooves seen on the posterior edge of some hard ticks and appear as a string of pearls (Fig 1).
Our proposed method for tick identification was developed specifically to aid in office identification of recovered ticks from humans. Identification is aided with a magnifying lens or microscope on low power. The six identifying features which include shape, size, mouth parts, color, dorsal shield, and festoons in combination with knowing endemic areas should allow determination of tick types affecting humans in the United States.
Ticks, mites, spiders and scorpions are arthropods of the class Arachnida which all have four pairs of legs as adults. Ticks belong to the subclass Acari, order Parasitiformes, and suborder Ixodida. Ixodida includes two major families, Ixodidae (hard ticks) with 13 genera and approximately 645 species and Argasidae (soft ticks) with 5 genera and approximately 170 species distributed worldwide. 13
Ticks go through a four-stage life-cycle which includes eggs, larva, nymphs and the adult. All stages except the egg require a blood meal from a host for transition to the next stage of development. Engorgement after a blood meal can make tick identification difficult. Hard ticks (
Host Attachment and Questing
Non-nidicolous ticks (species like
Once attached to the host, the tick will often take several hours to find the desired location for attachment. Attachment is then initiated with the use of toothed chelicerae which perform as cutting tools to allow penetration of a barbed hypostome which acts as a barbed hypodermic needle. A cement-like substance is secreted during the first few hours of the attachment process to hold the hypostome in place until feeding is complete over several days to two weeks. The tick will then detach, fall off and use the nutrition gained from the blood meal to advance to the next stage of development, or in the adult female, lay eggs.7,8,9,10
Tick-borne Illness Transmission Time
The ultimate outcome after a tick bite can include a local reaction, possible infection and rarely tick paralysis. Infectious agents can be transmitted to the host via infected saliva only after sufficient attachment time to the host. Stages of tick development (larva vs. nymph vs. adult) affect disease transmission.
The most prevalent tick-borne disease in the United States is Lyme disease or Lyme borreliosis. It is now reported in 48 states. 2 The hard tick,
Soft ticks like
Steven B. Jacobs, Dept of Entomology, Penn State Univ, 513 ASI Bldg, University Park, PA.
James E. Keirans, Professor and Curator, U.S. National Tick Collection, Institute of Arthropodology and Parasitology, Georgia Southern University, Statesboro, Georgia.
David R. Adams, Penn State Hershey Medical Center Dermatology HU-14 Hershey, PA 17033 (717) 531-8307 Fax: (717) 531-6516 E-mail: firstname.lastname@example.org