Digital Photographs of Life Stages of Amblyomma americanum, the Lone Star Tick, a Vector of Human and Veterinary Rickettsial Diseases
Images of Amblyomma americanum, an Ixodid tick vector of importance in transmitting several life-threatening rickettsial diseases, especially in the midwest and southeastern United States, are presented. Four digital images taken with either 65-mm or 180-mm lenses show stages in the life of the tick and the process of laying eggs. Illnesses are briefly discussed, as well as factors important in the increase in numbers of these emerging infectious diseases.
FIG. 1. Comparative sizes of life stages (l to r): engorged female with a few eggs already laid, approximately 15-mm long; adult female showing "star" marking on scutum (shield covering anterior portion of female and entire dorsum of male), body and mouth parts about 3-mm long; adult male lacking spot; nymph stage. A. americanumticks can be identified by the long capitulum (mouth parts) and by the white spot on the scutum of the female.
FIG. 2. Engorged female just beginning to lay eggs. Notice how the "star" now appears to be near the anterior end of the tick, as the abdomen has swollen with blood and development of eggs.
FIG. 3. Engorged female in the process of depositing her eggs, which are emerging at the anterior end of the body, just posterior to the capitulum. Several thousand eggs can be laid by one female.
FIG. 4. Egg mass from one female. The eggs will hatch into larvae in about 35 to 45 days. The six-legged larvae need a blood meal to transform into eight-legged nymphs, as do the nymphs in order to transform into adults. The adults also need a blood meal, frequently from a third host, in order to lay eggs.
All images were photographed using a Canon 20D digital camera and lenses. Figures 1 and 2 were taken with a 65-mm lens; figures 3 and 4 with a 180-mm lens. Engorged ticks were removed from cattle in Pulaski county, Missouri. They were incubated at room temperature in individual petri dishes lined with filter paper moistened with distilled water until egg laying. Other adult and nymph stages were collected by sweeping vegetation in rural areas of Missouri with a white muslin cloth attached to a dowel stick.
Tick-borne diseases are attracting growing attention from both public health officials and the research community because a tick may inject specific disease-causing bacteria into the temporary host as it gets a blood meal. Blood meals are required at each stage of its life (larvae, nymph, and adult). The Lone Star tick, Amblyomma americanum, is a vector for several life-threatening diseases, including Rocky Mountain spotted fever, caused by the bacterium Rickettsia rickettsii,and various forms of ehrlichiosis, including two human forms in the U.S. caused by Ehrlichia chaffeensis which infects monocytes and E. ewingiiwhich infects granulocytes. Flu-like symptoms, headache, nausea, fever, and muscle and joint pain, may be experienced within 1 to 2 weeks of a tick bite. A rash is often present in ehrlichiosis and is usually seen in Rocky Mountain spotted fever. Up to 4% of Rocky Mountain spotted fever patients and 10% of untreated patients with E. chaffeensis may die. Other diseases carried by A. americanum include tularemia (rabbit fever) caused by Francisella tularensis and southern-tick-associated rash illness and Q fever caused by Coxiella burnettii, which may infect cattle, pets, and humans.
The gram-negative, obligate intracellular bacteria are maintained in the environment in small rodents such as white-footed mice (Peromyscus leucopus) and the burgeoning population of white-tailed deer (Odocoileus virgineanus). Due to the decrease in numbers of carnivores that formerly kept the deer population under control, reforestation of land, movement of city-dwellers to a more rural lifestyle, and lack of controlled burning, ticks are an increasing threat to domestic and wild animals, as well as to humans. Research is currently focused on variations in the bacteria carried by ticks, areas of infectivity, and methods of detection, with PCR providing the quickest and most reliable method of identification. Early diagnosis is an important factor in lessening morbidity and mortality.
I wish to thank former William Woods University student Amanda Candee, now an Association of Public Health Laboratories and Centers for Disease Control Fellow at the Centers for Disease Control and Prevention in Atlanta, Georgia, for providing the engorged ticks.