(21 July 1994)
In response to questions resulting from the recent news reports of 'new tick-borne illness causing deaths in the Midwest,' the following information is provided:
Ehrlichioses are diseases caused by rickettsia-like organisms, which are extremely small, intracellular bacteria belonging to the family Rickettsiaceae, genus Ehrlichia (Tansill 1984). Ehrlichiosis was first described in Algerian dogs in 1935 (Donatien et al. 1935, Bakken 1994), and in the 1960s, a number of military guard dogs stationed in Vietnam died from complications of a hemorrhagic illness caused by Ehrlichia canis (Keefe et al. 1982).
Human ehrlichiosis is a more recently recognized disease. The first diagnosed case occurred in 1986 in a 51-yr-old man from Detroit who had been exposed to ticks in a rural area of Arkansas (Maeda et al. 1987). In 1990, the agent of human ehrlichiosis was isolated from the blood of a U.S. Army reservist at Fort Chaffee, AR (Dawson et al. 1991). The newly recognized organism was named E. chaffeensis (Anderson et al. 1991).
The genus Ehrlichia contains an emerging number of species. Prior to the discovery of E. chaffeensis, E. sennetsu was the only species known to infect humans. E. sennetsu causes Sennetsu fever, a mononucleosis-type illness first described in 1954, and occurring primarily in Japan (Misao et al. 1954, Bakken 1994). Sennetsu fever is very rare, and is usually benign, with no fatalities having ever been reported (Benenson 1990). The other species of Ehrlichia cause veterinary disease and include E. canis (canine ehrlichiosis), E. ewingii (canine granulocytic ehrlichiosis), E. risticii (Potomac Horse Fever), E. equi (disease in horses), E. phagocytophila (disease in sheep and cattle), as well as a small number of others (Rikihisa 1991, Tansill 1984).
Symptoms of human ehrlichiosis begin in 1-21 (average 7) days following infection and resemble those of Rocky Mountain spotted fever (RMSF). The spectrum of disease ranges anywhere from an illness so mild or asymptomatic that no medical attention is sought, to a severe, life-threatening condition. The characteristic clinical features are high fever and headache, but may also include malaise, myalgias, nausea, vomiting, and anorexia. A rash similar to that seen in RMSF is rare (only present in about 20-percent of cases). Since E. chaffeensis invades white blood cells, the body's immune system is adversely affected. This lessens the body's ability to fight other infections. In those patients with severe complications, acute renal or respiratory failure is most common. There have been a small number of fatalities (Spach et al. 1993, Benenson 1990, Eng 1990).
Human ehrlichiosis is not yet a notifiable disease, so the true incidence is unclear. Currently, via a very passive reporting mechanism, the Centers for Disease Control and Prevention (CDC) records approximately 50 cases per year. Through 1993, 339 cases were 'reported' from 27 states. The CDC offers free serological testing to physicians and state health departments for a panel of 4 rickettsial diseases: RMSF, typhus, Q-fever, and human ehrlichiosis. In order to meet the current case definition for human ehrlichiosis, a 4-fold rise or fall in antibody titer must be demonstrated. Most of the 'reported' cases are the result of this free testing service (personal communication with CDC, 1994).
A recent article in the Journal of the American Medical Association (Bakken et al. 1994) reports 12 cases of what may be a new type of human ehrlichiosis (termed 'human granulocytic ehrlichiosis,' or 'HGE'), occurring in Minnesota and Wisconsin from 1990 through 1993. These patients were all evaluated and treated at the Duluth (Minnesota) Clinic Ltd. Presenting symptoms included typical ehrlichiosis signs such as fever, headache, and severe myalgias. Two of the patients died from complications and secondary infections.
Serological and histological evidence points to a possible new species of Ehrlichia as the cause of the Duluth cases. It has not yet been named, but is apparently very closely related to both E. phagocytophila and E. equi, and distinct from E. chaffeensis. While E. chaffeensis is found predominantly in the monocytes, the new 'species' invades the granulocytes, giving rise to the distinction in the name of this latest form of human ehrlichiosis.
Both human ehrlichiosis and HGE appear to be arthropod- borne, with ticks thought to be the likely vectors. A possible vector for human ehrlichiosis is Amblyomma americanum (Lone Star tick) (Anderson et al. 1992). This tick is very prevalent in the south central and southeastern United States, where the majority of cases of human ehrlichiosis have been contracted. It is less clear which tick species might be involved in the transmission of the agent of HGE. Eight of the 12 Duluth patients had a history of tick bite by either Dermacentor variabilis (American dog tick) or Ixodes scapularis (black-legged tick, a.k.a. deer tick) prior to onset of symptoms (Bakken 1994).
Treatment of both human ehrlichiosis and HGE with doxycycline is strongly indicated. Patients generally respond quickly to prompt therapy. The death of the two Duluth patients 'despite intensive treatment efforts, underscores the potential gravity of HGE and the need for early diagnosis and specific tetracycline or doxycycline therapy' (Bakken et al. 1994).
References
1. Anderson, B.E., J.E. Dawson, D.C. Jones, and K.H. Wilson. 1991. Ehrlichia chaffeensis, a new species associated with human ehrlichiosis. J. Clin. Microbiol. 29(12):2838-2842.
2. Anderson, B.E., J.W. Sumner, J.E. Dawson, T. Tzianabos, C.R. Greene, J.G. Olson, D.B. Fishbein, M. Olsen-Rasmussen, B.P. Holloway, E.H. George, and A.F. Azad. 1992. Detection of the etiologic of human ehrlichiosis by polymerase chain reaction. J. Clin. Microbiol. 30(4):775-780.
3. Bakken, J.S. J. S. Dumier, S.-M. Chen, M. R. Eckman, L.L. Van Etta, and D.H. Walker. 1994. Human Granulocytic Ehrlichiosis in the upper midwest United States. JAMA. 272(3):212-218.
4. Benenson, A.S., ed. 1990. Control of Communicable Disease in Man, American Public Health Association, Washington, DC.
5. Dawson, J.E., B.E. Anderson, D.B. Fishbein, J.L. Sanchez, C. S. Goldsmith, K. H. Wilson, and C. W. Duntley. 1991. Isolation and characterization of an Ehrlichia sp. from a patient diagnosed with human ehrlichiosis. J. Clin. Microbiol. 29(12):2741-2745.
6. Donatien, A. and F. Lestoquard. 1935. Existence en Algerie d'une Rickettsia du chien. Bull Soc Pathol Exot. 28:418-419.
7. Eng, T.R., J.R. Harkess, D.B. Fishbein, J.E. Dawson, C.N. Greene, M.A. Redus, F.T. Satalowich. 1990. Epidemiologic, clinical, and laboratory findings of human ehrlichiosis in the United States. JAMA. 264(17):2251-2258.
8. Keefe, T.J., C.J. Holland, P.E. Salyer, and M. Ristic. 1982. Distribution of Ehrlichia canis among military working dogs in the world and selected civilian dogs in the United States. J. Am. Vet. Med. Assoc. 181:236-238.
9. Maeda, K., M.M. Markowitz, R.C. Hawley, M. Ristic, D. Cox, and J.E. McDade. 1987. Human infection with Ehrlichia canis, a leukocytic rickettsia. NEJM. 316(14):853-856.
10. Fukuda T., Y. Keida, and T. Kitao. 1954. Studies on causative agent of 'Hyuga netsu' disease. Med. Biol. 23:200-205.
11. Rikihisa, Y. 1991. The Tribe Ehrlichieae and Ehrlichial Diseases. Clin. Microbiol. Rev. 4(3):286-308.
12. Spach, D.H., W. C. Liles, G. L. Campbell, R. E. Quick, D. E. Anderson, and T. R. Fritsche. 1993. Tick-borne diseases in the United States. NEJM. 329(13):936-947.
13. Tansill, B., ed. Bergey's Manual of Systematic Bacteriology. 1984. Williams & Wilkins, Baltimore, MD.
For more information on Ehrlichiosis, please visit our human ehrlichiosis fact sheet at http://chppm-www.apgea.army.mil/ento/facts/ehrlichiosis.html
![[line]](http://chppm-www.apgea.army.mil/ento/images/line.gif){kind=small}
For additional technical questions, click here. For administrative information, click here
