Ehrlichiosis is a disease caused by several Gram-negative obligate intracellular bacteria that are transmitted by a tick vector. The frequency of ehrlichiosis is increasing, which is ascribed to the increased awareness and diagnostic availability, as well as the expansion of regions populated with the most common tick vector – Amblyomma americanum (also known as the Lone Star tick).
Lone Star Tick (Amblyomma americanum). Image Credit: Melinda Fawver / Shutterstock
Symptoms of the disease usually include fever, anorexia, headache and myalgia, with a relatively low incidence of rash (present only in 20% of affected individuals). The knowledge of disease epidemiology is vital, as early recognition may prevent a large number of cases (and thus avoid rare fatal outcomes).
Generally, the median age for acquiring ehrlichiosis is 51-53 years, while Caucasian males are the most commonly affected group. Even though cases of the disease are found year-round, the greatest number is observed between May and August, which coincides with periods of abundant populations of ticks and human open air recreation.
Epidemiology of Human Monocytic Ehrlichiosis
Human monocytic ehrlichiosis (caused by the species Ehrlichia chaffeensis and Ehrlichia canis) was initially described in 1986, and almost three thousand cases have been reported to the U.S. Centers for Disease Control and Prevention (CDC) in the last three decades. Even though the average incidence of this disease in the United States is 0.7 cases per million inhabitants, this estimation is based on passive surveillance and probably represents substantial underestimation of the actual incidence.
For example, in one seroprevalence study conducted on children it was demonstrated that 20% of infected individuals from endemic areas had detectable antibodies to Ehrlichia chaffeensis, without any previous history of apparent clinical disease. This is in line with the finding that more than two-thirds of all infections with causative agents of human monocytic ehrlichiosis are either without any symptoms or minimally symptomatic.
Akin to other tick-borne illnesses, the distribution of arthropod vectors and reservoirs in vertebrates highly correlates with the disease occurrence in humans. The predominant zoonotic cycle of Ehrlichia chaffeensis is comprised of infected white-tailed deer as the reservoir and Amblyomma americanum as the tick vector, both prevalent across southcentral and southeast United States.
This tick also has three feeding stages (known as larval, nymph and adult stages), and each developmental representative feeds only once. Transstadial transmission of Ehrlichia can be seen during nymph and adult feeding, because larvae are uninfected. Unlike some other species (most notably Rickettsia), Ehrlichia cannot be maintained via trans-ovarial transmission pathway.
Epidemiology of Human Granulocytic Anaplasmosis
In the 1990s certain patients from Wisconsin and Michigan with a history of tick bite presented with a febrile illness similar quite similar to human monocytic ehrlichiosis. These cases were characterized by inclusion bodies in granulocytes rather than agranulocytes (monocytes), which is why this syndrome was initially named human granulocytic ehrlichiosis. After recent reclassification of Ehrlichia phagocytophilum to the genus Anaplasma, the disease has been renamed to human granulocytic anaplasmosis.
The annual number of human granulocytic anaplasmosis cases exceeds those of human monocytic ehrlichiosis, with an incidence case rate of 1.6 per million inhabitants in the U.S. As with Ehrlichia chaffeensis, different serosurveillence studies evince that asymptomatic disease is quite common. The disease is primarily caused by Anaplasma phagocytophilum.
This disease is seen in Europe as well, where the majority of cases is found in central Europe (e.g. Slovenia) and Scandinavian countries (e.g. Sweden), though individual reports have come from other countries as well. Nonetheless, there is limited knowledge as to the exact epidemiological features and implicated animal reservoirs in Europe.
Different ticks transmit human granulocytic anaplasmosis in different continents; Ixodes scapularis and Ixodes pacificus are the predominant tick species in the United States, Ixodes ricinus in Europe, while Ixodes persulcatus transfers the infectious agent in Asia. Small mammals (such as White-footed mouse and Dusky-footed wood rat) are primary reservoirs of the disease.
Epidemiology of Ehrlichia ewingii
The epidemiology of Ehrlichia ewingii is usually addressed separately as there are certain problems with this species, such as the absence of specific serological tests and the lack of adequate reporting systems. What we do know is that most infections have been observed in immunosuppressed patients (after organ transplantation) and in those infected with human immunodeficiency virus (HIV).
The primary vector for Ehrlichia ewingii is the Lone Star tick, as for Ehrlichia chaffeensis. Most human cases of the disease have been documented in Missouri, Oklahoma and Tennessee in the United States, although infection has been described in dogs and deer throughout the range of the tick that transmits the disease, suggesting that the infection with this specific species might be even more widespread.