Co-infection of White-Tailed Deer with Multiple Strains ofEhrlichia chaffeensis

Ehrlichia chaffeensis infection in the reservoir host (white-tailed deer) and in an incidental host (dog) is impacted by its prior growth in macrophage and tick cell environments

Ehrlichia chaffeensis, transmitted from Amblyomma americanum ticks, causes human monocytic ehrlichiosis. It also infects white-tailed deer, dogs and several other vertebrates. Deer are its reservoir hosts, while humans and dogs are incidental hosts. E. chaffeensis protein expression is influenced by its growth in macrophages and tick cells. We report here infection progression in deer or dogs infected intravenously with macrophage- or tick cell-grown E. chaffeensis or by tick transmission in deer. Deer and dogs developed mild fever and persistent rickettsemia; the infection was detected more frequently in the blood of infected animals with macrophage inoculum compared to tick cell inoculum or tick transmission. Tick cell inoculum and tick transmission caused a drop in tick infection acquisition rates compared to infection rates in ticks fed on deer receiving macrophage inoculum. Independent of deer or dogs, IgG antibody response was higher in animals receiving macrophage inoculum against macrophage-derived Ehrlichia antigens, while it was significantly lower in the same animals against tick cell-derived Ehrlichia antigens. Deer infected with tick cell inoculum and tick transmission caused a higher antibody response to tick cell cultured bacterial antigens compared to the antibody response for macrophage cultured antigens for the same animals. The data demonstrate that the host cell-specific E. chaffeensis protein expression influences rickettsemia in a host and its acquisition by ticks. The data also reveal that tick cell-derived inoculum is similar to tick transmission with reduced rickettsemia, IgG response and tick acquisition of E. chaffeensis.

Strains of Ehrlichia chaffeensis in southern Indiana, Kentucky, Mississippi, and North Carolina

Rates of infection of Amblyomma americanum (L.) by Ehrlichia chaffeensis were compared in 100 ticks collected from sites in each of four states: Indiana, North Carolina, Kentucky, and Mississippi. The overall infection rates were similar among sites, ranging from 1 to 4%. Because pathogenic differences may exist between E. chaffeensis strains, nested polymerase chain reaction (PCR) amplification of the variable-length PCR target (VLPT), and sequencing of the amplicons were performed to differentiate between strains. The most common infecting strains at all sites exhibited a repeat profile of 1,2,3,4 (corresponding to the Arkansas/Jax/Osceola and Liberty strains). To determine whether the minimum infection rates (MIRs) or the most common infecting strain were changing over time in southern Indiana sites, 2765 ticks from six counties in 2000 and 837 ticks from seven counties in 2004 also were examined in pools of five ticks per pool. The MIRs for 2000 and 2004 were 3.5 and 4.2% respectively, suggesting that the overall MIRs remained low. At two sites, in Pike and Harrison counties, however, infection rates more than doubled from 2000 to 2004 (7 to 16% and 0.3 to 2.7% respectively). Across all sites, the most common infecting strains (Arkansas/Jax/Osceola and Liberty) did not significantly change (68% in 2000; 79% in 2004).

Natural history of Ehrlichia chaffeensis: vertebrate hosts and tick vectors from the United States and evidence for endemic transmission in other countries

Ehrlichia chaffeensis, an intracellular gram-negative zoonotic bacterium, is the causative agent of human monocytotropic ehrlichiosis (HME). In humans, the disease can range from a mild, non-specific illness with few to no clinical signs to a moderately severe to fatal disease, especially those with compromised immune systems. E. chaffeensis is maintained in a complex cycle involving white-tailed deer (WTD; Odocoileus virginianus) as a primary reservoir and the lone star tick (LST; Amblyomma americanum) as a primary vector. Numerous other species are naturally exposed to E. chaffeensis and disease has been documented in some domestic animals and wildlife including domestic dogs and ring-tailed lemurs. The organism has been found throughout the natural range of the LST and as the tick continues to expand its range, the geographic range of risk for E. chaffeensis infections will likely continue to expand. Recent data have indicated that E. chaffeensis, or a closely related organism, has been found in many species of ticks and vertebrate hosts in numerous countries.

Ecological havoc, the rise of white-tailed deer, and the emergence of Amblyomma americanum-associated zoonoses in the United States

Two infectious diseases, and one presumably infectious disease, each vectored by or associated with the bite of the lone star tick (Amblyomma americanum), were identified and characterized by clinicians and scientists in the United States during the 1980s and 1990s. These three conditions-human monocytic (or monocytotropic) ehrlichiosis (HME), Ehrlichia ewingii ehrlichiosis, and southern tick-associated rash illness (STARI)-undoubtedly existed in the United States prior to this time. However, the near-simultaneous recognition of these diseases is remarkable and suggests the involvement of a unifying process that thrust multiple pathogens into the sphere of human recognition. Previous works by other investigators have emphasized the pivotal role of white-tailed deer (Odocoileus virginianus) in the emergence of Lyme disease, human babesiosis, and human granulocytic anaplasmosis. Because whitetails serve as a keystone host for all stages of lone star ticks, and an important reservoir host for Ehrlichia chaffeensis, E. ewingii, and Borrelia lonestari, the near-exponential growth of white-tailed deer populations that occurred in the eastern United States during the twentieth century is likely to have dramatically affected the frequency and distribution of A. americanum-associated zoonoses. This chapter describes the natural histories of the pathogens definitively or putatively associated with HME, E. ewingii ehrlichiosis, and STARI; the role of white-tailed deer as hosts to lone star ticks and the agents of these diseases; and the cascade of ecologic disturbances to the landscape of the United States that have occurred during the last 200 years that provided critical leverage in the proliferation of white-tailed deer, and ultimately resulted in the emergence of these diseases in human populations.