Human Monocytic Ehrlichiosis, Mexico City, Mexico

Little information is available about human infections by the members of the genus Ehrlichia in Mexico. Only 2 species, Ehrlichia canis and E. chaffensis, are known to cause disease in this country. We report a fatal case of human monocytic ehrlichiosis in Mexico City in a man who was homeless.

During hospitalization, leukocytosis developed, which might have been associated with the multiple trauma. The patient initially had a platelet count within the reference range, but thrombocytosis developed, and the platelet count increased to 850,000/ μL. However, with persistent leukocytosis, the platelet count dropped, and thrombocytopenia (108,000/ μL) developed shortly before death.
Total serum protein and albumin concentrations at admission were within reference ranges, but dur-ing hospitalization they decreased, as would be expected for a diagnosis of ehrlichiosis. Alterations in liver enzyme levels and coagulation times also developed. Levels of alanine aminotransferase (ALT) and aspartate aminotransferase also increased after admission; ALT showed the largest increase. The level of γ-glutamyl transpeptidase increased by >10 times over its reference value (Table). The increase in the level of ALT could be related to hepatic alterations linked to ehrlichiosis.
Necropsy showed hepatosplenomegaly and pleural effusions. Molecular assays did not detect Bartonella or Rickettsia spp., but they did detect Ehrlichia/ Anaplasma spp. when primers Ehr1/Ehr2 were used. We isolated a 400-bp fragment of the 16S rRNA gene, which showed 99% identity with that of the E. chaffensis strain Arkansas. In addition, a 500-bp fragment of the dsb gene (present only in the members of the genus Ehrlichia) was amplified (10) and showed 100% identity with the sequence of E. chaffensis strain Arkansas.
Phylogenetic analysis of the isolated sequences grouped them with sequences of 2 strains of E. chaffensis (strain Arkansas and West Paces) detected in the United States; these sequences had a bootstrap value of 99. The sequences we obtained were deposited in GenBank under accession numbers MK351589 and MK370999 (Figure).

Conclusions
Ehrlichioses represent systemic infections that can cause damage to different organs and systems, affecting the liver, meninges, brain, heart, and lungs. Because the pathophysiology of this disease is not well established, the findings obtained by necropsy are useful. During his hospitalization, the patient acquired some bacterial infections, which could explain the paradox that he initially had thrombocytosis and leukocytosis instead of thrombocytopenia and leukopenia, as would be expected for ehrlichiosis. Leukopenia and thrombocytopenia are present in >50% of patients given a diagnosis of E. chaffeensis infection (5).
The patient might have been infected by blood transfusion because the long period between hospital admission and detection of thrombocytopenia is compatible with the incubation period for infection with E. chaffeensis. The patient received multiple blood transfusions from donors in Mexico City and the state of Mexico; in this state, a fatal case of ehrlichiosis caused by blood transfusion has been reported (5). A study in Costa Rica reported a large number of E. canis-infected healthy blood donors (11), and a study in the United States reported a patient who was infected by blood transfusion containing E. ewingii (12). More recently, a potential case of transfusion-transmitted human monocytic ehrlichiosis was reported in a 59-year-old woman in the United States after she received a blood stem cell transplant (13).
Although this possibility is less likely, it cannot be ruled out that ehrlichiosis could be acquired by tick bite. E. chaffeensis was previously detected in 3 hard tick species (Amblyomma americanum, A. mixtum, and Rhipicephalus sanguineus sensu 1ato) (14) in Mexico. Only the lone star tick (A. americanum) has been implicated as a primary vector of this pathogen in the United States; however, its presence in Mexico has only been recorded restricted to the Nearctic region (15). Conversely, no studies demonstrate the role of the other 2 tick species as potential vectors of this pathogen or whether DNA of this pathogen came from an infected host from which these ticks fed.
For this reason, it is essential to perform systematic surveillance of this and other tick-borne pathogens in blood donors in Mexico. Studies should also be conducted with questing ticks to identify the risk to which the population is exposed and establish the actual distribution of the pathogen.

About the Author
Dr. Alcántara-Rodríguez is chief of the Unit of Epidemiologic Surveillance for hospitals of the Health Ministry, Mexico City, Mexico. Her primary research interest is rickettsioses.