A previously well 58-year-old man came to the ER in early July with 3 days of fever (39.4 °C), severe headache, myalgias, fatigue, nausea, and diarrhea. This started a week after cutting brush at a lakeside cabin (surrounding event), but he had no recollection of a tick bite.

On exam, he appeared mildly ill, with conjunctival injection (pink eyes) and a rash on his abdomen of macules (small red spots). Labs then revealed leukopenia, thrombocytopenia, and elevated AST/ALT. He was started empirically (before lab confirmation) on doxycycline. Eventually, his fever resolved within a day and a half. A PCR was also initially sent, which later confirmed  E. muris eauclairensis.

“Ehrlichiosis” in the U.S. refers mainly to infections by E. chaffeensis (human monocytic ehrlichiosis, HME), E. ewingii, and—upper Midwest—E. Muris eauclairensis. Lone star ticks (Amblyomma americanum) transmit E. chaffeensis and E. ewingii across the south-central and eastern states. E. muris eauclairensis is carried by black-legged ticks (Ixodes scapularis) and has been reported in Minnesota and Wisconsin. 

Disease caused by these ticks is seasonal, peaking late spring through summer, in parallel with nymphal/adult tick activity. Incidence has risen over the past two decades with expanding tick ranges. Erlichiosis is the second most common disease transmitted by ticks, while lyme disease is the most common tick-borne illness.

Ehrlichia are obligate intracellular α-proteobacteria that preferentially infect monocytes/macrophages (E. chaffeensis) or granulocytes (E. ewingii). After entry via tick saliva, organisms reside in cytoplasmic vacuoles and replicate in clusters (morulae), subverting phagolysosomal killing (the body’s defense mechanism against pathogens in ticks). 

The host response—cytokine release and immune dysregulation—drives many systemic manifestations including fever, abnormally low counts of one or more circulating blood cell lines, and hepatitis. With prompt doxycycline, prognosis is excellent. Delay in therapy increases the risk of ICU admission, organ failure, and death. Severe outcomes are more likely in the elderly, immunocompromised (e.g., transplant recipients), or those with comorbidities. 

Typically, as in this case, onset is an acute, non-specific febrile illness: high fever, chills, headache (often severe), myalgias, malaise, and GI symptoms (nausea, vomiting, diarrhea) are common. Rash is more common in kids, but uncommon in adults, although our patient had it.

In this case, laboratory studies helped with the diagnosis. The blood work clues were leukopenia (low white cell count), thrombocytopenia (low platelet count), and mild transaminitis (elevated liver enzymes). Hyponatremia (low sodium) can also occur, but did not in this case. 

Lack of a known tick bite should not lower suspicion. It is important to note that we cannot afford to wait for PCR results in a suspected infection, as it can take several days to get the confirmation.

I encouraged the patient to use EPA-registered repellents, wear permethrin-treated long sleeves/pants, perform daily tick checks, and promptly remove attached ticks with a tick remover. There is no vaccine for this infection.

Fortunately, most tick bites do not lead to transmission of any infection, because only a minority of the ticks carry disease. Typically, the tick has to be attached for 24 hours. This is why we do not treat prophylactically with an antibiotic unless the patient shows some signs of infection.