Anaplasmosis and Ehrlichiosis
The characteristics, symptoms, diagnosis and treatment of anaplasmosis and ehrlichiosis.
Anaplasma phagocytophilum and Ehrlichia chaffeensis are related bacteria which are transmitted by ticks and cause infections in humans and some animal species.
Anaplasma has a wide distribution across North America and Eurasia. Anaplasmosis is the disease caused by A. phagocytophilum infection and is relatively mild in the majority of cases.
The main species of Ehrlichia is limited to North America. Ehrlichiosis, the disease resulting from infection with Ehrlichia spp., can cause significant and even life-threatening disease, requiring urgent treatment.
Anaplasmosis
Anaplasmosis, also known as Human Granulocytic Anaplasmosis (HGA) is spread through bites from Ixodes ticks. These are widespread across Europe, the UK and North America. Ixodes ricinus, the European species, can also transmit tick-borne encephalitis and other infections, but studies in Europe have found that infections with more than one tick-borne pathogen at a time are very rare. In North America, I. scapularis and I. pacificus transmit A. phagocytophilum and are also the vectors for Lyme disease.
A. phagocytophilum infects a wide range of animal hosts including wild and domestic ruminants, horses, birds, rodents and domesticated dogs and cats. It causes tick-borne fever in sheep, and pasture fever in cattle, with significant economic consequences.
The vast majority of anaplasmosis cases in humans are seen in North America. Cases are rarely reported in Europe and, in the UK, only one case of human anaplasmosis has been documented, acquired in Scotland where the pathogen is highly endemic in animals.
Symptoms
Most cases in humans go unnoticed. Those who develop illness generally have fever, muscle aches and pains, chills and joint pains. A few cases may have more significant symptoms including breathing difficulties and kidney failure. In these cases, urgent treatment is required.
Diagnosis
A blood film taken early after the onset of symptoms may show clusters (called morulae) of bacteria in granulocytic white blood cells.
Definitive diagnosis is by PCR testing, which is rapid and sensitive in the early stage of the illness. In the later stage, or after recovery, presumptive diagnosis may be made on the basis of antibody testing. Other infections that should be considered include Lyme disease, which is seen more frequently than anaplasmosis in humans in Europe.
PCR and IgG antibody testing are available at the UKHSA Rare and Imported Pathogens Laboratory (RIPL). Infection doctors wanting to test a suspected case should first discuss this with the RIPL clinicians, who will also provide advice on differential diagnoses and treatment options.
Treatment
The standard treatment is a course of doxycycline for 7 to 14 days, which is highly effective. If this is contra-indicated, other drugs may be considered. Fever usually resolves within 48 hours of starting treatment. Drug resistance and relapse after treatment have not been reported.
Prognosis
The majority of cases resolve asymptomatically or run a benign course. Disease may be more severe in the immunocompromised and older patients and hospitalisation may be needed. The overall case fatality rate reported in the USA is under 1%.
Ehrlichiosis
Ehrlichiosis is caused by Ehrlichia spp., usually E. chaffeensis and E. ewingii. It is almost entirely confined to North America, but imported cases do appear in the UK and should be considered in individuals who have been in risk areas and have relevant symptoms. In the USA, co-infections with Lyme disease and/or babesia are relatively frequent, so the diagnosis and management should take this into account. Co-infections are rarely seen elsewhere. The main vector is the Lone Star tick, Amblyomma americanum, and the main animal reservoirs are the white-tailed deer and the white footed mouse, which is widespread and also a significant reservoir for Lyme disease in the US.
Symptoms
Human Monocytic Ehrlichiosis (HME), caused by E. chaffeensis, is a significantly more harmful disease than anaplasmosis or Lyme disease. The initial symptoms are non-specific, with fever, gastro-intestinal symptoms, muscle aches and headaches. Most cases run a benign course, but a more serious picture can occur rapidly with signs of sepsis, multi-organ failure and death. HME should be considered in patients with a relevant exposure history who present with signs of sepsis, particularly if there are abnormal liver function tests, a fall in the platelet count and a fall in the white cell count.
E. ewingii causes Human Ewingii Ehrlichiosis (HEE). E. ewingii targets neutrophils rather than monocytes, and presents in a similar way to HME but is usually less severe.
Diagnosis
Because of its rarity, there is currently no direct molecular testing for ehrlichiosis in the UK. However, diagnosis can be made by careful evaluation of a peripheral blood smear, looking for characteristic morulae in infected leukocytes. Research assays may be available for confirmation, but if there is a compatible clinical syndrome and exposure history, treatment should be initiated on clinical suspicion.
Treatment
As for anaplasmosis, doxycycline is the best option for treatment and should be started if there is a credible suspicion of HME or HEE. Advice can be obtained from the UKHSA Rare and Imported Pathogens Laboratory.
Prognosis
The majority of infections resolve without complications. In symptomatic cases in the USA, the overall mortality rate is reported as 1% (higher for children under 5 years of age). Disease may be more severe in immunosuppressed patients or in the elderly. Chronic infection has not been reported.