January 2023 – Clinical Microbiology

A 70-year-old man presents with sudden-onset mental status change progressing to unconsciousness. He hikes and fishes regularly and had some recent “insect” bites. Lyme disease (caused by Borrelia burgdorferi) antibody screening by an enzyme immunoassay and immunoblots were positive (IgM: 3 proteins detected and IgG: 6 proteins detected; Figure 1). Anaplasma phagocytophilum IgG testing by immunofluorescence was also ordered and positive at a titer of 1:1024 (Figure 2).

Figure 1: Lyme
Figure 2: HGA

What is the best interpretation of these results?

  • Cross-reactive B. burgdorferi and A. phagocytophilum antibodies.
  • Recent/current Lyme disease and A. phagocytophilum infections.
  • Recent/current Lyme disease and remote A. phagocytophilum infection.
  • Recent/current A. phagocytophilum infection and remote Lyme disease.

The correct answer is ...

Recent/current Lyme disease and A. phagocytophilum infections.

Coinfection with tick-borne pathogens is a relatively common yet under-recognized occurrence. It is estimated that between 2% and 10% of patients with Lyme disease are concurrently affected by human granulocytic anaplasmosis (HGA). This is because the causative agents of Lyme disease (Borrelia burgdorferi, B. mayonii, etc.) and HGA (Anaplasma phagocytophilum) are transmitted by the same tick species (Ixodes scapularis). 

Lyme disease and HGA share overlapping symptoms, and therefore differentiating them on purely clinical grounds can be difficult. Infection with B. burgdorferi can be distinguished from HGA by the presence of erythema migrans (EM; target rash) and central nervous system involvement (neuroborreliosis). HGA, on the other hand, is more likely to cause transaminitis and cytopenias. In many cases, however, coinfections likely go undiagnosed.

Lyme disease can be diagnosed clinically based on tick exposure and EM, which is largely pathognomonic for the infection. Laboratory testing is principally serologic, and importantly, not necessary in patients with EM. For other suspected cases, a two-tiered testing approach is taken, including a screening test usually based on an enzymatic immunoassay (EIA). Positive screens are confirmed either using an EIA to detect antibodies to B. burgdorferi antigens different from the first EIA, or by immunoblot for IgM and IgG. An immunoblot is considered positive if either IgM (within 30 days of symptoms) or IgG (at any time) are positive. IgM and IgG immunoblots are considered positive if antibodies are detected to at least two out of a possible three antigens, and five out of a possible 10 antigens, respectively. Serology cannot reliably differentiate recent from remote infection, as IgM can persist beyond 30 days. Onset of symptoms must be used in conjunction with serologic results.

HGA can be diagnosed by polymerase chain reaction (PCR) during an acute infection. After the initial infection clears, however, serology by immunofluorescence assay (IFA) is the preferred method. Recent infection is suggested in patients with high IgG IFA titers (≥1:256) or by a fourfold increase in titers upon repeat testing.

1. Recent/current Lyme disease and remote A. phagocytophilum infection.

A recent or current Lyme disease infection, alongside remote Anaplasma infection would be indicated by a positive Lyme disease EIA screen and immunoblot results, with a low A. phagocytophilum IgG titers that do not increase with time. The A. phagocytophilum titer of 1:1024 is indicative of a very recent infection.

2. Recent/current A. phagocytophilum infection and remote Lyme disease.

In this patient’s case, a current or recent Lyme disease infection is suggested by confirmed two-tier serologic test results, recent symptoms, and CNS involvement. HGA infection alone would not account for the mental status change in this patient. Confirmation testing can be performed for neuroborreliosis performing a B. burgdorferi antibody index, which compares pathogen antibody levels in both CSF and serum.

3. Cross-reactive B. burgdorferi and A. phagocytophilum antibodies.

Antibodies to these two pathogens have not been shown to cross-react. 


  1. Hansmann Y, Jaulhac B, Kieffer P, et al. Value of PCR, serology, and blood smears for human granulocytic anaplasmosis diagnosis, France. Emerg Infect Dis. 2019;25(5):996-998.
  2. Horowitz HW, Aguero-Rosenfeld ME, Holmgren D, et al. Lyme disease and human granulocytic anaplasmosis coinfection: impact of case definition on coinfection rates and illness severity. Clin Infect Dis. 2013;56(1):93-99.
  3. Sanchez-Vicente S, Tagliafierro T, Coleman JL, Benach JL, Tokarz R. Polymicrobial nature of tick-borne diseases. mBio. 2019;10(5):e02055-19. Published 2019 Sep 10.
  4. Theel ES. The past, present and (possible) future of serologic testing for Lyme disease. J Clin Microbiol. 2016; May;54(5):1191-1196.

Jonathan Wilcock, D.O.

Fellow, Clinical Microbiology
Mayo Clinic

Photo of Elitza Theel, Ph.D.

Elitza Theel, Ph.D.

Consultant, Clinical Microbiology
Mayo Clinic
Professor of Laboratory Medicine and Pathology
Mayo Clinic College of Medicine and Science

MCL Education

This post was developed by our Education and Technical Publications Team.