Concomitant infection decreases the malaria burden but escalates relapsing fever borreliosis

Abstract

About 500 million cases of malaria occur annually. However, a substantial number of patients who actually have relapsing fever (RF) Borrelia infection can be misdiagnosed with malaria due to similar manifestations and geographic distributions of the two diseases. More alarmingly, a high prevalence of concomitant infections with malaria and RF Borrelia has been reported. Therefore, we used a mouse model to study the effects of such mixed infection. We observed a 21-fold increase in spirochete titers, whereas the numbers of parasitized erythrocytes were reduced 15-fold. This may be explained by polarization of the host immune response toward the intracellular malaria parasite, resulting in unaffected extracellular spirochetes and hosts that succumb to sepsis. Mixed infection also resulted in severe malaria anemia with low hemoglobin levels, even though the parasite counts were low. Overall, coinfected animals had a higher fatality rate and shorter time to death than those with either malaria or RF single infection. Furthermore, secondary malaria infection reactivated a quiescent RF brain infection, which is the first evidence of a clinically and biologically relevant cue for reactivation of RF Borrelia infection. Our study highlights the importance of investigating concomitant infections in vivo to elucidate the immune responses that are involved in the clinical outcome.

FIG. 1.

Coinfection leads to decreased malaria burden, increased relapsing fever, and decreased survival. (A) Percentage of erythrocytes containing malaria parasites (designated parasitemia) in mice infected with P. berghei (P.b) or coinfected with both P. berghei and B. duttonii (P.b/B.d) (n = 6; values are means ± standard deviations [SD]. The parasitemia was significantly lower (P = 0.024) in coinfected mice. (B) Numbers of B. duttonii spirochetes in the blood of mice infected with B. duttonii (B.d) or coinfected with both P. berghei and B. duttonii (P.b/B.d) (n = 6; values are means ± SD). Spirochete titers were significantly higher (P = 0.014) in coinfected mice. (C) Kaplan-Meier survival curve comparing mice infected with P. berghei (P.b) with those coinfected with both P. berghei and B. duttonii (P.b/B.d) (n = 6; values are means ± SD).

FIG. 2.

The host immune response is polarized toward malaria in coinfected mice. (A) Concentrations of IFN-γ in sera from mice infected with P. berghei (P.b), with B. duttonii (B.d), or with both parasites (P.b/B.d) (n = 6; values are means ± SD). (B) Concentrations of IL-4 in sera of mice infected with P. berghei (P.b), with B. duttonii (B.d), or with both parasites (P.b/B.d) (n = 6; values are means ± SD).

FIG. 3.

Latent residual RF attenuates malaria. Percentages of erythrocytes infected with malaria parasites (designated parasitemia) in mice with a residual Borrelia infection and a secondary malaria infection (P.b/B.d) or in mice with only a primary malaria infection (P.b) are shown (n = 10; values are means ± SD). The parasitemia in mice with latent residual RF was significantly lower during the first 18 days of malaria infection (P = 0.046) than that in age-matched control animals.

FIG. 4.

Plasmodium/Borrelia coinfection leads to severe anemia. Hemoglobin (Hb) concentrations in BALB/c mice are shown. Dashed lines represent 55% of the Hb concentration determined in uninfected mice based on the definition of severe anemia in BALB/c mice (15). (A) Mice infected with P. berghei (P.b) (n = 6; values are means ± SD). (B) Mice infected with B. duttonii (B.d) (n = 6; values are means ± SD). (C) Mice infected with both pathogens (P.b/B.d) (n = 6; values are means ± SD). There was no significant difference in hemoglobin concentration between coinfected and P. berghei-infected mice (P = 0.897).

FIG. 5.

Gross pathology in mice infected with both P. berghei and B. duttonii. Pictures are of internal organs from four representative BALB/c mice infected as follows: (A) uninfected; (B) infected with P. berghei; (C) infected with B. duttonii; and (D) infected with both pathogens. Abbreviations: S, spleen; N, necrotic area of the spleen; L, liver; PP, Peyer's patches. Note that the Peyer's patches in panel D are hemorrhagic.

FIG. 6.

Immunohistological staining of dendritic cells (A to D), B lymphocytes (E to H), and neutrophils (I to L) in murine spleen. The tissue architecture, with distinct germinal centers, is essentially normal in Borrelia-infected animals (arrow in panel G), while mice infected only with malaria (arrow in panel F) or with both pathogens (arrow in panel H) show a disordered structure. In the mice infected with P. berghei or B. duttonii or coinfected, immunohistochemical staining detected all three cell types, although the staining patterns could not be compared due to differences in splenic size and architecture. However, sections from the coinfected animals show severe necrotic tissue destruction (arrowheads in panels D, H, and L), which is not seen in mice infected with P. berghei or B. duttonii alone. Magnification, ×100. Scale bar, 100 μm.