Both Coinfection and Superinfection Drive Complex Anaplasma marginale Strain Structure in a Natural Transmission Setting

Abstract

Vector-borne pathogens commonly establish multistrain infections, also called complex infections. How complex infections are established, either before or after the development of an adaptive immune response, termed coinfection or superinfection, respectively, has broad implications for the maintenance of genetic diversity, pathogen phenotype, epidemiology, and disease control strategies. Anaplasma marginale, a genetically diverse, obligate, intracellular, tick-borne bacterial pathogen of cattle, commonly establishes complex infections, particularly in regions with high transmission rates. Both coinfection and superinfection can be established experimentally; however, it is unknown how complex infections develop in a natural transmission setting. To address this question, we introduced naive animals into a herd in southern Ghana with a high infection prevalence and high transmission pressure and tracked the strain acquisition of A. marginale through time using multilocus sequence typing. As expected, the genetic diversity among strains was high, and 97% of animals in the herd harbored multiple strains. All the introduced naive animals became infected, and three to four strains were typically detected in an individual animal prior to seroconversion, while one to two new strains were detected in an individual animal following seroconversion. On average, the number of strains acquired via superinfection was 16% lower than the number acquired via coinfection. Thus, while complex infections develop via both coinfection and superinfection, coinfection predominates in this setting. These findings have broad implications for the development of control strategies in high-transmission settings.

Keywords: Anaplasma marginale; bovine anaplasmosis; multistrain infections; superinfection; tick-borne disease.

FIG 1

Experimental design for tracking strain acquisition through time. Shown is the timeline for sampling individual naive animals that were introduced into an A. marginale-infected herd. The specific timeline varied for each animal, depending on the week at which seroconversion occurred. PCR was done weekly to detect A. marginale infection. Once an animal became PCR positive, MLST was done weekly to determine the number of A. marginale strains in each animal. Serum was collected every 2 weeks to detect seroconversion via a cELISA. Prior to seroconversion, all detected strains were defined as coinfecting strains. Following seroconversion, MLST was conducted every 2 weeks to determine the number of A. marginale strains per animal. All strains detected following seroconversion that were not previously detected were defined as superinfecting strains. The horizontal arrow represents the time period following seroconversion when samples were collected and PCR and MLST were performed to detect additional new strains. (−) indicates negative PCR. (+) indicates positive results by PCR or an msp5 cELISA.