The prevalence of infection and the amount of a pathogen in ticks are important factors when determining their ability to cause disease in humans. We have noticed a decline in the ability of Ixodes scapularis nymphs to infect mice with Anaplasma phagocytophilum (the agent of HGE) over time. Such a decline may result either from ticks clearing the infection (decrease in the prevalence), or a decrease in the amount of a pathogen in ticks, or both. BALB/c mice were infected with A. phagocytophilum through the bite of infected I. scapularis nymphs. Larval I. scapularis were placed on the infected mice for two consecutive weeks and allowed to feed to repletion. Freshly molted nymphs, 3-month old nymphs, and 6-month old nymphs were then tested by PCR to determine their prevalence of infection and their pathogen load. Prevalence of infection did not differ significantly between the three age groups. However, the amount of pathogen declined significantly in 6-month old nymphs when compared to freshly-molted and 3-month old nymphs. This “loss of infection” observed in I. scapularis nymphs infected with A. phagocytophilum could be an important measure of these ticks potential to cause human disease.