Infection by parasitic worms such as helminths can affect the immune response in ways that increase the susceptibility of hosts to infection by bacteria and viruses. Hence, treatment against helminths represents one way to combat microbial diseases. Ezenwa and Jolles (2015) investigated the effect of anthelmintic treatment on the acquisition of bovine tuberculosis (BTB) in a wild population of African buffalo and the effects on buffalo survival. The graph below shows Kaplan–Meier survival curves for the time to infection by BTB in treated and untreated (control) buffalo. Vertical ticks are censored observations. Confidence intervals for proportion uninfected are not shown, but they fully overlap between the two groups. The observed and expected number of infected individuals are shown below. Expected values are calculated under the null hypothesis of no difference in survival curves between groups.

a. Using the numbers in the table, estimate the infection hazard ratio, HR^ , for anthelmintic individuals compared to control individuals.

b. Using the numbers in the table, test whether antihelminth treatment affected the time to BTB infection of wild African buffalo.

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Treatment Anthelmintic Anthelmintic ControlControl An 100100 101101 Observed infected 3333 3636 Expected infected 32.8832.88 36.1236.12