If it is exposed to Ehrlichia ruminantium, which of the following animals is most likely to develop acute heartwater disease?

• A. A two-day old lamb in the United States
• B. A 3 year old cow in the United States
• C. A 7-year old cow in an endemic region of Africa
• D. A four-day old goat kid in an endemic region of Africa

Answer: (B)

Heartwater disease risk hinges on immunity from prior exposure. Animals acquire protection after being exposed to Ehrlichia ruminantium, and in regions where the disease is common, older animals often show less acute illness because of this partial immunity. Neonates may have maternal antibodies that offer short-term protection, and animals in endemic Africa may have had prior infections that blunt a new clinical outbreak.

Given those patterns, an adult cow in the United States would be most likely to develop acute heartwater if exposed, because she would have little to no prior exposure or immunity to the pathogen and would be highly susceptible when challenged. In contrast, animals in Africa—especially older cattle—are more likely to carry some immunity from previous infections; neonates and young kids in endemic regions are also more likely to benefit from maternal protection or early-life exposure patterns that reduce the chance of an acute, severe presentation.