Which animal is most likely to develop acute heartwater disease upon exposure?

The animal most likely to develop acute heartwater disease upon exposure is a 7-year old cow in an endemic region. Heartwater disease is caused by the bacterium Ehrlichia ruminantium and is transmitted primarily by ticks. This disease is predominantly found in certain regions of Africa and the Indian Ocean islands, as well as parts of the Caribbean and the southern United States.

A cow in an endemic area is more likely to be exposed to the ticks that carry the disease, and being a ruminant, the cow is also susceptible to the infection. In contrast, younger animals, such as a two-day old lamb or a four-day old goat kid, may not yet have the immunity or the level of exposure to develop the disease. Additionally, a three-year old cow in the United States would likely be in a region where heartwater disease is not prevalent, reducing the likelihood of developing the disease. Therefore, the 7-year old cow in an endemic region represents the greatest risk due to its age, exposure likelihood, and susceptibility as a ruminant in the correct geographical context.