Anaplasma infection in cows is most commonly seen in which age group?

• A. 0-1 year
• B. 2-4 years
• C. 5-7 years
• D. Adults over 8 years

Answer: (B)

Anaplasma marginale–induced disease in cattle most commonly appears in animals that have lost maternal antibodies but have not yet built strong immunity from prior exposure. Calves receive protective colostral antibodies, so they often harbor subclinical infections or have milder disease. As maternal immunity wanes and before many animals have been exposed enough to develop immunity, cows around two to four years old are the group where clinical disease is seen most often. Older cattle are more likely to be immune due to prior exposure or carry the infection subclinically, leading to fewer overt cases.