The correct answer is: A. Dog
Theilreia parva is a protozoan parasite that causes canine babesiosis, a serious disease of dogs. It is transmitted by ticks, and the most common tick vector is the brown dog tick, Rhipicephalus sanguineus. The parasite infects red blood cells, causing anemia and other symptoms. In severe cases, the disease can be fatal.
Buffalo are not susceptible to Theilreia parva infection. Cats are susceptible to Theilreia parva infection, but the disease is usually mild in cats. Horses are not susceptible to Theilreia parva infection.
Canine babesiosis is a serious disease that can be fatal in dogs. It is important to protect your dog from tick bites by using an effective tick control product. If you think your dog may have been infected with Theilreia parva, it is important to see a veterinarian right away.