What is the primary host for Plasmodium vivax?

Plasmodium vivax is a protozoan parasite known to cause malaria in humans. It has a complex life cycle that requires a definitive host for sexual reproduction and an intermediate host for asexual reproduction. Humans serve as the primary host, where the parasite undergoes its asexual development in the liver and red blood cells. The transmission typically occurs through the bite of an infected female Anopheles mosquito, which is the vector that aids in the parasite's lifecycle.

In contrast, the other options do not serve as primary hosts for P. vivax. Dogs, for instance, are not involved in the life cycle of P. vivax, nor are rats or cows. Hence, the correct answer highlights the critical role of humans in the transmission and propagation of this malaria-causing parasite. Understanding the host dynamics is crucial for developing effective control strategies against malaria.