ASCP Parasitology Practice Exam

Cryptosporidium oocysts are indeed immediately infective when passed in stool. This characteristic sets them apart from many other parasitic organisms, which often require additional time or specific environmental conditions to develop into an infective form.

Upon defecation, the oocysts of Cryptosporidium, which are robust and resistant to environmental stress, can immediately cause infection if ingested by a host. These oocysts can survive in the environment for significant periods, contributing to their transmission through contaminated water or food sources.

In terms of disease risk, this immediate infectivity underscores the importance of proper sanitation and water treatment to prevent outbreaks. The other listed organisms either do not have an immediate infective stage or must undergo further development after being excreted before they can infect a new host. For instance, Trichuris trichiura requires maturation in the environment to become infective, while Plasmodium ovale is transmitted via mosquito bites and requires further life cycle stages to become infective. Toxocara canis eggs also need time to develop in the environment before becoming infectious.