Maedi-Visna is the recognized alternative name for the same chronic ovine lentiviral infection. Ovine Progressive Pneumonia (OPP) refers to this disease in sheep, and the virus involved is the Maedi-Visna virus (MVV). Historically, two syndromes were described—Maedi, a progressive dyspnea from interstitial pneumonia, and Visna, a wasting or demyelinating form—together defining the disease entity. So the term Maedi-Visna is used interchangeably with OPP. The other options point to diseases in goats or cattle, or to generic terms not used as the official sheep disease name, which is why they aren’t correct.

Wild-type poliovirus circulation is now limited to only a couple of places, reflecting where transmission has not yet been interrupted. In this question, the two countries where wild poliovirus is still circulating are Afghanistan and Pakistan. Ongoing transmission in these neighboring nations has persisted due to challenges in achieving complete vaccination coverage, access issues in conflict-affected areas, and cross-border movement that helps spread the virus despite global eradication efforts. The other options refer to regions that have interrupted wild poliovirus transmission or have only experienced outbreaks from importations, not sustained endemic circulation. India cleared wild poliovirus years ago, and broader Africa is now polio-free for wild-type strains, with Nigeria also no longer sustaining wild-type transmission. Egypt and Somalia have faced outbreaks from imports but do not have ongoing endemic wild poliovirus circulation.

Seasonal patterns often drive when viral diseases in animals show up, because how and where animals are housed, how they interact, and environmental conditions change throughout the year. For Borna disease virus, outbreaks tend to cluster in specific times of the year—typically in temperate regions during autumn and winter—when horses are kept indoors in closer contact, stress from housing and handling is higher, and environmental conditions can help the virus persist in the environment or transmission opportunities increase. This makes cases rise during those seasons rather than remaining constant throughout the year. The idea is that the incidence follows the seasons due to management and environmental factors, not a uniform year‑round presence.

Antigenic diversity in foot-and-mouth disease virus is organized into distinct serotypes and many within-serotype variants. There are seven serotypes recognized globally, and immunity is serotype-specific, meaning protection against one serotype does not reliably protect against another. Within each serotype, there are numerous antigenic variants—subtypes or topotypes—driven by the virus’s high mutation rate. Taken together, seven serotypes with roughly sixty subtypes capture the wide diversity that makes vaccine design challenging, since vaccines must align with the circulating serotype and the prevalent subtypes within it. The other numbers understate this diversity, so this combination best reflects the real-world complexity of FMDV.