Under what condition are rat guards required on a ship?

Rat guards are specifically required on a ship when it is in a port where plague is endemic. This requirement is tied to the health risks associated with rodents, as they can carry disease, including the plague. In areas where the plague is present, having measures in place to prevent rats from accessing the ship is critical for maintaining sanitation and preventing the potential spread of disease to the crew and port personnel.

In contrast, while being in any port or in areas with high mosquito populations poses certain health risks, the direct requirement for rat guards is not specifically tied to those situations. Moreover, stating they should be present at all times abroad does not accurately reflect the targeted precaution needed for certain endemic areas. Hence, the requirement for rat guards is particularly emphasized in regions where the risk of disease transmission by rats, such as during an outbreak of plague, is significantly higher.