外寄生虫

例句：

1. 1. The dog has been bitten by an external parasite.
   这只狗被一种外部寄生虫咬伤了。
2. 2. External parasites can cause serious health problems.
   外部寄生虫可以引起严重的健康问题。
3. 3. Doctors are trying to develop a vaccine against external parasites.
   医生们正在尝试研发一种对抗外部寄生虫的疫苗。