A common cause of skin disease in dogs and cats is flea infestation. Traditionally we think of fleas as being a problem occurring during the summer months but with many homes having efficient heating it is common to see fleas all year round.
There are over 2000 species of flea in the world. Thankfully, only the cat flea and the dog flea (Ctenocephalides felis, Ctenocephalides canis) are important to dogs and cats. Despite the name, cat fleas infest dogs just as much as they infest cats.
Flea control can be very difficult as only the adult flea lives on your pet. The rest of the life cycle is completed in the environment. Female fleas can lay from 20 to 50 eggs a day and up to 2000 eggs during their lifetime, which then fall off your pet into the environment.
Depending upon temperature and humidity, the entire life cycle of the flea can be completed in as little as 12 to 14 days under ideal conditions, or be prolonged to 6 months and possibly as long as a year in rare instances. However, under average household conditions, fleas will complete their life cycle within 3 to 4 weeks. The adult fleas you see on your pet are only 5% of any flea infestation, the remaining 95% are living as immature stages in your home.
Pets that routinely go outdoors will come into contact with fleas from time to time, and require regular treatment. Many different products are available. (see the section on anti-parasitic treatments)
Finally, some pets are allergic to relatively small numbers of fleas, and may need particularly stringent flea control.