The presence and blood-feeding of fleas often results in allergic reactions (flea-related dermatitis, flea allergic dermatitis [FAD]) and can cause substantial blood loss, which may lead to anaemia in severe cases. Iron deficiency anaemia due to heavy infestations can be a particular problem in young animals.
Besides the direct effects of flea feeding, fleas are known to be transmitters of tapeworms as well as bacterial and viral pathogens to both animals and humans.