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Parasites and disease

As with other bird families, pelicans are susceptible to a variety of parasites. Avian malaria is carried by the mosquito Culex pipens, and high densities of these biting insects may force pelican colonies to be abandoned. Leeches may attach to the vent or sometimes the inside of the pouch. A study of the parasites of the American white pelican found 75 different species, including tapeworms, flukes, flies, fleas, ticks, and nematodes.

The brown pelican has a similarly extensive range of parasites. The nematodes Contracaecum multipapillatum and C. mexicanum and the trematode Ribeiroia ondatrae have caused illness and mortality in the Puerto Rican population, possibly endangering the pelican on this island.

Many pelican parasites are found in other bird groups, but several lice are very host-specific. Healthy pelicans can usually cope with their lice, but sick birds may carry hundreds of individuals, which hastens a sick bird's demise. The pouch louse Piagetiella peralis occurs in the pouch and so it cannot be removed by preening. While this is usually not a serious problem even when present in such numbers that it covers the whole interior of the pouch, sometimes inflammation and bleeding may occur from it and harm the host.

In May 2012, hundreds of Peruvian pelicans were reported to have perished in Peru from a combination of starvation and roundworm infestation.