While there has been a great deal of scientific research done on fish diseases, much of the information relates to the commercial aquaculture industry and not to the aquarium hobby. Furthermore, each species of aquarium fishes is unique in the diseases it is predisposed to contract. Any one who has been in the hobby for some period of time has been faced with the problem of trying to diagnose and treat a guppy ailment.

There is very little specific information relating to guppy disease available in the scientific community. Over the past twenty five years I have compiled a list of diseases that I have encountered most frequently in the hobby. Many of these diseases were diagnosed in fish sent to me for examination or from samples I submitted to University of California Veterinary Pathology Lab at Davis, CA.

A word of caution: definitive diagnosis of guppy diseases by microscopic examination is very difficult. It requires expensive equipment and extensive training in histology & pathobiology of fishes. A reliable university laboratory or private diagnostic laboratory can be used provided they are well versed in aquarium fish diseases, not diseases of fish used in aquaculture. Sending samples to a lab can be an expensive endeavour.

The following is a list of the most commonly encountered guppy diseases and a brief of the causative agent. The organisms have been arranged by taxonomic classification.

Bacterial Diseases

Aeromonas Hydrophila - a motile gram-negative pathogen. This is a common guppy pathogen that causes swim bladder infections, peritonitis (dropsy), septicemia, and ulcerative skin lesions. Guppies are highly suseptible to internal infections with this organism.
Treatment:   Kanamycin,enrofloxacin.

Pseudomonas fluorescens - a gram-negative pathogen, causes bacterial infection, fin rot, and hemorrhagic septicemia. This is a highly invasive organism that is difficult to treat. It can kill a tank of guppies in a matter of several hours!
Treatment:   Kanamycin or Neomycin, but Enroflozacin (expensive) is best.

Flexibacter Columnaris - a gram-negative bacteria seen in "haystack" configuration under the microscope, causes skin, gill, and fin rot, frequently seen with Aeromonas. This organism does not appear to be as common in guppies as literature suggests.
Treatment:   Trimethoprim-sulfa (TMZ) erithromycin or neomycin.

Streptococcaldiseases - a gram-positive cocci, the most common natural resident of the guppies mucous layer. Frequently causes secondary bacterial infections in response to injury, other disease organisms, or poor water conditions. Major cause of the white line finrot in guppies. They are the most common bacterial pathogens in guppies.
Treatment:   Trimethoprim-sulfa, furazolidin compounds or erythromycin.

Fungal Diseases

Saprolegnia Species - These are saprophytic water molds which are ubquituous to fresh water ecosystems. The organisms cause cotton-like growths on the skin, fins or mouth and usually affect damaged tissue. These are easy to identify under the microscope by their fungal hairlike hyphae. Treatments include Triple sulfa, Merbromin baths(mercurochrome), salt water baths and formaldehyde.

Ichthyophonus hoferi - This is an internal fungal disease that attacks the organs of the fish causing them to waste away. The intestines, ovary, and liver are infected. The females die first as the ovaries seem to be attacked first. The disease is contracted by eating contaminated fishfood, or the feces of affected fish. The disease is not treatable! All affected fish must be destroyed and their tanks thoroughly disinfected.

Parasitic Diseases

Protozoans

Ichthyophithirius multifilis - Ich, white spot disease, common aquarium pathogen.
Treatment:   formaldehyde, raise the temperature above 80 degrees for 7 days.

Icthyobodo necatrix - (costiasis) causes respiratory distress by attacking the gills. The fish may die without visible signs. Some will waste away. This is a flagellated protozoan under the microscope; Common in guppies.
Treatment:   Formaldehyde

Hexamita sp. - causes wasting away and white feces in guppies. Common in poorly maintained aquaria. Common in guppies.
Treatment:   Metronidazole in the food

Tetrahymena pyriformis - causes popeye, whirling disease, white muscle disease. Not highly contagious; Common in poor water conditions.
Treatment:   improve water conditions, formaldehyde, merbromin baths.

Myxosporidiosis - causes white cysts on the body, particularly near the peduncle in guppies. Unsightly but not very harmful.
Treatment:   merbromin baths.

Oodinum - Velvet or gold dust disease. Pear shaped protozoans attach to the gills and skin. These orgasnisms are salt tolerant.
Treatment:   Formaldehyde

Flukes

Dactylogyrid flukes, urocleid flukes, Gill flukes - They cause respiratory distress in high numbers. Common and frequently unnoticed.
Treatment:   Praziquantel.

Gyrodactylid flukes Skin Flukes - very common in guppies.
Treatment:   Praziquantel

Haplobothrium - Liver flukes - these are introduced to the aquaria via snails.
Difficult to treat.

Nematodes(worms) - three genera commonly affect guppies:   Eustrongyloides, Capillaria, and callamanus. They cause wasting away disease and white feces. They are highly contagious. They are the most common parasites of guppies and if left untreated will wipe out an entire fishroom.
Treatment:   Fenbendazole is the very best. It works well and does not harm the fish. Other treatments have been tried, including Levamisole and ivermectin. These compounds do destroy the worms but are very toxic to the fish as well. Examination of the fish's intestine post mortem will readily reveal the adult worms.

A note about antibiotics: Many guppy diseases can be treated effectively with several different antibiotics. The constant use of the same antibiotic will likely cause the development of resistant organisms rendering the medication useless. Rotating different antibiotics or using antibiotics in combination will help to prevent the development of resistant organisms.