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Aphanomyces as the cause of skin lesions in menhaden
Recent work by scientists at VIMS, the National Fish Health Research Laboratory, and the University of Stirling has shown that the water mold Aphanomyces invadans is the cause of the fish lesions that were formerly ascribed to a Pfiesteria toxin(s). Download a .pdf file of this work: Kiryu et al. (2002)
Aphanomyces-induced lesions like those shown here are invariably fatal to fishes. VIMS researchers hypothesize that the mold, which can not be transmitted at salinities above about 2 parts per thousand (normal seawater is around 35 parts per thousand), infects menhaden when juveniles move into low-salinity headwaters of Chesapeake Bay tributaries to feed on dense algal blooms.
The researchers identified Aphanomyces as the cause of the menhaden lesions by fulfilling Koch's Postulates. These are four criteria that pathologists use to determine whether a given infectious organism is the causative agent of a particular disease or disease syndrome:
| KOCH'S POSTULATES |
| Postulate 1: The organism must be found in diseased animals. |
| VIMS researchers and colleagues have found that more than 95% of menhaden lesions contain Aphanomyces invadans. |
| Postulate 2: The organism must be isolated from the diseased host and grown in pure culture. |
| VIMS researchers and colleagues were able to isolate Aphanomyces invadans from wild menhaden, and use these isolates to grow pure cultures of the mold on GP agar plates in the laboratory. |
| Postulate 3: The disease must be reproduced when a pure culture of the organism is inoculated into a healthy susceptible host. |
| Healthy menhaden inoculated with zoospores of A. invadans developed skin lesions identical to those afflicting wild menhaden. More importantly, healthy menhaden that were exposed in baths of water containing A. invadans zoospores also developed lesions and did so in the absence of Pfiesteria. |
| Postulate 4: The agent must be re-isolated from the experimentally infected animal. |
| VIMS researchers and colleagues were able to re-isolate A. invadans from several menhaden involved in the infection trials described under Postulate 3. |
Differences in the type of skin damage caused by A. invadans and Pfiesteria provide further proof that Aphanomyces is the cause of menhaden lesions. Tilapia and menhaden exposed to Pfiesteria shumwayae rapidly develop rash-like irritations of the skin surface that are consistent with localized loss of the epidermis (images to right show irritation of tilapia epidermis by Pfiesteria). Fish exposed to high densities of Pfiesteria eventually have much of their epidermis destroyed, which leads to osmotic shock and death (Noga et al. 1996, Vogelbein et al. 2000). However, they do not develop large ulcerous lesions. Wild menhaden exposed to Aphanomyces exhibit deeply penetrating, chronic lesions that penetrate the skin and erode underlying muscle. Interestingly, tilapia are resistant to Aphanomyces infections.