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Ulcerative Mycosis in Florida

Perspectives on Estuarine Fish Health - Lesions in Florida esturine fish are often caused by a fungal infection.

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In Florida, lesions in estuarine fish have been observed since the late 1970’s.  Unlike the rest of the eastern seaboard of the United States where menhaden are primarily affected, multiple species of estuarine and freshwater fish with ulcerative mycosis (UM) have been reported (Grier and Quintero 1987; Te Strake and Lim 1987; McGarey et al. 1990; FWC unpublished data).  In these prior studies, UM-type lesions appeared to be a mixed syndrome with a number of potential bacterial and fungal pathogens implicated.  The appearance of large numbers of fish with lesions in the spring of 1998 refocused scientific investigations into potential causative factors.

 

Since the outbreak in 1998, research conducted by the Florida Fish and Wildlife Conservation Commission (FWC) has implicated the water mold Aphanomyces invadans as a significant cause of lesions in Florida estuarine and freshwater fish.  Aphanomyces invadans has been found to be the causative agent of lesioned estuarine fish along the eastern seaboard of the United States and in Southeast Asia, Japan, and Australia.  Infections by this organism in other geographic areas have been termed “ulcerative mycosis,” “epizootic ulcerative syndrome,” “mycotic granulomatosis,” and “red spot disease.”

 

Aphanomyces invadans has a life cycle that consists of three different stages: hyphae, zoospore, and cyst (Figures 1 and 2). Changes in the aquatic environment (such as a change in salinity or water temperature) cause Aphanomyces invadans to undergo asexual reproduction. During sporulation (asexual reproduction) thousands of swimming zoospores are released into the water. These zoospores then search for a fish host. The zoospores infect the skin of the fish and cause the development of bloody skin ulcers.  If a host is not found, the zoospores become cysts and sink to the mud or sediment.

Isolates of Aphanomyces invadans from the Caloosahatchee and St. Johns rivers, and Hydrilla Lake were used to experimentally infect fish with the fungus in the laboratory.  The experiment successfully reproduced the same type of lesions that were found in wild fish.

Figure 1.  Life cycle of Aphanomyces invadans. (Lilley et al., 1998)

Photos: FWC Fish and Wildlife Research Institute
Figure 2.  Life cycle of Aphanomyces invadans showing hyphae, sporulation, and zoospore stages.

The definition of a lesion
A lesion (or ulcer) is defined as the localized shedding of necrotic skin tissue, with exposure of the underlying dermal or muscle tissue.  Lesions may have rough or raised irregular margins and will often appear bloody or necrotic (rotten) (Figure 3).

Photo: FWC Fish and Wildlife Research Institute
Figure 3.  Mullet with severe lesions.  Note irregular margins of lesion and exposed muscle tissue. 

Distributions of lesioned fish

Ulcerated estuarine fish have been collected in coastal areas throughout Florida.  Scientists at FWC’s Fish and Wildlife Research Institute (FWRI) were able to successfully identify Aphanomyces invadans from lesioned fish from the St. Lucie estuary, the Caloosahatchee River, Lake Teneroc (Hydrilla Lake), the Orange River, the Tomoka River, Tampa Bay, Cedar Key, and the Choctawhatchee River (Figure 4).  Ulcerated estuarine fish with fungal infections have been frequently collected in the St. Lucie Estuary, Caloosahatchee River, and the St. Johns River from areas of low salinity. However, Aphanomyces has been found statewide.

Figure 4.  Map of Florida showing the distribution of lesioned fish from 1980 to 2002.

Routine monitoring of the health of fish in Florida estuaries is conducted monthly by a partnership between FWRI’s Fisheries Independent Monitoring (FIM) group and Fish and Wildlife Health group.  Fish with gross external abnormalities, including lesions, fin rot, tumors, and skeletal abnormalities are culled by FIM during monthly random fisheries assessment sampling trips in Apalachicola Bay, Cedar Key, Tampa Bay, Charlotte Harbor, South Indian River Lagoon, North Indian River Lagoon, and the St. Johns River.  These culled fish are sent to FWRI headquarters in St. Petersburg for diagnosis.  Data from this random sampling show that out of the thousands of fish examined in the field, less than 0.1% of the fish in Florida estuaries suffer from Aphanomyces invadans infections (Figure 5), and less than 1% experience gross external abnormalities (Figure 6).

 

Figure 5.  Prevalence of fish infected with Aphanomyces invadans in the major estuaries of Florida.  Data points represent the number of fish positive for the fungus expressed as a percentage of the total fish sampled for each region.

 

Figure 6.  Prevalence of fish exhibiting gross external abnormalities in the major estuaries of Florida.  Data points represent the number of fish with abnormalities expressed as a percentage of the total fish sampled for each region. 

 

A variety of species are affected

The most commonly affected estuarine fish are striped mullet (Mugil cephalus), silver mullet (Mugil gyrans), and sheepshead (Archosargus probatocephalus) (Figure 7).  Other affected species have included black drum (Pogonias chromis), pinfish (Lagodon rhomboides), American shad (Alosa sapidissima), pigfish (Orthopristus chrysoptera), red drum (Sciaenops ocellatus), Atlantic croaker (Micropogonias undulatus), and gray snapper (Lutjanus griseus).

 

Aphanomyces invadans is a fish pathogen and there have been no reports of humans experiencing infections by this fungus.

 

Photos: FWC Fish and Wildlife Research Institute                              

Figure 7.  Examples of species affected by fungal lesions.  Left: Striped mullet (Mugil cephalus).  Right: American shad (Alosa sapidissimus), blue circles are drawn around the lesions.

 

Stressed fish are more likely to become infected with disease

Fish are very sensitive to changes in the aquatic environment.  Rapid changes in salinity, temperature, and/or water quality can cause fish to become stressed, leaving them open to infection.  Changes in salinity in estuaries can occur from natural occurrences such as local watershed basin run-off or run-off from severe storms and hurricanes, or from artificial manipulations of water levels in bodies of water.  In Florida, the St. Lucie Estuary and the Caloosahatchee River are connected to Lake Okeechobee by a canal system. When the level of Lake Okeechobee becomes too high, freshwater is released into the St. Lucie Estuary and the Caloosahatchee River.  This massive amount of freshwater influx (up to millions of gallons/day) can dramatically lower the natural salinity of the estuary. These rapid changes in the aquatic environment may stress the fish and increase their chances of infection by various diseases.  Along with stressing the fish, this influx of freshwater increases the distribution of suitable environments for Aphanomyces invadans, allowing it to encounter and infect estuarine fish hosts.  An increase in the prevalence of lesioned fish has been correlated with flow rates through the S-80 lock on the St. Lucie Canal (Figure 8).

 

Figure 8.  Water flow rates (cfs = cubic feet per second) through the S-80 lock on the St. Lucie Canal (pink) and the number of lesioned fish observed (blue) in the St. Lucie estuary from January 1999 to May 2000.  The total number of fish sampled during this period was approximately 30,000.

 

Mortality and fungal infections

Fish don’t necessarily die from infections by Aphanomyces invadans.  Scientists have observed lesions in various stages of the healing process in different species of fish.   These fish are frequently found in higher salinity water that may have helped, along with the fish’s immune system, to eliminate the fungus.  However, it is very difficult to obtain data on fish mortalities: weakened, dying, or dead fish may be eaten by predators or be found when decomposition interferes with determining the cause of death.  Secondary bacterial infections in the open lesion may increase the likelihood of mortality.

 

How can I help?

If you see or catch any ulcerated fish, please call the Fish Kill Hotline (1-800-636-0511) or fill out an online report at http://research.myfwc.com/fishkill/submit.asp . Report what type of fish had a lesion, how many fish have lesions, the location where you caught or saw the lesioned fish, and how to contact you for additional information.

 

Frozen samples can be used to identify this fungus using genetic techniques.  We will accept frozen lesioned fish in good condition that have the following information: location (GPS coordinates preferred) and date of capture, species, and standard length, total length, or fork length.  Contact the Fish Kill Hotline (above) for more information on providing samples.

 

Selected References

 

Blazer, V. S., J. H. Lilley, W. B. Schill, Y. Kiryu, C. L. Densmore, V. Panyawachira, and S. Chinabaut. 2002. Aphanomyces invadans in Atlantic menhaden along the east coast of the United States. Journal of Aquatic Animal Health 14:1-10.

 

Blazer, V. S., W. K. Vogelbein, C. L. Densmore, E. B. May, J. H. Lilley, and D. E. Zwerner. 1999. Aphanomyces as a cause of ulcerative skin lesions of menhaden from Chesapeake Bay tributaries. Journal of Aquatic Animal Health 11:340-349.

 

Dykstra, M. J., J. Levine, E. J. Noga, J. H. Hawkins, P. Gerdes, W. J. Hargis, H. J. Grier, and D. Te Strake. 1989. Ulcerative mycosis: a serious menhaden disease of the southeastern coastal fisheries of the United States. Journal of Fish Diseases 12:175-178.

 

Fraser, G. C., R. B. Callinan, and L. M. Calder. 1992. Aphanomyces species associated with red spot disease: an ulcerative disease of estuarine fish from eastern Australia. Journal of Fish Diseases 15:173-181.

 

Grier, H.G., and I. Quintero. 1987. A microscopic study of ulcerated fish in Florida. Florida Bureau of Marine Research WM-164, St. Petersburg, FL.

 

Hawke, J. P., A. M. Grooters, and A. C. Camus. 2003.  Ulcerative mycosis caused by Aphanomyces invadans in channel catfish, black bullhead, and bluegill from southeastern Louisiana.  Journal of Aquatic Animal Health 15:120-127.  

 

Johnson, R. A., J. Zabrecky, Y. Kiryu, and J. D. Shields.  2004.  Infection experiments with Aphanomyces invadans in four estuarine fish species.  Journal of Fish Diseases 27:1-9.

 

Kiryu, Y., J. D. Shields, W. K. Vogelbein, D. E. Zwerner, and H. Kator. 2002. Induction of skin ulcers in Atlantic menhaden by injection and aqueous exposure to Aphanomyces invadans. Journal of Aquatic Animal Health 14:11-24.

 

Kiryu, Y., J. D. Shields, W. K. Vogelbein, H. Kator, and V. S. Blazer.  2003. Infectivity and pathogenicity of the oomycete Aphanomyces invadans in Atlantic menhaden Brevoortia tyrannus.  Diseases of Aquatic Organisms 54:135-146.

 

Kiryu, Y., J. D. Shields, W. K. Vogelbein, H. Kator, and V. S. Blazer.  2005.  Factors influencing the sporulation and cyst formation of  Aphanomyces invadans, etiological agent of ulcerative mycosis in Atlantic menhaden, Brevoortia tyrannus.  Mycologia 97:569-575. 

 

Lilley, J. H., R. B. Callinan, S. Chinabut, S. Kanchanakhan, I. H. Macrae, and M. J. Phillips. 1998. Epizootic Ulcerative Syndrome (EUS) Technical Handbook.  The Aquatic Health Research Institute, Bangkok, Thailand.

 

Lio-Po, G. D., G. S. Traxler, L. J. Albright, and E. M. Leańo. 2000. Characterization of a virus obtained from snakeheads Ophicephalus striatus with epizootic ulcerative syndrome (EUS) in the Philippines. Diseases of Aquatic Organisms 43:191-198.

 

McGarey, D. J., T. Kraxberger Beatty, T., V. A. Alberts, D. Te Strake, and D. V. Lim.  1990. Investigations of potential microbial pathogens associated with ulcerative disease syndrome (UDS) of Florida fish. Pages 65-75 in T. O. Perkins and T. C. Cheng, editors. Pathology in marine science. Academic Press, San Diego.

 

Noga, E. J., J. F. Levine, M. J. Dykstra, and J. H. Hawkins. 1988. Pathology of ulcerative mycosis in Atlantic menhaden Brevoortia tyrannus. Diseases of Aquatic Organisms 4:189-197.

 

Te Strake, D. and D. V. Lim. 1987. Bacterial and fungal studies of ulcerative fish in the St. Johns River. Florida Department of Environmental Regulation Contract WM 138.