from infected fish revealed heavy white 

blood cell infiltration indicating an immune 

response. However, control and infected 

fish exhibited similar stress responses to 

the cold-shock experiment.

Previously, life cycle work with the Asian 

fish tapeworm demonstrated that for a 

fish to become infected it must eat an 

infected cyclopoid copepod (zooplankton 

intermediate host). However, studies at the 

NWHC indicate that fish that prey upon 

small infected fish can acquire infections 

as well. Thus, larger humpback chub that 

normally have few zooplankton in their 

diet can become infected by preying upon 

smaller infected fish. This evidence of the 

potential for post-cyclic transmission poses 

new questions of how infections with this 

parasite may affect older and larger fish.

These results demonstrate the importance 

of considering the role of Asian fish 

tapeworm infections when making 

management decisions focused on conservation of humpback chub. Furthermore, 

these data may be useful in developing 

ecological models concerning this fish and 

its environment. The results from this study 

provide insight into the potential implications of this parasite should it invade other 

ecosystems and fish species. 

U.S. Department of the Interior

U.S. Geological Survey

FS 2005–3057

August 2004

Endangered Fish Threatened 

by Asian Fish Tapeworm

The Asian fish tapeworm, an exotic 

parasite, has invaded the endangered 

humpback chub (Gila cypha) population 

from the Colorado and Little Colorado 

Rivers in Grand Canyon, Arizona. This 

parasite causes disease and death in carp 

in aquaculture settings and may retard 

growth in hatchery-reared roundtail chub 

(Gila robusta). Other consequences include 

destruction and dysfunction of the intestinal lining and adverse changes to certain 

blood parameters. Introduced into the U.S. 

in the 1970s with imported grass carp 

(Ctenopharyngodon idella), the Asian fish 

tapeworm (Bothriocephalus acheilognathi) 

was discovered in the Little Colorado River 

(LCR) by 1990. The LCR is the main 

tributary to the Colorado River in Grand 

Canyon and is an important spawning area 

for humpback chub. 

Research by the USGS National Wildlife 

Health Center (NWHC) investigated the 

extent that the humpback chub population 

within the LCR is infected with this 

tapeworm. Considering the negative 

growth and physiological effects associated 

with this parasite, scientists at the NWHC 

conducted experimental infection trials 

to investigate its effects on native chub. 

Small chub face thermal stress when they 

are washed from the LCR (23.5 °C) to the 

cold waters of the Colorado River (9.5 °C) 

during monsoon floods. Experiments also 

examined whether tapeworm infections 

exacerbate these cold-shock effects. 

Seasonal field parasite surveys of fish in 

the LCR during 1999 and 2000 revealed 

that of the 1,435 native and non-native fish 

examined, all species could be infected 

with the Asian fish tapeworm. However, 

humpback chub—while constituting only 8 

percent of the total fish sampled—harbored 

over 54 percent of the tapeworms recovered, indicating that humpback chub could 

maintain this parasite in the LCR without 

the presence of carp or other non-native 

fish species.

Experimental work initiated in 2001 

examined the effect of tapeworm infections 

on growth and physiological condition 

in bonytail chub (Gila elegans), closely 

related to humpback chub. Stressors 

such as cold shock and reduced food 

ration were introduced to examine their 

possible synergistic effects with tapeworm 

infections. Older juvenile fish (10 months 

old and 83 mm long) examined over 13 

weeks post-infection revealed no negative impact on growth or physiological 

condition in infected fish when compared 

to control fish. Subsequent experiments 

with younger and smaller fish (2.5 months 

old and 22 mm long) demonstrated 

infected fish length was reduced by as 

much as 9 percent on average compared 

to control fish by the end of the 24-week 

study. Additionally, when food ration was 

restricted, infected fish began dying 20 

days earlier and at nearly twice the rate 

of controls throughout the course of the 

trial. Infected fish were found to have 

significantly lower hematocrit (red blood 

cell volume) levels and a reduced amount 

of visceral fat when compared with control 

fish. Histological examination of gut tissue 

 Infected bonytail chub with distended abdomen 

(above) compared with uninfected chub (below)

The Asian fish tapeworm may inhibit the recovery of the humpback chub in the Grand Canyon.

Intermediate host copepod (Acanthocyclops robustus) infected with three larval tapeworms

Adult humpback chub

Intestine of bonytail chub infected with tapeworms

For more information on parasites in fish in the 

Grand Canyon, please contact: Rebecca A. Cole, 

USGS National Wildlife Health Center, 6006 

Schroeder Rd., Madison, WI 53711-6223, ph. (608) 

270-2468.