Figure 24. — A Louisiana muskrat heavily 

 infested with stomach worms. 



for such radical decline in the popu- 

 lation are disease and prolonged in- 

 tense drought. 



Drought brings about catastroph- 

 ic changes for the muskrat. Thou- 

 sands of small ponds, lakes, and 

 pools dry up, and food becomes 

 scarce. As water levels sink, the 

 muskrats are forced from their 

 homes and many die while search- 

 ing for more favorable conditions. 

 The supply of fresh water is re- 

 duced, and in the coastal marshes 

 salinity increases as the runoff from 

 the watershed fails. The normally 

 fresh or slightly saline water is re- 

 placed by undesirable salt water 

 from the sea. 



Disease, too, takes its toll, often 

 coupled with poor water conditions. 

 Despite its reputation as a generally 

 healthy animal, the muskrat is sub- 

 ject to numerous maladies, includ- 

 ing lobar pneumonia, septicemia, 

 enteritis, streptococcus infections, 

 tumors, coccidiosis, tularemia, leu- 

 kemia, lumpy jaw, and a skin ail- 

 ment of fungous origin. In addi- 

 tion to these diseases, the species is 

 host to some 65 parasites, mostly 

 nematodes (fig. 24), trematodes, 

 and cestodes. 



"Lousy" muskrats are often re- 

 ported by trappers, but examina- 



tion usually reveals the presence of 

 small mites instead of lice. Heavy 

 infestations during the breeding 

 season may result in the loss of 

 many new-born litters. Muskrats 

 with inflamed eyes are frequently 

 encountered, and this condition also 

 is caused by mites. If such animals 

 are held for a few days in a pen or 

 box, supplied with fresh water, and 

 dusted with an insecticide, such as 

 sulfur or rotenone, the condition 

 often clears. 



Coccidiosis is common in penned 

 muskrats. This disease may be rec- 

 ognized by the reddened and in- 

 flamed appearance of the intestines 

 and the presence of oocysts. The 

 oocysts may be seen in the diseased 

 tissue with the aid of a microscope. 

 Coccidiosis thrives in the pollution 

 that occurs on partly dried ponds 

 and still-water marshes during 

 droughts and it can greatly reduce 

 a muskrat population in a few 

 weeks. Where water levels can be 

 controlled and fresh water is avail- 

 able, it is never a serious factor. 



In some years epizootics, or out- 

 breaks of disease over widespread 

 areas, appear usually just before the 

 opening of the trapping season. 

 Thousands of muskrats die in a few 

 weeks, and trappers working over 

 such marsh areas may find numbers 

 of dead animals with no sign of in- 

 jury but in emaciated condition. 

 The majority of sick muskrats, 

 however, die unobserved in their 

 underground runs. Old burrows 

 and houses may serve as focal 

 points, or centers of infection, and 

 some marshes remain death traps 

 with recurrent epizootics over a pe- 

 riod of several years. Some of 



18 



