Description and Distribution
Roundworms include many parasitic and free-living forms. Parasitic roundworms (nematodes) are some of the most common worms found in wildlife species worldwide. Because many are usually large in size and tend to occur in large numbers, they are highly visible and are easily seen by hunters when cleaning game.
Nematodes generally exhibit long, unsegmented, cylindrical bodies with tapered, rounded or pointed ends. The worms have striations on the cuticle that may or may not be visible to the naked eye. The cuticle is capable of absorbing some materials but most food is taken in through the primitive alimentary tract. Nematodes have separate female and male sexes, with females usually being larger.
Table 1 is a listing of the common parasitic roundworms found in Michigan's wildlife.
Transmission and Development
Nematode parasites may have simple or complex life cycles and the explanation of those goes beyond the scope of this report. Most of the roundworms that hunters have questions about have a direct life cycle whereby the adult worm in the host passes eggs out with the fecal material. These eggs then contaminate vegetation where they may either embryonate or the larvae hatch, go through a series of developmental stages, reach the infective third-stage and are accidentally ingested by a susceptible host. The larvae mature to the adult stage in the host, usually in the region in which they will reside for the rest of their lives. If the infective stage larvae are accidentally ingested by an abnormal host, development is slowed and random migration may occur, causing tissue damage and inflammation. Abnormal hosts often die from the effects of these parasites. Examples of abnormal host-parasite relationships are the deer meningeal worm found in elk, and the raccoon ascarid that occasionally occurs in woodchucks, mice and squirrels. Both of these examples cause neurological problems resulting in abnormal behavior.
Clinical and Pathological Signs
Usually there are no clinical signs seen with roundworm infections unless the host is weakened by other stress conditions, such as winter, high density, human encroachment, immature animal or physical illness. Usually the gastrointestinal nematodes do not cause any harm to the host, but they may cause tissue proliferation and hemorrhages in young or severely infected individuals.
Lungworms may cause tissue damage and pneumonia, especially in severe weather conditions, high host densities, and in individuals in poor physical condition.
Parasitic nematode infections can be diagnosed by accurate identification of the egg, larval and/or adult stages. Egg and larval stages can be identified either by a direct smear of the feces (most of the parasites void their eggs or larvae this way) or by sugar or salt flotation of the fecal material.
Adult worms can be identified by various descriptive structures and anatomical features. Formalin preserved specimens may be cleared in various solutions to aid in their identification.
Treatment and Control
In wildlife species it is not feasible to try to treat individuals for nematode infections. If a wild animal is brought into captivity, it is possible to treat them with various drugs used in veterinary medicine to purge them of parasites. Usually only younger, older or weakened individuals have significant infections that would warrant any treatment.
Controlling wildlife population levels through proper management techniques (trapping and hunting) should prevent parasitic nematodes from attaining harmful levels.
Generally, parasitic nematodes do not cause any significant problems in their normal hosts. If large numbers of parasites are present or the host's health has been compromised by some extraneous factors, then nematode infections can be detrimental and significant. The general rules for nematode infections are: normal host - not many problems; abnormal host - severe problems if infection occurs; paratenic host (transport host) - normally no problems.
Usually the parasites are seen by hunters following the worms' random movements after the host has been killed. This explains why intestinal parasites are found in the body cavities or in subcutaneous spaces, locations where they normally would not be.
Generally, there is no human health significance seen with parasitic nematodes of wildlife species. Some of the ascarid species and the Trichinella spiralis worm may cause human health problems but these are rare in occurrence.
For questions about wildlife diseases, please contact the Michigan DNR Wildlife Disease Laboratory.