Listeriosis

NWDC is in the process of updating this fact sheet. 

Other names: Circling disease

Cause

Listeriosis is a disease of domestic and wild animals and occasionally humans, caused by bacteria from the Listeria group. Listeria monocytogenes is the species typically responsible for causing clinical illness. Listeriosis was first described in laboratory animals in 1926 and was first reported in wildlife in 1927 in South Africa. 

Significance

People occasionally contract listeriosis following the consumption of contaminated food or raw milk. The elderly, neonates, pregnant women, and those with compromised immune systems are more susceptible to this disease and it can be fatal to these groups. In 2011, a fatal outbreak of listeria caused illness in 146 people and the deaths of 30 people across 28 states in the United States. The outbreak was caused by the distribution and consumption of contaminated fruit. Contact with animals does not usually transmit Listeria to humans. 

Species Affected

L. monocytogenes has been isolated in many species of terrestrial and aquatic mammals, birds, reptiles, amphibians, and fish. Clinical illness has been observed in wild ruminants including white-tailed deer, moose, roe deer, fallow deer, giraffes, llamas, and alpacas. The disease has also been reported in wild carnivores, hares, rabbits, waterfowl, and primates. Domestic animals are susceptible to listeriosis, as are humans. 

Distribution

L. monocytogenes lives worldwide in soil and contaminated vegetation, but disease is more frequent in temperate and colder climates. Listeriosis occurs sporadically in wild populations and is usually associated with consumption of contaminated domestic animal feed stored outside. 

Transmission

Animals acquire listeriosis following the consumption of material from a contaminated environment. The bacteria can be found in soil, sewage, stream water, dust, vegetation, and silage. The bacteria may be introduced to the environment through the feces of an infected animal. New infections occur when animals ingest the bacteria in contaminated feed or water and it then either enters cuts or abrasions in the oral cavity or directly crosses the intestinal lining. Insects may be able to transmit Listeria, but this is likely not a significant cause of disease transmission. Humans may become infected by consuming food contaminated with the bacteria. 

Clinical Signs

Clinical listeriosis can have three different presentations: uterine infection, encephalitis (inflammation of the brain), and septicemia (infection of multiple organ systems). Uterine infection usually results in abortion with no other obvious clinical signs. Animals with encephalitis may exhibit depression, loss of coordination, and paralysis. These animals may also have a one-sided ear droop, walk in circles in one direction, and have inflammation of the eye. Animals with the septicemic form of listeriosis rarely show clinical signs and are often found dead.

Diagnosis

Listeriosis is diagnosed by isolating L. monocytogenes in samples of aborted placenta or fetus, brain tissue, and occasionally, other tissues or bodily fluids of infected animals. 

Treatment

Certain antibiotics are successful in treating listeriosis, but treatment is usually not attempted in wildlife. 

Management

It would be unrealistic to attempt to control L. monocytogenes in the environment. However, feeding silage to captive and free-ranging ruminants always presents the risk of causing listeriosis. Spoiled silage should not be fed to domestic animals and should not be discarded for wildlife. To avoid human infection, people are advised to cook meat and eggs thoroughly, rinse raw vegetables before eating, wash hands, surfaces, and utensils used for food preparation, and not to consume unpasteurized milk or cheeses.

Suggested Reading