In previous cases of listeriosis in cattle, veterinarians submitting samples to the Texas A&M Veterinary Medical Diagnostic Laboratory have reported profound neurological signs that have caused them to include rabies high on their list of differential diagnoses. The primary organism responsible is Listeria monocytogenes, an organism that is common in the environment both as free-living bacteria as well as contaminants shed by infected animals. The organism can persist for extended intervals in a wide range of temperatures. In addition to cattle, neurological signs of infection can be observed in other ruminant species such as sheep and goats. The organism can also cause food borne illness in humans through contamination of milk.
The disease is most commonly associated with poorly preserved silage with a pH greater than 4.0. Visible spoilage may or may not be present in the silage. Since soil can also harbor the organism, feeding silage is not a necessary factor in whether or not cattle are exposed to the organism.
Treatment of clinical cases involves the use of antibiotics, but it is very difficult to administer treatment before severe damage has been caused to the brain of the animal. Other causes of neurological disease should be considered as differentials for diagnosis including rabies, polioencephalomalacia, and thromboembolic meningoencephalitis. Tests available include culture or PCR testing of non-fixed brain tissue or cerebrospinal fluid as well as formalin fixed brain tissue for histopathological examination. Since rabies testing is also frequently requested in these cases, please inform the TVMDL if a rabies test is being performed at the Department of State Health Services laboratory concurrently with the TVMDL testing.
For more information on bovine testing at TVMDL, visit tvmdl.tamu.edu or call one of the agency’s full service laboratories in College Station or Canyon.
REFERENCES
Tyler, Jeffrey W., Blackwell’s Five-Minute Veterinary Consult: Ruminant; Scott R.R. Haskell, DVM, editor. 2008 pgs. 480-481.