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Understanding Central Nervous System Signs and Their Causes

Good observers in pig barns will often see pigs “not acting right.” This may involve individual animals or often will appear as outbreaks that involve many pigs. Although good clinical assessment is invaluable for establishing a proper diagnosis, it is often a challenge to sort out and effectively describe the subtle behavioral changes or clinical observations associated with central nervous system (CNS) signs.

Often, the observer lumps the clinical signs into terms that indicate something is wrong, but these terms may not be accurate or descriptive for what is actually wrong. Examples of terms that can mean many different things to different people include: “strep pigs,” “CNS pigs” or “downer pigs.”

To better establish an accurate diagnosis, more descriptive terms are helpful. Some examples of useful terms include: blind, uncontrolled eye movements, aware of surroundings, paddling movements, coma (unaware), tremors, seizures, incoordination, weakness, squealing or some evidence of pain. Being very specific in describing abnormal activity or behavior can help to narrow the long list of possible causes for these types of conditions (Table 1).

Reporting the number of pigs affected and number at risk (morbidity), as well as the number presumed dead from the condition (mortality), is also quite useful to starting to narrow down the possibilities.

Body temperature is useful to implicate infectious causes where fever is expected. Sometimes pigs are found dead with a few others showing signs of central nervous system disturbances. Differentiating the various maladies that can cause sudden deaths or central nervous system disturbances (i.e. affected brain or spinal cord) is very important for effective treatment or prevention (Table 2).

Usually, laboratory testing will be required to confirm a diagnosis. If whole animals cannot be delivered to the laboratory, then representative samples and tissues from typical cases need to be properly selected, collected, preserved and transported (Table 3).

Some of the recent noninfectious causes of CNS disease in grow-finish pigs include the following:

Example 1: Nine of 1,200 pigs that are 5 months of age are found dead and 12 are “dog-sitting” (Figure 1) with posterior paralysis, but otherwise are aware of their surroundings. No other pigs have clinical signs. Careful examination revealed broken vertebrae in the lower spine of affected pigs. A transient electrical surge (e.g. lightning or direct short) caused lethal electrocution and sublethal vertebral fractures.

Example 2: Approximately 3% of 8-week-old pigs develop ataxia (loss of coordination of the muscles, especially the extremities) posterior paresis (partial motor paralysis) in three successive groups. No gross lesions are detected, but microscopic and laboratory testing of the spinal cord reveals teschovirus (polio-like) disease.

Example 3: Approximately 5% of 1,200 pigs are wandering, dog-sitting or have seizures. No lesions are detected by necropsy, but microscopic examination of the brain reveals eosinophilic meningitis, the hallmark of sodium intoxication. Even though no one admitted that pigs were without water for two days previous to the abrupt onset of signs, the diagnosis was unequivocal when proper samples were taken and tested.

To determine the cause of CNS signs in pigs requires:

•Practicing diligent observation of pigs in their environment to identify disease.

•Providing useful descriptions of clinical signs, number affected, number at risk and mortality, to help identify the most likely malady that is present.

•Conducting careful selection and preservation of samples will usually allow rapid laboratory analysis to rule in or out a wide array of infectious and noninfectious diseases.

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Kent Schwartz, DVM
Iowa State University Veterinary Diagnostic Laboratory
[email protected]