Microscopical Evaluation of Smears of the Leptomeninges to Predict Meningitis in Piglets

SIMPLE SUMMARY: Meningitis (inflammation of the membranes covering the brain) is a common problem in 3- to 10-week-old piglets. It is often caused by bacteria that are called Streptococcus. Clinical signs such as problems with coordination of the limbs can be observed by the farmer, but it is also possible that piglets die suddenly, before clinical signs are detected. Therefore, examining dead piglets is often performed to establish a diagnosis. Unfortunately, meningitis is often difficult to recognise by the eye. To assist veterinarians in forming a quick diagnosis of meningitis and possible bacterial involvement, a microscopic evaluation of cells taken from the meninges was performed. This examination allowed the prediction of the likely presence of meningitis and the involvement of Streptococcus species in most of the cases. Microscopic findings were compared with bacteriological culture results, and based on this comparison, indications of meningitis and Streptococcal presence seemed correct in 89% and 100% of the cases, respectively. However, if only a few cells indicating meningitis were observed microscopically, no reliable prediction was possible. In conclusion, microscopic evaluation of cells from the meninges of piglets can help swine practitioners in establishing a tentative diagnosis of meningitis more quickly, which allows for better treatment, improves animal welfare, and reduces antimicrobial use. ABSTRACT: Meningitis, caused by bacterial infections such as Streptococcus spp., is a frequently observed cause of death in pigs. In order to implement effective treatment and avoid further losses, it is important to establish this diagnosis quickly. However, this is often difficult because macroscopic lesions may not be visible, and additional laboratory testing may take several days. The present study investigated whether microscopical examination of impression smears of the leptomeninges taken during necropsy can help in establishing a presumptive diagnosis of meningitis in pigs more quickly. In total, 54 pigs suffering from neurological signs and/or (acute) mortality were examined. They were 3 to 10 weeks old and originated from 16 farms. From each pig, bacterial culture was performed on one half of the brain using a swab from the leptomeninges. From the other half, paired cytological impression smears of the leptomeninges were stained with a commercial quick stain dye (Hemacolor(®)) and Gram stain and microscopically evaluated for the abundance of neutrophils and the presence of short-chain coccoid bacteria. Bacterial culture of the leptomeninges was positive in 36/54 cases, in 28 of which Streptococcus spp. were found. The numbers of smears with low, moderate, or high abundance of neutrophils were 19, 17, and 18, respectively. Short-chain coccoid bacteria were detected successfully in 12 pigs in the Gram-stained smear. The positive predictive value of smears with moderate or high abundance of neutrophils for bacterial presence and, therefore, likely meningitis was 89%, whereas the negative predictive value of smears with low abundance of neutrophils was 74%. The positive predictive value of smears with short chains of coccoid bacteria for diagnosis of Streptococcus spp. was 100%, whereas the negative predictive value was 62%. In conclusion, microscopical examination of impression smears of the leptomeninges of piglets with neurological signs and/or (acute) mortality is a feasible procedure that can help swine practitioners in establishing a tentative diagnosis of meningitis more quickly, especially if neutrophils are abundant, and short chains of coccoid bacteria are present.