Pathology in Practice

Devin von Stade Department of Microbiology, Immunology and Pathology, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO 80523.

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Brittany J. McHale Departments of Pathology, College of Veterinary Medicine, University of Georgia, Athens, GA 30602.

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Richard Gerhold Department of Biomedical and Diagnostic Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996.

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Lisa H. Williamson Large Animal Medicine, College of Veterinary Medicine, University of Georgia, Athens, GA 30602.

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Elizabeth W. Howerth Departments of Pathology, College of Veterinary Medicine, University of Georgia, Athens, GA 30602.

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History

A 4-year-old 150.5-kg (331.1-lb) castrated male llama (Lama glama) was evaluated because of a 3-year history of an intermittent gait abnormality. The lameness became apparent during periods of prolonged exercise, such as during participation in a parade or during a packing event on the trail. Treatment with meloxicam (1 mg/kg [0.45 mg/lb], PO, q 72 h) yielded no improvement. The llama was vaccinated annually with a rabies vaccine and a Clostridium perfringens types C and D–tetanus toxoid vaccine. Anthelmintics were administered periodically on the basis of clinical signs, such as conjunctival pallor, body condition, and loose feces. The animal was housed with other llamas on pasture, with free-choice access to pasture grass and hay. Camelid-specific minerals were provided on a free-choice basis, and a small amount of feed concentrate was provided daily.

Clinical and Gross Findings

The owners reported that the llama's locomotor problems had markedly worsened during the preceding 2 weeks and that it was now having difficulty rising from a cushed position. The attitude and appetite of the llama remained normal. An examination revealed moderate to severe (grade 3 to 4/5) proprioceptive deficits in both hind limbs. The forelimbs appeared unaffected, and cranial nerve deficits were not observed. Further diagnostic testing (analysis of a CSF sample or MRI of the vertebral column) was declined by the owners in favor of conservative management of the llama at the farm. The llama was treated presumptively with ivermectin (0.4 mg/kg [0.18 mg/lb], SC), meloxicam (1 mg/kg [0.45 mg/lb], PO, q 72 h for 4 treatments), and fenbendazole (20 mg/kg [9.1 mg/lb], PO, q 24 h for 5 consecutive days). Subsequently, anthelmintic treatments were administered on the basis of clinical signs, such as color of the ocular mucous membranes, body condition, and fecal consistency. The llama was kept in isolation and on level ground and was given assistance to rise from recumbency. Euthanasia was performed by rapid IV injection of barbiturate euthanasia solution a year later because of progression of clinical signs.

At the postmortem examination, all 4 hooves of the llama were overgrown beyond the solar-laminar junction and adipose stores were abundant. The spinal cord was removed and fixed in neutral-buffered 10% formalin. On cut section of the spinal cord at the level of C3, there was a firm, white to tan, 0.5-cm-diameter focal area that bulged slightly within the right lateral funiculus (Figure 1).

Figure 1
Figure 1

Photograph of a transverse section of the cervical spinal cord (at the level of C3) of a 4-year-old llama (Llama glama) that had a 3-year history of intermittent abnormal gait in the right hind limb. Within the right lateral funiculus, notice a firm, white to tan, 0.5-cm-diameter nodule that bulges slightly on cut section.

Citation: Journal of the American Veterinary Medical Association 258, 10; 10.2460/javma.258.10.1083

Histopathologic Findings

Representative tissue samples were collected, fixed in neutral-buffered 10% formalin, and routinely processed for histologic examination. Within the lateral funiculus of the spinal cord at the level of C3-C4, there was a circumscribed granuloma (Figure 2) containing a cross section of a degenerated subadult nematode. Surrounding the nematode was a layer of hypereosinophilic cellular debris, cholesterol clefts, and basophilic granular material (dystrophic mineral), all of which was further surrounded by a layer of foamy epithelioid macrophages and a thin capsule of fibrous connective tissue. The nematode was 120 μm in diameter with a 4- to 5-μm-thick eosinophilic cuticle and polymyarian-coelomyarian musculature (Figure 3). The white matter adjacent to the granuloma was rarified with axons that were swollen, rounded, and eosinophilic (spheroids); gitter cells; and reactive astrocytes. The adjacent gray matter of the ventral horn had scattered degenerated neurons, each of which had a peripheral nucleus and marginated Nissl substance (central chromatolysis) and was swollen with abundant pale eosinophilic cytoplasm. Capillaries were diffusely lined by hypertrophic endothelial cells. Within the ventral funiculus of the spinal cord at the level of C5, there was a focal area of rarefaction. The adjacent neurons were swollen with abundant pale eosinophilic cytoplasm. No changes were noted in the caudal cervical, thoracic, and lumbar spinal cord sections that were examined histologically.

Figure 2
Figure 2

Photomicrograph of a subgross section of spinal cord at the level of C3. There is a granuloma (arrowhead) in the right lateral funiculus of the spinal cord with pallor of the surrounding neuroparenchyma. H&E stain; bar = 200 μm.

Citation: Journal of the American Veterinary Medical Association 258, 10; 10.2460/javma.258.10.1083

Figure 3
Figure 3

Photomicrograph of the same section of spinal cord at the level of C3 as in Figure 2. The center of the granuloma contains a cross section of a degenerated subadult nematode (arrowhead) embedded in mineral. The surrounding neuroparenchyma has reactive astrocytes and swollen axons that are rounded and eosinophilic (spheroids). H&E stain; bar = 20 μm.

Citation: Journal of the American Veterinary Medical Association 258, 10; 10.2460/javma.258.10.1083

To identify the nematode, DNA was extracted from paraffin scrolls of the paraffin block containing the affected section of the spinal cord, and a portion of the internal transcribed spacer ITS 2 region of the rRNA of Parelaphostrongylus tenuis was amplified, cloned, and sequenced with previously described methods.1 The sequence obtained had 99% identity to 2 P tenuis sequences in GenBank and 98% identity to Parelaphostrongylus andersoni, which suggested that the nematode present was P tenuis.1 Parelaphostrongylus andersoni, or the muscleworm, was considered less likely to be associated with the clinical signs in this animal because that organism has not been identified in the spinal cord or brain of camelids.

Morphologic Diagnosis and Case Summary

Morphologic diagnosis and case summary: cervical spinal cord granuloma with mineralization and an intralesional subadult nematode (as result of P tenuis migration) in a 4-year-old llama.

Comments

For the llama of the present report, the cause of the clinical signs was aberrant spinal cord migration of the meningeal worm, P tenuis. The definitive host of P tenuis is white-tailed deer (Odocoileus virginianus), in which subadult nematodes reside in the dorsal and ventral sinuses and meninges of the cranial vault. Eggs are deposited into the venous circulation and mature into larvae (L1 stage) in the lungs; the larvae are then coughed up, swallowed, and excreted in the feces. Snails and slugs, the intermediate hosts, harbor the larvae as they develop to the infective L3 stage and are subsequently ingested by grazing ungulates. In the definitive host, the ingested L3 larvae migrate toward the spinal cord, where they mature into the 1- to 2-mm-diameter subadult stage (L5 stage) in the dorsal horns of the gray matter.2,3,4 Maturation of the larvae in the spinal cord can take up to 40 days. Once mature, the adult worms migrate through the cerebrospinal space to the brain and into the venous sinuses. Within an approximately 60-day period, the worms grow to 4 to 5 cm in length and then leave the CNS, moving into the cerebrospinal space between the dura and piarachnoid. Most white-tailed deer become infected within the first 2 years after birth, and almost all are infected by adulthood in some endemic regions.3

Although P tenuis infection is typically an incidental finding in white-tailed deer, infection with this nematode can cause clinical disease in domestic species and other dead-end hosts, such as equids, camelids, sheep, goats, cattle, and moose (Alces alces).2,3 In elk (Cervus elaphus), patent infections have been experimentally induced in individuals with and without development of clinical signs.5 In aberrant and dead-end hosts, the adult nematode will often begin migrating, as in white-tailed deer but upon reaching the spinal meninges, it can continue migrating into the spinal cord, resulting in local damage. Various histologic changes are associated with P tenuis infection in camelids, including mixed inflammation (eosinophils, neutrophils, macrophages, lymphocytes, and plasma cells), depending on the severity and chronicity of the lesions. Gliosis, hemorrhage, and Wallerian degeneration in the affected neuroparenchyma are often observed.6 Such changes throughout the CNS have been previously reported.1 However, in llamas, Rickard et al6 observed that most of the histologic changes occurred in the cervical portion of the spinal cord and the brain, and clinically, those lesions often caused hind limb paresis or paralysis. Examination of fecal samples with the Baermann technique can be performed to assess for dorsal-spined larvae.6 In llamas that were experimentally exposed to P tenuis, there were no consistent abnormalities in CSF total protein and glucose concentrations or aspartate aminotransferase or creatine kinase activities.6 However, eosinophilic pleocytosis was identified in all llamas infected with the meningeal worm.6 For P tenuis–infected llamas, treatment is targeted at killing the nematode to prevent the progression of clinical signs.7 Complete recovery from the infection or reversal of clinical signs is rare.2,8

For llamas, preventative measures against P tenuis infection are necessary because clinical signs associated with aberrant P tenuis migration are often permanent. Presently, there are 3 primary preventative techniques used for domestic animals, namely monthly SC administration of ivermectin to kill migrating larvae before they reach the CNS, environmental control to avoid intermediate host exposure, and environmental control of the definitive host (white-tailed deer) to limit local host population growth or prevent host proximity to or interface with domestic species.2,3,9

Because environmental control is often not an option or is subject to intermittent failure, it is recommended that cervids within a region in which P tenuis is endemic are not translocated to P tenuis–free regions.6,10 There is considerable concern about this parasite becoming established in the western United States and areas where caribou are present.11

References

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