Student Case Study

Erosive Polyarthritis in a Porcupine

Submitted by Rose Olsasky, Iowa State University

A juvenile (approximately 5 month old) female intact porcupine presented to the Wildlife Rehabilitation Center of Minnesota (WRC) following acute onset of lameness.  She was initially in the care of a different rehabilitation facility beginning at a few weeks old.  She was apparently healthy until 1 month ago when she began showing signs of lameness.  It was at this point that she was transferred to WRC. 

On physical exam, the porcupine was bright, alert, responsive, and had a normal appetite.  She walked with a stilted gait and there was concern for her favoring her left front leg.  A thorough orthopedic exam was difficult to perform without sedation.  It was unclear if she was just reactive to palpation versus spinal or long bone pain.  The remainder of her physical exam was within normal limits.  A CBC was ran but bloodwork was unremarkable.  The working differential diagnoses for her case included infectious (tick-borne), trauma, inflammatory (septic process), and degenerative/congenital. 

The porcupine was preemptively started on trimethoprim /sulfamethoxazole (TMPS) and doxycycline due to the concern for infectious and tick borne disease, such as Lyme.  There is no information about the validity of using the canine 4DX snap test for porcupines.  Additional diagnostics to rule out infectious causes would include a PCR tick panel but that was not pursued.  While rodents, including porcupines, are known reservoirs of Borrelia burgdorferi, they are typically not clinically affected1.  There are no specific reports of porcupines developing signs of infection that we see in our small animal patients such as shifting leg lameness or joint pain. However, there may be other infectious haemoparasites yet to be discovered in these wild rodents.  For example, a novel haemotropic Mycoplasma was recently identified in the orange-spined hairy dwarf porcupine but clinical effects of the organism remain unknown2.

Due to the fact that the porcupine appeared painful, she was started on pain medication (gabapentin). She had no history of trauma but her discomfort was difficult to localize.  After a couple days acclimating in hospital, she was sedated and radiographs were taken. There are few porcupine radiology reference images which makes interpretation difficult.  There were no overt abnormalities except possibly increased joint spaces which may be an entrapment due to her juvenile skeleton. Previous trauma was ruled out by the lack of fractures or soft tissue swelling [see the adjacent images]. Under sedation, a more thorough exam was performed and again, there were no obvious abnormalities. Radiographs were repeated a couple weeks later with similar images.

Through her stay, the porcupine was bright, alert, and responsive in hospital. She was eating well and appeared otherwise healthy.  Another issue developed while she was with WRC where she became acclimated to humans.  She would approach and greet people when they approached her enclosure, which makes re-release difficult.  Her ambulation also remained static or worsened slightly through the weeks.  Her front left leg was supinated and she seemed to have decreased range of motion.  She had difficulty climbing and porcupines need to be able to do so in the wild to find food3.  Due to lack of therapeutic response and poor long-term prognosis, humane euthanasia was elected. 

Any hospitalized patient that is euthanized receives a necropsy.  On gross exam, there were no obvious lesions. Her internal structures including heart, gastrointestinal tract, and abdominal organs appeared normal.  The only lesions were found in her joints.  When excising into the joint (both stifles and both cubital joints), there was copious amounts of suppurative effusion present (image to the left).  There was also evidence of articular erosions on all the joint surfaces with ranges of severity (see image on the right).  The total protein of the effusion was 7.0g/dL.  The synovial fluid was evaluated under the microscope and numerous nucleated cells were identified.  Majority of the effusion was made up of neutrophils as well as a decent amount of plasma cells. There was no bacteria noted on cytology.  An in-house culture was started but the results are not known at the time of this paper.

  

 There is not a lot of research or data on porcupine diseases.  Most of the conclusions from this report will be extrapolated from other rodents and small animal medicine cases.  Based on necropsy results, this porcupine had an erosive polyarthritis.  There are currently no published case reports of polyarthritis in porcupines.  Differentials for erosive polyarthritis include non-inflammatory (trauma, degenerative disease, gout), sepsis, infectious inflammation (Ehrlichia, Lyme, fungal), or immune-mediated (idiopathic, rheumatoid arthritis).  Based on fluid analysis, a non-inflammatory process was ruled out due to the elevated total protein.  Although culture results are not completed, the absence of a fever, normal white cell count, lack of systemic illness, and lack of bacteria seen on cytology helps to rule out sepsis.  As aforementioned, the lack of response to doxycycline makes tick-borne disease unlikely as well.  The last potential cause is immune-mediated.  In dogs, immune-mediated polyarthritis is caused by accumulation of immune complexes leading to chemotaxis of neutrophils into the joint5.  There are a number of different inciting causes including idiopathic, secondary to infection (non-joint), secondary to GI illness, or secondary to neoplasia.  There is no similar immune-mediated polyarthritis documented in rodents, however, mice have been experimentally used as a model for rheumatoid arthritis.  This can be done through intra-articular antigen-induction, collagen-induction, or transgenic mice lacking interleukin-336,7.  The mice in these experiments are meant to mimic the human disease and there is little literature on natural occurring rodent arthritis.  It remains unclear what the underlying etiology is for this porcupine’s polyarthritis.

In reviewing the case, arthrocentesis and fluid analysis would have been a useful antemortem diagnostic.  However, retrospectively knowing the results would not have changed the prognosis or end decision to euthanize.  Something else that could have been done is immunosuppressive medications or glucocorticoid therapy to treat an immune-mediated process.  Based on the author’s research, this is the first documented case of a non-septic erosive polyarthritis in a porcupine. Given the multifocal nature and extent of the lesions, humane euthanasia was ultimately the best choice in this case.

References

1. Radolf, Justin D et al. “Of ticks, mice and men: understanding the dual-host lifestyle of Lyme disease spirochaetes.” Nature reviews. Microbiology vol. 10,2. 9 Jan 2012; 87-99.

2. Valente JDM, Saldanha A, Martini R, Lange RR, Baggio RA, Martins TF, Dos Santos LP, de Sousa RS, Vieira TSWJ, Vieira RFC. 'Candidatus Mycoplasma haemosphiggurus' a novel haemoplasma species in orange-spined hairy dwarf porcupines (Sphiggurus villosus) from Southern Brazil. Transbound Emerg Dis. 2021 May;68(3):1054-1061. 

3. Uldis R. The North American Porcupine. 2nd ed. Comstock publishing associates: Cornell University Press, 2009; 218-222. 

4. MacWilliams PS, Friedrichs KR. Laboratory evaluation and interpretation of synovial fluid. Vet Clin North Am Small Anim Pract 2003;33:153-178.

5. Rondeau MP, Walton RM, Bissett S, et al. Suppurative, nonseptic polyarthropathy in dogs. J Vet Intern Med 2005;19:654-662.

6. Williams, R O. “Rodent models of arthritis: relevance for human disease.” Clinical and experimental immunology vol. 114,3. 1998; 330-2. 

7. Talabot-Ayer, D., Martin, P., Seemayer, C.A., et al. Immune-mediated experimental arthritis in IL-33 deficient mice. CYTOKINE 2014; 69, 68-74