Case Study: Mycoplasmosis in Six Rats

Signalment and Case Presentation

Six 12-week-old male rats from the same litter were presented with sneezing and excessive porphyrin secretion in their tears (chromodacryorrhea). They were housed in a small enclosure, and the owner reported that the rats fought frequently during the four weeks preceding display of the clinical signs. All six rats were seen sneezing during physical examination; all lung fields were clear on auscultation. The rat that weighed the least also had a head tilt and circled to the right. The rest of the physical examination was unremarkable.

Diagnostic and therapeutic plan

Based on the signalment and clinical signs, a presumptive diagnosis of murine mycoplasmosis was made. A radiograph of the lightest rat was taken, and mild thickening of the right tympanic bulla was visible (Figure 1), which was consistent with the observed head tilt. Other diagnostic tests available include culture and isolation, enzyme-linked immunosorbent assay (ELISA) and polymerase chain reaction (PCR) tests. A two-week course of doxycycline—which specifically targets Mycoplasma pulmonis—and enrofloxacin—which targets opportunistic bacteria-causing secondary infections—were prescribed. A two-week course of meloxicam, a nonsteroidal anti-inflammatory drug (NSAID), was prescribed to reduce inflammation in the upper respiratory tract. The rats were then separated into pairs and housed in three enclosures during treatment but were eventually re-homed together.

Figure 1. Dorsoventral radiograph of the rat showing clinical signs of sneezing and a head tilt. Note the thickened right tympanic membrane (circled).

Case outcome

Sneezing and excessive porphyrin production improved tremendously in all six rats after two weeks, but the lightest rat still had a slight head tilt. By this time, a new home had been found for all of them, where they could be housed in one large enclosure with sufficient food and space for each rat. It was decided that they would be surgically castrated before moving to their new home to reduce aggression and competition for food.

Rats can be castrated using either an open or closed technique, but closure of the inguinal ring is indicated with open castrations to avoid herniation. Open castrations were performed for all six rats.

One week post-surgery, the rats were moved to their new home. They remained asymptomatic the majority of the time, however the disease recurred periodically and was treated with short courses of doxycycline with or without another antibiotic. The lightest rat’s head tilt did not resolve, but his quality of life did not seem to be adversely affected.

Discussion

Murine respiratory mycoplasmosis has been well-described and studied in laboratory rats since as early as 1969.1 Clinical signs of the disease include dyspnea, sneezing, ocular and nasal discharge, chromodacryorrhea and head tilt.2 Transmission of the disease is usually by direct contact.3

Previously asymptomatic rats frequently succumb to infections with Mycoplasma pulmonis when placed under stressful conditions. In this case, six rats competing for limited food and space at an age when they are starting to become sexually active is definitely a stressful situation. Treatment of the disease includes addressing the causative organisms and associated inflammation.4 Eliminating or reducing stressors is also an important aspect of disease management.

This report describes a typical case of mycoplasmosis in rats, which is a very common problem when husbandry is not ideal. Discussions with owners about proper husbandry should be part of any rat consult, especially when there is excessive porphyrin secretion in the tears, which is indicative of stress. Owners should also be aware that the signs might return at some point in future, and that a full “cure” is very difficult to achieve. However, most rats with mycoplasmosis can still lead a happy, healthy life with appropriate veterinary care and commitment from the owner.

It is unfortunate that the head tilt displayed by the lightest rat did not resolve completely, and the lack of more advanced diagnostic imaging modalities precluded the ability to better assess the tympanic bullae in this rat. Higher resolution radiography using a dental radiography unit and mammography films may yield clearer radiographs, but magnetic resonance imaging (MRI) remains the gold standard for assessing pathological changes in the tympanic bullae. If budget limitations do not preclude their use, CAT scan (CT) and MRI can be very useful in helping to decide whether additional therapy, such as myringotomy, flushing/curettage of the tympanic bulla or lateral/ventral bulla osteotomy, is warranted. These additional therapies would aim to address the changes present in the right tympanic bulla and could be indicated to help resolve the residual head tilt.

References

  1. Kohn, DF and Kirk, CE. Pathogenicity of mycoplasma pulmonis in laboratory rats. Laboratory Animal Care. 1969; 19:321-330
  2. Kohn, DF and Clifford, CB. Biology and diseases of rats. In: Fox, JG. (Ed.) Laboratory Animal Medicine. San Diego, Calif. Elsevier. 2002.
  3. Lindsey, JR, Davidson, MK, Schoeb, TR and Cassell, GH. Mycoplasma pulmonis-host relationships in a breeding colony of Sprague-Dawley rats with enzootic murine respiratory mycoplasmosis. Laboratory Animal Science. 1985; 35:597-608.
  4. Graham, JE and Schoeb, TR. Mycoplasma pulmonis in Rats. J Exot Pet Med. 2011; 20:270-276.

Bio

Shangzhe Xie, DVM, graduated in 2008 from Murdoch University in Perth, Western Australia, and completed a Master’s of Veterinary Studies in Conservation Medicine from Murdoch University in 2010. Dr. Xie joined Banfield Pet Hospital of Burbank, Ill., in July 2010 and has expanded the clientele to include exotic species. He also served on the Banfield Exotic Pets Care Guidelines Committee in 2011.