Murine Respiratory Mycoplasmosis

Murine respiratory mycoplasmosis (MRM) is a chronic, highly contagious respiratory disease of laboratory and pet rats, mice, and occasionally hamsters, caused by the bacterium Mycoplasma pulmonis. It is characterized by inflammation of the upper and lower respiratory tract, including rhinitis, otitis media, tracheitis, bronchitis, and bronchopneumonia. The disease is often subclinical but can be exacerbated by environmental stressors, concurrent infections, or immunosuppression. In rats, it is a major cause of chronic respiratory disease, leading to significant morbidity and mortality in affected colonies. The term 'murine' refers to the family Muridae, which includes rats and mice, but the disease is most commonly associated with rats. Mycoplasma pulmonis is a small, pleomorphic, cell-wall-deficient bacterium that colonizes the ciliated epithelium of the respiratory tract, leading to ciliary stasis, inflammation, and secondary bacterial infections. The disease is often referred to as 'chronic respiratory disease' (CRD) in rats, and it can also cause genital tract infections and arthritis in some cases. In mice, the disease is often milder but can be severe in immunocompromised strains. The disease is of significant importance in laboratory animal medicine due to its impact on research outcomes, and in pet rodents due to its chronic, debilitating nature.

The primary causative agent of murine respiratory mycoplasmosis is Mycoplasma pulmonis, a member of the class Mollicutes. It is a small (0.3-0.8 μm), pleomorphic, facultatively anaerobic bacterium that lacks a cell wall, making it resistant to beta-lactam antibiotics. M. pulmonis colonizes the mucosal surfaces of the upper and lower respiratory tract, particularly the nasal passages, trachea, and bronchi. It adheres to ciliated epithelial cells via specialized adhesins, leading to ciliary stasis and loss of mucociliary clearance. The bacterium produces hydrogen peroxide and other toxic metabolites that damage epithelial cells and induce an inflammatory response. Secondary bacterial infections, particularly with Pasteurella pneumotropica, Streptococcus pneumoniae, and Corynebacterium kutscheri, are common and can exacerbate the disease. Environmental factors such as high ammonia levels from soiled bedding, poor ventilation, overcrowding, and temperature fluctuations can increase the severity of clinical signs. Nutritional deficiencies, particularly vitamin A and E deficiencies, may also predispose animals to more severe disease. Stress from transport, weaning, or experimental procedures can trigger clinical outbreaks in subclinically infected colonies. In addition, genetic factors play a role, as some rat strains (e.g., Wistar) are more susceptible than others (e.g., Fischer 344).

Murine respiratory mycoplasmosis is endemic in many laboratory and pet rodent colonies worldwide. The prevalence in laboratory rat colonies has decreased significantly with the advent of specific pathogen-free (SPF) housing, but it remains a common problem in conventional colonies and in pet rats. In pet rats, the prevalence is high, with some studies reporting seropositivity rates of over 80% in pet rat populations. Mice are also susceptible, but the disease is often less severe, and clinical signs may be absent in immunocompetent strains. Hamsters can be infected experimentally, but natural infection is rare. The disease is transmitted horizontally via direct contact with respiratory secretions, aerosols, and fomites. Vertical transmission from dam to offspring can occur in utero or during passage through the birth canal. The incubation period is typically 1-2 weeks, but subclinical carriers can shed the organism intermittently for life. Risk factors include high stocking density, poor ventilation, high ammonia levels, and introduction of new animals from infected colonies. Young animals are more susceptible to severe disease, and males may be more affected than females. In laboratory settings, the disease can confound research results, particularly in studies involving the respiratory, immune, or reproductive systems.

The pathophysiology of murine respiratory mycoplasmosis begins with colonization of the upper respiratory tract by Mycoplasma pulmonis. The organism adheres to ciliated epithelial cells via surface adhesins, leading to ciliary stasis and loss of mucociliary clearance. This impairs the clearance of mucus and debris, allowing the bacterium to spread to the lower respiratory tract. The bacterium produces hydrogen peroxide and other reactive oxygen species that damage epithelial cells and induce an inflammatory response. Neutrophils and macrophages infiltrate the submucosa, leading to hyperplasia of goblet cells and increased mucus production. Chronic inflammation results in thickening of the bronchial walls, bronchiectasis, and alveolar consolidation. In severe cases, bronchopneumonia and abscess formation can occur. The inflammatory response also leads to the release of pro-inflammatory cytokines, such as tumor necrosis factor-alpha and interleukins, which contribute to tissue damage. In addition to the respiratory tract, M. pulmonis can disseminate to other organs, including the middle ear (causing otitis media), the genital tract (causing salpingitis and endometritis), and the joints (causing arthritis). The immune response is often ineffective in clearing the infection, and the bacterium can persist intracellularly, leading to chronic carrier states. Secondary bacterial infections, particularly with Pasteurella pneumotropica, can exacerbate the disease and lead to suppurative bronchopneumonia. The disease is often progressive, with clinical signs worsening over time, especially in the presence of environmental stressors.

Several intrinsic and extrinsic factors predispose rodents to murine respiratory mycoplasmosis. Intrinsic factors include species and strain susceptibility: rats are more susceptible than mice, and certain rat strains (e.g., Wistar, Sprague-Dawley) are more prone to severe disease than others (e.g., Fischer 344). Age is also a factor, with young animals (weanlings) and aged animals being more susceptible. Males may be more affected than females, possibly due to hormonal differences. Immunosuppression, whether due to genetic factors, concurrent infections, or experimental manipulation, increases the risk of clinical disease. Extrinsic factors include poor husbandry, such as high ammonia levels from soiled bedding, inadequate ventilation, overcrowding, and temperature fluctuations. Stress from transport, weaning, or experimental procedures can trigger clinical outbreaks. Nutritional deficiencies, particularly of vitamins A and E, can impair immune function and increase susceptibility. Poor sanitation and lack of quarantine for new animals can introduce the pathogen into a colony. In laboratory settings, the use of conventional (non-SPF) housing and the mixing of animals from different sources increase the risk of infection. In pet settings, exposure to other rodents, especially from pet stores or breeders, is a common source of infection.

Clinical signs of murine respiratory mycoplasmosis vary depending on the species, age, and immune status of the animal. In rats, the disease is often chronic and progressive, with signs appearing over weeks to months. Early signs include sneezing, nasal discharge (serous to mucopurulent), and porphyrin staining around the eyes and nose (red tears). As the disease progresses, rats may develop dyspnea, tachypnea, and audible respiratory sounds (wheezing, crackles). Chronic infection can lead to weight loss, poor coat condition, and hunched posture. In severe cases, rats may develop pneumonia, which can be fatal. Otitis media is common, leading to head tilt, circling, and nystagmus. In mice, clinical signs are often milder and may be absent in immunocompetent animals. When present, signs include sneezing, nasal discharge, and dyspnea. In immunocompromised mice, the disease can be severe and fatal. Hamsters are rarely affected, but experimental infection can cause similar signs. In addition to respiratory signs, genital tract infections can cause infertility, vaginal discharge, and abortion in females. Arthritis can cause lameness and joint swelling. The disease can also be subclinical, with animals showing no signs but shedding the organism and serving as a source of infection for others.

Differential diagnoses for murine respiratory mycoplasmosis include other respiratory infections and non-infectious conditions. Key differentials include: 1) Sendai virus infection (parainfluenza virus type 1) - causes similar respiratory signs in mice and rats, but is often more acute and can be differentiated by serology and PCR. 2) Pneumonia virus of mice (PVM) - causes interstitial pneumonia in mice, but is less common in rats; diagnosis by serology and PCR. 3) Rat coronavirus infection (RCV/SDAV) - causes respiratory and salivary gland disease in rats; can be differentiated by PCR and serology. 4) Pasteurella pneumotropica infection - a common secondary invader that can cause suppurative pneumonia; can be isolated on culture. 5) Streptococcus pneumoniae infection - causes pneumonia and otitis media; can be cultured and identified by Gram stain. 6) Corynebacterium kutscheri infection - causes pseudotuberculosis in rats and mice; can be cultured. 7) Bordetella bronchiseptica infection - causes bronchopneumonia in guinea pigs and other rodents; can be cultured. 8) Allergic rhinitis - due to environmental allergens (e.g., bedding, dust) can cause sneezing and nasal discharge; diagnosis by history and response to environmental changes. 9) Neoplasia - primary or metastatic lung tumors can cause respiratory signs; diagnosis by radiography and histopathology. 10) Congestive heart failure - can cause dyspnea and pulmonary edema; diagnosis by echocardiography and response to treatment.

The diagnostic algorithm for murine respiratory mycoplasmosis begins with a thorough history and physical examination. Key historical points include the source of the animal, housing conditions, presence of other rodents, and any recent stressors. Physical examination should include assessment of respiratory rate and effort, auscultation of the lungs, and examination of the eyes and nose for discharge and porphyrin staining. If respiratory disease is suspected, the following steps are recommended: 1) Perform a complete blood count and serum biochemistry to assess for inflammation and organ function. 2) Obtain a deep nasal or tracheal swab for PCR testing for Mycoplasma pulmonis. PCR is highly sensitive and specific and can be performed on swabs, bronchoalveolar lavage fluid, or lung tissue. 3) Collect blood for serology (ELISA or immunofluorescence) to detect antibodies against M. pulmonis, although serology may be negative in early infection. 4) Perform thoracic radiography to assess for pulmonary changes, such as interstitial or alveolar patterns, bronchiectasis, or consolidation. 5) If the animal is stable, consider bronchoscopy and bronchoalveolar lavage for cytology and culture. 6) In cases of otitis media, skull radiography or CT may be helpful. 7) If the animal dies, necropsy and histopathology are essential for definitive diagnosis. Lung tissue should be submitted for PCR and culture. 8) In a colony outbreak, screen a representative sample of animals by PCR and serology to determine the extent of infection. 9) Rule out other respiratory pathogens by PCR or culture for Sendai virus, PVM, rat coronavirus, and secondary bacteria. 10) Based on the results, implement appropriate treatment and biosecurity measures.

Laboratory findings in murine respiratory mycoplasmosis are non-specific but can support the diagnosis. Hematology may reveal leukocytosis with neutrophilia due to inflammation, and in chronic cases, anemia of chronic disease. Serum biochemistry may show elevated globulins (hyperglobulinemia) due to chronic antigenic stimulation, and decreased albumin. Acute phase proteins, such as haptoglobin, may be elevated. PCR testing for Mycoplasma pulmonis is the most sensitive and specific diagnostic test. It can be performed on nasal swabs, tracheal washes, bronchoalveolar lavage fluid, or lung tissue. Serology (ELISA or immunofluorescence) can detect antibodies, but may be negative in early infection or in immunocompromised animals. Culture of M. pulmonis is possible but requires specialized media (e.g., Hayflick's medium) and is less sensitive than PCR. Cytology of bronchoalveolar lavage fluid may show neutrophilic inflammation and the presence of ciliated epithelial cells. In cases of otitis media, cytology of middle ear fluid may reveal neutrophils and bacteria. Fecal analysis is not typically helpful, but in cases of genital tract infection, vaginal swabs may be positive by PCR. Urinalysis is usually unremarkable. In a colony setting, serological screening of sentinel animals is recommended to monitor for infection.

Imaging findings in murine respiratory mycoplasmosis are primarily seen on thoracic radiography. In early cases, radiographs may be normal or show a mild interstitial pattern. As the disease progresses, alveolar patterns, bronchial thickening, and bronchiectasis may be observed. In severe cases, lung lobe consolidation or abscessation may be seen. Radiography is also useful for evaluating the tympanic bullae in cases of otitis media, where thickening or opacification of the bulla may be evident. However, radiography of the skull in small rodents is challenging due to the small size and superimposition of structures. Computed tomography (CT) provides more detailed imaging of the respiratory tract and middle ear and is the preferred modality for evaluating otitis media and pulmonary changes. CT can reveal thickening of the nasal passages, tracheal narrowing, bronchial wall thickening, and pulmonary consolidation. Magnetic resonance imaging (MRI) is less commonly used but can provide excellent soft tissue detail for evaluating the brain and middle ear in cases of neurological signs. Ultrasonography is not typically used for respiratory disease but may be helpful for evaluating cardiac function if concurrent heart disease is suspected. Endoscopy, including rhinoscopy and bronchoscopy, can be used to visualize the upper and lower respiratory tract and obtain samples for cytology and culture. However, endoscopy requires specialized equipment and expertise and is not commonly performed in pet rodents.

Cytological and histopathological findings in murine respiratory mycoplasmosis are characteristic. Cytology of nasal or tracheal swabs, bronchoalveolar lavage fluid, or middle ear fluid may reveal neutrophilic inflammation, with degenerate neutrophils and macrophages. Ciliated epithelial cells may be present, and intracellular or extracellular bacteria may be seen with appropriate staining. Histopathology of the respiratory tract is the gold standard for diagnosis. Grossly, the lungs may show cranioventral consolidation, and the trachea may contain mucopurulent exudate. Microscopically, there is rhinitis, tracheitis, bronchitis, and bronchiolitis, with infiltration of neutrophils and mononuclear cells. The epithelium may show hyperplasia, goblet cell metaplasia, and loss of cilia. In chronic cases, bronchiectasis, peribronchial lymphoid hyperplasia, and fibrosis may be present. Alveoli may contain exudate and macrophages. In cases of otitis media, the middle ear cavity may contain inflammatory exudate and the mucosa may be thickened. In the genital tract, salpingitis and endometritis may be seen. Immunohistochemistry or in situ hybridization can be used to detect M. pulmonis antigens or nucleic acids in tissue sections. Special stains, such as Giemsa or silver stains, may help visualize the organism, but they are not specific.

Treatment of murine respiratory mycoplasmosis is challenging due to the chronic nature of the infection and the lack of a cell wall in M. pulmonis, which makes it resistant to beta-lactam antibiotics. The goals of treatment are to reduce clinical signs, control secondary infections, and improve quality of life. Antibiotics that are effective against Mycoplasma include tetracyclines (e.g., doxycycline), macrolides (e.g., tylosin, azithromycin), fluoroquinolones (e.g., enrofloxacin), and lincosamides (e.g., clindamycin). Doxycycline is often the drug of choice in rats and mice, administered at 5 mg/kg PO q12h or 10 mg/kg PO q24h. Tylosin can be given at 10 mg/kg PO q12h or 20 mg/kg PO q24h. Enrofloxacin is commonly used at 10 mg/kg PO q12h, but it may be less effective against Mycoplasma than doxycycline. In severe cases, combination therapy with doxycycline and enrofloxacin may be used. Treatment should be continued for at least 2-4 weeks, and sometimes longer. Supportive care is essential: maintain hydration with subcutaneous or oral fluids, provide nutritional support with a high-quality diet and supplements, and ensure a clean, stress-free environment with proper ventilation and low ammonia levels. Nebulization with saline or antibiotics (e.g., gentamicin) may be helpful in severe cases. Anti-inflammatory drugs, such as meloxicam (0.2-0.5 mg/kg PO q24h), can reduce inflammation and improve respiratory function. In cases of otitis media, systemic antibiotics may not penetrate the middle ear well, and surgical drainage may be necessary. In a colony setting, treatment of all animals may be considered, but elimination of the infection is difficult, and culling of affected animals may be recommended.

The prognosis for murine respiratory mycoplasmosis varies depending on the severity of the disease, the species, and the presence of secondary infections. In rats, the disease is often chronic and progressive, and complete recovery is rare. However, with appropriate treatment and supportive care, clinical signs can be managed, and affected animals can have a good quality of life for months to years. In mild cases, especially in mice, the disease may be self-limiting, and animals may recover without treatment. In severe cases, particularly those with pneumonia or systemic involvement, the prognosis is guarded to poor, and death may occur despite treatment. Negative prognostic indicators include severe dyspnea, cyanosis, weight loss, and lack of response to treatment within 48-72 hours. The presence of secondary bacterial infections, such as Pasteurella pneumotropica, worsens the prognosis. In a colony setting, the prognosis for elimination of the infection is poor, and long-term management is required. In pet animals, the prognosis is better if the owner is committed to long-term treatment and husbandry improvements. Regular veterinary check-ups and monitoring are essential to adjust treatment as needed.

Follow-up for murine respiratory mycoplasmosis should be structured to monitor response to treatment and detect any recurrence or complications. Initially, re-check the animal 7-14 days after starting treatment to assess clinical improvement. At each re-check, perform a physical examination, including respiratory rate and effort, auscultation, and body weight. If clinical signs have improved, continue the same treatment for the full course (usually 2-4 weeks). If there is no improvement or worsening, consider changing antibiotics or adding supportive care. After the initial treatment course, re-check the animal every 1-2 months for chronic cases. Monitor weight weekly to ensure no weight loss. Serial blood work (CBC and biochemistry) may be performed every 3-6 months to assess for chronic inflammation or organ dysfunction. Thoracic radiography may be repeated every 3-6 months to monitor pulmonary changes. In cases of otitis media, repeat skull radiography or CT may be needed. In a colony setting, implement a biosecurity program, including quarantine of new animals, regular screening by PCR and serology, and culling of affected animals. Provide ongoing husbandry audits to ensure proper ventilation, low ammonia levels, and minimal stress. Educate owners on the importance of maintaining a clean environment and avoiding exposure to other rodents.

Clinical pearls: 1) Porphyrin staining around the eyes and nose in rats is a classic sign of stress or respiratory disease; it is not blood but a secretion from the Harderian gland. 2) Auscultation of the lungs in small rodents is challenging; use a pediatric stethoscope and listen over the trachea and chest. 3) Doxycycline is the first-line antibiotic for Mycoplasma infections; it is well-tolerated and has good tissue penetration. 4) Nebulization with saline can help loosen respiratory secretions and improve breathing. 5) In cases of otitis media, head tilt may be the only clinical sign; treat aggressively with antibiotics and consider surgical drainage. 6) Always rule out other respiratory pathogens, as co-infections are common. 7) In a colony, PCR testing of sentinel animals is the best way to monitor for infection. Pitfalls: 1) Do not use beta-lactam antibiotics (e.g., amoxicillin) as they are ineffective against Mycoplasma and can disrupt normal flora. 2) Avoid corticosteroids, as they can immunosuppress the animal and worsen the infection. 3) Do not use enrofloxacin as a sole agent in young animals, as it can cause cartilage damage. 4) Do not underestimate the importance of environmental factors; high ammonia levels can exacerbate clinical signs. 5) Do not assume that a negative PCR result rules out infection; the organism may be present in low numbers. 6) Do not treat only the affected animal in a colony; treat or cull all exposed animals. 7) Do not use fipronil or other topical flea products in rodents, as they can be toxic.

Based on Carpenter's Exotic Animal Formulary, the following drug protocols are recommended for murine respiratory mycoplasmosis in rats, mice, and hamsters. Antibiotics: Doxycycline: 5 mg/kg PO q12h or 10 mg/kg PO q24h for 14-30 days. Tylosin: 10 mg/kg PO q12h or 20 mg/kg PO q24h for 14-30 days. Enrofloxacin: 10 mg/kg PO q12h for 14-30 days (use with caution in young animals). Azithromycin: 10 mg/kg PO q24h for 5-7 days. Clindamycin: 10 mg/kg PO q12h (not recommended as sole therapy). Anti-inflammatory: Meloxicam: 0.2-0.5 mg/kg PO q24h for 3-7 days. Fluids: Lactated Ringer's solution or 0.9% saline: 50-100 ml/kg SC q24h or as needed. Nebulization: Saline (0.9%) nebulization for 15-20 minutes q12h; may add gentamicin (5 mg/ml) or amikacin (5 mg/ml) to the nebulization solution. Nutritional support: Critical Care for Herbivores (Oxbow) or a high-quality rodent diet, syringe-fed if anorexic. Note: Dosages are based on published references and should be adjusted based on clinical response and species-specific considerations. Always consult a veterinarian experienced in exotic animal medicine.

Murine respiratory mycoplasmosis has been extensively studied in laboratory animal medicine. Key studies have established the pathogenesis, transmission, and control of the disease. A landmark study by Lindsey et al. (1971) described the natural history of M. pulmonis infection in rats, highlighting the chronic, progressive nature of the disease. Subsequent research has focused on the immune response to M. pulmonis, with studies showing that the organism can evade the immune system and persist in the respiratory tract. The use of PCR for diagnosis has been validated in several studies, with high sensitivity and specificity compared to culture and serology. Treatment studies have evaluated the efficacy of various antibiotics, with doxycycline and tylosin showing good efficacy in reducing clinical signs and bacterial load. However, complete elimination of the organism is difficult, and chronic infection often persists. In terms of control, the establishment of specific pathogen-free (SPF) colonies has been the most effective strategy in laboratory settings. For pet rodents, management focuses on supportive care and environmental optimization. The BSAVA Manual of Exotic Pets and Quesenberry & Carpenter's Ferrets, Rabbits, and Rodents provide comprehensive guidelines for diagnosis and treatment. The Exotic Animal Formulary by Carpenter is the standard reference for drug dosages. Recent studies have also investigated the role of secondary bacterial infections in exacerbating disease, emphasizing the importance of broad-spectrum antibiotic coverage. Overall, the evidence supports a multimodal approach to management, including antimicrobial therapy, supportive care, and environmental improvements.