Mycoplasmosis in Rats: Signs and Diagnosis

Understanding Mycoplasma in Rats

What is Mycoplasma pulmonis?

Mycoplasma pulmonis is a cell‑wall‑deficient bacterium belonging to the class Mollicutes and the genus Mycoplasma. It is the primary etiologic agent of respiratory mycoplasmosis in laboratory and pet rats. The organism measures 0.2–0.3 µm in diameter, lacks a peptidoglycan layer, and relies on sterols obtained from the host for membrane stability. Its genome is small (≈0.9 Mb) and encodes limited metabolic pathways, reflecting an obligate parasitic lifestyle.

Key biological features include:

Host specificity: Predominantly infects Rattus norvegicus and Rattus rattus; occasional isolation from other rodents.
Transmission: Direct contact, aerosols, and contaminated bedding facilitate spread among colony members.
Colonization sites: Upper and lower respiratory tract epithelium, with occasional dissemination to the middle ear and genital tract.
Virulence factors: Adhesion proteins (P1, P30) enable attachment to ciliated cells; lipoproteins trigger inflammatory responses.

In infected rats, M. pulmonis establishes persistent colonization, leading to chronic rhinitis, otitis media, and bronchopneumonia. Laboratory diagnosis typically relies on culture in specialized mycoplasma medium, polymerase chain reaction targeting the 16S rRNA gene, or serologic detection of specific antibodies. Understanding the organism’s taxonomy, morphology, and transmission dynamics is essential for accurate identification and effective control of respiratory disease in rat colonies.

How Rats Contract Mycoplasma

Transmission Routes

Mycoplasma infection spreads among laboratory and wild rats through several well‑documented pathways. Direct contact between animals transmits the organism via mucosal surfaces, nasal secretions, and saliva. Contaminated bedding, food, and water provide indirect exposure; the pathogen survives on these substrates long enough to infect new hosts. Vertical transmission occurs when infected females pass the organism to offspring during gestation or through lactation. Parasitic vectors such as ectoparasites (mites, fleas) can carry mycoplasmas from one rat to another, facilitating spread within dense colonies. Aerosolized droplets generated by coughing or sneezing enable airborne dissemination, especially in poorly ventilated facilities.

Key transmission routes include:

Direct animal‑to‑animal contact (nose‑to‑nose, grooming)
Contaminated environmental materials (bedding, feed, water)
Maternal passage (in utero, milk)
Ectoparasite‑mediated transfer
Airborne particles (aerosol spread)

Risk Factors for Infection

Mycoplasma infection in laboratory rats develops when environmental, biological, and management variables converge to compromise host defenses.

High animal density increases contact rates and aerosol transmission.
Inadequate ventilation elevates airborne pathogen load.
Chronic stress from handling, transport, or environmental disturbances suppresses immunity.
Young or geriatric rats exhibit reduced immune competence, heightening susceptibility.
Immunosuppressive agents or experimental protocols that alter immune function create favorable conditions for colonization.
Contaminated bedding, water, or feed act as reservoirs for viable organisms.
Introduction of untested animals without a quarantine period enables direct introduction of the pathogen.
Deficient cage cleaning and disinfection protocols permit accumulation of infectious material.
Co‑infection with bacteria, viruses, or parasites can disrupt mucosal barriers and facilitate Mycoplasma establishment.
Genetic strains with inherent immune deficiencies display increased infection rates.

Recognition of these factors allows targeted mitigation strategies to reduce the incidence of Mycoplasma disease in rat colonies.

Recognizing the Signs of Mycoplasmosis

Respiratory Symptoms

Sneezing and Nasal Discharge

Sneezing and nasal discharge are frequent manifestations of mycoplasma infection in laboratory rats. The respiratory epithelium becomes inflamed, leading to abrupt, repetitive sneezes that may occur in clusters. Nasal secretions range from clear serous fluid to thick, muco‑purulent exudate, reflecting the progression from early irritation to secondary bacterial involvement.

Key diagnostic considerations for these signs include:

Observation of sneezing frequency and pattern during routine health monitoring.
Assessment of discharge consistency, color, and volume to differentiate primary mycoplasma effects from opportunistic pathogens.
Collection of nasal swabs or lavage fluid for polymerase chain reaction (PCR) targeting Mycoplasma pulmonis DNA.
Cytological examination of discharge to identify inflammatory cells and exclude fungal elements.
Correlation with additional respiratory signs such as dyspnea or audible wheezing to strengthen the presumptive diagnosis.

Prompt recognition of sneezing and nasal discharge, combined with targeted laboratory testing, enables early intervention and reduces the risk of outbreak spread within a colony.

Difficulty Breathing and Wheezing

Mycoplasma pulmonis infection commonly produces respiratory compromise in laboratory and pet rats. The pathogen colonizes the upper airway, spreads to the lower respiratory tract, and induces inflammation that narrows air passages.

Difficulty breathing manifests as increased respiratory rate, shallow thoracic movements, and audible wheezing during both inspiration and expiration. Rats may adopt a hunched posture, exhibit reduced activity, and show signs of hypoxia such as cyanotic mucous membranes. Wheezing results from turbulent airflow through inflamed bronchi and can be detected by careful auscultation.

Accurate identification of these respiratory signs requires systematic evaluation:

Observe respiratory rate and pattern in a calm environment.
Perform auscultation with a pediatric stethoscope; note high‑pitched or musical sounds.
Capture thoracic radiographs to reveal interstitial infiltrates, pleural effusion, or lung consolidation.
Collect nasal or tracheal swabs for polymerase chain reaction targeting Mycoplasma DNA.
Culture samples on specialized agar under microaerophilic conditions for definitive organism recovery.

Correlation of clinical signs with imaging and laboratory findings confirms the presence of mycoplasmal pneumonia and guides therapeutic decisions. Early detection of breathing difficulty and wheezing reduces morbidity and improves outcomes in affected rodents.

Head Tilting and Balance Issues

Head tilting frequently appears in laboratory rats infected with Mycoplasma spp., indicating disruption of the vestibular system. The abnormal posture results from impaired signal transmission between the inner ear, cranial nerves, and cerebellum, leading to a persistent deviation of the skull toward the affected side. Accompanying balance deficits manifest as unsteady gait, frequent falls, and inability to maintain a horizontal position.

Clinical observation should record the direction and degree of tilt, the presence of circling behavior, and any asymmetry in ear positioning. Neurological assessment must include:

Evaluation of righting reflexes and ability to navigate a straight runway.
Observation of spontaneous nystagmus with a video otoscope.
Testing of proprioceptive responses by gently rotating the animal on a flat surface.

Laboratory confirmation relies on detecting Mycoplasma organisms in cerebrospinal fluid or inner‑ear tissue. Recommended procedures:

Collect CSF via cisterna magna puncture under anesthesia.
Perform polymerase chain reaction targeting Mycoplasma-specific 16S rRNA genes.
Culture samples on specialized agar (e.g., SP4) for isolation and antibiotic susceptibility testing.
Apply immunohistochemistry on vestibular epithelium sections to visualize bacterial infiltration.

Imaging, such as high‑resolution micro‑CT, may reveal ossicular erosion or fluid accumulation, supporting the diagnosis. Differential considerations include ototoxic drug exposure, viral encephalitis, and traumatic injury; each can be excluded through history, serology, and the specific molecular signature of Mycoplasma infection. Prompt identification of vestibular involvement enables targeted antimicrobial therapy and reduces morbidity in affected colonies.

Other Clinical Signs

Porphyrin Staining Around Eyes and Nose

Porphyrin staining appears as reddish‑brown secretions that accumulate on the periorbital and nasal regions of rats. The discoloration results from the excretion of porphyrin pigments in the tear and nasal glands, a response commonly triggered by stress, irritation, or systemic infection. In the context of Mycoplasma‑induced disease, the staining often intensifies as the organism proliferates in the respiratory tract, causing inflammation and discomfort that stimulate glandular secretion.

Recognition of porphyrin staining contributes to early detection of mycoplasmal respiratory disease. Key diagnostic considerations include:

Localization: staining confined to the eyes and nose, without spread to the fur coat.
Consistency: wet, glossy secretions that dry to a crusted rim.
Correlation: presence alongside sneezing, nasal discharge, or labored breathing.
Exclusion: absence of similar staining in control colonies without Mycoplasma exposure.

Confirmatory diagnosis requires laboratory testing of respiratory samples (e.g., PCR, culture) to identify Mycoplasma species. The presence of porphyrin staining should prompt immediate sampling, as it often precedes overt respiratory signs and can indicate subclinical infection within a colony.

Lethargy and Weight Loss

Mycoplasma infection in laboratory rats frequently presents with reduced activity and a measurable decline in body mass. Affected animals move less, display diminished grooming, and show weak responses to handling or environmental stimuli. The decrease in locomotion often progresses over several days, leading to a state of apparent apathy that can be mistaken for stress or aging.

Weight loss accompanies the lethargic state. Rats may lose 5–15 % of their initial body weight within a week, accompanied by a lower body‑condition score and reduced food intake. The loss is typically gradual, correlating with the onset of anorexia and the catabolic effects of the pathogen on respiratory and systemic tissues.

Accurate identification relies on systematic observation and laboratory confirmation. Key steps include:

Daily recording of activity levels and food consumption.
Weekly weighing to detect trends exceeding normal growth variation.
Collection of oropharyngeal or nasal swabs for polymerase‑chain‑reaction assays targeting Mycoplasma species.
Serological testing for specific antibodies to assess exposure history.
Culture on specialized mycoplasma broth or agar, recognizing the organism’s fastidious growth requirements.
Post‑mortem examination of lung and tracheal tissue, with histopathology revealing inflammatory infiltrates and epithelial damage.

Combining quantitative monitoring of lethargy and weight loss with targeted molecular and serological diagnostics provides a reliable framework for confirming mycoplasma infection in rat colonies.

Rough Coat and Hunched Posture

Rats infected with mycoplasma species often develop a coarse, unkempt fur coat. The texture becomes gritty, and the hair may appear clumped or broken. This alteration reflects poor grooming behavior and reduced skin health, both of which are common in systemic infections.

A pronounced curvature of the spine accompanies the coat changes. Affected animals adopt a hunched posture, with the thoracic region elevated and the abdomen drawn inward. The stance indicates discomfort and possible musculoskeletal involvement caused by the pathogen’s inflammatory response.

These external signs aid in early detection. When observed together, they suggest the presence of a mycoplasmal disease and warrant further investigation:

Physical examination confirming rough fur and spinal curvature.
Laboratory testing of nasal or oropharyngeal swabs for Mycoplasma DNA via PCR.
Serological assays to detect specific antibodies.
Radiographic imaging to assess vertebral changes if severe hunching persists.

Prompt recognition of the coat and posture abnormalities enables timely confirmation of infection and initiation of appropriate antimicrobial therapy.

Diagnosing Mycoplasmosis

Clinical Examination and History

Clinical assessment of a rat suspected of mycoplasma infection begins with a thorough history. Record the animal’s age, sex, strain, and housing conditions, noting recent introductions of new conspecifics, stressors such as overcrowding, temperature fluctuations, or changes in diet. Ask about prior respiratory disease, antibiotic treatments, and any recent outbreaks within the colony. Documentation of these factors guides interpretation of physical findings and informs containment measures.

During physical examination, observe the animal in a calm environment to reduce stress‑induced artifacts. Key observations include:

Respiratory rate and pattern; tachypnea or labored breathing suggests pulmonary involvement.
Nasal and ocular discharge; serous to mucoid secretions are common.
Audible sounds; crackles or wheezes detected with a stethoscope indicate alveolar or airway inflammation.
Body condition; weight loss or reduced grooming may reflect chronic disease.
Temperature; hypothermia can accompany severe infection.

Palpation of the thorax should assess for pain or resistance, while gentle abdominal examination rules out concurrent gastrointestinal issues. Collecting a detailed history and performing systematic clinical checks provide the foundation for targeted diagnostic testing, such as PCR or culture, and enable timely intervention.

Diagnostic Tests

Polymerase Chain Reaction (PCR) Testing

Polymerase Chain Reaction (PCR) offers rapid, specific detection of Mycoplasma species responsible for rat infections. By amplifying conserved genetic regions, PCR distinguishes pathogenic strains from commensal flora, enabling accurate confirmation of clinical suspicion.

Key aspects of PCR testing for rat mycoplasmosis:

Sample collection: lung tissue, tracheal swabs, or blood, preserved in nucleic‑acid‑stabilizing buffer.
DNA extraction: silica‑column or magnetic‑bead kits produce high‑purity templates suitable for amplification.
Primer design: primers target the 16S rRNA gene or species‑specific virulence genes, ensuring sensitivity down to a few copies per reaction.
Thermocycling conditions: initial denaturation at 95 °C, followed by 35–40 cycles of denaturation, annealing (55–60 °C), and extension at 72 °C.
Detection: agarose‑gel electrophoresis, real‑time fluorescence, or melt‑curve analysis provides qualitative or quantitative results.

Interpretation relies on the presence of a specific amplicon of expected size or fluorescence threshold crossing. Positive PCR confirms infection even when serology or culture yields ambiguous data, supporting timely therapeutic decisions and epidemiological monitoring.

Culture and Sensitivity

Mycoplasma cultures from rat specimens require specialized media lacking a cell wall. Commonly used broth contains SP-4 base enriched with horse serum, yeast extract, and a pH indicator. Solid media such as agar plates are supplemented with sterols to sustain growth. Inoculum should be taken from lung tissue, trachea, or nasal swabs and placed in a humidified incubator at 37 °C with 5 % CO₂. Growth appears as a diffuse, “fried‑egg” colony after 2–7 days; microscopic examination shows characteristic filamentous forms lacking a rigid wall.

Sensitivity testing follows isolation. The broth microdilution method, adapted for Mycoplasma, determines minimum inhibitory concentrations (MICs) for tetracyclines, macrolides, fluoroquinolones, and pleuromutilins. Procedure:

Prepare serial dilutions of each antimicrobial in SP‑4 broth.
Add a standardized inoculum (≈10⁴ CFU/mL) to each well.
Incubate under the same conditions as culture.
Record the lowest concentration that prevents color change, indicating inhibition.

Interpretation relies on established breakpoints for rodent Mycoplasma spp.; values above the breakpoint denote resistance. Quality control strains, such as Mycoplasma pulmonis ATCC 23633, validate assay performance. Results guide therapeutic choices and inform epidemiological monitoring of resistant isolates.

Serological Testing

Serological testing provides a rapid means of confirming rat mycoplasma infection when clinical signs are ambiguous. Antibody detection assays identify host immune responses to Mycoplasma spp., allowing differentiation between exposed and naïve animals.

Common serological methods include:

Enzyme‑linked immunosorbent assay (ELISA): quantifies IgG or IgM antibodies; high throughput; sensitivity typically 85‑95 % and specificity 90‑98 %.
Indirect immunofluorescence assay (IFA): visualizes antibody binding on fixed mycoplasma antigens; useful for confirming ELISA results; requires fluorescence microscopy.
Complement fixation test (CFT): detects complement‑binding antibodies; less sensitive than ELISA but valuable for historical comparison with archived sera.
Microscopic agglutination test (MAT): measures agglutination of mycoplasma cells by serum antibodies; simple to perform, limited by subjective interpretation.

Sample collection must follow strict aseptic technique. Blood is drawn from the tail vein or retro‑orbital sinus, allowed to clot, and serum separated by centrifugation. Ideal sampling occurs 2–3 weeks post‑exposure, when antibody titers peak; earlier collection may yield false‑negative results due to the seroconversion window.

Interpretation of serological data requires awareness of cross‑reactivity among Mycoplasma species. Positive results should be corroborated with molecular diagnostics, such as PCR, to confirm the specific pathogen. Repeated testing at 2‑week intervals can track seroconversion dynamics and assess the efficacy of eradication measures.

Differentiating from Other Respiratory Illnesses

Mycoplasma pulmonis infection in laboratory rats produces a respiratory syndrome that can resemble bacterial pneumonia, viral bronchiolitis, fungal invasion, or allergic airway disease. Accurate differentiation relies on a combination of clinical observation, pathology, and laboratory testing.

Clinically, Mycoplasma infection often presents with a gradual onset of nasal discharge, sneezing, and mild dyspnea. The discharge is typically serous to mucoid and lacks the purulent character seen in Pasteurella‑induced pneumonia. Viral agents such as Sendai virus produce more abrupt fever spikes and systemic signs, while fungal infections generate weight loss and severe respiratory distress that progress rapidly. Allergic bronchitis manifests as intermittent wheezing without the persistent nasal secretions common to Mycoplasma disease.

Pathological assessment further separates these conditions. Mycoplasma lesions are confined to the upper airway and peribronchial lymphoid tissue, showing hyperplasia of bronchus‑associated lymphoid tissue (BALT) without extensive alveolar consolidation. In contrast, bacterial pneumonia displays lobar or multifocal consolidation with neutrophilic exudate. Viral infections generate bronchiolar epithelial necrosis and interstitial inflammation, whereas fungal disease shows granulomatous lesions with hyphal elements.

Definitive diagnosis employs targeted laboratory techniques:

Polymerase chain reaction (PCR) on nasal swabs or lung tissue: high sensitivity for Mycoplasma DNA, negative in most bacterial and viral infections.
Culture on specialized media (e.g., SP4 broth): confirms viable Mycoplasma organisms; bacterial pathogens grow on standard agar, fungi on Sabouraud medium.
Serology (ELISA or indirect immunofluorescence): detects specific antibodies; viral serology distinguishes Sendai or rat coronavirus infections.
Radiography or computed tomography: reveals peribronchial thickening in Mycoplasma cases, while bacterial pneumonia shows dense infiltrates and fungal disease may present cavitary lesions.

By integrating these clinical, pathological, and diagnostic criteria, investigators can reliably separate Mycoplasma‑related respiratory disease from other common respiratory illnesses affecting rats.