Respiratory Issues in Mice (Mycoplasmosis)

Respiratory disease is the most common and significant health problem affecting pet mice. The vast majority of these infections are caused by a bacterium called Mycoplasma pulmonis. It is so pervasive in the pet mouse population that it's assumed nearly all fancy mice are born already carrying the organism.

This leads to a chronic, lifelong respiratory disease, often referred to as mycoplasmosis or simply "myco." While there is no cure, understanding the disease allows owners to manage it effectively, minimize flare-ups, and keep their mice comfortable.

The Nature of Mycoplasma

Mycoplasma pulmonis is a unique bacterium that lacks a cell wall, which makes it resistant to certain types of antibiotics. It colonizes the respiratory tract (nasal passages, airways, and lungs) of mice. In a young, healthy, and unstressed mouse, the immune system is often able to suppress the bacteria, leading to no outward signs of illness. However, the disease is always lurking, ready to flare up when conditions are right.

Triggers for a Flare-Up

Any form of stress can weaken the immune system and allow the mycoplasma infection to take hold and cause active disease. Common triggers include:

• Poor Air Quality: This is the number one trigger. High levels of ammonia from urine buildup in a dirty cage are extremely damaging to the respiratory lining.
• Improper Bedding: Dusty beddings can irritate the airways, while aromatic beddings like cedar and non-kiln-dried pine release toxic oils that compromise lung function.
• Stress: Social stress from incompatible cagemates, a barren environment, or nutritional deficiencies can all trigger a flare-up.
• Other Illnesses: If a mouse is fighting another illness, its immune system is weakened, giving the mycoplasma an advantage.

Signs of a Mycoplasma Flare-Up

The signs can range from subtle to severe and tend to worsen over time.

• Porphyrin Staining: The first sign is often the presence of a red-brown discharge around the nose and eyes. This substance, called porphyrin, is not blood but is a pigment secreted from a gland behind the eye during times of stress or illness.
• Sneezing: Persistent sneezing is a common early sign.
• Audible Breathing ("Chattering"): The most characteristic sign is a clicking, rattling, or chattering sound when the mouse breathes. This indicates inflammation and fluid in the airways.
• Labored Breathing: In advanced stages, you may see the mouse's sides heaving with the effort of breathing.
• Lethargy and a Puffed-Up Coat: A sick mouse will be less active and its fur may appear unkempt.
• Head Tilt: If the infection spreads to the inner ear, it can cause a persistent head tilt and loss of balance.

Management, Not a Cure

It is crucial to understand that treatment for mycoplasmosis is about management, not a cure. The goal is to get the active infection under control and make the mouse comfortable.

• Veterinary Care: A veterinarian will prescribe antibiotics to fight the flare-up. A combination of drugs, such as doxycycline and enrofloxacin (Baytril), is often used for best results. Anti-inflammatories may also be prescribed.
• Supportive Care: During a flare-up, ensure the mouse is kept warm and has easy access to food and water. A temporary move to a smaller, single-level "hospital cage" can be helpful.
• Husbandry is Key: The most important part of management is providing impeccable husbandry to prevent future flare-ups. This means:
  • Excellent Ventilation: The cage must have good airflow.
  • Fastidious Cleanliness: The cage must be kept scrupulously clean to prevent any ammonia buildup.
  • Safe Bedding: Use paper-based or aspen bedding with low dust.

By managing their environment carefully, you can minimize the impact of this chronic disease and provide your pet mouse with a long and happy life.