Respiratory tract infections in the guinea pig can be manifested by nasal and ocular discharges, sneezing, abnormal respiratory sounds, difficulty in breathing and increased depth of breathing; these manifestations are often accompanied by systemic signs such as altered appetite or anorexia, lethargy and poor condition of the animal’s coat. Sudden death may occur sometimes. In the case of severe dyspnoea the animal should be handled with great care to avoid worsening the condition and, if necessary, can be given oxygen before carrying out any diagnostic or therapeutic manoeuvres.
Bilateral nasal swabs or tracheal lavage (inoculating 2 ml/kg of sterile saline solution), carried out with the animal anaesthetised, can be useful to determine the aetiological cause of the infection. Chest X-rays in the two standard projections enable better evaluation of the extent and severity of any lesions and, thereby, help to make the prognosis more accurate. X-rays of the skull are useful for assessing the tympanic bullae, which may become involved by the infection.
Bacteria are the main causes of respiratory tract infections in guinea pigs. Predisposing factors are poor environmental conditions and states of stress, such as overcrowding, poor hygiene, scarce ventilation and vitamin C deficiency. The most common bacterium responsible for respiratory tract infections in the guinea pig is Bordetella bronchiseptica; this microbe can cause catarrhal or mucopurulent rhinitis, ocular and nasal discharges, tracheitis and pneumonia with consolidation of the lungs. The infection may be fatal or be the cause of miscarriage and stillbirths. Infected guinea pigs may also develop immunity or become chronic carriers of the infection.
Chlamydophila caviaeis a common bacterium in guinea pigs which are infected by horizontal transmission (presumably via inhalation or sexual contact) at 2-8 weeks old. The incubation period is 2-4 days. This microbe usually causes asymptomatic, self-limiting infections. The clinical signs generally include conjunctivitis and, more rarely, rhinitis and abortion. The diagnosis is made from the finding of intracytoplasmic inclusion bodies in cells in a conjunctival scrape. The treatment consists of locally applied ophthalmic products containing tetracycline or parenterally administered enrofloxacin.
Other microbes isolated from guinea pigs with respiratory tract infections are: Streptococcus pneumoniae, Streptococcus zooepidemicus, Staphylococcus aureus, Pseudomonas aeruginosa, Klebsiella pneumonia, Pasteurella pneumotropica and Pasteurella multocida.
There are documented cases of viral pneumonia in guinea pigs: these are caused by a species-specific adenovirus and are characterized by low morbidity but high mortality. The transmission is horizontal and the incubation period is 5-10 days. The infection is clinically apparent in adults in good condition, but can be fatal in young and elderly subjects. The signs include breathing disorders (tachypnoea and dyspnoea), a nasal discharge, altered appetite, loss of weight, disheveled coat and dejection. Sudden death, without any premonitory signs, may also occur. In vivo diagnosis is difficult because it is hard to grow the virus in culture, although there is a polymerase chain reaction test. Post-mortem examination reveals emphysema, consolidation of the lungs, petechiae of the lung parenchyma; histological examination shows bronchitis and necrotising bronchiolitis as well as intranuclear inclusion bodies.
Since guinea pigs can be infected by adenovirus without having any clinical manifestations and experimental infection causes signs only in guinea pigs of 2-3 days old, it is presumed that the presence of concomitant factors, such as an immunodepressed state, is necessary in order for the infection to be clinically evident.
It is worth remembering that healthy guinea pigs cough and sneeze occasionally without this having any clinical significance. Sometimes rhinitis is due to irritant substances or allergies; loss of blood from the nose (epistaxis) can be caused by vitamin K deficiency or trauma. A bronchogenic papillary adenoma can occur in guinea pigs: this tumour accounts for a third of all neoplasms reported in guinea pigs. The tumours may be multiple and can, therefore, be mistaken for metastases, abscesses or granulomata. Bronchogenic and alveolar adenocarcinomas are also possible. Chest X-rays are useful for differentiating neoplastic lesions from pneumonia, particularly in cases that do not respond to antibiotic treatment.
Treatment may not be effective if the patient is brought for assessment when the disease is already advanced, or it may relieve symptoms without curing the patient. While waiting for the results of the culture studies, broad-spectrum antibiotics should be administered. Agents that are particularly indicated are:
- Chloramphenicol (50 mg/kg per os every 12 hours)
- Enrofloxacin (5-10 mg/kg per os every 12 hours)
- Trimethoprim/sulphonamide (30 mg/kg per os every 12 hours)
Supportive measures (keeping the animal warm and the environment clean and dry, administration of fluids and vitamins, forced feeding) are important. In some cases a diuretic (furosemide) may be indicated. Subjects with dyspnoea should be cared for in an oxygen cage. An aerosol, for 30-45 minutes two or three times a day, may be useful to remove mucus from the airways. Saline solution, with one or more of the following drugs added empirically, can be used:
- F10 diluted 1:250
- Acetylcysteine 2%
- Gentamicin 5 mg/ml
- Enrofloxacin 10 mg/ml
- Tylosin 10 mg/ml
- Aminophylline 3 mg/ml
Oral administration of a mucolytic (for example, bromhexine) may also be useful; the dose is extrapolated from that used in dogs and cats.
Prevention is based on avoiding contact between guinea pigs and other animals (rats, rabbits, dogs and cats) that can transmit the bacteria responsible and on taking care of the environmental conditions (humidity and temperature). Sudden changes in temperature should be avoided and the animal’s cage must be well ventilated and kept scrupulously clean.
Respiratory tract infections in guinea pigs sometimes lead to infections of the middle and inner ear; in these cases the animal may develop incoordination, torticollis, circling or rolling to one side.