Parasites are major causes of respiratory tract disease in the dog and cat. Recent advances in therapy of these diseases have been made providing the practicing veterinarian with a more rational treatment modality. This review will discuss the biology, diagnosis, disease, and treatment of respiratory parasites (protozoan, nematode, trematode, and arthropods) of the dog and cat. Emphasis will be placed on the use of modern chemotherapeutic agents in their control. The parasites will be discussed based on their location within the respiratory system; nasal mucosa and sinuses, lung parenchyma, and airways.
Nasal mucosa and sinuses
Epidemiology: Arthropod dipteran parasite; some 34 different species in North America.
- Large maggots seen in the skin of dogs and cats and represent the larva of the rodent and rabbit bots. Adult flies lay eggs around the entrances to rodent burrows, and when host passes, the small first-stage maggot hatches and jumps onto the passing host.
- Larva capable of entering the host through the mouth, nose, eyes, or anus.
- In rodent hosts, larvae remain as 1st-stage larvae within the nasopharyngeal region near the posterior end of the soft palate and in the nasal passages. Maturation time from infection to when the larva leaves the warble is between 3 to 8 weeks.
- Seasonality: Adult flies have single emergence in late spring in the more temperate parts of the United States.
- Cats: Infected as they hunt. Kittens may be infected by larvae brought back on the fur of the queen.
- How larvae enter cats unknown, but probably through the mouth, nose, or anus as they do in the rodent and lagomorph hosts.
Clinical signs: Depend mainly on where the larva locates.
- Skin lesions; (warble) containing a single larvae usual present on the cheek, neck, or back, but other sites are reported.
- Acute upper respiratory tract distress; severe sneezing, unilateral initially serous then mucopurulent nasal discharge. Sneezing and nasal discharge can persist for a week.
- May be accompanied by unilateral facial swelling especially over the nose.
- Bloody nasal discharge, soft palate and pharyngeal swelling have been reported.
- Laryngeal edema (larval migration in the cervical neck) may cause laryngeal edema and arrest. Respiratory signs may persist for few days to several weeks, then often recede in severity occasionally to be followed by neurologic signs
- If neurologic signs develop, they will do so one to two weeks after respiratory signs, although respiratory signs have been reported to occur as long as 4 to 10 weeks before the onset of neurologic signs.
Diagnosis: Viewing the larvae within the respiratory tract, or made on circumstantial evidence of acute rhinitis (sometimes progressing to neurologic disease) in an outdoor cat during late summer and fall. The larvae or its migration tracts through the brain may be identified on CT scan or MRI.
Treatment: Ivermectin (0.1 to 0.3 mg/kg, PO, q24h, 3 days) very effective.
- Prednisone (1 mg/kg, PO, q12h, 3 weeks, then 1 mg/kg, PO, q24h, 3 weeks). [Diagnosis based on clinical signs, but usually improve). Few develop neurologic disease. Some neurologic cats improve clinically with this treatment but outcome unchanged.
Pneumonyssoides caninum (canine nasal mite)
Epidemiology: Arthropod parasite of nasal sinuses of dogs. Occurs in the USA, Canada, Japan, Australia, Sth. Africa and Europe.
- Adult mites (1 mm X 0.5 mm in size) are easily identified by their leg morphology; the first pair of legs each terminate in a pair of large hooks, while legs two, three, and four each terminate in a sucker armed with a pair of smaller hooks.
- Believed that dog-to-dog transmission is by the direct transfer of larvae from one infested dog to another.
Clinical signs: Sneezing, although can present with facial pruritis, snuffling, snorting + nasal discharge and excessive lacrimation.
Treatment: Ivermectin (200 µm/kg body weight, PO or SC) or Milbemycin (0.5 mg/kg, PO) once weekly for 3 weeks are effective.