Objectives

The primary objectives of this session are to review the known bacterial and viral causes of the canine infectious respiratory disease complex CIDC followed by a discussion of optimal diagnostic tests, treatments, and preventions.

The most common agents associated with CIRDC include canine adenovirus 2, canine respiratory coronavirus, canine influenza viruses (H3N8 and H3N2), canine herpesvirus, canine pneumovirus, canine parainfluenza virus, B. bronchiseptica, Streptococcus equi subspecies zooepidemicus, and Mycoplasma spp. Dogs with CIRDC generally present with an acute onset of cough with or without sneezing. Nasal and ocular discharges can also occur depending on the causative infectious agent. Some agents like the H3N2 influenza virus induce fever. A very acute course of disease resulting in hemorrhagic pneumonia and death seems to be most common with S. equi var. zooepidemicus. If the affected dog has not completed a routine distemper, parvovirus, adenovirus 2, and parainfluenza vaccine series, canine distemper virus infection (CDV) can also be a cause of CIRDC. This virus also induces diarrhea and can cause mucopurulent ocular and nasal discharge.

Each of the CIRDC agents can be harbored by normal dogs making it difficult to interpret molecular diagnostic assay results in some cases. Coinfections can occur in some dogs and may potentiate clinical signs of disease. While vaccines are available in some countries for many of the causes of CIRDC, with the exception of CDV, immunity is not permanent and booster vaccines are needed yearly (2011 AAHA canine vaccination guidelines; www.aaha.org). In addition, even when dogs are vaccinated against canine parainfluenza virus, canine adenovirus 2, H3N8 canine influenza virus, H3N2 influenza virus, or B. bronchiseptica, clinical signs of disease can still occur if the dog is exposed. However, the clinical disease should be less severe and of shorter duration than in dogs that have not been vaccinated.

All dogs with a history consistent with acute cough should have a thorough physical examination performed preferably in the parking lot of the hospital or an examination room that is easy to isolate and disinfect. If pulmonary crackles are auscultated, thoracic radiographs are indicated; for biosafety reasons, the dog should be transported through the hospital on a gurney that can be disinfected and the radiographs made at the end of the day so that the radiology room can be disinfected. Respiratory treatment guidelines are available from ISCAID which also discuss the diagnostic issues with CIRDC (Lappin et al. 2017). Cytology of discharges to attempt to diagnosis bacterial CIRDC and pick antibiotics was not recommended. However, if fungal diseases are suspected, cytology of nasal discharges can still be of diagnostic benefit. If the affected dog is not febrile, is eating, and has no evidence of pneumonia, no specific tests are needed and the dog can just be treated as indicated by the history and physical examination findings.

If the dog has significant clinical illness or an outbreak is suspected, aerobic bacterial culture and antimicrobial susceptibility testing, Mycoplasma spp. culture (or PCR assay), and molecular diagnostic procedures for canine parainfluenza virus, canine adenovirus 2, canine distemper virus, canine respiratory coronavirus, canine influenza viruses, canine herpesvirus, pneumovirus, B. bronchiseptica, and Mycoplasma spp. (or M. cynos alone) can be performed. However, as discussed, nucleic acids of all of the CIRDC agents can be amplified from both healthy and diseased dogs and modified live vaccine strains of the organisms can also be amplified.

Thus, a positive molecular assay result may not prove causation. Molecular assays also can give negative results by the time dogs are presented since viral shedding rates tend to peak very early in disease. For the influenza viruses, if the dog has not been vaccinated, a rising titer can also be used to prove an acute exposure and so saving serum from the first presentation to compare to a second serological test results 2–3 later can be beneficial.

Most dogs with CIRDC have viral causes and so antibiotics are usually not indicated. Most dogs maintain normal appetite and attitude and clinical signs generally resolve spontaneously within 10 days without antimicrobial therapy. The ISCAID working group recommended that antimicrobial therapy only be considered within the 10-day observation period only if fever, lethargy, or inappetence is present together with mucopurulent discharges (Lappin et al. 2017).

When dogs with suspected bacterial CIRDC are identified based on mucopurulent nasal discharge, fever, lethargy, or inappetence but no clinical evidence of pneumonia, the ISCAID working group recommended the administration of doxycycline (5 mg/kg PO q12h or 10 mg/kg PO q24h) empirically for 7–10 days as the first line antimicrobial option. This antibiotic should be effective for most CIRDC cases caused by B. bronchiseptica or Mycoplasma spp. The other first line drug options recommended by the ISCAID working group were amoxicillin or amoxicillin-clavulanate (Lappin et al. 2017). Amoxicillin should be effective for most secondary bacterial infections (to primary viral infections) and S. equi var. zooepidemicus, but not Mycoplasma spp. which lack cell walls. Amoxicillin-clavulanate may be required for some B. bronchiseptica isolates. Inhalational aminoglycoside therapy has also been anecdotally mentioned as a treatment for B. bronchiseptica-associated CIRDC. If the first drug chosen is ineffective and bacterial disease is still suspected after the first 7 days, the ISCAID working group recommended that a more extensive diagnostic workup should be considered prior to considering use of other drug classes like fluoroquinolones or azithromycin. Repeated diagnostic tests are not needed in dogs with CIRDC that respond clinically.

Dogs with cough due to uncomplicated CIRDC should have rest enforced, be handed with a harness, not a collar, fed soft or canned food if showing signs of discomfort when swallowing, and have the cough controlled with anti-tussive agents.

References

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2.  Lappin MR, Blondeau J, Boothe D, et al. Antimicrobial use guidelines for treatment of respiratory tract disease in dogs and cats: antimicrobial guidelines working group of the international society for companion animal infectious diseases. J Vet Intern Med. 2017;31:279–294.

3.  Priestnall S, Erles K. Streptococcus zooepidemicus: an emerging canine pathogen. Vet J. 2011;88:142–148.

4.  Priestnall SL, Mitchell JA, Walker CA, et al. New and emerging pathogens in canine infectious respiratory disease. Vet Pathol. 2014;51:492–504.

5.  Renshaw RW, Zylich NC, Laverack MA, et al. Pneumovirus in dogs with acute respiratory disease. Emerg Inf Dis J. 2010;16:993–995.

6.  Ruch-Galle R, Moroff S, Lappin MR. Adenovirus 2, Bordetella bronchiseptica, and parainfluenza molecular diagnostic assay results in puppies after vaccination with modified live vaccines. J Vet Intern Med. 2016;30:164–166.