Acute respiratory infection can affect structures from the nasal passages to the alveoli. In some cases the infection can be self-limiting which it others it can be life-threatening. For others still it can become persistent and chronic. The types of infectious agents can have a significant geographical spread and only certain infections are seen in certain areas. For some it is the coexistence of two or more infections or an underlying comorbidity that gives the optimal conditions for disease to appear. Most classes of infectious agents can cause respiratory disease in dogs and cats including viruses, rickettsia, bacteria, fungi and nematodes.
Rhinitis is an all-encompassing term describing a condition where there is an active inflammatory reaction in the nasal passages causing sneezing, nasal discharge and nasal discomfort. Typically it involves secondary bacterial infections, and this can be so deep-seated as to involve the adjacent bony structures. Primary infectious causes include viral (feline rhinotracheitis, feline calici and reo virus), rickettsial (Chlamydia psittaci var felis), bacterial i (Bordetella bronchiseptica, Staphylococcus and non-haemolytic Streptococcus spp. and Mycoplasma spp.), and fungal (Aspergillus fumigatus [dogs and cats], A. felis and Cryptococcus neoformans (cats). Consideration needs to be given to FeLV and FIV, environment and exposure situations (catteries, kennels, etc.). Capillaria boehmi is an emerging nasal nematode of dogs in parts of Europe.
For dogs nonspecific rhinitis (inflammatory) tends to predominate and in that situation bacteria contribute as secondary pathogens. In cats FHV and B. bronchiseptica are the most important pathogens in shelter cats, either clinical affected or not and the tendency to develop clinical signs of disease in catteries is dependent on the type of management model and is reduced with reduction in stress.
Clinical signs typical of acute rhinitis caused by infection include nasal discharge and sneezing. Nasal discharges can be unilateral or bilateral, serous, mucoid or mucopurulent. As the condition becomes more chronic discharges can be blood tinged, and epistaxis, stertor and facial deformity and facial pain and epiphora and conjunctivitis may be apparent.
Infections of the major conducting airways typically are caused by viruses and bacteria. Parasitic infection will result in larval migration in the larger airways causing coughing, but Oslerus osleri is a nematode parasite that is specific to the trachea and tracheal bifurcation and causes parasitic tracheobronchitis. It is transmitted horizontally from dam to off-spring, but is less commonly seen due to widespread use of anthelmintics in pregnant bitches.
The single most common cause of coughing in the dog is kennel cough and it is presumed to involve infection with Bordetella bronchiseptica or canine parainfluenza virus (CPiV). Other viruses (CAV-2, CDV) and Mycoplasma spp. may be implicated, and in complicated cases secondary bacterial proliferation can result in bronchopneumonia. Coinfection appears common with B. bronchiseptica tending to have virus coinfection in 50% of cases, but is even greater for CPiV (> 80%). What is surprising is that 40% of healthy dogs are infected with B. bronchiseptica. It is more likely that viral infections need secondary bacterial involvement to become clinical significant and that viral infection alone will cause subclinical over very mild disease. Infection is probably by inhalation of infected aerosolised sputum, but close physical contact with oronasal secretions and fomites might also be a route of transmissions. The history of the dog being in a suitably infective environment is highly suggestive, but not always a requirement for diagnosis. Clinical signs typically include acute onset coughing that can be mild to severe, paroxysmal and is often described as harsh or hacking. There may be mild systemic signs of pyrexia, inappetence and lethargy. Signs typically appear 3–10 days after exposure.
The reports in recent years of transmission of equine influenza virus to kenneled dogs and the possibility that transmission has also occurred between dogs (in the USA outbreaks, but not the Newmarket, UK outbreak) is a worrying development and may indicate the emergence of a new canine infectious disease. While there is no clear evidence that this virus has entered the pet population in all geographical locations, the presence of respiratory infection with clinical signs varying from mild (kennel cough-like) to severe (pneumonia) might raise suspicion of canine influenza infection. In dogs in China seroprevalence for avian, pandemic and human seasonal flu virus can be as high as 5%. This raises issues of public health with the presence of these viruses in pet dogs. However, in the cases of canine flu seen so far the potential key involvement of Streptococcus spp. in the generation of severe disease needs to be considered, in particular in the context of a kenneled population. S. zooepidemicus has been reported in severe cases of bronchopneumonia and in some of these cases pulmonary haemorrhage has resulted in the expectoration of blood. There is experimental evidence that H5N2 can be transmitted to cats and can cause respiratory signs, but no reports to date of natural infection.
Lung Parenchymal Infections
Lung inflammatory disease is more properly known as "pneumonia." The predominant cell type will often determine the likely cause with neutrophils being associated with bacteria and eosinophils with parasitic diseases. In many cases the underlying cause of pneumonia cannot be determined, but the effect is the same with neutrophil infiltration of the lung and airways and secondary proliferation of bacteria, most of which are part of the normal local flora (typically gram-negative anaerobes). A major cause of pneumonia in the dog is aspiration, and there appears to be an increased prevalence in the Irish Wolfhound, but in general bacterial bronchopneumonia is relatively rare, and even rarer in cats. The rare inherited condition primary ciliary dyskinesia typically has recurrent bacterial bronchopneumonia as part of the clinical presentation.
Bacteria that can be regarded as not part of the normal flora include Pseudomonas and micro-aerophilic saprophytic organisms such as bacteroides, Nocardia and Actinomyces. In certain geographical areas mycotic pneumonia is important. The emergence of S. zooepidemicus as a causal agent in haemorrhagic bronchopneumonia has been noted in the last few years. Mycobacterial infections should be considered in cats and can be M. bovis (now very rare) or M. microti and are presumed to be picked up because of hunting behaviour. While TB can affect the lungs, GIT or be disseminated, it is mainly a skin disease. For respiratory infection with TB the route of infection is more typically haematogenous rather than by inhalation resulting in a more interstitial and diffuse appearance on radiography rather than classic granulomas (see neoplasia).
Respiratory parasites of cats and dogs are identified world-wide. The type of parasites involved will depend on geographical location but can include Filaroides spp. (Oslerus osleri, Filaroides hirthi), Crenosoma vulpis, Aelurostrongylus abstrusus, Capillaria aerophila. Cardiovascular parasites (Dirofilaria immitis, Angiostrongylus vasorum) can affect the lung as part of the normal parasite life cycle as can some gut parasites such as the Ascarids (visceral larval migrans). Considering parasites will have a prepatent period it could be argued they are chronic infections, but once they affect the animal the clinical signs have an acute onset and presentation. All these parasites result in eosinophilic pneumonitis and/or eosinophilic bronchitis, but some can cause pulmonary eosinophilic granulomatosis. Most respiratory parasitic infections follow a predictable transmission method using an intermediary host (typically molluscs) or a paratenic host (small mammals, bird, reptiles). For many of these parasites the dog is not the definitive host, but rather wild canids such as fox, coyote and wolf. This might explain why affected dogs are common show clinical signs and in some cases can develop life threatening illness. By contrast the cat is likely the definitive host for Aelurostrongylus abstrusus and this might explain why many infected cats remain asymptomatic.
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