Canine influenza  

Andrew Vince

Canine influenza is caused by an H3N8 influenza A virus, and has been recognized intermittently since an outbreak of respiratory disease in racing Greyhounds in 2004. In that first outbreak, initial cases occurred in Florida and eventually spread to involve 30 states and Washington DC.

In 2015, a new outbreak of respiratory disease in Chicago led to the identification of an H3N2 strain of influenza A virus, apparently related to strains of canine influenza circulating in Korea, China, and Thailand since 2006, notably responsible for an outbreak of respiratory disease in dogs in Thailand in 2012. Current strains in Illinois and Iowa are reportedly closely related genetically to Asian strains, although a clear point of introduction to the United States has not been identified. There is no indication of an outbreak of canine influenza in Canada as of this report.

Clinical signs of canine influenza infection include fever, sneezing, nasal discharge, dry cough, and loss of appetite lasting 5-7 days.

The virus is extremely contagious; the American Veterinary Medical Association has estimated that as many as 80% of dogs exposed to the virus will be successfully infected. However, case fatality rates have been very low, with rare mortalities limited to those dogs with other co-morbidities. These viruses are typically shed for 7-10 days, beginning 24 hours prior to the onset of clinical signs.

Several recommendations have been posed by the CVMA and AVMA, including:

  • If you think your dog has influenza, contact your veterinarian before bringing your dog to the clinic. Most infected dogs will not require veterinary care. If your dog does become severely ill, it may need medical treatment, but contacting your veterinarian allows them to take steps to mitigate risk for other animals
  • If possible, keep your dog away from other dogs if it is ill or has been exposed to ill/infected dogs, for 7-14 days.
  • If you live in an area where the virus becomes endemic, diminish dog-dog contact while the virus is spreading (such as dog parks), and avoid travelling with your dog outside that area.

Avoid purchasing/importing dogs/puppies from endemic areas.

Influenza A matrix PCR testing is optimal for the diagnosis of canine influenza in both antemortem and post-mortem samples, requiring only submission of a nasopharyngeal swab in virus transport medium; tissues (principally lung) are also a useful postmortem sample. It is strongly suspected that the AHL immunohistochemical stain could aid in the postmortem diagnosis of canine influenza (based on prior detection of related viruses in other species), although the lack of Canadian cases has left this untested. Hemagglutination inhibition (HI) testing is of limited utility in diagnosis of acute disease, as it is most limited by strain variations and depends on the presence of antibodies, which may not be present until later in the progression of disease.  

Clostridial myositis resembling pseudoblackleg in a cat

Andrew Vince

The body of a 6-year-old cat was received for postmortem investigation at the Animal Health Laboratory. The cat had been seen by a veterinarian with an initial 1-hour history of repeated vomition and lethargy, had a fever of 40.5oC, but otherwise no significant abnormalities on physical examination. Bloodwork was recommended and declined, and the cat was treated with Convenia and Cerenia and sent home for monitoring and recheck the day following. The cat was examined again the following day , was still febrile, and rapidly became recumbent and stuporous. The cat began to act painfully and started dragging its right hindlimb, and there was palpable swelling and gas crepitation extending from the toes to the pelvis. The cat progressed rapidly to coma and cardiopulmonary arrest.

At postmortem, the right hindlimb was diffusely edematous, congested, and palpably emphysematous. The skeletal muscle of the right hind limb was irregularly dark red-black, dry, and friable (Fig. 1), with a characteristic odor (typically described as “rancid butter”). The lungs were diffusely firm, rubbery, and wet. Histologically, there was generalized hemorrhage and edema throughout the muscle and soft tissues of the right hindlimb; skeletal myocytes were swollen and hypereosinophilic, with rare sarcoplasmic fragmentation and vacuolation. Small numbers of neutrophils were noted in the interstitium. There was also generalized pulmonary edema, focal adrenal necrosis, and limited emphysema within the adrenal and spleen. Muscle from the right thigh was submitted for bacterial culture and clostridial fluorescent antibody testing (FAT), both of which demonstrated large numbers of Clostridium septicum in pure culture.

Clostridium septicum is a gram-positive, spore-forming, obligate anaerobic bacterium that can cause cellulitis and myositis without the need for percutaneous inoculation. In most species, it is postulated that infection is established through hematogenous spread from the gastrointestinal tract; myonecrosis then occurs as a result of the release of several exotoxins, including alpha, beta, gamma, and delta toxins.

Clostridium septicum is best described as a cause of acute fatal myositis and septicemia (called “pseudoblackleg”) in cattle, small ruminants, horses, and swine, with only a few cases described in companion animal and zoo/wildlife species. Pseudoblackleg is characterized by the absence of a detectable inoculation wound. In this case, and in most similar companion animal reports, pure culture of this bacterium and lack of obvious wounds resulting in infection strongly suggests a hematogenous origin, although the possibility of latent infection and the factors that might activate such an infection are poorly understood. In similar cases, death is typically rapid after onset of clinical signs, even with prompt treatment. Culture and/or FAT on tissue with consistent lesions of myonecrosis and emphysematous myositis is an effective way of confirming this diagnosis. Clostridial myositis should be suspected with the triumvirate of necrosis, hemorrhage, and tissue emphysema in soft tissues and muscle, regardless of animal species affected.   

Proximal right thigh of a cat with clostridial myositis. Muscle is regionally purple-black, and edema fluid is pooling in fascia.

Figure 1. Proximal right thigh of a cat with clostridial myositis. Muscle is regionally purple-black, and edema fluid is pooling in fascia.

Encephalitozoon cuniculi microsporidiosis in a 10 week old puppy

Margaret Stalker, Jennifer Lillywhite, Hugh Cai, Pat Bell-Rogers, Rebeccah McDowall, Qiumei You


A 10-week-old female Boxer puppy was received for postmortem examination, following a brief period of illness characterized by vomiting, laboured breathing, pale mucous membranes, hypothermia, and collapse, with rapid clinical deterioration culminating in death. The pup had been present in its new home for 3 days prior to the onset of clinical signs, and had received DAPPV vaccination 5 days prior, as well as periodic deworming with Strongid. The remaining 8 littermates were reported to be healthy.

On postmortem examination, the heart had faintly visible irregular areas of pallor visible on the epicardial surface, with moderate dilation of the right atrium and ventricle. Histologic examination revealed a widespread, multisystemic inflammatory disease characterized by infiltrates of macrophages, lymphocytes, plasma cells, and in some areas neutrophils, particularly evident in the myocardium, kidneys, and brain. Kidney sections had occasional medullary tubular epithelial cells with clear cytoplasmic vacuoles containing multiple, faintly staining ovoid organisms, ~1-3 µm long (Fig. 1). These organisms did not stain with antibodies to Neospora caninum or Toxoplasma gondii, and were PAS-negative, however, they did stain with Brown & Brenn tissue Gram stain (Fig. 2). Two gel-based PCR assays for Encephalitozoon cuniculi (1, 2) were strongly positive. The PCR product of each assay was sequenced and found to have 100% and 99% identity with the E. cuniculi small subunit ribosomal RNA gene. Further strain characterization using primers designed to amplify the internal transcribed spacer (ITS) region of the rRNA allowed further identification of the organism as E. cuniculi genotype II.

E. cuniculi is an obligate intracellular spore-forming parasitic fungal organism belonging to the phylum Microsporidia, and is best known as a relatively common parasite of rabbits and rodents, where it typically causes subclinical infections. A recent case series documented 19 cases of fatal encephalitozoonosis in puppies from Texas (3), and confirmed the identity of the organism as E. cuniculi strain III. Affected puppies ranged in age from 4-10 weeks, and had brief clinical histories of depression, inappetence, and neurologic signs. On postmortem, renal lesions were consistently found, characterized by lymphoplasmacytic and histiocytic interstitial nephritis with organisms visible in renal tubular epithelial cells, as in this case. Brain lesions were also typically found, with E. cuniculi spores in vascular endothelium in the brain parenchyma.

E. cuniculi has recently been divided into 3 strains: strain I infects rabbits and humans; strain II infects mice, rats, and blue foxes; and strain III infects dogs, swine, and humans, although the full range of susceptible species continues to expand. Infections in humans are typically in immunocompromised individuals, highlighting the potential zoonotic risk of these organisms. The life cycle is direct, and the primary route of transmission is through ingestion of infective spores shed in urine by infected hosts. Horizontal transmission is implicated in the Texas study, where most infected animals were associated with breeding facilities containing numerous dogs. Transplacental transmission from asymptomatic dams has also been documented experimentally in dogs. Clinically significant infections are generally seen in puppies, likely acquired from the dam. No effective treatment has been reported for dogs. In this case, the source of infection was undetermined in this puppy, but thought likely to be from an asymptomatic infection in the dam. This case represents the first report, to our knowledge, of E. cuniculi infection in a dog in Canada, and the first report of E. cuniculi strain II infection in a dog.   

Organisms in kidney, H&E stain.

Organisms in kidney, B&B Gram stain.

Figure 1: Organisms in kidney, H&E stain.

Figure 2: Organisms in kidney, B&B Gram stain.