First reported in January 2004 in racing greyhounds in Florida, the current outbreak of canine influenza originates from the
equine H3N8 virus.1 This virus is transmitted directly dog-to-dog. In fact, it is the first time that influenza has demonstrated efficient horizontal
dog-to-dog transmission. As of April 2006, the Cornell University Animal Health Diagnostic Center has identified anti-influenza
antibodies in dogs not affiliated with greyhound racetracks from 20 states and the District of Columbia. More than 10% of
the samples were seropositive in Arizona, Colorado, Connecticut, Delaware, Florida, Massachusetts, New York, North Carolina,
and Washington, D.C. As these were spontaneous submissions, it isn't known what proportion of the samples were from healthy
or clinically affected dogs.
VIRULENCE AND CLINICAL SIGNS
All dogs are susceptible,2 and in a naïve dog population, influenza spread is rapid, with an infection rate of nearly 100% and 80% morbidity. Mortality
has reached 5% in high-risk populations (racing greyhounds), but the mortality rate in the general pet and shelter population
is probably less than 1%.2
The disease has a two- to five-day incubation period. Twenty percent of dogs show no clinical signs of illness but may be
carriers that can shed the virus.3 The clinical syndrome in most dogs is similar to infectious tracheobronchitis, and clinicians should use the same precautions
and management.
In the mild form of the disease, dogs have a mild fever and develop a moist and soft or dry and loud cough that may last up
to three weeks (even as long as 45 days). A greenish nasal discharge may be present from a viral rhinitis and secondary bacterial
infection.
In the severe form of the disease, dogs have a high fever (104 to 106 F) and may have tracheitis, bronchitis, suppurative
bronchopneumonia, and hemorrhage in the respiratory tract. The peracute form seen in greyhounds results in up to 36% mortality.1 Death may occur as little as four hours after infection.4 Extensive hemorrhage is seen in the lungs, pleural cavity, and mediastinum.
DIAGNOSIS AND TREATMENT
Detecting antibodies against the influenza virus confirms the diagnosis. Submit acute (current) and convalescent samples;
a single serum sample may be used in dogs that have recovered from a respiratory infection. Send samples to a laboratory that
tests for canine influenza virus such as Cornell University's Animal Health Diagnostic Center (
http://www.diaglab.vet.cornell.edu/issues/civ.asp). A pcr test can detect the virus in nasal swab samples, but this test has a high rate of false negative results. A positive
pcr test result indicates influenza infection.
Being an enveloped RNA virus, the influenza virus is easy to kill with disinfectants. Isolate affected dogs, and thoroughly
disinfect clothes, hands, surfaces, and equipment after exposure. The secondary bacterial nasal infection is responsive to
first-generation cephalosporins but is poorly responsive to doxycycline, amoxicillin trihydrate-clavulanate potassium, and
quinolones. In patients with bronchopneumonia, use aggressive antibiotic therapy and fluid therapy to maintain hydration.4 Antiviral medications (e.g. oseltamivir; Tamiflu—Roche) have not been adequately evaluated in treating canine influenza but are unlikely to substantially
change the course of the disease.
PREVENTION AND ZOONOTIC POTENTIAL
Canine influenza vaccines are being developed but are not currently available; the equine influenza vaccine does not protect
dogs. Vaccinations against other respiratory diseases should be kept current so that clinicians can reach an early and accurate
diagnosis of influenza infection. Eventually canine influenza will reach an enzootic pattern from the current pandemic pattern.
The H3N8 equine influenza virus has been around people for more than 40 years with no documented human cases. Thus, there
is virtually 0% risk of canine influenza virus transmission to people.3
Helio Autran de Morais, DVM, PhD, DACVIM
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Helio Autran de Morais, DVM, PhD, DACVIM (internal medicine and cardiology)
Department of Medical Sciences
School of Veterinary Medicine
University of Wisconsin
Madison, WI 53706
REFERENCES
1. Crawford PC, Dubovi EJ, Castleman WL, et al. Transmission of equine influenza virus to dogs. Science 2005;310:482-485.
2. Rishniw M. Canine influenza. 2005. Available at: http://
http://www.vin.com/Members/Associate/Associate.plx?DiseaseId=2647.
3. Matrosovich MN, Matrosovich TY, Gray T, et al. Human and avian influenza viruses target different cell types in cultures
of human airway epithelium. Proc Natl Acad Sci USA 2004;4620-4624.
4. Centers for Disease Control and Prevention. Media briefing on canine influenza. Available at:
http://www.cdc.gov/od//oc/media/transcripts/t050926.htm.
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