The dogs in this highly trained sled-dog pack competed in a sledge dog race after prolonged transportation and were additionally moderately exercised before the race. Three days after the sled race the dogs were given a commercial polyvalent canine vaccine containing CDV, CAV-2, CPV, CPIV, CCoV and leptospira. Clinical signs of respiratory disease were observed four days after vaccination. Immunosuppression in dogs immunised with a similar polyvalent vaccine was reported by Phillips and others . They demonstrated that the absolute lymphocyte count in blood was suppressed on day five and seven following immunisation and that one group of dogs immunised with CDV and CAV-2 also had decreased lymphocyte count on day three post vaccination. All the dogs produced neutralising antibodies to canine distemper virus and some parts of the natural immunity were not influenced by the vaccination. However, the immunisation with polyvalent vaccines significantly suppressed the lymphocyte response to phytohaemagglutinin, a mitogen that induce proliferation in T-lymphocytes in vitro on day 5, 7 and 11 post inoculation, but returned to normal response level by day 14 following immunisation. The combination of CDV/canine adenovirus type 1 (CAV-1) or CDV/CAV-2 was believed to be responsible for the suppression in lymphocyte responsiveness. The duration of the suppressed lymphocyte response was at least seven days and was most pronounced on day 7 post inoculation with up to 90% suppression of mitogen-induced lymphocyte response compared to control dogs. In the present study, the use of modified live polyvalent canine vaccine combined with stress associated with training, competition and prolonged transportation may have suppressed the innate immune response to S. equi subsp. zooepidemicus, which seldom causes pathology in dogs. The possibility of a co-infection with viral pathogens cannot be excluded and may have contributed to this outbreak.
Outbreaks of haemorrhagic pneumonia in dogs due to S. equi subsp. zooepidemicus have been described following transportation to a race track associated with sudden change in the weather , in newly arrived densely housed research dogs [4, 6, 7] and in kennelled dogs [3, 10–12]. In most of these reports the cause of the outbreak of CIRD is believed to be primary viral agents alone or in combination with environmental factors that induce secondary peracute pneumonia. Some of these papers report the use of CDV/CAV-2 vaccine prior to infection with S. equi subsp. zooepidemicus or a possible dual infection with CDV , though do not postulate that immunosuppression due to vaccination with modified live polyvalent canine vaccine is significant in this pathogenesis.
In a study of experimental parvovirus infection in dogs, Potgieter and others  observed that dogs vaccinated with modified live CDV and CAV-1 five days before challenge with virulent canine parvovirus resulted in disease caused by canine parvovirus whereas unvaccinated dogs remained healthy. This may indicate that these animals had reduced immunity associated with recent vaccination.
Histopathologically, the pneumonia in Case 1 was severe with the presence of inflammatory cells, leakage of blood into the lung tissue, and intralesional gram positive cocci, which were likely to be S. equi subsp. zooepidemicus (Figures 2 and 3). Similar histopathological findings were also described in the paper of Priestnall and others , who specified that S. equi subsp. zooepidemicus is associated with acute and often fatal clinical disease in dogs.
A possible source of S. equi subsp. zooepidemicus in the outbreak described in this paper was neighboring horses. However, there may have been sub-clinical carrier animals within the pack as Chalker and others  isolated this pathogen from 9.7% of clinically healthy kennel dogs, but where the kennel had a history of endemic CIRD.
S. equi subsp. zooepidemicus is not normally carried by dogs, but it can probably cause disease in a situation where there is high level exposure from diseased dogs or other sources in the environment, an on-going viral infection, temperature stress, transport stress, intense exercise associated with training and competition and vaccine induced immunosuppression. In a retrospective study of 393 cases of streptococcal infections in dogs, four cases (1%) were found to be caused by S. equi subsp. zooepidemicus. In humans, S. equi subsp. zooepidemicus represents a potential virulent zoonotic pathogen, with infection inducing a toxic shock syndrome following contact with horses [16–18].
One report used a study population from a re-homing shelter with an endemic respiratory disease, including 39 dogs positive for S. equi subsp. zooepidemicus culture on post-mortem lung wash and 16 negative control dogs. An objective scoring system based on histopathological lung examination demonstrated that 26 (67%) of dogs with a positive culture had evidence of pneumonia caused by S. equi subsp. zooepidemicus. The lungs of the dogs affected with severe pneumonia caused by S. equi subsp. zooepidemicus had significantly elevated levels of expression of the genes of pro-inflammatory cytokines indicating a strong innate immune response caused by the invading pathogen.
Paillot and others  demonstrated that three novel genes (szeF, N and P) for production of superantigens in S. equi subsp. zooepidemicus are occurring in half of the isolates from cases of disease. The authors indicate that the introduction of these genes to the population of S. equi subsp. zooepidemicus may be the cause of an emerging trend of severe pneumonia caused by this pathogen. However, in the study of Priestnall and others  there was no indication that the carriage of these superantigen encoding genes is related to the severity of the histopathological changes. The authors emphasise that further knowledge and alertness of early clinical signs will contribute to enhance the treatment success and avoid transmission of the causative strain of S. equi subsp. zooepidemicus to other animals and humans.
The rate of resolution of pneumonia in humans has been defined as the resolution of radiographic abnormalities associated with the disease . Patients with moderately severe pneumonia have a radiological clearance rate of 70% after 1 month, and more delayed if the pneumonia is severe . A multicenter cohort study that included adult patients classified as at low risk of short-term mortality concluded that full resolution of symptoms from community-acquired pneumonia (CAP) may take more than 28 days . This study was not specific to athletes, as they are thought to have a better physical health status compared with the general population. When to recommence training for athletes following pulmonary infection depends upon the underlying microbial aetiology and should be individualised . There is no reason to believe that dog athletes should be advised differently. The long-term rehabilitation of high performance sled dogs may be complicated because of the dual challenge of strenuous exercise and low temperature.
As a general rule, caution should be taken to prevent athletes from returning to competition too fast. In human athletes, potential complications such as pneumothorax, bronchiectasis and hemoptysis has been recorded, as well as acute respiratory failure. Furthermore, intense exercise too early in the rehabilitation process can increase susceptibility to viral illness and weaken the muscle performance .
We may speculate that the environmental stress related to the transportation, high-end competition, time of vaccination and too early exercise all contributed to the immunosuppression and consequently the susceptibility to virus- and/or bacterial infection, and where potentially more aggressive and opportunistic bacteria like S. equi subsp. zooepidemicus can progress quickly. It may also be advisable to choose a later moment for vaccination than close up to a challenging physical competition, and to avoid any strenuous exercise for at least ten to 14 days after immunisation.
Return to play is a hot issue within the top-level sports, but without any specific guidelines post pneumonia. These dogs were in a very good physical shape at the start of the outbreak of haemorrhagic pneumonia, and the two surviving individuals recovered relatively fast and returned to their previous performance level by setting time track records in their first race after recovery.