Despite being a well-known disease, influenza is still a significant contributor to mortality in humans as well as animals. The Influenza virus is a major cause of the equine viral respiratory tract disease which is a big killer and makes a significant economic impact every year in the horse industry.
The outbreaks of the disease are more likely when the animals are sheltered in close quarters next to one another. Such arrangements are common at horse shows, racetracks, and even sales barns. The virus is spread by direct contact with the infected animals or through contact with body fluids in the form of virus-contaminated droplets produced by coughing or sneezing, and it is also spread through objects like clothing, water buckets, etc.
Symptoms that relate to the disease include a significant loss of appetite, fever, lethargy, coughing and nasal discharge, which changes from watery at first to pus and mucus later. Under normal cases, these symptoms can be resolved in 7 to 14 days with a cough usually persisting for longer. More severe complications can include secondary bacterial pneumonia, myocarditis or inflammation of the heart muscles, myositis or muscle inflammation and limb edema or fluid swelling.
Vaccination is one of the most effective ways to protect horses against the infection. It is known that commercially produced vaccines effectively protect horses against equine influenza but it is still a challenge, being a continuous threat to our horse's health.
The primary reason is that influenza viruses undergo a process called the antigenic drift, which means they change slowly over time. This process can result in new strains of the virus over time that the immune system fails to recognize. This results in people as well as animals getting the flu more than once or in spite of vaccination. This also results in influenza vaccines needing to be updated periodically. The vaccine needs to consider and protect against all currently circulating strains of the virus.
Another important factor contributing to the wide variety of circulating influenza virus strains is the fact that they can cross over the species barrier. This essentially means that a particular virus that only infected horses or other animals may later change to infect humans as well and vice versa.
Common examples of viruses crossing the species barrier include the transmission of the H5N1 and H7N9 or 'bird flu' virus in humans, which earlier only infected birds. Another common example is the crossing over of the equine H3N8 influenza virus that only infected horses to infect dogs in the US. Other cases of transfer of the virus from horses to dogs have been seen in the UK and Australia. This also raises important doubts whether the reverse of this can also happen. However, so far, it has been seen that the canine version of the virus has only very mild effect on the horses.
In conclusion, due to the influenza viruses’ ability to commonly infect a broad range of species or hosts and their ability to cross the species barrier at times and finally their continuous antigenic evolution, it represents a serious infectious threat and challenge to both humans and animals such as dogs, horses, and others.
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