Why Can't We Eradicate Equine Herpesvirus?
Eradication of infectious diseases is a great goal, but it's rarely practical. The best known (and perhaps only) example of infectious disease eradication is the elimination of smallpox. So, why is it so hard to do?
The following general criteria need to be in place to eradicate a disease:
- It must have a clearly defined host range and that range is ideally only one species. A disease that can affect multiple species is very hard to control.
- It must predictably cause disease in individuals that are infected.
- There must be no long-term carriage state. Once a person/animal gets over the illness, he/she/it must get rid of the infection completely in a defined and predictable period of time.
- A highly effective vaccine should be available.
- There must be a commitment to put in lots of time, money and effort everywhere the disease exists.
This isn't the case with most diseases, and equine herpesvirus (EHV) has many characteristics that make eradication impossible:
- Unpredictable disease: EHV infection doesn't always cause signs of disease. When it does cause disease the signs can be quite variable and difficult to easily differentiate from other infections.
- Longterm carriage: This is the biggest problem with herpesviruses. EHV is able to survive in a latent (dormant) state in the body after infection. It can lie dormant for a long period of time, but infected animals can always start shedding the virus again. A large percentage of horses are carrying EHV in their bodies and there's no way to get rid of it.
- Vaccine: Vaccines are available but they are by no means 100% effective at preventing infection.
- Time, effort, money and cooperation are terms that are not commonly associated with disease control in horses. Getting everyone to follow a standard recommendation (if one were able to control disease) would be difficult to impossible. The entire horse-owning population would not be willing to spend the money for broad control measures, and there's no real impetus for governments to do so. Even getting people to agree to follow basic vaccination and infection control recommendations is difficult. If there is any negative impact on use of horses, ease of management or any other minor inconvenience, 100% compliance with any recommendation becomes impossible to obtain.
We have to live with EHV. It will always be a risk to horses. Good infection control measures and vaccination of certain groups (e.g. pregnant mares) can help control the impact of the virus.
