By Patty Wilber
I got vaccines last week. Three shots: shingles, COVID, and a combined shot (Tdap) for tetanus, diphtheria, and pertussis. Just for the record, they made me feel pretty puny for a couple days. And led me to researching vaccines for horses.
The shingles is a two-shot series (second one in July) and when the series is complete, should protect me from that scourge for the rest of my life. The Tdap is good for 10 years, and the COVID, unfortunately, is only good for around six months. Many other human vaccines are good for our lifetimes.
This got me thinking about vaccines in horses. The rabies vaccine can protect a horse for three years, but most other horse vaccines are only good for six to 12 months. Why is horse vaccine effectiveness so short-lived?
The immune system is incredibly complicated but here is a simplified version of how vaccines activate the Acquired Immune System to create long-term immunity in humans.
The shot delivers an inactive version or portion of the pathogen. White blood cells called macrophages engulf the pathogen part, process it, and then “present” a molecular fragment on their surfaces. Another white cell, called a T-helper cell, recognizes the fragment on the macrophage and binds to it. This interaction between the macrophage and the T-helper cells “activates” the T-helper cell.
In the meantime, B-cells (ANOTHER white blood cell) also bring the pathogen inside, process it, and present the very same fragments on their surfaces. The activated T-helper cells bind to that fragment attached to the B-cell, and this interaction causes the B-cell to become a plasma/memory cell. The plasma cells produce the antibodies that can attack that specific type of pathogen.
The plasma cells are also called memory cells because some of them continue to circulate in our blood for years and if that pathogen is encountered, the plasma cells recognize it, divide, and produce millions of antibodies in a short amount of time. All those antibodies neutralize the pathogen and prevent us from being sick from that pathogen! So many vaccines for humans work this way and provide long-term immunity created by our own cells. Vaccines, honestly, are a miracle of modern medicine.
The horse Acquired Immune System works as I described for humans, except that the memory cells don’t last all that long, so long-term antibody-mediated immunity is not achieved. Instead, we have to restimulate the system every six to 12 months by re- vaccinating. In New Mexico, that usually means a shot or shots for Eastern and Western Encephalomyelitis, Equine Influenza, Tetanus, Rhinopneumonitis (equine herpesvirus), and West Nile is/are given once or twice a year to prevent morbidity and mortality from those diseases.
There is no specific research I could find to explain why horses do not have a strong memory cell response. Some untested hypotheses include: 1. Horses have a more robust innate immune system than humans so are born with the ability to fight off a lot of pathogens, and don’t need to acquire protection. 2. Horses evolved as relatively short-lived species, so the cost of producing memory cells that lasted a long time wasn’t worth it. 3. Horses move around a lot, so would be expected to encounter novel pathogens in each new location, so why waste energy remembering last pasture’s pathogens when the new location has all new illnesses? 4. The immune system can consume a lot of energy, and horses need to prioritize that energy for running away from predators instead.
Happy Friday!
