When an animal first makes contact with a foreign substance, its body prepares a range of protein molecules (called immuno-globulins or antibodies) to attack the “invader” or infectious agent and then inactivate it.
These immuno-globulins come in five basic forms: IgG, IgA, IgD, IgE and IgM. Each has a specific role in protecting the body against one particular pathogen or foreign substance. For horses, the IgG molecules are the predominant type in the blood and colostrum.
As newborn foals have not had contact with pathogens in utero, they have not prepared their arsenal of IgG antibody “weapons” the immune system makes when an attack occurs.
In short, the body of a newborn foal knows what to do with any “invader,” but has not done this before, so there is no IgG in their blood to fight infection.
From this state, it takes three to six weeks to develop strong antibodies to any particular disease-causing agent. In this period any new born is susceptible to infection.
The first milk a mare makes, called colostrum, is needed by the foal to overcome this and to protect it from infections.
Colostrum is a rich mixture of essential nutrients including vitamins, sugars, and a range of important proteins, including a high concentration of antibodies (IgG) to infectious bacteria and viruses that the mother has been exposed to in her environment during pregnancy.
These viruses, which the mother has already built up immunity to with her IgG antibodies, may be an immediate problem for the foal that hasn’t yet had its first “fight” against these viruses, and therefore has no antibodies in its arsenal.
So, the sooner a foal takes colostrum from its mother, the sooner it has immediate protection from any disease-causing organisms. This transfer of IgG from the mother to the newborn is called “passive transfer” of immunity.
Foals must receive the IgG in the first six to twelve hours of birth as it is only during this time that the specialized cells in their small intestine are able to absorb the IgG supplied in the colostrum.
Foals not suckling early will have low levels of IgG in their blood, called “failure of passive transfer” (FPT). As a result, they are more likely to suffer from infections of the gut and lungs and also septic arthritis, which can be life threatening.
