What is Failure of Passive Transfer?
Foals are born without immunoglobulins (antibodies) and have a very immature immune system. Colostrum from the mare is essential for the survival of a foal since it contains immunoglobulins, known as IgGs.
Foals that do not receive adequate colostrum in the first 12-24 hours of life do not have a sufficient level of IgGs to fight off pathogens (bacteria, viruses etc) and are therefore susceptible to infection until their immune system is able to develop its own antibodies. Antibody production by a foal’s own immune system is detectable around 10-14 days of life but true adult levels take up to 12-14 weeks to achieve.
Causes of Failure of Passive Transfer
Causes of FPT can be either attributed to the mare or the foal. Mares may not produce enough good quality colostrum or it may be due to milk leakage prior to foaling. Foals may not be able to suckle adequately due to a variety of reasons (Weakness, Neonatal Maladjustment Syndrome, Hard Palate deformities etc), or even from being rejected by the mare and not being allowed to suckle. Other issues may arise from the foal having an inability to absorb colostrum within their intestines, and this can occur if a foal ingests large amounts of bacteria orally either during birth or whilst trying to feed.
Diagnosis of Failure of Passive Transfer
FPT can be measured by collecting a blood sample from a foal, (usually) before 24 hours of age, but can be taken up to 7 days of age.
Laboratory testing using an immunoglobulin assay, which gives an accurate and quantitative result, is considered the gold standard. This test takes 24-48 hours to receive a result.
Alternatively, a convenient foal-side test is available which gives a semi-quantitative result (See Figure 1). The main advantage being that a result is obtained at the time of testing. This means that action can be taken immediately if the foal is considered to have FPT.
Complete FPT is determined when a foal has levels of IgGs less than 200mg/dL, and partial FTP is determined when a foal has levels of IgGs between 200 and 800mg/dL. Therefore, a foal that has greater than 800 mg/dL of IgGs is considered to have adequate immunity.
The image below is of a foal side test (Idexx SNAP foal IgG) obtained from a foal that’s dam had leaked milk prior to birth. It was observed to be drinking regularly from its mother but as can be seen from the test has only partial immunity and therefore has partial FPT. In this case, leakage of milk prior to birth likely resulted in poor quality colostrum being consumed by the foal.
Figure 1: Result of an IgG foal side test which is between 400-800mg/dL indicating partial FTP.
It is important to know that IgGs derived from the dam (through colostrum intake) start to wane at around 4-6 weeks of life, and we also know that a foals immune system is not producing enough IgGs to be considered as protected on its own until around 12 weeks. This means that if a foal has partial FPT (<800mg/dL) within their circulatory system they may be susceptible to infection around 6-8 weeks of age.
What to do if a foal has low levels of IgGs
Unfortunately once a foal is 24 hours old it has limited ability to absorb IgGs from colostrum. Therefore, once they have reached this age the only way of guaranteeing that they have adequate levels of IgGs within their system is to administer hyperimmune plasma. This is commercially known as Equiplas and contains large amounts of IgGs (between 20 and 30g per litre) harvested from donor mares (healthy, vaccinated adults). This can be administered to foals via slow intravenous infusion by a veterinarian, and can be conducted either in a hospital or on farm, as long as a clean dry environment is available.
It should be noted that even if your foal is diagnosed with FPT at this stage of life, feeding colostrum is still really beneficial for their health, as it is full of valuable nutrients, vitamins, and minerals, which are often higher in quality and quantity than in post-colostral milk.
Management of foals during the first phase of their life can be a tricky business, so ensuring they have adequate colostral immunity will go a long way to helping them lead a long and happy life!