Passive transfer of immunity – testing calves

Passive Transfer - Testing Calves

Passive Transfer – The process of ensuring that a newly born calf receives colostrum from its mother within the first twenty-four hours of life, thereby receiving maternal antibodies which are absorbed into the small intestine, is known as ‘passive transfer’.


The importance of the passive transfer of immunity

So why is passive transfer so important in the calving process? A calf, while born with a functional immune system, has limited immune resources. Colostrum should be fed to the calf as soon as possible after its birth in order to give the calf a fighting chance of getting some immunity protection from its mother to sustain it until it can develop its own antibodies.

It has been shown that calves that have received the correct amount of colostrum by the age of four weeks will reduce the producer’s costs compared with those that did not receive the correct amount. Treatments for health issues in calves are reduced and mortality rates are commensurately improved by upwards of 10%.


What is the test for passive transfer of immunity?

In order for testing to take place, a sample of blood must be taken from the calf and the blood separated into red blood cells and serum. The total blood serum protein (BSTP) is calculated from the sample using a refractometer which has been specially calibrated.


When should sampling take place?

There are a number of conditions that should be in place before sampling in order to ensure that a reliable BSTP figure can be identified. The first is to take the sample from a calf that is fully hydrated, which should be 60 to 90 minutes after its last feed. If it is not possible to take a sample within this time frame, you should aim to take the samples at the same time each day as hydration levels can vary within the 24 hour period. The calf should also be at least 24 hours old, and peak values may actually be reached 48 hours after colostrum feeding, according to some recent data.


How to read BSTP results

The BSTP values decrease the more at risk a calf is from infection. The following is the point-based scoring for guidance.

Under 4.5: The calf is at high risk of infection.
4.5 – 4.9: There is minimal protection from infection.
5.0 – 5.4: The calf has moderate protection when its exposure to pathogens is average.
5.5 – 6.0: The calf is well protected unless there has been heavy exposure to pathogens.
Over 6.0: The calf has absorbed a very large quantity of antibodies and is therefore well protected and has good passive transfer of immunity. The calf should be monitored to ensure that the reading is not distorted because the animal was dehydrated when the sample was taken or that it has scours.

Best practice suggests that farms should be aiming for BSTP scores reaching 5.0 in 80 per cent of calves with 50 per cent being over 5.5. Monitoring the feeding regime of new-born calves should ensure that these levels can be reached to promote the essential passive transfer of immunity.


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