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What’s the role of calf immunity in calf health?

Jim Quigley Published on 07 October 2009

Because calves are born without circulating immunoglobulins (Ig), particularly IgG, in their blood, it’s essential that they consume colostrum as soon as possible after birth. Absorption of IgG depends on many factors – quality of colostrum, age at feeding, level of stress, bacterial contamination in the colostrum, weather, and many others.

Therefore, there’s considerable variation in serum IgG in calves after 24 hours of age.

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After 24 hours, we can monitor the degree to which calves have achieved success of (or failure of) passive transfer by monitoring plasma or serum IgG concentration. Alternatively, serum total protein may be used to estimate serum IgG concentration.

Many farmers and veterinarians use serum total protein to estimate serum IgG concentration in young calves. Serum total protein can be quickly and conveniently measured with a clinical refractometer.

There is a good relationship between serum total protein and serum IgG, particularly in very young calves just a few days of age. More recently, the development of colostrum replacers allows producers to provide IgG to calves when colostrum is unavailable or of poor quality.

These products provide a consistent source of readily absorbable IgG for calves that can be fed with minimal delay and may reduce variation in overall passive immunity on a farm.

Serum IgG concentration
We can view the calf’s circulating IgG as an equation: Plasma IgG = IgG intake (grams) × AEA ( percent) ÷ Plasma volume (L)
Where:
• IgG intake (grams) = liters of colostrum consumed × colostrum quality (grams of IgG/L)
• AEA = apparent efficiency of IgG absorption, which generally ranges from 20-40 percent
• Plasma volume (L) = estimate of total liters of plasma in the animal; normally estimated as a percentage of bodyweight (often 7-9 percent of BW in kilograms).

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Proper colostrum management implies that we attempt to maximize intake by the grams of IgG from colostrum (feeding greater volume and/or higher quality) as well as efficiency of absorption (feeding early, minimizing stress, etc.) and minimize stress and bacterial contamination, which can reduce AEA.

Because serum IgG (or total protein) is closely related to passive immunity in the newborn calf, it’s common to think of calves with higher protein concentrations as healthier than calves with lower proteins. Not necessarily so. Let’s look at an example.

Observations on the farm
Recently, a study was conducted on a large California calf ranch wherein young calves (~1 day of age) were brought to the farm to be raised to 60 days. A subset of 678 calves from 51 farms were evaluated to determine if there was a relationship between serum total protein (measured within 72 hours of arrival using an on-farm refractometer) and morbidity (number of veterinary treatments recorded per animal) to 14 days.

Farms provided from 1 to 60 calves to the calf ranch during the trial; however, farms that provided fewer than five calves were deleted from the data, which left 629 calves from 31 farms. As can be seen in Figure 1*, there is a clear relationship between serum total protein and the number of veterinary treatments per farm.

Generally, the lower the serum total protein, the more treatments per calf. We can use the regression line to predict the extent to which increasing serum total protein would reduce treatments on this farm.

In this example, calves from farms that averaged a serum total protein of 5.0 g/dl would average 4.3 treatments per calf, whereas calves from farms averaging 6.0 would have 2.7 treatments per calf. If we assume that an “all-in” cost of a veterinary treatment is $3, then reducing treatments from 4.3 to 2.7 would save the calf raiser $4.80 per calf.

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Although these numbers make sense (i.e., greater serum protein, lower treatments), why is there so much variation in the data?

Let’s look at two farms as an example. Farm A provided calves with total proteins that averaged 5.31 g/dl on arrival but they were treated an average of 7 times per calf. Farm B provided calves with average total protein of 5.32 on arrival; calves were treated an average of only 2.8 times per calf.

Clearly, some factors other than total protein are important to whether or not calves become sick. In the case of Farm A, 85 percent of the calves arrived as the calf ranch classified as “dirty” (partially or mostly covered with mud and/or manure), whereas only 11 percent of calves were “dirty” when they came from Farm B.

The ‘health equation’
At the risk of oversimplifying the concept of health versus disease, we’ll focus on infectious disease, since most research suggests that intestinal and respiratory infections are responsible for most morbidity and mortality in young calves.

For the sake of this discussion, we’ll ignore other forms of disease – nutritional disease, metabolic disease, etc. We can define health as a balance between the animal’s ability to resist disease-causing pathogens and the level of those pathogens in the environment: Health = immunity / pathogenic exposure

When the animal’s ability to resist pathogens (i.e., immunity) is greater than the level of exposure, the animal remains healthy. On the other hand, when the level of exposure overwhelms the calf’s ability to resist, the calf becomes sick.

We generally use serum IgG as an indicator of calf immune competence. That’s because IgG in the blood is essential for the animal to recognize pathogens. Immunoglobulins bind to specific proteins on a pathogen (called the antigen) much like a key fits into a lock.

However, immune competence of the calf should be viewed as the sum of all of the animal’s defensive systems and their relative ability to identify and negate the effects of invading pathogens.

Overall immune competence is not only circulating IgG concentration, it also includes physical barriers against infection, numbers and activity of immune cells (leukocytes), and the complement system. The degree of stress to which the calf is exposed will impact immunity.

Calves exposed to transportation, cold or hot weather, dystocial calving, etc. may have an impaired immune response. Stress hormones interact with most aspects of the immune system and can dramatically depress calf immunity.

Exposure to pathogens
The second factor in the calf health equation is the degree of pathogenic exposure. Calves are exposed to different numbers and types of pathogens – viruses, bacteria, protozoa, etc. – throughout their life.

The degree of exposure varies depending on many factors – contact with contaminated animals, movement of pathogens through water, milk, air, human contact, contact with contaminated buckets, feeders, nipples, waste milk, etc.

In large part, exposure is a function of the overall level of management on the farm – understanding potential sources of infection, establishing procedures to minimize the risk of infection and monitoring key measurements on the farm to ensure that procedures are working. The degree of exposure on a farm changes constantly.

Calves that become sick will often shed large numbers of pathogens (viral particles, oocysts) into the environment, increasing herd exposure. Removal of contaminated bedding and proper cleaning reduces exposure.

All-in/all-out procedures reduce the overall exposure level as there is not a continuous introduction of contaminated animals onto the farm. Proper farm management can help keep overall exposure to a level that allows calves to resist infection.

Bottom line
Sometimes we can become obsessed with the idea that circulating IgG concentrations are the only factor when it comes to calf health. Achieving a certain concentration of serum IgG (10 g/L or higher) or total protein (5.2 g/dl or higher) becomes the goal in and of itself, rather than a component of the larger equation.

Rather, calf immunity is only one portion of the overall calf health equation and the level of immunity required to maintain good health depends on the farm and level of exposure.

Calves with failure of passive transfer of immunity can survive and grow well if they are raised in an environment with minimal pathogen exposure. Though serum total protein concentration is important, it is only one part of the calf health equation.

Equal attention should be paid to reducing the level of pathogen exposure on the farm. Remember the old saying: “It’s possible to do more with a shovel than with a needle.” PD

*References and figures omitted but are available upon request to

Jim Quigley
Vice President & Director of Calf Operations APC, Inc.

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