June 7 2024 03:05 PM

Tracking subclinical pneumonia reveals key practices for preventing calfhood respiratory disease.

These petrifilms are used to visualize the amount of air contamination in the calf microenvironment. Each blue dot represents an airborne bacteria collected over 10 minutes. The film on the left was collected in a calf barn with open front and back panels, lower stocking density, and better ventilation than the calf barn on the right.

Respiratory disease in the preweaning period remains a challenge within the dairy industry. This diagnosis has two categories: clinical pneumonia and subclinical pneumonia. Clinical pneumonia is characterized by physically observable signs of illness such as cough, elevated respiratory rate, and nasal or ocular discharge. Subclinical pneumonia occurs when calves appear healthy but have varying levels of lung consolidation that is only detectable with advanced imaging such as lung ultrasound.

In a 2014 USDA National Animal Health Monitoring System (NAHMS) study, it was determined that approximately 9.5% of calves experience clinical pneumonia before weaning. Research has shown that for every clinical case of pneumonia, there are about two to four subclinical cases of pneumonia. When extrapolating from this data, we can determine that about 30% to 50% of calves experience respiratory disease in the preweaning period.

The use of ultrasound to aid in the diagnosis of calf respiratory disease has become an increasingly popular practice as it is the only way to detect subclinical pneumonia with a high level of accuracy. Strategic, routine lung ultrasound allows us to find calves with lung consolidation and provide them with the treatments they may need.

Our practice has been doing routine lung scanning for nearly six years and has conducted a peer group study to gather data and compare calf rearing practices with health outcomes. With the aid of lung ultrasound, we have a few points of discussion from the collected data that we hope can help prevent calf pneumonia.

Start out on the right foot

Any discussion about calf health begins with colostrum. High-quality colostrum containing at least 200 grams of IgG delivered within the first couple hours of birth is necessary to achieve successful passive transfer.

The most recent industry goals for successful colostral passive transfer are to have at least 70% of calves with a serum total protein (STP) above 5.8 grams per deciliter (g/dL) and at least 40% with STP above 6.2 g/dL (Table 1). A higher STP indicates that more antibodies were transferred to the calf to protect it from disease until its immune system begins to produce antibodies on its own.

It is interesting to reflect on how these goals have evolved over the years — consider that in 2011, calves with an STP above 5.2 g/dL were considered to have received adequate passive transfer! We believe it is reasonable to speculate that these goals will continue to rise, especially in regard to protection against calf pneumonia. 

When these new industry standards for passive transfer were established in 2020, the risk of disease associated with each category of STP was broadly analyzed. More recent studies have shown that the risk of clinical pneumonia is only reduced for calves that receive excellent passive transfer.

There has not been published research on the impact of passive transfer category on the risk for subclinical pneumonia diagnosed with lung ultrasonography. As a clinic, we have found that calves with an STP 6.2+ g/dL are far less likely to have subclinical pneumonia compared to lower PT categories (Figure 1). This also happens to be the highest and newest industry standard goal.

Considering the recent research looking more closely at certain diseases’ relationships with each passive transfer category, and with our clinic’s data, we conclude that we may need to aim higher on passive transfer to protect against respiratory disease specifically. Whether that new goal is a larger proportion of calves or a higher threshold of STP in the excellent category, this improvement could help reduce the risk of respiratory disease in preweaned calves.

Air in the microenvironment 

The quality of air a calf breathes has a tremendous impact on its risk of developing pneumonia. Air quality has several contributing factors, including building ventilation, individual calf stall ventilation, and the level of air contamination. 

Adequate building ventilation brings fresh air into the barn (via positive pressure tubes, natural ventilation, and so on) and allows contaminated air to escape. For building ventilation to be impactful, we must consider the air at the level of the calf, which brings the idea of the calf microenvironment to the forefront. Consider a calf barn with one calf in an individual stall with four solid panels surrounding it versus another calf in an individual stall with four open fence panels. Which microenvironment would be easier to fully replace the stall’s contaminated air with fresh air?

Our peer group data showed that when there is a higher number of solid stall panels that potentially block fresh air exchange at the calf level, there is a rise in the proportion of calves with lung lesions. Our data showed that going from four solid panels to three solid panels translated to 14 fewer calves per 100 calves with treatable lung lesions. Going from four solid panels to two solid panels led to 28 fewer calves with treatable lung lesions per 100 calves.

These findings are not surprising, are supported by research, and further reinforce what an ideal calf stall should be. When stalls allow air to flow through them by having fewer solid panels, the exchange of dirty air for fresh air is much more successful and helps the designed calf barn ventilation, natural and mechanical, to perform more effectively. 

Stocking impacts air quality

The level of air contamination can be impacted by several factors, but it can largely be tied to the number and size of calves per square foot of building space, or the stocking density. Each calf will produce and expire a given amount of humid, dirty air at a constant rate, and this amount climbs as calves grow. A calf barn has a finite amount of space, and the more calves stocked into it, the more contaminated the air has the potential to become. Our peer group showed a positive relationship between cubic feet per calf and the proportion of calves with subclinical lung lesions. As barns became more densely stocked, the proportion of calves with lung lesions rose.

Most calf barns are built based on the monthly average number of replacements needed, typically not to accommodate calving slugs. So, statistically speaking, the calf barn will be overstocked 50% of the time.

What can we do to aid in preventing respiratory disease with the variability of stocking density? Much like how there are recommendations for calculating the size of transition cow pens to optimize fresh cow health, we believe there need to be updated recommendations for calf barn sizing that help deal with fluctuations in stocking rates.

Without having university research data to guide a recommendation at this time, we need a starting point. Therefore, with respiratory disease in mind, we want to start the discussion that calf barns be built at a minimum of 125% of the targeted average goal of calves in the barn. For example, if a dairy is striving for 30 heifer calves per month, they will have roughly 60 preweaned calves in their barn. We are recommending that this barn needs to have a minimum space for 75 calves. This provides space for spikes in heifer calves coming in the barn and time to properly clean, disinfect, and rest pens between calves before restocking.

We believe the higher cost of building a bigger calf barn will be offset by the improvement in calf health and the many established financial benefits of reduced calf respiratory disease. These benefits include approximately 1,200 pounds more milk in the first lactation, improved heifer reproduction, reduced mortality, greater longevity in the herd, and the list goes on. 

Where do we go from here?

We are just one veterinary clinic speaking on our data and experiences. Calf lung ultrasound has allowed us to have greater involvement in calf health and management on several dairies, and we think we can use this experience to provide an even larger impact rather than just diagnosing and treating individual calves with subclinical pneumonia. Our practice hopes to lead this discussion to improve our industry’s standards for raising calves and to reduce the incidence of respiratory disease in the preweaning period.