Dairy calf respiratory disease most commonly occurs between 2–6 months of age, although it can occur earlier. The National Animal Health Monitoring System’s Dairy 2007 study indicates that 12.4% of preweaned heifers have respiratory disease, and of those preweaned calves dying, 22.5% were from respiratory disease.

The most common viral respiratory pathogens in dairy calves are BRSV and PI3, and the most common bacterial respiratory pathogens in dairy calves are Pasteurella bovis, Mycoplasma bovis, and Mycoplasma dispar, says Amelia Woolums, DVM, MVSc, Dipl. ACVIM, Dipl. ACVM, University of Georgia. While Mannheimia haemolytica can cause respiratory disease in dairy calves, it is not commonly found. “From the few reports available, it appears that M. haemolytica may be more commonly associated with respiratory disease in beef calves than in dairy calves,” she says.

Neonatal immune function

The immune system of the neonatal calf is generally well functioning, with a couple of important caveats. “On the first day or two after birth, many immune cell functions are diminished because of the effect of high levels of circulating cortisol resulting from the surge in maternal and fetal cortisol near the onset of parturition, meaning calves are relatively more immunosuppressed on the first day or two of life than they will be later that same week,” Woolums explains.

Secondly, while the immune cells of the calf in the first few weeks of life can perform all the same functions as adult cattle and can respond immunologically to infection, vaccination or other immune stimuli, they do not perform quite as strongly or as quickly as adult cells do. Related to this is the fact that a newborn calf is naïve to the immune stimuli it experiences in the first few weeks of life, while an adult animal may have experienced the same immune stimuli in the past, and thus have immune memory. “Immune memory, if present, gives an older animal an even stronger immune response than it will have if it is responding to a challenge the very first time,” Woolums says. “It is reasonable to assume that responses in the neonatal lung are not as strong and rapid as in the adult lung.”

Stimulating immunity

Stimulation of appropriate immunity in calves is best approached through a combination of management to promote good general health and rationally applied efforts to induce specific immunity to agents that the calf is at risk for encountering. A calf is unlikely to develop adequate immunity if it is poorly nourished and overwhelmed by challenge with high numbers of pathogens; this is why good general management is the first step to ensure good respiratory immunity.

Woolums notes that efforts to improve specific immunity also should include ensuring that calves receive an adequate amount of colostrum (all respiratory pathogens cause less severe disease in calves that have adequate passive antibody transfer, as compared to calves that do not have adequate passive transfer), and that cows be vaccinated regularly against pathogens that are a problem for calves so that colostral antibody titers to these specific agents stay high. In terms of respiratory disease, boosting cows one to two months before calving (or at least at pregnancy check for beef cows) with a vaccine against IBR, BVDV,  BRSV, and PI3 should increase colostral titers to these viruses and help calves resist disease.

Vaccinating calves against respir-atory pathogens can help prevent respiratory disease before weaning, and can prime them for an anamnestic response when they are exposed to respiratory pathogens post-weaning. “If calves have good passive transfer of antibody from excellent colostrum management, and if cows are vaccinated regularly to help provide high levels of colostral antibodies against respiratory pathogens, then it is probably not necessary to vaccinate calves in the first two months of life,” Woolums says. “However, calves with inadequate or uncertain passive antibody transfer may benefit from vaccination against viral pathogens — and possibly also Mannheimia haemolytica +/- Pasteurella multocida — in the first month or two of life.

Woolums suggests that if calves have excellent passive antibody transfer and no history of preweaning respiratory disease, then it is most rational to vaccinate them after maternal antibodies have had some time to decline. However, it has been shown that calves with low to moderate levels of maternal antibody can respond to vaccination. “Calves with excellent maternal antibody transfer and no history of preweaning respiratory disease can be vaccinated approximately one month prior to weaning, with a booster at weaning,” she says. “This should prepare them to have a memory response to challenge that may occur after weaning, when their maternal antibodies have disappeared.”

Derailing the calf’s system

Calves exposed to overwhelming challenge with infectious agents, or calves with inadequate nutrition, may not be able to resist respiratory infection even if they have been vaccinated and receive adequate colostrum. Additionally, poor air quality can overwhelm respiratory defenses. High counts of bacteria in the air in calf barns, which are an indicator of poor ventilation, are related to increased risk of calf respiratory disease, Woolums says.

Dust and other particulates and irritating gases or fumes such as ammonia from urine, chlorine from bleach treatment of the environment, etc., can cause dysfunction of the mucociliary apparatus that keeps viruses and bacteria out of the lower respiratory tract. “These insults can also cause dysfunction of alveolar macrophages, which are the most important component of defense in the lowest part of the lung,” Woolums explains. “If the mucociliary apparatus and alveolar macrophages are not functioning normally, then viruses and bacteria that would normally be restricted to the upper respiratory tract can advance into the lower respiratory tract and cause pneumonia.”

Bad weather is often anecdotally related to respiratory disease, says Woolums, but the link has not consistently been found. However, specific instances of extreme weather could serve as a stressor which, when compounded with other stressors mentioned above, could contribute to occurrence of respiratory disease in certain cases. 

The FPT/dystocia calf

If a calf has failure of passive transfer (FPT) or dystocia, immune function can definitely be compromised. “FPT leads to absence or very low levels of serum antibody in the calf,” says Amelia Woolums, DVM, MVSc, Dipl. ACVIM, Dipl. ACVM. “And since serum antibody is also secreted back across epithelial surfaces, antibodies on the mucosal surfaces, which protect calves from respiratory and gastrointestinal infection, are also likely to be lower in calves with FPT. Thus, FPT calves are more susceptible to disease in the first days to weeks of life from lack of circulating antibody and because of suboptimal levels of antibody on mucosal surfaces.

When an immune cell identifies and kills an infectious agent, it becomes activated, and then it activates other immune cells. “Since lack of antibody decreases the efficiency of this process, calves with FPT may be slower to develop some cellular immune functions than if antibody was present,” explains Woolums.

However, notes Woolums, it has also been shown that calves with FPT will develop their own antibodies faster than calves with normal passive transfer when they are exposed to infection or vaccination. So while antibody provided by adequate passive transfer protects calves from infection by binding to infectious agents, which may amplify the activation of immune cells and speed development of immunity, it appears to slow the production of antibodies by the calf itself to infections or vaccinations.

Calves known to have FPT can be vaccinated as early as the first week of life. “When calves are vaccinated in the presence of low to moderate levels of maternal antibody, they appear to respond best if they are given a booster dose two to four weeks after the first dose,” Woolums says. “This is true even if modified live viral vaccines are given to calves, which can sometimes induce adequate immunity after only a single dose in adult cows.”