Bovine respiratory disease (BRD) cases continue to be a major problem in both beef and dairy operations. The morbidity and mortality impact and the loss of production often continue in spite of vaccinations against other infectious agents including bovine herpesvirus (BHV)-1, bovine viral diarrhea virus (BVDV), parainfluenza-3 virus (PI3V), bovine respiratory syncytial virus (BRSV), Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni, says Robert Fulton, DVM, PhD, Oklahoma State University.
“Veterinarians and producers then are looking for other agents and management factors besides those that they are vaccinating against,” Fulton explains. “Since there are no USDA licensed bovine coronavirus (BCV) vaccines for BRD, some look for this to be the next infection of the respiratory tract for vaccine development and approval.”
All of the aforementioned pathogens have an impact on BRD cases; however, BHV-1 (IBR) and BVDV also affect reproduction by fetal infections, Fulton says. PI3V, bovine adenovirus (BAdV), BRSV, and BCV do not cause fetal disease. “It’s too early to rank BCV among the other agents, but there is growing evidence that BCV is a player in beef and dairy BRD. The point to be made is that initially some thought BCV was an ‘infection looking for a disease’. However, veterinarians serving their clients and diagnostic laboratories are now concerned about BCV and there is growing evidence for its role in BRD.”
BCV has been associated with BRD in neonates, stockers and now in feedlot animals, Fulton says. “Veterinarians have reported BCV enteric disease in beef calves, with BRD signs later in the calves’ life. In the stocker and feedlot operations it is found in animals shortly after arrival. Calves under commingling conditions such as auction markets and shipping are often infected with BCV. BRD cases in the first week after arrival may occur due to BCV and complicating agents.” Maternal transmission could be manifested by both enteric shedding by the dam or by infectious virus in the respiratory tract secretions.
Fulton says the virus can be found in nasal swabs and lung washing fluids in extremely high infectious titers. Likewise, early work in BCV enteric infections found virus in fecal samples. “It is an enveloped RNA virus and is susceptible to common disinfectants. Enveloped viruses traditionally have been thought susceptible to the environment. However, virus-laden feces from infected animals should be considered important in transmission in addition to the respiratory secretions.”
In Fulton’s experience, most all home-raised calves he tested direct from the ranch and without exposure to other cattle were negative for virus in nasal swabs. “However, in at least three studies of sale barn cattle, 40% to 85% of the calves at Day 0 (entry) were shedding virus in the nasal swabs. But, by Day 8, they had cleared the infections. As animals seroconvert, they clear the infections.”
What BCV does
BCV may infect the bovine respiratory tract throughout, from the nasal mucosa, trachea, and the lungs. “The clinical signs are similar to an uncomplicated viral infection such as primary BVDV, PI3V, BRSV or BAdV, elevated body temperature, and nasal and ocular discharge,” Fulton says.
Experimentally the BCV may infect the trachea and lungs and the primary infection may not be grossly observed, he adds. Other agents such as M. haemolytica, P. multocida and Mycoplasmas may be more evident by their lesions.
The association of BCV with BRD has been primarily by the isolation of virus from nasal swabs of cattle with BRD signs and seroconversions to BCV. The clinical signs of upper respiratory disease with BCV are not unlike other viruses such as BVDV, PI3V, and BRSV with coughing, fever, and nasal and ocular discharges. Typically, says Fulton, these BCV seroconversions and BCV isolations occur shortly after arrival in the feedlot. “There are reports of naturally occurring disease in cattle with primary infections (single BCV) or in cases of multiple infections including viruses such as BHV-1, BVDV, PI3V, BRSV, and bacteria including M. haemolytica, P. multocida, and Mycoplasma spp.”
The virus can be found in swabs from the nasal tract, lung lavage washings from live animals as well as tissues collected at necropsy. BCV can be found in healthy-appearing animals as well as those manifesting BRD clinical signs and lesions. “This statement is qualified as other bovine viruses and bacteria can be found in healthy animals also,” Fulton notes. “It’s not unusual for healthy cattle to be carriers of BRD pathogens and expose other susceptible cattle that eventually get sick and may die.”
Fulton says to remember that viruses may compromise the host defense mechanisms of the bovine by permitting bacterial colonization, damage to the mucociliary system of the nasal muosa and trachea, and the lung by inhibiting phagocytosis. “Some viruses are immunosuppressive, affecting systemic immunity of the cell-mediated and humoral immune systems.”
Detecting BCV infections can be done in a variety of ways including viral isolation. For infectious virus isolations, a special cell line is used, human rectal tumor cells, which are very susceptible to the BCV. Viral RNA may be detected by gel-based or real time PCR as these tests, especially real time PCR, are often used by diagnostic laboratories in the U.S., Fulton says. “The ‘devil in the details’ is the interpretation of the PCR tests and viral isolation from live animal samples and seroconversions.”
Active infections may be confirmed by serology using paired serums, especially from several animals in the group with rising antibody levels as confirmation when the serums were tested against BCV. Fulton says a recent study utilized a virus neutralization test with varying serum dilutions against a fixed amount of BCV in a microtiter assay. “Active infections were confirmed with four-fold or higher rises in antibody titer in serums of cattle, both healthy and clinically ill cattle with BRD under feedlot conditions.”
For tissues collected at necropsy, BCV can be identified by antigen detection systems in tissues, fluorescent antibody on the lung and other respiratory tissues. “In our hands we have found immunohistochemistry (IHC) on fixed respiratory tissues to be the best way to identify BCV in postmortem tissues,” Fulton adds. “Positive IHC results in necropsy tissue along with histopathology positive lesions are the most rewarding for BCV in diseased animals.”
There are different strains of BCV as detected by sequencing of the viral genome. “There appears to be at least three groups of BCV, two respiratory groups and the enteric group. Studies are in progress to determine if the genetic differences are also involved with antigenic differences,” Fulton explains.
At the present time, there are no USDA BCV vaccines for respiratory disease. “There are some anecdotal reports of the current vaccines for use in cattle, however these are off label,” Fulton says. “There appears to be interest in vaccine research, both for USDA approval as well as autogenous vaccines.”
In light of that, Fulton says the best defense is to monitor the cattle for illness, most likely with antimicrobials to be used to control the complicating bacteria and mycoplasmas. The infected calves will clear the infection as they develop antibodies to BCV.
Coronavirus challenge studies
Robert Fulton, DVM, PhD, says there are several studies where bovine coronavirus (BCV) has been shown to have a negative effect. After experimental challenge in young calves, the virus could be found in the feces of diarrheic calves and nasal swabs up to 5 days, Fulton says. “Respiratory disease signs occurred in only a few calves. Lesions of emphysema and interstitial pneumonia were evident in only a few calves.”
For other studies, there were mixed reports of BCV detected in lung tissues of cattle with BRD, one report with no BCV detection in the lung, and another detecting BCV antigen by immunofluorescence in respiratory tissues.
Another study used the winter dysentery strain of BCV to infect 2- to 4-day-old dairy calves by oral challenge, and the calves developed diarrhea which persisted until the calves were necropsied or euthanized. “The infected calves were febrile; however, no calves exhibited respiratory signs,” Fulton says. “The calves had lesions in the small and large intestine, but also had epithelial damage in the nasal turbinates, trachea, and interstitial pneumonia.” BCV antigen was detected in the epithelium of the small and large intestine and was also detected in the nasal turbinates, trachea, and lungs.
“This study demonstrated the dual tropism of BCV for the digestive and respiratory tracts,” Fulton says. “This latter study indicates the potential for a model to demonstrate the pathogenicity of BCV respiratory isolates.”
The Oklahoma studies
Fulton conducted three different studies in a retained ownership program from the ranch to the feedlot, and found that the level of BCV neutralizing antibodies varied. Calves with low levels of BCV neutralizing antibodies, 16 or less, were more likely to be treated than those with higher titers.
“In three studies of commingled mixed source calves, BCV was recovered from calves at entry and the infections were cleared by Day 8,” he says. The BCV was identified in the lung samples (bronchoalveloar lavage collections) along with the nasal swabs. Calves with low levels of BCV antibodies at entry were most likely to be shedding BCV. BCV was isolated from both healthy and sick calves, but was not isolated from sick calves after 4 days postarrival.
“BCV should be considered along with other bovine respiratory viruses in the diagnosis of etiologies in bovine respiratory diseases, especially for those animals ill shortly after arrival,” Fulton explains.