Viral Respiratory Tract Infections in Cattle
Parainfluenza-3 virus (PI-3) is an RNA virus classified in the Paramyxovirus family. Infections caused by PI-3 are common in cattle. Although PI-3 is capable of causing disease, it is usually associated with mild to subclinical infections. The most important role of PI-3 is to serve as an initiator that can lead to development of secondary bacterial pneumonia.
Clinical signs include pyrexia, cough, serous nasal and lacrimal discharge, increased respiratory rate, and increased breath sounds. The severity of signs worsens with the onset of bacterial pneumonia. Fatalities from uncomplicated PI-3 pneumonia are rare. Lesions include cranioventral lung consolidation, bronchiolitis, and alveolitis with marked congestion and hemorrhage. Inclusion bodies may be identified. Most fatal cases have a concurrent bacterial bronchopneumonia.
Diagnostic procedures for PI-3 are similar to those for Bovine Respiratory Syncytial Virus.
Treatment focuses on the antimicrobial therapy directed toward bacterial pneumonia (see Bacterial Pneumonia in Cattle). NSAIDs are also a therapeutic consideration.
PI-3 vaccines are available and are almost always combined with bovine herpesvirus 1 (infectious bovine rhinotracheitis). Modified-live and inactivated vaccines are available for IM administration. Vaccines containing temperature-sensitive mutants for intranasal administration are also available.
Bovine respiratory syncytial virus (BRSV) is an RNA virus classified as a pneumovirus in the Paramyxovirus family. This virus was named for its characteristic cytopathic effect—the formation of syncytial cells. In additional to cattle, sheep and goats can also be infected by respiratory syncytial viruses. Human respiratory syncytial virus (HRSV) is an important respiratory pathogen in infants and young children. Antigenic subtypes are known to exist for HRSV, and preliminary evidence suggests there may be antigenic subtypes of BRSV. BRSV is distributed worldwide, and the virus is indigenous in the cattle population.
BRSV infections associated with respiratory disease occur predominantly in young beef and dairy cattle. BRSV can be considered as a primary BRD pathogen and is also a component of the bovine respiratory disease complex. Passively derived immunity does not appear to prevent BRSV infections but reduces the severity of disease. Initial exposures to the virus are associated with severe respiratory disease; subsequent exposures result in mild to subclinical disease. BRSV is an important virus in the bovine respiratory disease complex because of its frequency of occurrence, predilection for the lower respiratory tract, and ability to predispose the respiratory tract to secondary bacterial infection. In outbreaks, morbidity tends to be high, and the case fatality rate can be 0–20%.
Fever (104°–108°F [40°–42°C]), depression, decreased feed intake, increased respiratory rate, cough, and nasal and lacrimal discharge are common. Dyspnea, possibly with open-mouthed breathing, may become pronounced in the later stages of the disease. Subcutaneous emphysema may occur. Secondary bacterial pneumonia is a frequent occurrence. A biphasic disease pattern has been described but is not consistent.
Gross lesions include a diffuse interstitial pneumonia with subpleural and interstitial emphysema along with interstitial edema. These lesions are similar to and must be differentiated from other causes of interstitial pneumonia (see also Interstitial Pneumonia in Cattle.) Bronchopneumonia of bacterial origin is usually present. Histologic examination reveals syncytial cells in bronchiolar epithelium and lung parenchyma, intracytoplasmic inclusion bodies, proliferation and/or degeneration of bronchiolar epithelium, alveolar epithelialization, edema, and hyaline membrane formation.
A diagnosis of BRSV requires laboratory confirmation. BRSV is a difficult virus to detect, although chances of isolation may improve when sampling animals in the incubation or acute phases of infection. Although virus isolation is difficult, PCR is a useful and rapid method commonly used to detect the antigen. Other procedures that have proved useful in detection of BRSV antigen are fluorescent antibody and immunoperoxidase staining.
Paired serum samples can be used to establish a diagnosis. However, the antibody titer of animals with well-developed clinical disease may be higher in the acute sample than in the sample taken 2–3 wk later, because the antibody response often develops rapidly, and clinical signs follow virus infection by up to 7–10 days. Single serum samples with high antibody titers from a number of animals in a respiratory outbreak may help diagnosis if coupled with clinical signs. Calves that become infected with BRSV in the presence of passively derived antibody may not seroconvert.
Treatment focuses on antimicrobial therapy to control secondary bacterial pneumonia (see Bacterial Pneumonia in Cattle). There is no specific treatment for the viral interstitial pneumonia. Supportive therapy and correction of dehydration may be necessary. There are anecdotal reports of treatment with antihistamines and/or corticosteroids being of benefit. Most animals will recover in several days without treatment.
General control and prevention are discussed under Enzootic Pneumonia of Calves and Shipping Fever Pneumonia. Inactivated and modified-live vaccines are available and may serve to reduce losses associated with BRSV; however, there is a paucity of field trials to evaluate the efficacy of these vaccines.
(Infectious bovine rhinotracheitis virus, Infectious pustular vulvovaginitis, and associated diseases)
Bovine herpesvirus 1 (BHV-1) is associated with several diseases in cattle: infectious bovine rhinotracheitis (IBR), infectious pustular vulvovaginitis (IPV), balanoposthitis, conjunctivitis, abortion, encephalomyelitis, and mastitis. Only a single serotype of BHV-1 is recognized; however, three subtypes of BHV-1 have been described on the basis of endonuclease cleavage patterns of viral DNA: BHV-1.1 (respiratory subtype), BHV-1.2 (genital subtype), and BHV-1.3 (encephalitic subtype). BHV-1.3 has been reclassified as a distinct herpesvirus designated BHV-5.
BHV-1 infections are widespread in the cattle population. In feedlot cattle, the respiratory form is most common. The viral infection alone is not life-threatening but predisposes to secondary bacterial pneumonia, which may result in death. In breeding cattle, abortion or genital infections are more common. Genital infections can occur in bulls (infectious pustular balanoposthitis) and cows (IPV) within 1–3 days of mating or close contact with an infected animal. Transmission can occur in the absence of visible lesions and through artificial insemination with semen from subclinically infected bulls. Cattle with latent BHV-1 infections generally show no clinical signs when the virus is reactivated, but they serve as a source of infection for other susceptible animals.
The incubation period for the respiratory and genital forms is 2–6 days. In the respiratory form, clinical signs range from mild to severe, depending on the presence of secondary bacterial pneumonia. Clinical signs include high fever, anorexia, coughing, excessive salivation, nasal discharge that progresses from serous to mucopurulent, conjunctivitis with lacrimal discharge, inflamed nares (hence the common name “red nose”), and dyspnea if the larynx becomes occluded with purulent material. Nasal lesions consist of numerous clusters of grayish necrotic foci on the mucous membrane of the septal mucosa, just visible inside the external nares. They may later be accompanied by pseudodiphtheritic yellowish plaques. Conjunctivitis with corneal opacity may occur as the only manifestation of BHV-1 infection. In the absence of bacterial pneumonia, recovery generally occurs 4–5 days after the onset of signs.
Abortions may occur concurrently with respiratory disease but may be seen up to 100 days after infection. They can occur regardless of the severity of disease in the dam. Abortions generally occur during the second half of pregnancy, but early embryonic death is possible.
In genital infections, the first signs are frequent urination, elevation of the tailhead, and a mild vaginal discharge. The vulva is swollen, and small papules, then erosions and ulcers, are present on the mucosal surface. If secondary bacterial infections do not occur, animals recover in 10–14 days. With bacterial infection, there may be inflammation of the uterus and transient infertility, with purulent vaginal discharge for several weeks. In bulls, similar lesions occur on the penis and prepuce. (See also Vulvitis and Vaginitis in Large Animals.)
BHV-1 infection can be severe in young calves and cause a generalized disease. Pyrexia, ocular and nasal discharges, respiratory distress, diarrhea, incoordination, and eventually convulsions and death may occur in a short period after generalized viral infection.
In uncomplicated IBR infections, most lesions are restricted to the upper respiratory tract and trachea. Petechial to ecchymotic hemorrhages may be found in the mucous membranes of the nasal cavity and the paranasal sinuses. Focal areas of necrosis develop in the nose, pharynx, larynx, and trachea. The lesions may coalesce to form plaques.
The sinuses are often filled with a serous or serofibrinous exudate. As the disease progresses, the pharynx becomes covered with a serofibrinous exudate, and blood-tinged fluid may be found in the trachea. The pharyngeal and pulmonary lymph nodes may be acutely swollen and hemorrhagic. The tracheitis may extend into the bronchi and bronchioles; when this occurs, epithelium is sloughed in the airways. The viral lesions are often masked by secondary bacterial infections. In young animals with generalized BHV-1 infection, erosions and ulcers overlaid with debris may be found in the nose, esophagus, and forestomachs. In addition, white foci may be found in the liver, kidney, spleen, and lymph nodes. Aborted fetuses may have pale, focal, necrotic lesions in all tissues, which are especially visible in the liver.
Uncomplicated BHV-1 infections can be diagnosed based on the characteristic signs and lesions. However, because the severity of disease can vary, it is best to differentiate BHV-1 from other viral infections by viral isolation. Samples should be taken early in the disease, and a diagnosis should be possible in 2–3 days. A rise in serum antibody titer also can be used to confirm a diagnosis. It is not possible to detect a rising antibody titer in abortions, because infection generally occurs a considerable length of time before the abortion, and titers are already maximal. BHV-1 abortion can be diagnosed by identifying characteristic lesions and demonstrating the virus in fetal tissues by PCR, virus isolation, immunoperoxidase, or fluorescent antibody staining. Gross and microscopic lesions detected shortly after death may help to establish a diagnosis. PCR methods can be used to identify antigen in a variety of tissues or exudates.
Antimicrobial therapy is indicated to prevent or treat secondary bacterial pneumonia. General recommendations for control are discussed under Shipping Fever Pneumonia. Immunization with modified-live or inactivated virus vaccines generally provides adequate protection against clinical disease. Both IM and intranasal modified-live vaccines are available, but the IM types may cause abortion in pregnant cattle. The intranasal vaccines can be used in pregnant cattle. The IM vaccines are easier to use and often are the vaccines of choice in feedlots. Breeding and replacement heifers and bulls should be immunized when 6–8 mo old, before breeding, and yearly thereafter. Some recommend that young bulls not be vaccinated, because they may be discriminated against when sold for breeding if they have antibody titers. Feeder calves should be immunized 2–3 wk before entry into the feedlot. A number of western European countries have eradicated or are attempting to eradicate BHV-1 from their domestic cattle populations. Eradication of the virus is possible by a combination of serologic surveillance, culling of reactors, biosecurity, and vaccination. To aid in eradication, deletion mutant vaccines have been developed that permit discrimination between antibody produced in response to the vaccine and antibody produced in response to natural exposure.
Bovine viral diarrhea virus (BVDV) is an RNA virus classified as a Pestivirus in the family Flaviviridae (see Bovine Viral Diarrhea and Mucosal Disease Complex). The role of BVDV in BRD as a primary pathogen has been controversial but appears to be that of a virus capable of inducing immunosuppression, which allows for development of secondary bacterial pneumonia. Seroconversion to BVDV after arriving in the feedlot has been reported to be the occurrence of respiratory disease in feedlot calves. Calves that arrive at the feedlot with high titers to BVDV have also been shown to be less likely to develop respiratory disease, and BVDV has been reported to be the virus most frequently associated with multiple viral infections of the respiratory tract of calves. Some studies have shown that the presence of a calf persistently infected with BVDV in a feedlot pen increases the risk of respiratory disease within that pen.
Treatment for acute BVDV infection is supportive and includes antimicrobials to prevent or treat bacterial pneumonia. General principles of control are discussed under Enzootic Pneumonia of Calves and Shipping Fever Pneumonia. Inactivated and modified-live vaccines are available for IM administration. Recently, vaccines containing both the type I and type II genotypes have become available. Vaccination of cows before breeding with modified-live vaccines is an important strategy to prevent the occurrence of persistently infected calves. Testing for persistently infected calves and removing them from the pen has been used as a strategy to reduce the risk of disease within feedlots in high-risk groups.
Several other viruses may potentially be involved in BRD. Bovine herpesvirus 4 has been implicated in several diseases, including BRD. Bovine adenovirus has been associated with a wide spectrum of diseases, with bovine adenovirus type 3 being the serotype most often associated with BRD. Two serotypes of bovine rhinovirus have been recognized to cause respiratory tract infections in cattle. Other viruses reported to be associated with BRD include bovine reovirus, enterovirus, and coronavirus. Evidence is growing that bovine coronavirus may have a more important role in BRD than previously recognized. Bovine coronavirus may play a role in some outbreaks of calf pneumonia on pasture in beef cow-calf operations.
These viruses have a role similar to that of the other viruses previously discussed; ie, in combination with other stressors, they can serve as initiators of bacterial pneumonia. Vaccines are not available for prevention of these viral respiratory diseases.