Infectious Bovine Rhinotracheitis (IBR)

Family - Herpesviridae
Subfamily - Alphaherpesvirinae
Genus - Vericellovirus

    Bovine Herpes Virus - 1 (BHV-1) is a herpesvirus and all herpesviruses are composed of relatively large double-stranded, linear DNA genomes encoding 100-200 genes encased within an icosahedral protein cage called the capsid which is itself wrapped in a lipid bilayer membrane called the envelope. This particle is known as the virion. 18

           
IBR is caused by BHV-1 and it is also known as Red Nose.  It has different subtypes that produce different lesions.
    -BHV-1.1 causes IBR
    -BHV-1.2 causes infectious pustular vuvlovaginitis (IPV)
    -BHV-1.2a causes  abortion, whereas BHV-1.2b does not cause abortions
    -BHV-1.3 (also known as BHV-5) causes an encephalitis of inflammation of the brain 1


Clinical signs: 
    The virulence of several strains of one
genotype of virus can vary greatly so the disease can vary from being subclinical to causing systemic fatal disease.  1

Typical lesions of IBR include:
-sudden onset of anorexia
-loud coughing (
tracheitis)
-fever
-severe rhinitis with typical grayish necrotic foci of the mucus membranes of the nasal septum
-serous discharge from eyes and nose
-increased salivation
-sometimes a slight hyperexcitability
-loss of milk production often first clinical sign in dairy cattle
-shallow and increased respirations
-respiratory distress evident with exercise
-sudden death can sometimes occur within 24 hours after the first signs appear from extensive obstructive bronchiolitis
-conjunctivitis may or may not be present, one or both eyes may be affected
-abortion is a common sequel in breeding stock that occurs some weeks after clinical illness or vaccination of non-immune pregnant cows with the MLV vaccine (most common in cows 6-8 months pregnant)
-endometritis, poor conception, and short estrus can occur after insemination with infected semen 1

*It is unlikely that the respiratory and genital forms occur together 1

Typical lesions of IPV:
-frequent urination
-elevation of tail
-mild vaginal discharge
-swollen vulva
-small papules, then erosions and ulcers on the mucosal surface of the vagina 1

Typical lesions of balanoposthitis:
-similar lesions as IPV only on glans penis and preputial mucosa. 
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<>Epidemiology:
    Bovine Herpes Virus - 1 occurs throughout the world, but the incidence of disease is lower than one would expect given the high prevalence of infection.  Being that the virus belongs to the Herpesviridae family it characteristically causes a latent infection.  The virus often becomes reactivated from its typical location in the trigeminal ganglion following stress or administration of corticosteroids which are immunosuppressive.  This reactivation results in re-excretion of the virus and a rise in neutralizing antibodies. 1


Transmission:

    BHV-1 can transmit directly between cattle, mainly through bodily secretions.  These secretions may include nasal exudate, coughed up droplets, genital secretions, fetal fluids and tissues.  The respiratory disease is spread by aerosol transmission whereas the genital disease is spread venereally.  The virus has also been found in wild ruminants in Canada such as the white tailed deer, mule deer, pronghorn antelope, which suggests that these animals may serve as a reservoir for the virus.  1

 

Morbidity and Mortality:
    When the respiratory disease is uncomplicated, fatalities due to BHV-1 are quite rare.  However, when secondary bacterial infections occur, the incidence of mortality increases due to the development of bacterial bronchopneumonia.  It should be noted that the incidence of morbidity and mortality is highest in feedlot cattle because of the frequent introduction of susceptible animals into the feedlot.  These animals may either bring in the virus, or become infected from others within the feedlot.   

Risk Factors for the Development of Disease Caused by BHV-1:

-all ages and breeds are susceptible, but it is most common to see IBR in animals over 6 months of age
-perhaps a seasonal increase in disease in feedlots in fall and winter when new susceptible animals introduced

-animals that are unvaccinated are highly susceptible to epidemics of respiratory disease and abortion
-newborns are highly susceptible to systemic form of the disease if there is failure of passive transfer of maternal antibodies

-genetics play a role in the severity of the disease caused by BHV-1 1


Immunity:
    Immunity to BHV-1 is very complex.  It involves local and systemic antibody production (humoral immunity) and cell-mediated immunity.  The level of humoral antibody has been used as an indicator of previous infection, but the levels of antibodies are not indicative of resistance to disease because the cell mediated immunity may provide protection even if the antibody levels are low.  Notably, calves acquire colostral antibodies that offer some protection to the virus from their dams.  It is possible that this maternal antibody may interfere with successful vaccination of the calf as the passive immunity may last anywhere from 1 to 6 months.   Nutrition also plays a role in the calves ability to mount an immune response.  For example, if the calf is deficient in B vitamins, it's humoral response may be depressed, therefore giving injections to feedlot animals infected with BHV-1 can be beneficial.1  


<>Economic impact:
    BHV-1 can have a major economic impact on breeding herds and feedlot animals due to losses from deaths, abortions,  infertility, and loss of production and cost of treatment when secondary bacterial infections in the respiratory tract occur.   Also, latent infection of breeding animals causes international trade problems and entry into AI units. 1


Pathogenesis:

    This virus causes disease via several different routes including a primary infection restricted to the respiratory tract, eyes, and the reproductive tract.  As well, neuronal spread can cause a systemic disease that can be fatal in young calves.1

Respiratory disease:

    BHV-1 infects the nasal cavities and upper respiratory tract causing rhinitis, laryngitis, and tracheitis.  The virus then spreads from the nasal cavity to the eyes likely via the lacrimal duct to cause a conjunctivitis and other ocular lesions.  It is also thought that the virus plays a role in predisposing the lung to pneumonic pasteurellosis by damaging the protective clearance mechanisms of the lung.  1


Encephalitis:
    It is likely that the BHV-1 virus spreads from the nasal mucosa to the brain via the trigeminal nerve resulting in a non-suppurative encephalitis, alternatively it has been suggested that it may gain access to the brain via the blood.1


Abortion:
    The virus may be transported into the placenta and fetus by peripheral leukocytes to cause abortion.  The fetus is highly susceptible to BHV-1, which causes a peracute infection that is often fatal resulting in mummification, abortion, stillbirth, or weak calves with the usual IBR lesions, as well as lesions of the stomach and intestines. 1


Systemic Infection of Neonates:
    This disease is characterized by severe inflammation and necrosis of the respiratory tract and alimentary tracts, including the pharynx, esophagus, lungs, larynx, lymph nodes, liver, kidney and brain.  The common outcome of these lesions are severe laryngeal edema and respiratory distress that leads to difficulty swallowing and aspiration pneumonia.  This syndrome has often also involved diffuse, severe erosion and ulceration of the upper alimentary tract1

Diagnosis:
-ELISA
-PCR
-Direct of indirect immunofluorescence techniques
-immunoperoxidase staining
-electron microscope examination
-virus isolation
-immunohistochemistry
-serology
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Treatment:
    Broad-spectrum antimicrobials are indicated to treat secondary bacterial infections
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Control:
    Vaccination is recommended in areas where the prevalence of infection is high and eradication is not feasible.1



Vaccinations: 
    The current rationale for vaccinations is as follows:
    -the virus is ubiquitous and the occurrence of disease is unpredictable
    -economic losses can be high
    -colostral immunity in calves wanes by 4-6 months of age
    -vaccination will prevent abortion due to the virus and provide protection against respiratory disease if given at least 10 days before natural
    exposure
   
    MLV, inactivated, and subunit vaccines are available.  There are two types of MLV vaccines; parenteral and intranasal.  MLV vaccines have three advantages over inactivated vaccines.  These are 1) induction of a rapid immune response, 2) relatively long duration of immunity, and 3) the induction of local immunity.  Both the parenteral and intranasal MLV vaccines stimulate the production of humoral antibody, however the intranasal vaccine stimulates the production of local immunity in the nasal mucosa.  This type of vaccine is safe to use and highly effective to prevent abortions in pregnant cows.  The intranasal vaccine is also efficacious in preventing respiratory disease caused by BHV-1.  Currently parenteral MLV vaccines for BHV-1 cannot be administered to pregnant cattle as it will likely trigger abortion.  Another concern with using MLV vaccines is that the vaccinated animal may still shed the virus and it may be transmitted to a pregnant cow causing abortion. 
If newborn calves under 3 days of age are vaccinated with a parenteral MLV vaccine for IBR and PI-3 they may develop the highly fatal systemic form of IBR so it is important to avoid vaccination in very young calves.  The vaccine virus can remain in a latent stage and vaccination does not provide protection against the establishment of latent infection with a wild strain.  As well, vaccination also does not inhibit re-excretion of a wild strain that was latent at the time the vaccine was administered.

    Inactivated vaccines do not cause abortion, immunosuppression or latency, although they do not prevent the development of a latent infection with the wild strain.  They also do not cause shedding and are safe for use in and around pregnant animals.  Another advantage is their stability in storage.  However, a disadvantage of the inactivated vaccines is that they are less efficacious than the MLV vaccines. 
    Subunit vaccines offer some advantages which include 1) they do not contain live virus and therefore can not be shed to other animals, cause abortion or latent infection, 2) they prevent infection and disease, 3) they are not immunosuppressive, 4) serological assays, based on one or more antigen, provide a potential to differentiate vaccinated animals from those naturally infected.  However, they also have disadvantages such as 1) two or more immunizations may be required because the amount of antigen in the vaccine may not be sufficient to cause an immune response, 2) the vaccines must be compatible with other commonly available multivalent vaccines, and finally 3) the efficacy of the vaccine is highly dependent upon the use of an effective adjuvant. 
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