Infection and Clinical Signs
Bovine Viral Diarrhea Virus


Epidemiology and Pathogenesis

Unless otherwise noted, epidemiology and pathogenesis by BVDV are taken from Merck (2005).

Cattle are the primary hosts for BVDV however, the virus may also infect sheep, goats, water buffalo and wild ruminants (Carter 2005). Infection is spread directly and indirectly via host secretions. Infection therefore can be spread via ingestion or inhalation of the virus. Artificial insemination or coitus also facilitates the spread of BVDV as it is also shed in semen of bovine hosts. Bovine viral diarrhea is caused by BVDV and can exist in a variety of forms with a wide range of clinical manifestations. These range from gastroenteric, respiratory and hemorrhagic syndromes to acute viral diarrhea and/or fatal mucosal disease. These syndromes are discussed in more detail below.

BVDV has a world wide distribution and therefore is a world wide disease. Regular vaccination programs are commonly performed however bovine viral diarrhea continues to play a significant role in bovine disease. Infection can occur at any age butclinical signs are most commonly seen in cattle between 6 months and 2 years of age.

With the many manifestations that BVDV infection can cause, the majority of the concern is oriented towards the effects the virus has on the reproductive system. BVDV has the ability to cross the placenta and infect fetuses at any time during gestation. The effects of fetal infection however are dictated mostly by time of infection during gestation. If a pregnant cow contracts BVDV during her first trimester early in gestation, fetal death may occur and the fetus will be reabsorbed (Carter 2005). This can lead to infertility and repeat breeding. If fetal death does not occur, calves could abort later on in gestation, be stillborn, have malformations or be weak and unthrifty calves (Carter 2005, Merck 2005). Infection between 4 to 120 days of gestation most often results in immunotolerant calves with persistent BVDV infections. These calves can be in good or poor health but often have enteric or respiratory problems (Carter 2005). Secondary bacterial infections are common due to the immunocomprimization of the calf as the virus targets lymphoid tissue and impairs immunity (Carter 2005). Identification of persistently infected calves is very important for two reasons. These calves shed massive amounts of virus into the environment and if bred, they will infect their fetus and produce more persistently infected calves (Carter 2005). Eventhough most persistently infected calves die within the first 6 to 24 months of life,  they act as a continual source of infection for the rest of the herd. 


Aborted BVDV Fetus
http://www.vet.uga.edu/vpp/
ivm/eng/Modes/routes02.htm


Sick calf with BVDV
http://www.scielo.br/scielo.php?pid=
S0100-736X2000000200001&script=sci_arttext


Malformed Calf
http://www.holsteinusa.
com/html/cvm.html


Clinical Diseases

Unless noted otherwise, description of disease by BVDV are taken from Merck (2005).

Reproductive Disease

Literature suggests that bovine viral diarrhea is the  most common diagnostic explanation with respect to bovine abortions. As described above, infection prior to 125 days of gestation may result in fetal death, abortion, resorption, mummification and malformations while infection after 125 days of gestation is more likely to result in persistently infected calves. These calves develop leucopenia, anorexia, pyrexia, loss of condition and mucoid bloody diarrhea. Salivation is often prominent,  thick and stringy nasal secretions are present along with eroded and crusty skin of the muzzle area. Upon necropsy, ulceration and erosion of the upper respiratory and digestive tract is evident and due to BVDV stong affinity for lymphoid tissue, ulceration and erosion are most often seen within the payer’s patches and esophagus of infected animals.

         
                                 Erosions of the esogphagus from BVDV infection                                      Erosion of the payer's patches of the intestine
                                                    http://w3.vet.cornell.edu/nst/nst.asp?Fun=F_Ssrch                                                      http://w3.vet.cornell.edu/nst/nst.asp?Fun=F_Ssrch

Respiratory Disease


There exists controversy in the literature concerning the exact role of BVDV in bovine respiratory disease. BVDV is often included within the etiologies of bovine respiratory disease complex.  It has been suggested that due to the immunosuppression and alteration of normal defense systems from BVDV infection, cattle become susceptible to secondary bacterial pneumonia. BVDV has also been reported to be the most common virus associated with other viral infections of the respiratory tract.

Bovine respiratory disease complex  often arises in situations of stress,  poor management, poor environmental conditions and malnutrition.  Failure of passive transfer may also significantly contribute to the development of bovine respiratory disease.  These additive factors depress the overal health and immunity of calves resulting in increased susceptibility to viral and/or bacterial infections. Commonly involved bacterial species include Pasteurella multocida, Mannheimia haemolytica and Mycoplasma bovis. Viral species commonly associated with bovine respiratory disease complex include  Bovine  Respiratory Syncytial Virus, Parainfluenza Virus 3, and Bovine  Herpesvirus-1.


                                           Secondary bacterial pneumonia of a bovine
                                                    http://w3.vet.cornell.edu/nst/nst.asp?Fun=F_Ssrch

Enteric Disease

BVDV infection commonly causes mucosal disease in calves that are 6 months to 2 years of age. Acute diarrhea disease can result from both cytopathic and noncytopathic BVDV strains.  Clinical signs often include a biphasic fever, depression, transient inappetance, rapid respiration,  excessive nasal secretions and lacrimation, and diarrhea.  The incubation period for acute bovine viral diarrhea is 6 to 12 days with clinical signs lasting 1 to 3 days. Clinical signs begin  to resolve once neutralizing antibodies are produced in the host. If however, infection has significantly affected lymphoid tissue, immunosuppression becomes evident and clinical signs and disease become more severe.

Severe BVDV infections are often associated with concurrent bacterial or viral infections. Clinical signs include high fever, oral ulceration,  coronary band and interdigital ulceration or erosion, diarrhea, dehydration, leukopenia and thrombocytopenia. Extensive lymphoid depletion as well as petechial hemorrhage of mucous membranes is commonly observed.  Severe bovine viral diarrhea often causes high morbidity with moderate mortality and clinical signs usually last 3 to 7 days.

      
                                                                                                                                              BVDV infected bovine with severe diarrhea
                                                                                                                        http://www.vetmed.ucdavis.edu/vetext/INF-DA/INF-DA_Rinderpest.html                                           



ABBREVIATIONS