Bovine Viral Diarrhea (BVD) is a widespread viral disease impacting cattle globally, caused by the Bovine Viral Diarrhea Virus (BVDV). This disease presents a significant economic challenge to livestock producers due to its varied and often subtle manifestations within a herd. The virus can affect multiple body systems and lead to substantial financial losses through reduced productivity and increased animal health costs.
Acute and Chronic Manifestations
Individual cattle infected with BVDV can display a range of observable signs, reflecting either acute or chronic forms of the disease. In acute cases, animals may show general signs of being unwell, including fever, lethargy, and a reduced appetite. Nasal and eye discharge, coupled with diarrhea, are also common symptoms, and lactating cows might experience a noticeable drop in milk production. Acute infections are often mild, with animals recovering within a week or two.
The virus can also lead to chronic issues, particularly in young animals, manifesting as persistent poor growth or a general failure to thrive. Chronic infections may also present as recurrent respiratory problems or ongoing digestive disturbances like persistent diarrhea. These clinical signs are not exclusive to BVD and can mimic symptoms of other common bovine illnesses, making accurate diagnosis challenging.
Mucosal Disease
A distinct and often fatal manifestation of BVD is Mucosal Disease (MD), characterized by severe and specific clinical signs. This severe form typically arises when a persistently infected (PI) animal, carrying the BVD virus from birth, becomes superinfected with a different, more damaging strain of BVDV. Affected cattle develop extensive erosions and ulcers in the mouth, on the muzzle, in the nose, and throughout the digestive tract, including the esophagus and intestines.
Animals suffering from MD often exhibit profuse and sometimes bloody diarrhea, leading to rapid dehydration and significant weight loss. Lameness can also be observed due to lesions and inflammation between the hooves and around the coronary band. The disease progresses rapidly, usually resulting in death within days or weeks.
The Persistently Infected Carrier
The most significant and often deceptive aspect of BVD lies with the Persistently Infected (PI) carrier animal. These animals acquire the BVDV infection while still in the womb, before their immune system fully develops. As a result, PI animals are born with the virus and continuously shed large quantities of it throughout their lives, acting as the primary source of infection for other cattle.
Many PI animals appear outwardly healthy and may not show any overt signs of illness, making their identification challenging without specific testing. However, some subtle indicators might suggest their presence, such as being “poor doers,” meaning they fail to grow or thrive at the same rate as their herd mates. They may also exhibit a dull coat or a general unthrifty appearance, even if they are not acutely sick. Identifying these animals is crucial for controlling BVD spread within a herd.
Reproductive Signs in the Herd
BVD also impacts the reproductive performance of a cattle herd, leading to visible collective signs. The virus can cause increased rates of early embryonic death, which may result in cows returning to estrus (heat) at irregular intervals. Abortions and stillbirths are common consequences of BVDV infection in pregnant animals, potentially occurring at any stage of gestation.
Newborn calves can also show signs of BVD exposure through the birth of weak or malformed individuals. Specific congenital defects observed in calves include cerebellar hypoplasia, which causes tremors, poor coordination, and an unsteady gait. Ocular defects, such as small eyes or blindness, are another visual cue of BVDV infection during fetal development. These reproductive issues collectively contribute to reduced conception rates and an overall decline in herd productivity.