Bovine Viral Diarrhea Virus (BVDV) poses a significant threat to cattle health worldwide. The disease it induces varies greatly, ranging from mild, transient infections to severe, even fatal conditions. Accurate Bovine Viral Diarrhea Diagnosis is crucial for effective management and control strategies. This article delves into the complexities of BVDV infections, focusing on diagnosis and the different clinical presentations of the disease.
Acute or transient BVD, resulting from BVDV infection in immunocompetent animals, is often characterized by subclinical infection. In many cases, cattle experience inapparent infections followed by seroconversion without showing obvious clinical signs. However, acute clinical disease can manifest in varying degrees of severity. Mild forms are marked by high morbidity but low mortality, while severe enteric disease can lead to considerable mortality. Typical signs of acute clinical BVD include biphasic fever (around 104°F or 40°C), depression, reduced milk production, transient loss of appetite, rapid breathing, excessive nasal discharge, increased tear production, and diarrhea.
Clinical signs typically emerge 6–12 days post-infection and persist for 1–3 days. Transient leukopenia, a decrease in white blood cell count, may coincide with the onset of clinical signs. Recovery is usually rapid, aligning with the production of viral neutralizing antibodies. Gross lesions are uncommon in mild cases. BVDV primarily targets lymphoid tissue, which can result in immunosuppression, increasing susceptibility to secondary infections and exacerbating their severity.
Certain BVDV isolates, particularly BVD type 2, are associated with severe clinical presentations. These severe forms are characterized by high fever (around 107°F or 41–42°C), oral ulcerations, eruptive lesions affecting the coronary band and interdigital cleft, diarrhea, dehydration, leukopenia, and thrombocytopenia (reduced platelet count). In cattle with thrombocytopenia, petechial hemorrhages, small pinpoint bleeding spots, may be observed on the conjunctiva, sclera, nictitating membrane of the eyes, and mucosal surfaces of the mouth and vulva. Prolonged bleeding from injection sites is also common. Severe acute BVD is further associated with swollen lymph nodes, erosions and ulcerations throughout the gastrointestinal (GI) tract, petechial and ecchymotic hemorrhages on visceral serosal surfaces, and extensive lymphoid depletion. The duration of overt disease in severe cases can extend from 3–7 days, with high morbidity and mortality rates reaching 25% or higher. The severity of acute BVD is primarily determined by the virulence of the infecting viral strain, irrespective of the viral biotype.
BVDV poses significant risks to pregnant cattle. The virus can cross the placental barrier and infect the fetus. The consequences of fetal infection are varied and depend on the stage of gestation at the time of infection and the specific BVDV strain involved. Infection near fertilization can lead to reduced conception rates. During the first four months of fetal development, infection may result in embryonic resorption, abortion, growth retardation, or the establishment of persistent infection (PI). Congenital malformations, particularly affecting the eye and central nervous system (CNS), can arise from fetal infections occurring between months 4–6 of gestation. Fetal mummification, premature birth, stillbirth, and the birth of weak calves are also potential outcomes of fetal BVDV infection.
Persistent infection is a critical sequel of fetal infection with noncytopathic BVDV. Calves persistently infected with BVDV may appear healthy and of normal size or exhibit stunted growth and increased susceptibility to respiratory and enteric diseases. PI calves often have a shortened lifespan, with death being common before two years of age. Persistently infected cows invariably give birth to PI calves. While bulls can also be persistently infected, the risk of transmitting the infection to calves in utero is lower. Gross lesions directly attributable to BVDV are often absent in PI cattle at necropsy, complicating diagnosis based solely on post-mortem findings. Antibody against BVDV is typically not detected in PI cattle unless they have been vaccinated or superinfected with an antigenically distinct BVDV strain. This immunological tolerance means that standard serologic tests designed to detect the absence of antiviral antibodies may fail to identify some PI animals. However, PI cattle exposed to antigenically different BVDV can mount an antibody response. Therefore, a comprehensive approach to bovine viral diarrhea diagnosis, especially for persistent infection, is necessary, often involving direct virus detection methods rather than solely relying on antibody tests.
Mucosal Disease (MD) represents another severe manifestation of BVDV infection, specifically arising in persistently infected animals. Clinical signs of acute MD include fever, leukopenia, dysenteric diarrhea (diarrhea with blood and mucus), inappetence, dehydration, and erosive lesions affecting the nares and mouth. Death typically occurs within a few days of symptom onset. Necropsy findings in acute MD reveal erosions and ulcerations throughout the GI tract. The mucosa overlying Peyer’s patches, lymphoid tissues in the small intestine, may be hemorrhagic and necrotic. Microscopic examination reveals extensive necrosis of lymphoid tissues, particularly gut-associated lymphoid tissue.
Chronic MD presents with less severe clinical signs than acute MD, but its course can extend over several weeks to months. Intermittent diarrhea and progressive weight loss (wasting) are common features. Coronitis, inflammation of the coronary band of the hoof, and eruptive skin lesions in the interdigital cleft can cause lameness. Necropsy lesions in chronic MD are similar to, but less pronounced than, those observed in acute MD. Often, the only gross lesions are focal ulcerations in the mucosa of the cecum, proximal colon, or rectum. The mucosa over Peyer’s patches in the small intestine may appear sunken.
In conclusion, bovine viral diarrhea diagnosis requires a multifaceted approach due to the varied clinical presentations of BVDV infection, ranging from subclinical to acute and chronic mucosal disease. Understanding the different forms of BVDV infection, associated clinical signs, and pathological findings is essential for accurate and timely diagnosis, ultimately contributing to effective BVD control and improved cattle health management.
