An oculo-cerebellar syndrome caused by congenital bovine viral diarrhoea virus-infection

Virus-induced congenital malformations in cattle

Diagnosing the cause of bovine congenital malformations (BCMs) is challenging for bovine veterinary practitioners and laboratory diagnosticians as many known as well as a large number of not-yet reported syndromes exist. Foetal infection with certain viruses, including bovine virus diarrhea virus (BVDV), Schmallenberg virus (SBV), blue tongue virus (BTV), Akabane virus (AKAV), or Aino virus (AV), is associated with a range of congenital malformations. It is tempting for veterinary practitioners to diagnose such infections based only on the morphology of the defective offspring. However, diagnosing a virus as a cause of BCMs usually requires laboratory examination and even in such cases, interpretation of findings may be challenging due to lack of experience regarding genetic defects causing similar lesions, even in cases where virus or congenital antibodies are present. Intrauterine infection of the foetus during the susceptible periods of development, i.e. around gestation days 60–180, by BVDV, SBV, BTV, AKAV and AV may cause malformations in the central nervous system, especially in the brain. Brain lesions typically consist of hydranencephaly, porencephaly, hydrocephalus and cerebellar hypoplasia, which in case of SBV, AKAV and AV infections may be associated by malformation of the axial and appendicular skeleton, e.g. arthrogryposis multiplex congenita. Doming of the calvarium is present in some, but not all, cases. None of these lesions are pathognomonic so diagnosing a viral cause based on gross lesions is uncertain. Several genetic defects share morphology with virus induced congenital malformations, so expert advice should be sought when BCMs are encountered.

Neuro-invasion by a 'Trojan Horse' strategy and vasculopathy during intrauterine flavivirus infection

The central nervous system (CNS) is a major target of several important human and animal viral pathogens causing congenital infections. However, despite the importance of neuropathological outcomes, for humans in particular, the pathogenesis, including mode of neuro-invasion, remains unresolved for most congenital virus infections. Using a natural model of congenital infection with an RNA virus, bovine viral diarrhoea virus in pregnant cattle, we sought to delineate the timing and mode of virus neuro-invasion of and spread within the brain of foetuses following experimental respiratory tract infection of the dams at day 75 of pregnancy, a time of maximal risk of tissue pathology without foetal death. Virus antigen was first detected in the foetal brains 14 days postinfection of dams and was initially restricted to amoeboid microglial cells in the periventricular germinal layer. The appearance of these cells was preceded by or concurrent with vasculopathy in the same region. While the affected microvessels were negative for virus antigen, they expressed high levels of the type I interferon-stimulated protein ISG15 and eventually disappeared in parallel with the appearance of microcavitary lesions. Subsequently, the virus spread to neurons and other glial cells. Our findings suggest that the virus enters the CNS via infected microglial precursors, the amoeboid microglial cells, in a 'Trojan horse' mode of invasion and that the microcavitary lesions are associated with loss of periventricular microvasculature, perhaps as a consequence of high, unrestricted induction of interferon-regulated proteins.

Congenital defects of the ruminant nervous system

Abnormalities of the nervous system are common occurrences among congenital defects and have been reported in most ruminant species. From a clinical standpoint, the signs of such defects create difficulty in arriving at an antemortem etiology through historical and physical examination alone. By first localizing clinical signs to their point of origin in the nervous system, however, a narrower differential list can be generated so that the clinician can pursue a definitive diagnosis. This article categorizes defects of the ruminant nervous system by location of salient clinical signs into dysfunction of one of more of the following regions: cerebrum, cerebellum,and spinal cord. A brief review of some of the more recognized etiologies of these defects is also provided. It is important to make every attempt to determine the cause of nervous system defects because of the impact that an inherited condition would have on a breeding program and for prevention of defects caused by infectious or toxic teratogen exposure.

Reproductive consequences of infection with bovine viral diarrhea virus

Reproductive efficiency is imperative for the maintenance of profitability in both dairy and cow-calf enterprises. Bovine viral diarrhea virus is an important infectious disease agent of cattle that can potentially have a negative effect on all phases of reproduction. Reduced conception rates,early embryonic deaths, abortions, congenital defects, and weak calves have all been associated BVDV infection of susceptible females. In addition, the birth of calves PI with BVDV as a result of in utero fetal exposure is extremely important in the perpetuation of the virus in an infected herd or spread to other susceptible herds. Bulls acutely or PI with BVDV may bea source of viral spread through either natural service or semen used in artificial insemination. Management practices including elimination of PI cattle, biosecurity measures and strategic use of vaccination can be implemented to reduce the risk of BVDV related reproductive losses. Development of vaccines and vaccine strategies capable of providing better protection against fetal infection would be of benefit.

Pathogenesis of transplacental virus infection: pestivirus replication in the placenta and fetus following respiratory infection

Although transplacental virus infections account for considerable morbidity and mortality in both animals and humans, very little is so far known about the pathways whereby virus reaches the conceptus, the subsequent virus-host interactions in the early phases of the infections, and the establishment of persistent non-lethal infection. Using a natural animal model we recently demonstrated that bovine pestivirus can spread from the site of infection to the ovine fetus within 72 h, despite the expression of interferon in the reproductive tract [1]. In the present study we demonstrate that pestivirus first establishes infection and spread within the allantoic and amniotic membranes and then the fetus, followed several days later by infection of the uterine glands. However, virus replication and spread within the fetus is, at least in part, controlled by fetal developmental factors. In fetuses less than 25 days of gestational age, the virus remains restricted to the bulbis cordis, the first brachial pouch and occasionally the aorta. Over the next few days the virus spreads to multiple tissues, in addition to becoming more widespread and pronounced within the initially infected tissues. A potential role for the binucleated cells of the allantochorion in the spread of the virus from the fetal to the maternal tissues was also found. These cells expressed high levels of viral antigen just prior to and during the time period in which virus antigen became detectable in the epithelial cells of the uterine glands, in endothelial cells of uterine vessels and in scattered macrophage-like cells in the uterine stroma. Most likely this relatively late virus transfer is inconsequential for the mother, since it occurs at a time when a maternal virus-specific antibody response is becoming measurable. This is in contrast to the fetus, where the infection will have established itself widely prior to the development of lymphoid tissues and a functional immune response, thus setting the scenario for development of specific tolerance to the persisting virus.

The clinical manifestations of bovine viral diarrhea infection

BVDV infections may result in a wide range of clinical manifestations ranging from subclinical infections to a severe, highly fatal form referred to as MD. In recent years, a better understanding of pathogenesis has been achieved that has allowed for a better understanding of the different clinical forms of BVDV infection. Knowledge in this area continues to evolve. The clinical forms of BVDV infections are best understood by breaking them into categories that include infections in immunocompetent cattle, fetal infections, and infections in cattle that are immunotolerant to BVDV.

The pathologies of bovine viral diarrhea virus infection. A window on the pathogenesis

Pathologic lesions caused by bovine viral diarrhea virus (BVDV) infections comprise a wide spectrum of type, degree, and, by implication, pathogenesis, including congenital defects, necroticerosive lesions in mucosal epithelia and skin, and reactive as well as degenerative changes in lymphoid tissues. At least some of the pathology may not be solely due to BVDV replication per se, but rather caused by a host response to the virus, particularly the production of pro-inflammatory cytokines.

Brain lesions in calves following transplacental infection with bovine-virus diarrhoea virus

In 33 calves and subadult cattle of the Holstein-Friesian breed ranging from 1 to 210 days of age, the spectrum of brain lesions induced by intra-uterine infection with bovine-virus diarrhoea virus (BVDV) was retrospectively analysed. Of these, 27 animals originated from herds with a long history of BVD. Six calves were derived from dams vaccinated between the 90th and 118th day of gestation with a BVD live vaccine. The most frequent lesion was cerebellar hypoplasia, being present in 25 out of 33 (76%) of the animals. In most of these cases, cerebellar hypoplasia was associated with hydranencephaly, internal hydrocephalus, microencephaly or porencephaly. In cases with hydranencephaly, the fluid-filled cavities were devoid of ependymal lining. The lumina of the lateral ventricles of these cases were surrounded by glial fibrillary acidic protein (GFAP)-positive cells and a dense layer of immunoreactive cell processes. In the white matter adjacent to the dilated ventricular lumina, a reactive astrocytosis was present. Porencephalic cysts were surrounded by astrocytes with increased expression of GFAP and vimentin-positive cells and cell processes. In hydranencephalic brains, staining for neuron-specific enolase (NSE) revealed a marked reduction of NSE-positive nerve cells in cortical areas. In all six experimental cases and in several field cases with hydranencephaly or internal hydrocephalus, small groups of heterotopic NSE-positive neurons were present in the white matter of the cerebral hemispheres. In markedly hypoplastic cerebella, reduction of the cortical cell layers and degenerative changes in, and heterotopia of, Purkinje cells were found. In these cases, NSE- and neurofilament-positive cell processes were either markedly diminished or only remnants of immunoreactive cell processes were present. In five animals without significant gross cerebellar abnormalities, degenerative changes of Purkinje cells were found. Immunohistochemical staining using antibodies against glial and neuron-specific proteins on these brains, which represent postnatal end-stage lesions of BVDV-induced disturbances of the normal brain development, did not provide any insight into the possible pathogenetic mechanisms of these alterations. Application of immunohistochemistry, however, revealed changes, such as reactive astrocytosis and loss of nerve cell processes, which were not obvious on haematoxylin and eosin (H&E) stained sections.

Cache Valley virus

Cache Valley Virus (CVV) is a causative agent of a mosquito-borne disease syndrome of sheep and, possibly, of all ruminants, characterized by embryonic and fetal death, stillbirths, and multiple congenital malformations. CVV is endemic in Canada, Mexico, and the United States. Several related Bunyaviruses also may play a role in syndromes of congenital malformations and embryonic losses in North America.

Impact of bovine viral diarrhea virus on reproductive performance in cattle

The major economic impact of BVDV infections is in producing reproductive dysfunctions. The virus can have a detrimental impact on the developing fetus at all stages, but the most severe consequences occur early in gestation. The virus is maintained in the bovine population through the production of PI offspring. Only the NCB biotype of the virus can produce PI calves. The NCB biotypes is the one that produces the vast majority of BVDV infections and, as a consequence, the greatest economic losses. Biotype and antigenic variation are two independent variables and vaccines should emphasise antigenic composition, not biotype. A BVDV control program that does not emphasize heifers is doomed to failure.

New concepts in the pathogenesis, diagnosis and control of diseases caused by the bovine viral diarrhea virus

The new information on the pathogenesis and epidemiology of mucosal disease of cattle is reviewed. It is now known that clinical mucosal disease occurs only in cattle which were infected with a pestivirus in early gestation and were born with persistent viral infection and specific immunotolerance. These animals may be clinically normal at birth but may develop fatal mucosal disease, perhaps following superinfection with another pestivirus, usually between 6 and 24 months of age. They may also remain clinically normal indefinitely and breed successfully. The progeny from persistently infected females will similarly be persistently viremic, and maternal families of such animals may be established.Congenital defects may occur when infection of the fetus occurs in mid-gestation. Although fetuses may be infected in utero in late gestation, the infections do not persist, the fetuses develop antibodies, and they appear to suffer no ill-effects. Postnatal infection can result in subclinical disease (bovine viral diarrhea) with a normal immune response; the virus may also be responsible for enhanced susceptibility to other infections, diarrhea in newborn calves, and reproductive failure.Prevention of the economically important diseases caused by the virus is dependent upon the identification and elimination of persistently viremic animals, which are reservoirs of infection, and the vaccination of immunocompetent females at least three weeks before breeding. However, because of serotypic differences between strains, there is some doubt whether vaccination will reliably provide protection against the transplacental fetal infections that are important in the pathogenesis of this disease. There is no substantial evidence to warrant the vaccination of feedlot cattle.

Congenital defects of the bovine central nervous system

Congenital brain defects in cattle are usually obvious at birth, but detection may depend on the nature and extent of the defect. The cause may be genetic or environmental, or the cause may be unknown. Many central nervous system defects are inherited as a simple autosomal recessive trait and may have a significant economic impact on purebred and commercial cattle operations. A common structural defect of genetic nature is internal hydrocephalus. A functional defect of simple autosomal recessive inheritance is bovine progressive degenerative myeloencephalopathy. The most significant teratogenic agents causing central nervous system defects are prenatal viral infections such as bovine virus diarrhea and bluetongue viruses. Many other defects of the central nervous system are reviewed.