Etiology, Epizootiology and Control of Maedi-Visna in Dairy Sheep: A Review

Abstract

Maedi-visna (MV) in sheep is caused by maedi-visna virus (MVV), a small ruminant lentivirus (SRLV) that causes chronic infection and inflammatory lesions in infected animals. Pneumonia and mastitis are its predominant clinical manifestations, and the tissues infected by MVV are mainly the lungs, the mammary gland, the nervous system and the joints. MV has a worldwide distribution with distinct MVV transmission patterns depending on circulating strains and regionally applied control/eradication schemes. Nevertheless, the prevalence rate of MV universally increases. Currently, gaps in understanding the epizootiology of MV, the continuous mutation of existing and the emergence of new small ruminant lentiviruses (SRLVs) strains, lack of an effective detection protocol and the inefficiency of currently applied preventive measures render elimination of MV an unrealistic target. Therefore, modifications on the existing MV surveillance and control schemes on an evidentiary basis are necessary. Updated control schemes require the development of diagnostic protocols for the early and definitive diagnosis of MVV infections. The objectives of this review are to summarize the current knowledge in the epizootiology and control of MV in dairy sheep, to describe the research framework and to cover existing gaps in understanding future challenges regarding MV.

Accurate Diagnosis of Small Ruminant Lentivirus Infection Is Needed for Selection of Resistant Sheep through TMEM154 E35K Genotyping

“…Once infection occurs, immune responses result in production of antibodies that generally remain detectable, for the whole animal’s life, which is at the basis of the current control programs [ 4 , 5 ]. Strategies to control SRLV are based on the identification of seropositive animals since no vaccine is available, despite profuse trials [ 6 ]. However, antigenic heterogeneity of circulating strains may be wider than the covered by available ELISA tests [ 7 , 8 , 9 ] making serological response not always detectable.…”

Toxic Wasting Disorders in Sheep

“…This review highlights the broad variety of toxic organic/inorganic compounds and plants that may be involved in states of emaciation, wasting, and loss of productivity in sheep. These conditions should be thus considered as differential diagnoses, especially in those animals and flocks where the classic infectious and parasitic wasting diseases have been ruled out [ 1 , 2 , 3 , 4 ]. Diagnosis may be fastidious in many cases, since lesions could be minimal and/or unspecific, and a good anamnesis focused on revealing a history of exposure should be the cornerstone of the diagnostic work-up.…”

“…There are several diseases of sheep that may cause wasting, leading to decreased productivity and economic losses in sheep flocks as a result. Johne’s disease, maedi-visna, caseous lymphadenitis, and several parasitic diseases, amongst others, have been traditionally included under the umbrella of the so-called “thin ewe syndrome”, a concept that encompasses several diseases with wasting as the main clinical sign [ 1 , 2 , 3 , 4 ]. Hence, this concept has included mostly infectious and parasitic disorders, probably underestimating the role of other disease causes.…”

Serological evidence and spatial analysis of small ruminant lentiviruses in herds in Maranhão, Brazil

“…The low occurrences of illness are also associated with the short time of permanence of animals in flocks, being intended for own consumption, sale or marketed with other creators. According to Kalogianni et al (2020), the type of exploration appears as a risk factor for dissemination of the agent. It is understood that the less time an animal remains on a property, the less the chances of it being infected and, if it is, not showing clinical signs or other manifestations, such as a drop in meat and milk production, as they tend to be belatedly (GOMEZ-LUCIA; BARQUERO; DOMENECH, 2018).…”

“…In infected flocks, the virus elimination occurs mainly by means of secretions, more specifically, defense cells containing viral particles. The transmission occurs by the animal's direct contact, especially in intensive creations (KALOGIANNI et al, 2020).…”