Читать книгу Principles of Virology, Volume 2 - Jane Flint, S. Jane Flint - Страница 52

Approaches to prevent crop infections are shown at top, and include vector control, policy changes, and development of resistant species. When crops do become infected, incineration of the population is usually the only real alternative.

Natural physical barriers of plants, such as the cuticle and cell wall, normally preclude viruses from gaining access to permissive cells. Viruses are delivered into plants when these barriers are breached, through wounds or the action of vectors (insects, nematodes, fungi) that feed on the plants. Agricultural viruses can cause epidemics with far-reaching implications for both food security and the economy. For example, cassava mosaic begomoviruses cause more than 25 million tons of losses per year in Africa, India, and Sri Lanka. Because the cassava crop represents the daily staple for more than 500 million people, epidemics are often linked to famine events.

Viral diseases are difficult to control once they have begun. Although farmers are well educated to detect signs of infections, because the plants are sown in close proximity, by the time an infection is noted, it has often already spread throughout the crop. When infections occur, destruction of the entire crop (including both infected and uninfected plants, usually by burning) is the only certain strategy to end an agricultural viral epidemic. Prophylactic control measures are therefore crucial to prevent or restrict these infections. Historically, farming strategies have included pesticide management of vector insects and the use of non-host “trap plants” that attract the vector but cannot be infected. Because epidemics can arise from viruses that spill over from adjacent reservoir species, parcel placement and meticulous weeding restrict viral spread. The use of genetically resistant plants is one of the most efficient and sustainable strategies to control virus infections in fields. With increased understanding about the reproduction cycles of plant viruses, resistant plant varieties can be developed. For example, as RNA interference is a major immune strategy for plants, developing crop species that encode a viral gene allows complexes to be formed between RNA transcribed from the inserted gene and RNA of the invading virus, resulting in the degradation of the latter.

Climate-based variations in viral disease may also be caused by bodily changes in the host that influence susceptibility. Such changes might be linked to circadian rhythms, or be governed by alterations in the thicknesses of mucosal surfaces, production of virus receptors, or immune fitness. For example, if the mucosa is thinner in the winter or the skin is drier and cracked, the protective barriers that normally block viral entry into a host can become compromised.

Although this text focuses primarily on those viruses that cause disease in humans and animals, plants and crops, and the viruses that infect them, are subjected to many of the same variables (Box 1.12).