Читать книгу Principles of Virology, Volume 2 - Jane Flint, S. Jane Flint - Страница 22

In the 1930s, a vaccine was developed for yellow fever virus that dramatically reduced the mortality associated with infection by this virus. Nevertheless, mosquitos remain a primary vector for transmission to humans of viruses for which vaccines do not exist, including Zika and chikungunya viruses, as well as the parasite that causes malaria. Consequently, mosquito control remains a major public health initiative worldwide, but these ubiquitous flying syringes pose a formidable challenge to such efforts.

As mosquitos breed in standing water, reducing the prevalence of seemingly innocuous water traps such as old tires, inflatable pools, birdbaths, clogged gutters, and dis carded soda bottle caps can have a substantial impact on mosquito populations. However, such strategies are likely to be only moderately effective in humid, rainy, or swampy environments. Mosquito netting, with a maximum effective mesh size of 1.2 millimeters, has proven effective when hung over beds or incorporated into tents, and variants of this physical barrier were in effect even in the time of Cleopatra. Similarly, the widespread use of insecticides and repellents has reduced spread of mosquito-borne infections.

Recently, more-creative strategies for mosquito control have been added to the anti-mosquito arsenal, including biocontrol, the use of natural enemies to manage mosquito populations. For example, certain fish, lizards, and other insects, such as dragonflies, feed on mosquito larvae. Their presence may thus help to limit populations naturally, although careless introduction of these species into mosquito-rich environments could de stabilize fragile ecosystems. Genetic manipulation of the mosquitos themselves is an active area of research: studies are ongoing to breed and then release large numbers of sterile male mosquitos; females that mate with a sterile male produce no offspring, thus reducing the next generation’s population size. An even more sophisticated control has been the development of genetically modified strains that require an antibiotic to develop beyond the larval stage. Modified males develop normally when provided with the antibiotic in nurseries. However, when the males are released into the wild and mate with normal females, the genetic vulnerability is transferred to future generations in an environment where the antibiotic is not available. As a result, progeny maturation cannot occur. In April 2014, Brazil’s National Technical Commission for Biosecurity approved the commercial release of a genetically modified mosquito, and the U.S. Food and Drug Administration is considering such measures in the United States.

Other successful, and creative, mosquito control campaigns have been waged. For example, to reduce transmission of dengue virus, a community in Australia released millions of mosquitos infected with a bacterial species, Wolbachia, which prevents transmission of viruses such as dengue. When Wolbachia-infected mosquitos were released, they bred with others, infecting them with the bacteria and, in turn, preventing the infected mosquitos from transmitting viruses.

Figure 1.2 The pace of discovery of new infectious agents in the dawn of virology. Koch’s introduction of efficient bacteriological techniques spawned an explosion of new discoveries of bacterial agents in the early 1880s. Similarly, the discovery of filterable agents launched the field of virology in the early 1900s. Despite an early surge of virus discovery, only 19 distinct human viruses had been reported by 1935. Data from Burdon KL. 1939. Medical Microbiology (MacMillan Co., New York, NY), with permission.

Despite many efforts, a human influenza virus was not isolated until 1933, when Wilson Smith, Christopher Andrewes, and Patrick Laidlaw serendipitously found that the virus could be propagated in an unusual host. Laidlaw and his colleagues at Mill Hill in England were using ferrets in studies of canine distemper virus, a paramyxovirus unrelated to influenza. These ferrets were secluded from the environment and other pathogens (for example, all ferrets were housed separately, and all laboratory personnel had to disinfect themselves before and after entering a room). Despite such precautions, it is thought that an infected lab worker transmitted the influenza virus to a ferret. When this ferret developed a disease very similar to influenza in humans, Laidlaw and colleagues realized its implications. These researchers then infected naïve ferrets with throat washings from sick individuals and isolated the virus now known as influenza A virus. (Note the effective use of Koch’s postulates in this study!) Subsequently, influenza A virus was shown to also infect adult mice and chicken embryos. The latter proved to be an especially valuable host system, as vast quantities of the virus are produced in the allantoic sac. Chicken eggs are still used today to produce most influenza virus vaccines.