Silkworm

A sick silkworm caterpillar: the characteristic black spots of the disease (pepper in Provencal dialect is pèbre, hence the name of the disease, pebrine, given by de Quatrefages)

From: Innate Immunity , 2019

Polymers in Biology and Medicine

D.N. Breslauer , D.L. Kaplan , in Polymer Science: A Comprehensive Reference, 2012

9.04.2.1 Silkworm Silk

Silkworms spin composites of two silk fibers out of two converging silk glands. 12 These fibers are surrounded by a glue-like sericin protein coating that holds the fibers and thus the cocoons together. The individual silkworm silk fibers (brin) are 10–12   μm in diameter with a triangular cross section, resulting in a composite fiber (bave) of up to 65   μm in diameter. For most uses of silk, the sericin layer is boiled off with an alkaline or soap solution. Silkworm silk is the most commonly used silk material due to the domestication of this source of the protein for textile manufacturing.

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Household Technologies

Edited by, ... Helen King , in Field Guide to Appropriate Technology, 2003

BREEDING SILKWORMS

Silkworms eat only mulberry leaves. The entire process can be controlled by keeping the worms in a controlled environment; protecting them from ants, mice, and disease; and feeding them mulberry leaves. The silk that is produced is called "real silk." Wild silkworms will eat other kinds of leaves, as well as mulberry, and fend for themselves. The silk from wild silkworms is thicker and less lustrous, and is called "tussah" silk.

Cultivation starts with eggs collected from the previous year. These eggs are incubated until spring, when fresh mulberry leaves are available. The eggs hatch into worms, which are fed leaves for about five weeks. At the end of this time they are about two inches long, and then they spin a cocoon. A protected place—like a basket—is provided for the worms when making the cocoons. Most of the cocoons are collected for silk; a few are left so the moths will emerge and breed eggs.

Wild moths breed in a free for all. Cultivated moths are bred by putting only two in an aluminum case.

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MEASURING THE ABILITY OF FOOD TO FUEL WORK IN ECOSYSTEMS

Steven H. Cousins , ... Kevin Attree , in Dynamic Food Webs, 2005

The Herbivorous Food Step

We investigated the silkworm ( Bombyx mori Linnaeus) food chain as the reference herbivorous interaction and looked at the transformation of mulberry leaf into silkworm and silkworm faeces (frass). In a similar attempt to maintain evolutionary familiarity with the test substrates we extracted soil bacteria from soil taken from underneath our local mulberry tree (Morus alba Linnaeus) and grew them with the test substrates in digesters. The extraction process detached bacteria from soil by sonication in a mild detergent, then washed, filtered and centrifuged them to a pellet (Attree, 1998). The bacteria were re-suspended in a stock solution and added to flasks containing equal masses of powdered materials; leaf, silkworm, and frass plus bacteria-only controls. The respiration of the soil bacteria was measured over five days by passing carbon dioxide free air into the digester flasks and the CO2 produced was measured by a precipitation of barium carbonate in the outlet flasks. This experiment has developed some basic techniques to measure the power output per unit mass of the materials in the food chain step and shows that while the energy in the system is conserved, the power density of the materials is not conserved. The power density of the silkworm is raised relative to the leaf and that of the frass lowered relative to both. These are, however, very early results only.

Muthukrishnan (1978) give energy accounting (first law) data for final instar silkworm larvae free-feeding on mulberry leaf and show: 1000 gcal of fresh leaf results in 462 gcal assimilated and 538 gcal frass and a gross conversion efficiency of 16% into silkworm mass. We approximate this to lg leaf = 0.16g larva + 0.54g frass + 0.3g CO2. The power densities of the materials produced and as measured above have been transformed from; leaf 12.6 W.kg-1 (estimated peak 16 W.kg-1) to larvae 13.0 W.kg-1 (estimated peak 22.0 W.kg-1) to frass 6 W.kg-1 (estimated peak 7.0 W.kg-1) (Attree, 1998). Thus a unit mass of leaf is transformed into a small quantity of material with a higher power density, the silkworm, and a large quantity of low power density material, the frass, while, by the first law, the energy contents are conserved.

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Louis Pasteur and Silkworm Disease (1865–1870)

Yves Carton , in Innate Immunity, 2019

1.2 Current knowledge about silkworms and pebrine disease

Silkworm farming, which began in Turkey in AD 552 (two monks fraudulently brought the graine, i.e. the silkworm eggs of this butterfly, from Tibet), has developed in Europe, mainly in Andalusia (Spain) and Italy. This presupposes that the mulberry tree – the only plant on which the caterpillar feeds – has also been cultivated. It was Olivier de Serres (1539–1619), an agronomist, who truly established the mulberry crop in France, hoping that this industry would bring more activities to the French regions, and freely France from a reliance on external suppliers of silk. The breeding of silkworms (called éducation in French) then developed on a wide scale in France, in specialized structures in silkworm farms. The development cycle from egg to adult lasts about 50 days. The five instar larvae (separated by four molts) develop over 30 days, ending with the "migration" of the caterpillar on the heather shoots (which have been deposited for this purpose), weaving its cocoon and transforming into a pupa. The adult butterfly emerges after 20 days. For the production of cocoons and their shipment to the spinning manufactures, they are boiled to kill the pupae.

Pebrine is a disease found in the silkworm, Bombyx mori (Lepidoptera), caused by the microsporidium Nosema bombycis, with a poorly defined taxonomic status, but with similarities to fungi. The genus Nosema, described in 1857 by Professor von Nägeli of the University of Zurich, includes 32 species, all parasites of Insects and various arthropods. This disease – with no apparent symptoms at the beginning of contamination – results at an advanced stage, in small black spots on the skin of the caterpillar, as well as on the butterfly, which has the appearance of peppercorns. First called "maladie de la tache" (silkworm spot disease), Armand de Quatrefages 2 (1860a) gave the name pebrine to this disease, referring to peppercorns, whose name in Provençal dialect is pebre.

Figure 1.2

Figure 1.2. A sick silkworm caterpillar: the characteristic black spots of the disease (pepper in Provencal dialect is pèbre, hence the name of the disease, pebrine, given by de Quatrefages)

 (source: © Pasteur L., Etudes sur la maladie des vers à soie, tome 1, la pébrine et la flacherie, ed. Gauthier-Villars, Paris, 1870). For a color version of this figure, see www.iste.co.uk/carton/innate.zip

Figure 1.3

Figure 1.3. Appearance of the spots under the cuticle of a sick caterpillar

 (source: © de Quatrefages A., Etudes sur les maladies actuelles du wor à soie, Comptes rendus de l Académie des Sciences, XXX, 1860a)

Figure 1.4

Figure 1.4. Life cycle model of Nosema bombycis

 (source: text and drawing by C. Wang Jian-Yang, PhD 2007, with the kind permission of C. Texier, Clermont-Ferrand University)

The life cycle of N. bombycis is divided into the environmental (infective) phase and intracellular phases (merogony and sporogony). All stages of development of N. bombycis are diplocaryotic. (1). In the environmental infective phase, the proper environmental conditions are required to active mature spores, resulting in polar tube extrusion and sporoplasm deposition into the host cell cytoplasm. (2). In the intracellular phases, N. bombycis sporoplasm is in direct contact with the host cell cytoplasm and matures into meront, which multiplies by binary fission (merogony). The plasmalemma thickening is the beginning of the sporogony stage. Each sporont produced two sporoblasts, and each sporoblast produced two mature spores (sporogony). (3). Spore dimorphism: N bombycis completes its relatively simple life cycle with two sporulation sequences forming two types of spore respectively: "primary spore, internal spore or FC (few coils of polar filament) spore", which can germinate quickly after formation (autoinfection) and "environmental spore or external spore" (N, nuclear; PT, polar tube) 3 .

The microsporidium cycle 4 includes an infectious phase in the surrounding environment and an intracellular phase in the silkworm. The infectious phase is represented by spores, which, in contact with the caterpillar stage of the Insect, will germinate, releasing a polar tube that penetrates into directly accessible cells: the epidermis located under an abraded cuticle, the middle intestine and the deep part of the tracheoli (the terminal tracheal cells): the silk gland, Malpighi tubes, the nervous system, the dorsal vessel and gonads. This leads to systemic, i.e. generalized, auto-infection. At the end, i.e. at the death of the caterpillar, the spores formed will be released into the external environment. Here is a very brief summary of the cycle of this microsporidium. On the other hand, organs with an external cuticle, the trachea (which is an invagination of the epidermis, therefore bordered by a cuticle), the anterior and posterior intestines (bordered by a cuticle) and the chitinized parts of the mouth cannot be infested.

The spore, once germinated (sporoplasm) in the potentially attackable cells, each generates a meront, which by binary fission, gives two sporonts. Each of its sporonts divides again twice, giving successively two sporoblasts and four mature spores, in each infested cell. Each of these mature spores can then infest a new cell with its germline tube. These various stages are all diplokaryotic, i.e. they have two nuclei. It has been shown that blood (hemolymph) and blood cells (hemocytes) are also carriers of spores, which is certainly the main vector of infection of the different organs. Hemocyte infection occurs in two modes: (1) an active mode by germination of the polar tube entering the hemocytic cell and (2) a passive mode, the hemocyte phagocyting Nosema's spore, which then becomes directly intracellular. This "germination" is rapid and leads to the spread of infection of most internal tissues or organs, easily accessible: the silk gland, Malpighi tubes, the nervous system, the dorsal vessel and gonads.

Figure 1.5

Figure 1.5. (a). Schematic structure of an infesting spore, with the system injecting its contents through the polar tube. (b) Electronic image of a dikaryotic spore, with two nuclei

 (source: © Wang J.-Y., Interactions Microsporidies-insectes in vivo: Dissémination de Nosema bombycis (Microsporidia) dans son hôte Bombyx mori (Lepidoptera) et caractérisation de protéines structurales majeures de N. bombycis impliquées dans l'invasion, PhD thesis, Université d'Auvergne, 2007)

It is useful to specify some more characteristics of the disease, which have been at the origin of so much speculation among the authors who have studied this disease. At each new instar larva, the infested caterpillars (four molting are present, for five larval stages) carrying black spots under the cuticle, these spots disappear on the larvae that have recently molted. On the other hand, the so-called "horizontal" infection (which can occur at any stage of the caterpillar) in the intestine or trachea can spread throughout the body more or less rapidly, with the various organs of the larva or adult being successively affected. In fact, if the infection occurs in the first stage by the mouth, it takes about 30 days for the infection to spread to all organs. However, by injection, the time is shortened to 10–12 days (again depending on the dose of spores used). This will be important in adults for the reproductive organs, especially the female gonads. If they are ultimately affected, some or all of the eggs produced will carry this parasite, again in varying amounts. We then have a "vertical" infestation by "heredity".

It should be recalled that at the time of this temporarily prevalent silkworm, malady that affected silkworms, the authors interested in the disease had no information about this agent and even about its existence, and of course about the complex cycle of this microsporidium that we have just described. However, as we will see below, Pasteur, very dedicated to his discoveries on fermentations, studied pebrine disease with a renewed approach, sometimes, even with some preconceptions, by encouraging experimentation. Following this work on silkworm and animal diseases, Pasteur demonstrated the possible role of pathogens present in the environment in their spread by infection and/or contagion.

It is also worth mentioning that other diseases affected silkworms at that time: muscardine and flacherie disease or "morts-flats" disease. Muscardine is caused by the parasitic fungus Beauveria bassiana, while flacherie disease is caused by several types of cytoplasmic or nucleic viruses (BmCPV, BmNPV and BmIFV), leading to digestive tract infection by pathogenic bacteria, Serratia marcescens, Streptococcus bombycis or Bacillus sp. It was only in 1927 that A. Paillot 5 recognized three distinct flacherie diseases: a pathology due to a sporulating Bacillus, chronic flacherie disease due to Streptococcus and the typical flacherie caused by filtering viruses. Flacherie disease often occurs in silkworm farms. Pasteur also had to take an interest in this pathology, without knowing exactly all its causes, because very often it was concomitant with pebrine, and sometimes confused with it.

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Medical Biotechnology and Healthcare

Osnat Hakimi , ... Andrew Carr , in Comprehensive Biotechnology (Third Edition), 2019

5.37.4.2 Chemical Characterization of Silk

Regulatory bodies require chemical characterization of implants. 8 Native mulberry silkworm silk fibers are known to be primarily composed of the fibrous protein fibroin, which is well characterized in terms of its sequence and secondary structure. 99–102 Work has also been done to characterize the, glycoprotein family sericin, which forms the gum-like coating of silk fibers. 103,104

However, there may still be other, unknown components in silk which are of importance in the clinical setting. A few studies isolated and characterized additional molecules in native silk fibers and hypothesized that these may take part in pigmentation 105 or defense against predators. 35,106 Spider silks, moreover, can contain a wide range of surprising compounds, ranging from glycoproteins to neurotransmitters. 107,108

Thus, a complete characterization of all silk fiber components from different populations of silkworms and spiders remains an important challenge.

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Biopolymers

Christopher Brigham , in Green Chemistry, 2018

3.22.2.2 Silk

Silks are protein polymers that are spun into fibers by some insects. Commonly, we think of silkworms and spiders as silk producers, but scorpions as well as some fly and mite larvae are also capable of making silk fibers. 14,15 Silk fibers, like those of collagen and gelatin, are biodegradable and biocompatible. Silk possesses superior mechanical properties (e.g., tensile strength, toughness, elasticity). Several types of silk can be produced by spiders. Each different silk has a different composition, structure, and properties, depending on the insect producing it and the function for which it is used by the producing organism. The most-well-characterized silk is known as major ampullate, or dragline silk. Dragline silk is used as lifeline support for some spiders.

Silks, like collagen, are characterized by a repetitive primary amino acid sequence. Like collagen, the amino acid sequence of silks tends to be glycine rich. The glycine-rich regions of spider silk proteins are characterized by the tripeptide motif Gly-Gly-X, where X represents one of a subset of nonglycine amino acids. These glycine-rich regions are followed by stretches of the amino acid alanine (poly-A regions). Several Gly-Gly-X motifs and poly-A stretches are repeated on a silk polypeptide, sandwiched between N- and C-terminal nonrepeated regions (Fig. 3.22.3). These poly-A regions are thought to form crystalline, hydrophobic domains that are responsible for the high tensile strength of silk fiber. 16,17 Secondary structures of silk include β-sheet structures separated by random coils. 15 Spider silk proteins (often called spidroins) have very HMW, from 200,000–350,000 mass units.

Figure 3.22.3. Schematic drawing of a silk polypeptide. The nonrepetitive N-terminus (N, blue) consisting of ∼130 amino acid residues is followed by alternating regions of Gly-Gly-X motifs and poly-Ala regions (yellow and orange), followed by a nonrepetitive C-terminus (C, red) of ∼110 amino acids.

 Adapted from Rising A, Johansson J. Toward spinning artificial spider silk. Nat Chem Biol 2015;11:309–15.

Given the heterogeneity of silk proteins synthesized by insects and arachnids, a recombinant source of silk proteins has been sought. Recombinant silk proteins also exhibit more processability and can be formed into morphologies like films, hydrogels, particles, or nonwoven meshes. 15 Silk is commonly used as a material for clothing manufacture, but other, more specialized uses exist, due to the high strength of silk. One such use that relies on the strength of silk is its use in parachutes.

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Environmentally Transmitted Pathogens

Charles P. Gerba , in Environmental Microbiology (Third Edition), 2015

22.3.1.6 Microsporidia

Microsporidia, the nontaxonomic name to describe organisms belonging to the phylum Microspora, were first described in 1857, when Nägeli identified Nosema bombycis , a microsporidian responsible for destruction of the silkworm industry. To date, over 1000 species of microsporidia infecting insects, invertebrates and all five phyla of vertebrate hosts have been described. Microsporidia are for the most part considered to be opportunistic pathogens in humans. There were only a handful of documented cases before the advent of the AIDS epidemic. Since then there have been hundreds of documented cases in immunocompromised patients. However, there have also been cases documented among the immunocompetent. Five genera have been associated with the majority of human infections: Enterocytozoon bieneusi, Encephalitozoon hellem, Encephalitozoon cuniculi, Encephalitozoon intestinalis, Pleistophora spp. and Nosema corneum. The first four have the potential to be waterborne because they are shed in feces and urine. E. bieneusi, E. hellem and E. intestinalis are the most common cause of microsporidian infections in patients with AIDS (Curry and Canning, 1993). In addition, they are much smaller (0.8×1.5   μm depending on species) than other parasites, and potentially more difficult to remove by water treatment filtration.

The microsporidian spore has the potential of being transmitted by water. The life cycle of microsporidia contains three stages: the environmentally resistant spore, merogony and sporogony. The spore is ingested by a host or possibly inhaled in some cases. Once in the body, it infects cells and goes through merogony followed by sporogony, which results in production of resistant infective spores (Figure 22.14). The spores are then shed via bodily fluids such as urine and excreta. Once in the environment they have a strong potential to enter water sources. E. intestinalis spores have been identified in sewage, surface and ground waters, supporting the notion of environmental transmission. The spores are highly resistant to heat inactivation and drying. Waller (1979) found that E. cuniculi survived 98 days at 4°C and 6 days at 22°C.

Figure 22.14. Microsporidium spore with tube by which it penetrates host cells.

 Courtesy Centers for Disease Control.

Enterocytozoon bieneusi, Encephalitozoon hellem, Encephalitozoon cuniculi and Encephalitozoon intestinalis cause a variety of illnesses. E. bieneusi causes diarrhea and wasting disease. It is the most important cause of microsporidiosis in AIDS patients. Several surveys have determined that 7 to 30% of AIDS patients who have unexplained chronic diarrhea are infected with E. bieneusi (Weber et al., 1994). E. intestinalis is similar to E. bieneusi in that it infects the intestines and causes diarrhea, but it can also infect kidneys and bronchial and nasal cells. It can infect macrophages, which allows it to disseminate throughout the body. It is secreted in feces and urine, which supports the notion of water transmission. E. cuniculi is not an intestinal parasite but it can be shed in urine (Zeman and Baskin, 1985), and therefore environmental transmission is a possibility. It has also been described infecting many different mammals, which means that there could be many animal reservoirs that can contaminate the environment. E. hellem has been recognized and shown to cause eye infections (keratoconjunctivitis) and disseminated infections such as ureteritis and pneumonia. It does not invade the intestine, but it can be shed in the urine (Schwartz et al., 1994). Both water- and foodborne outbreaks have been documented (Cotte et al., 1999; Decraene et al., 2012).

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Recombination

Abraham B. Korol , in Encyclopedia of Biodiversity, 2001

III.A. Selection for Changed Recombination, Genetic Modifiers of Recombination

Selection for altered recombination proved an important tool in analyzing genetic control of recombination. Such experiments have been conducted on dozens of organisms, including Drosophila, flour beetle Tribolium castaneum, grasshopper Schistocerca gregaria , silkworm Bombix mori, lima bean, and fungi Neurospora and Schizophyllum (Korol et al., 1994). These experiments complemented the numerous findings on the existence of a considerable amount of genetic variation in RF in natural and laboratory populations (see Section IV.B) available for selection to act on. The effectiveness of directional selection for altered RF has been shown to depend on the segment under study, the size, structure, and origin of the start population; the breeding system; the estimation procedure; and the intensity and duration of selection. Genetic analysis of the accumulated differences between selected lines enabled the question of the genetic basis of variation in recombination to be addressed. In as few as 10 generations, divergent selection for RF in the p–Y region of silkworm chromosome 2 succeeded to produce lines with RF = 37–39 and 5–7%, starting from 25.6%. The obtained evidence suggests that recombination (frequency and distribution) in eukaryotic genomes is under complex control of polymorphic modifiers with small to moderate effect. The rate of recombination in a given region may depend on both linked modifiers and genes located on other chromosomes.

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Earthquakes and Coseismic Surface Faulting on the Iranian Plateau

Manuel Berberian , in Developments in Earth Surface Processes, 2014

5.5 Bursting of the Haftvād's Worm at the Bam Citadel and Shaking the Whole Region

The Bam Citadel [Arg-e Bam (see Figure 14.6 for the location)] may have been destroyed during the reign of King Ardeshir Bābakān Sāsānid (r. 224–241), the founder of the Sāsānian dynasty. Since then, there have been at least 10 phases of documented reconstruction at the citadel (Berberian, 2005). The oldest recorded partially standing structure in the Bam Citadel was the 1751 reconstructed mosque (Gaube, 1979). Much of the citadel dates from the late eighteenth and nineteenth centuries. The citadel and the city of Bam, located by the Bam fault (see Figure 14.6), was completely destroyed during the 26 September 2003 Mw 6.6 earthquake (Berberian, 2005).

Local legend has it that the Bam Citadel was the place where the gigantic "Haftvād Worm" was housed in the old times, 3 or Haftānbokht [in the Pahlavi text of Kārnāmak Artakhshir Pāpakān; 272–273] during the reign of the Pārthian governor of Kermān and Bam. The oldest reference to this legendary gigantic Worm is found in the Kārnāmak Artakhshir Pāpakān (Anklesaria, 1935; Mashkur, 1950; avesta.org; Markus-Takeshita, 2001; Shahbāzi, 2003a; Shāki, 2002). According to local legend, the Bam Citadel [Arg-e Bam, Qal'eh Haftvād, or Qal'eh Bahman] was the capital of Haftvād of Kermān. It is said that Haftvād resided in Kerman's Qal'eh Dokhtar, near Qal'eh Ardeshir, built on the mountainous outskirts of the city of Kermān. These toponyms might be remnants of the Haftvād's legend. However, according to a second folk etymology, the Worm burst open with noise at the Bam Citadel and shook the region (Sanjana, 1896; Shahbāzi, 2003a; iranicaonline.org).

In accordance with the Kārnāmak, the fortress was destroyed by the order of Ardeshir Bābakān Sāsānid [r. 224–241], and not by bursting of the Worm. Ferdowsi Tusi in Shāhnāmeh (1010), first "symbolically" noted that the entire region was shaken by a very loud rumble that had burst from the Worm's throat when two youths accompanying Ardeshir poured molten metal (lead and bronze) into the pool where the Worm lay. Tabari (915), followed by Bal'ami (d. 996), does not mention the story of the Worm; however, he addressed Ardeshir's campaign in Kermān and the area to the south (Levy, 1967; Pāyandeh, 1975; Bosworth, 1999) (Table 5.1).

Table 5.1. Summary of the Haftvād/Haftānbokht Worm Legend and Destruction of the Bam Citadel in 224   AD

Author Book Cause of Destruction of the Bam Citadel Cited Section
Mostaufi Qazvini Nozhat al-Qolub [1340] – Chapter XIII, Bam
Anonymous writer from Asadābād, Hamédān Mojmal al-Tavārikh val Qesas [520/1126] Summary of the legend based on the Shāhnāmeh Bahār (1318/1939, p. 60)
Ferdowsi Tusi Shāhnāmeh [1010] A rumble burst forth from its [worm's] throat so heavy that it shook the pool and the whole region about it ["Tarāki bar āmad ze holqum-e-oui; Ke larzān shod ān kondeh-o- boom-e oui"] Ashkānian/744
Bal'ami [d. 996] Tārikh Bal'ami The Persian text of Tabari ed. Bahār (pp. 820, 879)
Tabari [839–923] Tārikh al-Rasul wal-Moluk (Tabari) [915] Ardeshir defeated Balāsh, the last Ārsācid ruler at Kermān, then marched to the coastland and cut the ruler in half with his sword ed. Bosworth, (1999) V, 10; ed. Pāyandeh (1975)
Anonymous Kārnāmak Ardeshir Pābakān [272–273] Ardeshir Bābakaān commanded the destruction of the Fortress Chapter 8/16

It seems that at that time, Ardeshir Bābakān Sāssānid [r. 224–239], by conquering Kermān and Bam, killed the Kerm-e Haftvād at the Bam Citadel ["the worm burst with a big bang noise which rocked the area"], completely destroyed the citadel and killed most of its inhabitants, put an end to the rule of Haftvād, built the new village of Kolālān or Kojārān [possibly the old quarter of Kurzān in western Bam], and brought the "seven fires of Bahrām" to the new village. The entire episode of the Haftvād Worm rests on the rationalization of historical events of an unknown nature; the legendary element could be a mixed metaphoric reference to a "destructive earthquake" or even a "conquering battle" by King Ardeshir against the ancient city of Bam and its Pārthian governor, Haftvād (iranicaonline.org; Berberian, 2005).

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Animal Health: Foot-and-Mouth Disease

F. Diaz-San Segundo , ... T. de los Santos , in Encyclopedia of Agriculture and Food Systems, 2014

New subunit vaccine candidates

The initial attempts to produce vaccines that addressed some of the limitations of the inactivated vaccine included inoculation of susceptible animals with viral structural protein VP1 or a fragment of VP1 (Bachrach et al., 1975; Kleid et al., 1981; Strohmaier et al., 1982; see Grubman and Baxt, 2004 for references). This approach and its limitations have been extensively reviewed (Grubman, 2003; Taboga et al., 1997). In another approach researchers have attempted to utilize empty virus particles as the immunogen. Empty viral capsids or virus-like particles contain all of the epitopes present on intact virus, but lack the infectious nucleic acid. These capsids are natural products of FMDV infection, are antigenically similar to virions, and are as immunogenic as infectious virus in animals (Grubman et al., 1985; Rowlands et al., 1974; Rweyemamu et al., 1979). The empty viral capsids can be produced in various expression systems and then delivered to the animal. Alternatively, the DNA encoding the capsid-coding region and the 3Cpro required for capsid polypeptide processing can be delivered to the animal via DNA or viral vectors and the capsids synthesized in the animal. This latter approach has a potential advantage as compared with delivery of preformed empty capsids to the animal as the synthesis of the immunogen in the animal should induce both antibody as well as cell-mediated immune responses.

Li et al. (2008) used a silkworm–baculovirus expression system to produce FMDV Asia-1 empty capsids and vaccinated cattle with the hemolymph from infected silkworms. Four of the five animals were protected from homologous virus challenge. One potential limitation in the use of empty capsids as vaccine candidates is that in the absence of virion RNA they are less thermostable than the natural virus particle and therefore may be less immunogenic ( Curry et al., 1997). Porta et al. (2013) attempted to enhance stability of baculovirus-expressed FMDV A22 empty capsids and also reduce expression of the toxic 3Cpro. The mutated capsids were more stable than the WT capsids to both heat and acid and 3C expression and catalytic activity was reduced. Two of the four cattle vaccinated twice with WT empty capsids and an oil adjuvant and three of the four animals inoculated with mutant capsids were protected from challenge with homologous virulent virus (Porta et al., 2013).

Other investigators have used viral vectors, including poxvirus, pseudorabies virus, and replication-defective human adenovirus (Ad5), as delivery vehicles (Grubman and Baxt, 2004; Zhang et al., 2011). Thus far, the most efficacious viral-vectored delivery vehicle is the Ad5 system. The Ad5-FMDV A24 (Ad5-A24) vector constructed by Grubman and colleagues (Moraes et al., 2001) is protective, after one inoculation, as early as 7 days postvaccination in both swine (Moraes et al., 2002) and cattle (Grubman et al., 2010; Pacheco et al., 2005). More recent studies with second-generation vectors containing the NS protein 2B coding region demonstrated enhanced efficacy in challenge studies in swine (Pena et al., 2008) and cattle (Moraes et al., 2011). Furthermore, by changing the route and sites of inoculation the protective dose could be significantly reduced (Grubman et al., 2012b). The US Department of Agriculture (USDA) in collaboration with the US Department of Homeland Security and GenVec Inc. have produced this vaccine on the US mainland in BSL2 facilities and tested it in cattle following the requirements of the Center for Veterinary Biologics of the Animal Plant and Health Inspection Service, USDA, which included safety testing of the vaccine in cattle at three sites on the US mainland. In June 2012 this vaccine was granted a conditional license for inclusion in the US National Veterinary Vaccine Stockpile for use in cattle in the United States in the event of an emergency situation.

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