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Emerging Trends in Veterinary Virology
Muhammad Abubakar, Jonas Johansson Wensman
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eBook - ePub
Emerging Trends in Veterinary Virology
Muhammad Abubakar, Jonas Johansson Wensman
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Advances in biochemistry, molecular biology, virology, and structural biology have enabled the researchers in veterinary medicine to make many exciting discoveries that have, in some cases, conceptually revolutionized our understanding of the discipline. Emerging Trends in Veterinary Virology is a review of selected topics about viral infections in animals. 11 chapters cover recent findings about specific viruses that infect a variety of hosts. The contents cover several types of veterinary infections in birds (Infectious laryngotracheitis (ILT), avian leucosis), cats (feline rabies), dogs (canine distemper), bovines (viral leukemia) and equines (hendra virus disease). Additionally, special topics such as the epidemiology of veterinary zoonoses and SARS are also covered. The book provides updated information for researchers (virologists, microbiologists), students and veterinarians, alike.
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Medicina veterinariaInfectious Laryngotracheitis Virus: Molecular Biology, Pathobiology, and Control Strategies
Muhammad A. Shahid1, *, Ahmad J. Sabir2, Muhammad I. Arshad3, Sadeeq ur Rahman4, Muhammad F. Tahir5, Muhammad N. Zahid6, Muhammad A. Zahoor7
1 Department of Pathobiology, Faculty of Veterinary Sciences, Bahauddin Zakariya University, Multan, Punjab, Pakistan
2 Asia Pacific Centre for Animal Health, The University of Melbourne, Werribee, Victoria, Australia
3 Institute of Microbiology, University of Agriculture, Faisalabad, Punjab, Pakistan
4 College of Veterinary Sciences and Animal Husbandry, Abdul Wali Khan University, Mardan, Khyber Pakhtunkhwa, Pakistan
5 Poultry Research Institute, Rawalpindi, Punjab, Pakistan
6 Department of Biology, College of Science, University of Bahrain, Kingdom of Bahrain
7 Department of Microbiology, Government College University, Faisalabad, Punjab, Pakistan
Abstract
Infectious laryngotracheitis (ILT) is an economically important respiratory disease of chickens that is prevalent throughout the world. It is caused by an infectious laryngotracheitis virus (ILTV), also named Gallid alphaherpesvirus 1 (GaHV-1). It is a member of the genus Iltovirus, subfamily Alphaherpesvirinae, family Herpesviridae and order Herpesvirales. The ILTV genome is a linear double-stranded DNA molecule with an average genome length of 151,607 nt. Twelve herpes simplex virus -1 homologue genes have been identified in the ILTV genome, with seven of them, glycoproteins B, C, H, K, L, M and N, present in the UL region, while glycoproteins D, E, G, I and J are present in the US region of the ILTV genome. Although chicken is the natural host of ILTV, infections have also been reported in pheasants, pheasant-chicken crosses, peafowls, turkeys, and ducks. An incubation period of 3–12 days is followed by an acute phase of the infection which lasts 1–2 weeks. During this phase, the virus replication occurs in the conjunctiva, trachea, and larynx, resulting in conjunctivitis, gasping, coughing, and expectoration of blood-mixed mucus. ILTV infection results in decreased weight gain and egg production. It causes 0 to 80% mortality depending on the virulence of the strain involved. Like other herpesviruses, ILTV establishes latent infection in trigeminal ganglia and virus reactivation and shedding occur following various stress factors.
ILTV infections are generally diagnosed by the typical clinical signs and detection of intranuclear inclusion bodies in the affected tissues. Furthermore, the detection of virus-specific antigen by fluorescent antibody, immunohistochemical staining of smears and tissues, detection of DNA by a polymerase chain reaction, and virus isolation by inoculating embryonated chicken eggs or cell cultures can also be performed. Virus neutralization assays and different types of ELISAs have also been established. To control ILTV infections, a combined effort is required encompassing prompt disease diagnosis, the use of geographic information system technology, biosecurity, vaccination, differentiation of infected from vaccinated (DIVA), and eradication of reservoir hosts.
Keywords: Alphaherpesvirinae, Coughing, Expectoration of blood, GaHV-1, Gallid alphaherpesvirus 1, Gasping, Herpesviridae, Herpesvirales, Infectious laryngotracheitis (ILT), Infectious laryngotracheitis virus (ILTV), Iltovirus, Respiratory disease.
* Corresponding author Muhammad A. Shahid: Department of Pathobiology, Faculty of Veterinary Sciences, Bahauddin Zakariya University, Multan, Pakistan; Tel:+923012187006; E-mail: [email protected]
INTRODUCTION
Infectious laryngotracheitis (ILT) is an economically important disease that affects the respiratory tissues of chickens. It is caused by Gallid alphaherpesvirus-1 (GaHV-1) that is generally known as the infectious laryngotracheitis virus (ILTV) [1]. The virus inflicts heavy production losses because of mortality and reduced production of the chickens. An average mortality range is approximately 10-20%; however, in severe cases of the disease, the mortality and morbidity can reach up to 100% and 70%, respectively [2]. The morbidity and mortality can be as low as 0.5 and 0.1%, respectively, in the milder type of disease [2]. In developed poultry industries, the milder enzootic forms of the disease are frequently encountered and are seen as mucoid tracheitis, sinusitis, with low mortality [3]. Flocks with persistent infection can exist alongside flocks that are free from ILTV and serious disease outbreaks can occur periodically whenever ILTV spreads from persistently infected flocks into the flocks of unvaccinated chickens [2]. A recently published report suggests that ILTV is evolving fast with greater transmissibility and capacity to spread into ILTV-free areas [4].
Transmission of the ILTV virus usually occurs by the horizontal route, and it mainly infects the upper respiratory system of the chickens; however, it can invade the trigeminal ganglia during the initial period of replication. After an experimental inoculation, the virus or viral DNA is detectable after 2 days of infection, and the highest titers of the virus are observed between 4 to 6 days [5, 6]. The virus is detectable at sites other than the respiratory tract including the liver and caecal tonsils. It is assumed that the infection of the macrophages is involved in this spread [6, 7].
Clinical signs associated with the severe form of the disease are dyspnoea, expectoration of blood-mixed mucus from the trachea, and a variable degree of conjunctivitis. Clinical signs associated with the milder form of the disease include wet eyes, conjunctivitis, swelling of the infraorbital sinuses, constant nasal discharge, mucoid tracheitis, respiratory rales, unthriftiness, and decline in egg production [3, 8]. As observed in other herpesviruses, the establishment of latent infections and reactivation of infections, followed by virus shedding, has been observed in the tracheal-organ- cultures taken from latently infected birds with ILTV [7]. Certain stresses, for example, housing with unknown birds, are linked with the reactivation of the infection [9-11]. Mortality and decrease in egg production caused by ILTV inflict multimillion-dollar losses to the U.S. poultry industry every year, and similar effects are likely to be seen in other intensive poultry industries of the world. Infectious laryngotracheitis is the first avian-viral disease for which an effective vaccine has been developed. The disease is considered to be of no public health concern [3].
MOLECULAR BIOLOGY OF GALLID ALPHAHERPESVIRUS-1
Virion Structure, Replication, and Morphogenesis
Electron microscopy of chicken embryo cells infected with ILTV has demonstrated the presence of nucleocapsids with icosahedral symmetry (Fig. 1). The spherical virion consists of a core, capsid, tegument, and an envelope. Packed into a capsid protein, the genome of the virus is comprised of a linear, single, double-stranded DNA (ds DNA) molecule [12]. The nucleocapsid of ILTV is comprised of 162 capsomeres, including 150 hexons and 12 pentons. The triangulation number (T) is 16. There are 960 copies of the capsid. The diameter of the ILT virion is approximately 195-250 nm. A lipid-bilayer envelope surrounding the nucleocapsid is connected with the outer surface of the tegument. As observed in other avian herpesviruses, ILTV infects a narrow range of hosts, both in-vivo and in-vitro. Besides chickens, natural infection of ILTV has been observed in pheasants only [13]. The virus can be grown on the chorioallantoic membranes of the embryonated chicken eggs [14] and the primary chicken embryo cells of the kidney and liver [15]. The virus also grows on leghorn male hepatoma (LMH) stable cell lines; however, these cell lines are not much sensitive to ILTV [16]. The replication cycle of ILTV is not well-understood; however, it seems similar to that of other alphaherpesviruses, for example, herpes simplex virus type 1 (HSV-1) [17]. One-step grow...