Page 168 - Avian Virology: Current Research and Future Trends

Infectious Bronchitis Virus | 161

It is important to note that the incidence and distribution of IBV Virus stability to environmental inactivation
variants are complex and unpredictable. While some viruses, such IBV is temperature sensitive; most strains can be inactivated at
as the Mass strain, are distributed worldwide, other IBV variants temperatures of 56°C for a minimum 15 minutes to 45°C for 90
are restricted to certain regions of the country (Arkansas) or minutes (Cavanagh and Gelb, 2009). The virus will survive for
unique to a continent (D274, Europe only). While many IBV only a few days at room temperature, at which their viral infectiv-
variants have come and gone over the past four decades, some ity will exhibit a gradual decrease over time. On the other hand,
variants which have persisted for a long time still haunt the poul- virus residing in litter containing faeces can survive for a consider-
try industry, such as the Arkansas, California (cousin of Arkansas able amount of time (Animas et al., 1994; de Wit et al., 2010a).
virus) and Delaware viruses (Jackwood, 2012). However, lyophilized virus can retain its infectivity for at least 21
months when stored at 4°C (Hofstad and Yoder, 1963). Com-
Sources of infection, principal routes and mercially available common disinfectants can easily inactivate the
duration of excretion virus (Bengtong et al., 2013).
IBV is distributed worldwide, particularly in countries which
have an intensive poultry industry. Being a highly infectious Risk and consequences of importing disease
virus, the virus has a short incubation period of 36 hours and can through poultry and associated products
spread rapidly in unvaccinated chickens within one to two days IBV is listed as one of the avian diseases notifiable under the OIE
(Ignjatovic and Sapats, 2000). The major sources of IBV were list (World Organization for Animal Health, http://www.oie.
from contaminated feed and drinking water, tracheal-bronchial int/en/animal-health-in-the-world/oie-listed-diseases-2017/),
exudate and faeces of chickens (ignjatovic and Sapats, 2000). which implies that countries importing poultry should perform
Direct contact with these bodily fluids from infected chickens is tests to minimize the risk of importing IBV into the country.
the most likely source of infection. IBV can also spread horizon- While IBV is globally distributed, the introduction of an exotic
tally via aerosol or ingestion. strain can increase the genetic pool of viruses that circulate on site
While vertical transmission of the virus has been suspected in (Ignjatovic and Sapats, 2000; Toro et al., 2015). This increases the
one study (McFerran et al., 1971), it was recently demonstrated likelihood of generating more and newer variants through spon-
in two studies where viral RNA was detected in the allantoic fluid taneous recombination (Toro et al., 2015).
of viable embryos and infected eggs through natural infection and
experimental inoculation, respectively (Cook, 1971; Pereira et al.,
2016). IBV was recovered from the semen of cockerels, and IBV Diagnosis
RNA was detected in the trachea of hens artificially inseminated Several laboratory tests are available to confirm diagnosis, and
with semen of IBV infected roosters, providing experimental they can be divided into two broad categories – agent identifica-
evidence for venereal transmission (Cook, 1971; Gallardo et al., tion and detection of immune response.
2011).
For the respiratory forms of IBV, the virus is copiously shed Virus isolation and identification through antigen
from the respiratory tract of the birds and into the environ- detection
ment through coughing (Ignjatovic and Sapats, 2000). Thus, Confirmation of IB is often based on serology-assisted virus
high titres of IBV can be recovered from the trachea and lungs isolation, and this is usually performed on 9- to 10-day-old
of infected chickens from 1 to 7 days post infection. IBV can embryonated SPF chicken eggs. Typical IBV clinical signs include
also be recovered from cloacal contents within 1 month post lesions such as curling and dwarfing of the embryos, clubbing of
infection. the down or urate deposits in the kidneys 5 to 7 days post inocula-
tion (Momayez et al., 2002). Conversely, IBV can also be isolated
Methods of spread from TOCs within 48 to 72 hours post inoculation, through the
Airborne transmission is the most common form of virus spread visualization of ciliostasis and damage to the tracheal epithelium
among chickens, houses and farms. Airborne transmission via under microscopy (Nicholas et al., 1983). Immunofluorescence
aerosols can occur readily among birds kept within 1.5 m radius. test containing fluorescein-conjugated antibodies that bind to
Movement of live birds is another source of virus transmission, as IBV in tracheal smears is an alternative method of isolating the
it may introduce IBV to a different flock. Experimental evidence virus through detection of fluorescent signals (Yagyu and Ohta,
has also revealed IBV persistence and re-excretion as a potential 1990; Ahmed et al., 2007).
risk factor for IB (Alexander and Gough, 1977).
Virus isolation and identification through
Species susceptible to IBV infection genome detection
Apart from avian species, no other species are susceptible to natu- CAMs from infected eggs are first homogenized and subjected
ral IBV infection. However, IBV can be propagated in suckling to tests such as immunodiffusion, immuno-histochemistry or
mice, rabbits and guinea pigs when inoculated via the intracer- RT-PCR (Hironao et al., 1970; Jackwood et al., 1992; Abdel-
ebral route (McIntosh et al., 1967). Even so, such virus passage Moneim et al., 2009). In addition to these methods, genetic based
only appears to result in the selection of a non-pathogenic strain tests such as RT-PCR and RT-RFLP (reverse transcription restric-
for chicks (Yachida et al., 1979). tion fragment length polymorphism) are commonly used to