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Avian Metapneumoviruses | 125

to recover within seven to ten days after the initial infection. In and Easton, 1994; Hafez et al., 2000). Subgroup B viruses were
laying ducks, respiratory signs are followed by a drop in egg pro- not reported in the UK until 1994 (Naylor et al., 1997). Also,
duction and shell quality. In Europe the drop in egg production in the 1980s a non-A/non-B virus was circulating in France
has been reported to be in the order of 30% however in China and was classified as subgroup D (Bayon-Auboyer et al., 2000).
this was reported to be between 40% and 85% (Sun et al., 2014). To date France is still the only country to have isolated a virus
belonging to this subgroup. Subgroups A and B are now broadly
distributed across most part of the world (Lu et al., 1994; Tanaka
Immune responses et al., 1995; Jones, 1996; Banet-Noach et al., 2005; Gharaibeh
Studies suggest that the cell mediated response is the main line of and Algharaibeh, 2007; Owoade et al., 2008; Chacon et al., 2011;
defence against AMPV infection in the respiratory tract. A study Rivera-Benitez et al., 2014; Franzo et al., 2017; Mayahi et al.,
in 1992 showed that vaccinated turkey poults incapable of pro- 2017; Tucciarone et al., 2017). Surprisingly subgroups A and B
ducing antibodies due to chemical bursectomisation remained have not been detected in North America.
resistant to challenge with a virulent virus (Jones et al., 1992). It AMPV was first reported in the USA in the state of Colorado
has also been reported that turkey poults that were vaccinated at in 1996 (Senne et al., 1997) during an outbreak of respiratory
12 days of age were resistant to challenge at 22 weeks of age even disease in turkeys and the isolated viruses were later classified
though they had very little circulating ELISA antibody at that as a new subgroup, subgroup C (Cook et al., 1999; Panigrahy et
time (Williams et al., 1991b). More evidence of the importance al., 2000; Seal, 2000). In 1997 a virus of the same subgroup was
of the cell mediated response has been show in chemically T-cell- detected in the state of Minnesota (Goyal et al., 2000; Panigrahy
compromised turkey poults. These birds were slower to recover et al., 2000; Lwamba et al., 2002a) and was a major problem for
from infection compared with non-T-cell-compromised birds in the Minnesota turkey industry and neighbouring states between
terms of clinical signs and histological lesions. Viral RNA was also 1997 and 2000 and was estimated to have cost the industry in
detected for a longer period in T-cell-compromised birds. It has the order of $15 million. There have been no recent reports of
been suggested that variations in T-cell responses may play a role AMPV outbreaks in the US however AMPV C seropositive
in altering AMPV-pathogenesis in turkeys and chickens (Jones turkey breeders are occasionally reported (Qingzhong and Cook,
and Rautenschlein, 2013). It would be very interesting to see if 2016). AMPV C viruses have also recently been isolated from
this extends to ducks. pheasants in South Korea, although it should be noted that these
Humoral antibody responses following AMPV infection samples came from a live bird market where the risk of sample
are detectable by virus neutralization tests (VNT), ELISA and contamination from other species is high. AMPV C has also
immunofluorescence as early as 5 days after the appearance of recently been isolated from broiler chickens in China (Lee et
clinical signs. Virus neutralizing antibodies have been shown to al., 2007; Wei et al., 2013). In 1999 a subgroup C virus was also
be at their highest between 10 and 14 days after infection and detected in France (Toquin et al., 1999a) however this virus was
correlate well with the clearance of infectious virus but then they isolated from diseased Muscovy ducks and is not pathogenic for
quickly decline (Baxter-Jones et al., 1989). Antibodies that react turkeys under experimental conditions (Toquin et al., 2006a)
in immunofluorescence were shown to have a similar profile. As (author’s unpublished work, 2014–2017). Other duck specific
discussed later under the heading ‘diagnosis’ antibodies that react AMPV-C viruses have now been reported in China (Sun et al.,
in ELISA continue to be detected up to 6–7 weeks post infection. 2014). No AMPV C turkey viruses have been detected outside
Local immune responses have also been demonstrated in turkeys of the US. Australasia seems to be still the only continent free of
following AMPV infection by the accumulation of B cells and IgA AMPV however no reports since 1990 have been published (Bell
antibody in nasal turbinates (Cha et al., 2007). Local IgA antibody and Alexander, 1990).
has also been detected in bile, lachrymal fluid and tracheal washes
(Khehra, 1998; Cha et al., 2007; Rautenschlein et al., 2011). In
VNT these antibodies have the same profile of duration as those Diagnosis
detected in serum (Liman and Rautenschlein, 2007; Rauten- Definitive diagnosis of AMPV cannot be made simply based on
schlein et al., 2011). Despite the detection of humoral antibodies, observed clinical signs or gross lesions as they are not pathogno-
it has been shown that they do not necessarily lead to resistance monic. Therefore, laboratory analyses are required.
to infection (Kapczynski et al., 2008) further demonstrating the
importance of the cell mediated branch of the immune response. Virus isolation
Firstly, the isolation of AMPV from field samples is not straight
forward and thus should not be the first point of call for AMPV
Epizootiology diagnosis. However, it is fundamental for characterizing the virus.
AMPV was first detected in South Africa in 1978 in turkeys (Buys Sample timing and type are critical when considering virus
and Du Prees, 1980) and quickly spread across Europe (Giraud isolation. Known AMPVs primarily replicate in the upper respira-
et al., 1986; McDougall and Cook, 1986; Wilding et al., 1986; tory tract thus, samples from the trachea, nasal turbinates, choanal
Wyeth et al., 1986; Hafez and Woernle, 1989; Redmann et al., cleft as well as ocular and nasal secretions can be considered. Sam-
1991; Cook et al., 1993b). With the exception of the UK, both ples should be taken as early as possible after infection as under
subgroups A and B were present in Europe in the 1980s (Juhasz experimental conditions peak excretion of infectious virus has