Formación

The family Adenoviridae

Family Adenoviridae Genus Genus (Murphy et al. 1 995) Mastadenovirus A viadenovirus

Equine adenoviru -I ( EAdV -I)

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Literature Review ₃₅

Group characteristics

Adenoviruses are non-enveloped icosahedral v iruses 80 to 1 10 nm in d iameter with a double stranded DNA genome. The capsid consists of subunits arranged i nto 240

hexamers and 1 2 pentamers with penton fibres projecting from apices (Murphy et af.

1 995). The penton bases are composed from group-specific antigens that are common to most members of the family. Antibodies raised against these proteins possess only low levels of neutralizing activ ity. The hexons and fibres are major antigenic sites. The fibre protei n i s respons ible for attachment of the viru s to RBC i n a haemagglutination reaction. This reaction is type-specific w ith some subgroup speci ficity. Hexon proteins elicit a heterologous population of antibodies, some of which are family cross-reactive in a CF test and others show marked type specificity in SN tests (Horwitz 1 996; Murphy et (If. 1 995).

Adenoviruses are usual l y confined to one host speCIes or c losely rel ated speCIes (Horwitz 1 996). S imilarly, they usuall y replicate onl y in cell cultures derived from their natural host. The CPE consists of bunches of rounded, refractile cells w ith characteristic basophilic intranuclear inclusion bodies (Hsiung 1 982). Human adenoviruses were

i solated from healthy people for 24 months after i nitial infection, suggesting the ability

of the virus to establish persi stent infection in its host ( Horwitz 1 996). Also, it has been suggested that adenoviruses can establish l atent infection in l ymphocytes and l ymphoid tissue. The exact mechanisms of adenovirus latency, however, have not been established ( Horwitz 1 996). It has been shown that three adenovirus genes encode products that have the potential to interact with the host' s immune response, and therefore could

facilitate persistence or latency of the virus. Cellular responses to a and

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are inhibited by E t A proteins and VA RNAs, while E3 protein protects infected cel l s from l ysis mediated b y cytotoxic T l ymphocytes (CTL) and tumor necrosi s factor a

(TNFa) (Mahr & Gooding 1 999; Shenk 1 996).

Although there i s no evidence of associ ation between adenoviruses and any human neoplasm, several human serotypes can produce tumours in rodents (Shenk 1 996). Tumour formation is associated with integration of adenovirus DNA into the host cel l ' s chromosome with n o production o f mature viruses (Horwitz 1 996).

Chapter 1 _{3 6} Adenoviruses have been isolated from many species. They are reasonably stable in the environment. Human adenoviruses usual l y produce asymptomatic infections, but have also been associated with clinical i nfections affecting a variety of organs i nc luding

respiratory, ocul ar, urinary and gastrointestinal systems (Horwitz 1 996; Murphy et al.

1 995). Typical ly, adenoviral disease i s short-lived, self-limi ted and with no serious consequences in i mmunocompetent persons. However, adenoviruses can cause more serious respiratory disease in young or i mmunocompromised hosts. Additionally, i n such patients, they can disseminate t o other organs causing acute systemic i nfection (Horwitz 1 996).

Equine adenovirus

Virus properties

The biological and physiochemical properties of equine adenov irus have been described

by Wilks & Studdert ( 1 973), E ngland et al. ( 1 973), Fatemie-Nainie & Marusyk ( 1 979),

Ardans et al. ( 1 973), Moorthy & Spradbrow ( 1 978a) , Konishi et al. ( 1 977), Dutta

( 1 975), and Harden ( 1 974) . Equine adenovirus is not sensitive to chloroform or ether

and is stable within the pH range 4 to 7, but slightly i nactivated at pH 3 ( Konishi et al.

1 977). However, there are some differences between isolates, as equine adenoviruses

described by Ardans et al. ( 1 973) and Dutta ( 1 975) were stable at pH 3, while the

i solate described by H arden ( 1 974) w as i nactivated at pH 3. Additionally, other characteristics reported for different isolates differed slightly. The cell l ines used to propagate the virus were usual l y primary cell s of equine origin , as these were the only cells that supported the growth of all the i solates characterised. Other cells supported the

growth of some isolates, but not others ( Konishi et al. 1 977; Harden 1 974; England et

al. 1 973). Different isolates also differed in their haemagglutination characteristics.

Most of the equine adenoviruses described haemagglutinated human type 0 and equine

RBC. However, Moorthy and Spradbrow's i solate was not tested against human 0 RBC

and Dutta's i solate haemagglutinated only human 0 RBC, and not equine ones. All

i solates that were tested for haemagglutination w ith rat RBC gave positive reactions and the titres were much higher than those obtained w ith RBC from other species (Moorthy

& Spradbrow 1 978a; Harden 1 974) . The virion s ize reported by different authors ranged from 70 to 82 nm, and the buoyant density in CsCI ranged from 1 .27 glcc to 1 . 34 glcc

Literature Review ₃₇

( Konishi et al. 1 977; Kamada et al. 1 977; Dutta 1 975; Harden 1 974; Ardans et al. 1 973;

England et al. 1 973; Wilks & Studdert 1 973).

Antigenic serotypes

Adenovirus serotypes are identified on the basis of S N and HI assays (Murphy et al.

1 995). Two viruses are defined as separate serotypes if they show either no cross­ reaction or an homologous/heterologous titre ratio greater than 1 6 in both directions i n the SN test. For heterologous/homologous titre ratios o f 8 and 1 6, a serotype assignment is made on the basis of lack of cross-reaction in HI tests and/or the existence of major

biophysical or biochemical differences (Murphy et al. 1 995).

Most of the equine adenoviruses compared were closely related and constituted a single

serotype designated as EAdV - 1 (Studdert 1 978; Thompson et al. 1 976), although there

were slight anti genic differences between i solates (Fatemie-Nainie & Marusyk 1 979;

Studdert et al. 1 974) . In 1 982, the i solation of an adenovirus antigenical l y different to

EAdV - 1 was reported (Studdert & Blackney 1 982). The i solate was obtained from

diarrhoeic foal faeces and was found to be totall y unrelated to EAdV- l by the SN test. Furthermore, it did not haemagglutinate any RBC tested including human, monkey, equine, porcine, guinea pig and chicken. A new serotype was designated EAdV-2

( Homer & Hunter 1 982; Studdert & B lackney 1 982). Both EAdV - 1 and EAdV -2 were

i solated i n New Zealand from young Thoroughbred horses with clinical signs of ill­

thrift, upper respiratory disease or diarrhoea (Homer & Hunter 1 982).

Viral DNA

DNA restriction analysis of the genome of EAdV - 1 has been publi shed (Sheppard et al.

1 992; H igashi & Harasawa 1 989; Ishiyama et al. 1 986). The comparison of restriction

maps of three different i solates of EAdV -1 confirmed the homogeneity among different i solates of EAdV - 1 , despite some polymorphism that could be detected with appropriate

restriction enzymes (Higashi & Harasawa 1 989). Several genes from EAdV- l and

EAdV-2 have been cloned and sequenced (Reubel & Studdert 1 997b; Reubel &

Studdert 1 997a). Sequence data confirmed that EAdV- l and EAdV-2 are two separate viruses. Phylogenetic analysis indicated that both EAdV - 1 and E AdV -2 evolved

separately from each other and from other adenoviruses (Reubel & Studdert 1 997b;

Chapter J ₃₈

Disease

Equine adenovirus usually produces subclinical or mild respiratory tract i nfection i n immunocompetent horses (Studdert 1 996a).

Adenoviruses were i solated from cases of fatal respiratory disease in Arab foals

(Thompson et al. 1 976; Ardans et al. 1 973; McChesney et al. 1 970; Todd 1 969), as well

as from Arab and non-Arab foals and older horses with milder, non-fatal respiratory

disease (Moorthy & Spradbrow 1 978a; Kamada et al. 1 977; Dutta 1 975; England et at.

1 973) and healthy ani mals (Wilks & S tu ddert 1 972; Harden et al. 1 972; Petzoldt &

Schmidt 1 97 1 ). Adenovirus was also i solated from the bronchus and lung of a 3-month­

old Thoroughbred colt with fatal mucopurulent pneumonia, although Corynebacterium

SI'. was also i solated from pneumonic lung tissue of the foal and was probably the cause

of death ( Konishi et al. 1 977).

Equine adenovirus was also isolated from two of three cases of cauda equina neuritis. The adenovirus infection in neural tissue was considered to be l atent as determined by the fai lure to demonstrate virus in tissue sections by immunofluorescence and the need to co-cultivate andlor passage the virus several times before the occurrence of any visible CPE in cell culture. None of the three ani mals had antibody to EAdV - I as

determined by immunofluorescence, HI and S N ( Edington et al. 1 984).

Fatal adenovirus pneumoni a III Arab foals occurs in foals with pnmary, severe

combined i mmunodeficiency disease (PSCID ) . As maternal antibody wanes, these foals become susceptible to infection with many pathogens among which adenovirus seems to p lay a predominant role (Studdert 1 996a). In these foals, EAdV can be i solated from many different organs. The outcome of i nfection is usually fatal ( Studdert 1 996a). The disease signs and pathology of adenoviral infection in PSCID foals has been reviewed ( Studdert 1 996a) and will not be discussed in detail .

C linical signs observed i n non-fatal respiratory disease caused by equi ne adenovirus are variable and include rhinitis, serous to mucopurulent nasal discharge, cough, enlarged

l ymph nodes or stunted growth (Moorthy & Spradbrow 1 9780.; Kamada et al. 1 977;

Dutta 1 975). It i s possible that adenoviral infection at the time of trans ient immunosuppression caused by factors such as stress, cold or other infections, can lead

Literature Review ₃₉

to more severe clinical signs than infection in immunocompetent hosts. For example, the adenovirus i solated by Dutta ( 1 97 5 ) came from a young foal w ith severe respiratory tract disease. Equine herpesvirus- 1 I4 was i solated from other foals with mil der respiratory disease on the same farm. As EHV - 1 infection causes transient

i mmunosuppression in the horse (Charan et al. 1 997; H annant et al. 1 99 1 ; Dutta et al.

1 980), adenovirus i nfection at the time of this transient i mmunossuppression may have lead to more severe disease in this foal .

Experimental infection

The pathogenicity of equine adenovi rus has been studied in experimentall y infected

animals (Glee son et al. 1 978; Pascoe et af. 1 974; McChesney et al. 1 974). Clin ical signs

i n both conventional and SPF foals experimentall y infected with adenovirus varied from subclinical infection to severe respiratory disease. In a l arge-scale experimental infection of 35 foals, the clinical signs observed i ncluded intermittent fever, nasal and

ocul ar discharges, polypnea and cough ( McChesney et al. 1 974). Colostrum-deprived

foal s showed more apparent clinical s igns and more extensive post-mortem lesions than the colostrum fed foals. Clin ical signs disappeared spontaneously by 1 0 days post infection. Only two colostrum-deprived foals died, and both of them had concurrent bacterial infections. Foals that were not killed for necropsy grew and developed normall y. Experimental infection of foetuses resulted in death and abortion. Pathological changes on post-mortem examination consisted of pulmonic lesions involving the cardiac region of the lungs, characterised as atelectasis, suppurative bronchopneumonia and i nterstitial pneumonia. H i stologicall y, hyperplasia, swelling, necrosi s and i ntranuclear i nclusions of epithelial cells were observed. Two-month-old and 4-month-old foals exposed to equine adenovirus developed only mild respiratory disease with few or no clinical signs. Similarly, post-mortem examination revealed no gross lesions and much milder histological changes in epithelial cells, as compared with

younger foals (McChesney et a/. 1 974).

Epizootiology

Several serological surveys conducted throughout the world have indicated that adenoviral infection is common among horses and often seroconversion occurs w ithout

any clinical s igns ( Herbst et al. 1 992; H omer & H unter 1 982; S tuddert & Blackney

Chapter I ₄₀

Serological evidence of adenoviral infection in hor e was first presented by Darbyshire

& Pereira ( 1 964), who reported that 24. 1 % of 1 78 horse sera tested in the United Kingdom had precipitating antibody to group-specific adenoviral antigens. The results of similar surveys in Iran (Afshar 1 969), Ireland (Timoney 1 97 1 ), New Zealand ( Fu et

al. 1 986), Nigeria (Obi & Taylor 1 984) and Italy ( Farina et al. 1 978) were 1 0.9% of 73 horses, 1 0.6% of 227 horses, 39% of 1 83 , 4.5% of 44 and 1 8.5% of 694 horses, respectively. Also, 88.6% of hor es imported to Japan from different countries had neutral ising antibodies to EAdV - 1 ( Harasawa et al. 1 977) and 39% of horse sera tested in The Netherlands had CF antibodies to EAdV ( Moraillon & Morai llon 1 978). Of 63 1 equine serum tested in another study (Studdert et al . 1 974), 73. 1 % had H I antibody to

EAdV - 1 . Sera included in this survey originated from horses from different countries, different breeds and age groups. There wa no significant difference observed in antibody prevalence between horses from different countries or different breeds. The prevalence was slightly higher in horses 1 year of age and older than in younger foals. A similar age distribution was reported by others ( Homer & Hunter 1 982; Harden et al.

1 974) .

I t has been demonstrated, that the AGI D test is relatively less sensitive than SN and H I tests for detection o f antibodies to EAdV ( Kamada 1 978; Harden e t al. 1 974; Pascoe et

al. 1 974) . In one study, 77% of 433 erum samples tested had neutralising antibodies to equine adenovirus, whereas only 1 4% had precipitating antibodies detectable by AG ID and 53% of the same sera were positive by CF ( H arden et al. 1 974) . Complement fixation titres were two to three dilutions lower than neutrali ing ones. However, CF had the advantage of detecting common group antigen and on 1 6 occasions it detected anti-adenoviral antibody in the absence of type specific neutralising antibodies to EAdV - 1 , indicating for the first time the existence of the econd type of EAdV 10

Austral ia ( Harden et al. 1 974) .

A more recent survey conducted in Australia distinguished the prevalence o f infection with EAdV - 1 and EAdV -2. About 80% of 339 horse sera tested were positive for SN antibodies to EAdV-2 and 86% for antibodies to EAdV- l . Most positive horses were positive for antibodies to both viral serotypes. There were, however, horses positive for antibodies to only one of the viruses (Studdert & Blackney 1 982).

Literature Review _{4 1}