it has now been discovered that canine herpesvirus passaged in a canine herpesvirus growth supporting tissue culture at suboptimal temperatures process a small plaque variant which lacks pathogenicity yet imparts resistance to virulent chv , when employed as a vaccine in bitches or new born pups . analysis of the plaque characteristics of 14 chv strains isolated at various times and from different geographical areas reveals an overall mean plaque size of 1 . 50 ± 0 . 09 mm at 35 ° c . plaques produced by the different field strains did not differ significantly in size ( p & lt ; 0 . 01 ). table 1 . __________________________________________________________________________history and mean plaque size of different chv strains in dkc monolayercultures after 5 days of growth at 30 ° and 35 ° under a 1 % methylcelluloseoverlay medium plaque size ( mm ) strain geographical source ( yr ) tissue source . sup . a dkc cultures 30 ° c . 35 ° c . __________________________________________________________________________f - 205 new york (&# 39 ; 61 ) lung 2 0 . 6 ± 0 . 05 1 . 50 ± 0 . 12f - 205 - mp new york (&# 39 ; 61 ) lung 312 0 . 6 ± 0 . 09 1 . 50 ± 0 . 15f - 205 - mp new york (&# 39 ; 61 ) lung --. sup . b 0 . 65 ± 0 . 03 0 . 75 ± 0 . 04g 4 / 66 georgia (&# 39 ; 66 ) spleen 3 0 . 57 ± 0 . 07 1 . 56 ± 0 . 05m 4 / 66 maryland (&# 39 ; 66 ) kidney 2 0 . 62 ± 0 . 05 1 . 48 ± 0 . 04o 3 / 66 ontario (&# 39 ; 66 ) urine ? 0 . 58 ± 0 . 07 1 . 50 ± 0 . 08s 4 / 63 washington , d . c . (&# 39 ; 63 ) tc ? 0 . 60 ± 0 . 10 1 . 52 ± 0 . 22k 9 / 67 kentucky (&# 39 ; 67 ) kidney 3 nd . sup . c 1 . 46 ± 0 . 07n . j . 2 / 68 new jersey (&# 39 ; 68 ) kidney 2 nd 1 . 52 ± 0 . 10n . y . 6 / 69 new york (&# 39 ; 69 ) vagina 4 0 . 48 ± 0 . 12 1 . 50 ± 0 . 06sl / 18 maryland (&# 39 ; 65 ) spleen 2 nd 1 . 49 ± 0 . 10m 9 / 68 maine (&# 39 ; 68 ) kidney 2 nd 1 . 54 ± 0 . 08f 8 / 73 new yori (&# 39 ; 73 ) lung 2 0 . 56 ± 0 . 07 1 . 45 ± 0 . 12b 10 / 73 massachusetts (&# 39 ; 73 ) kidney 3 0 . 62 ± 0 . 03 1 . 46 ± 0 . 10n . j . 2 / 74 new jersey (&# 39 ; 74 ) kidney 3 nd 1 . 52 ± 0 . 10pr / 1 missouri (&# 39 ; 67 ) tc 4 nd 1 . 49 ± 0 . 22__________________________________________________________________________ . sup . a samples were neonatal pup tissues received for diagnosis or infected dkc cultures . the exceptions were strains 0 3 / 66 and n . y . 6 / 69 , which were isolated from mature female dogs . tc indicates canine kidney cell cultures . . sup . b see text for natural history of mp variant of strain f205 . . sup . c nd , not done . when the wild ( mp ) strains of canine herpesvirus are tissue culture passaged at optimal temperatures neither a reduction in virulence for newborn pups nor the emergence of plaque variants is observed . however , when the mp strains are tissue culture passaged in a canine herpesvirus growth supporting medium at suboptimal growth sustaining temperatures , i . e . & lt ; 33 ° c ., preferably between about 28 ° c . and about 33 ° c ., there appears in a very few passages , i . e . usually less than ten and frequently 2 or 3 , a substantial number of small plaques which contain a canine herpevirus small plaque ( mp ) variant which lacks pathogenicity for newborn pups , but which serves as a vaccine to impart resistance to dogs against virulent mp canine herpesvirus strains . the small plaque ( mp ) variant can be cloned and is temperature and tissue culture passage stable . the mp variant has a plaque size at 35 ° c . less than about 0 . 7 times the diameter of the mp strain from which it is derived . for example , in contrast to the wild type mp virus , ( table 1 supra ) the mp variant ( e . g . strain f - 205 ) produced plaques at 35 ° c . that were approximately one - half the size of the wild - type ( mp ) virus plaques ( fig1 ). similar consistency in plaque size and character was observed at the 30 ° c . growth temperature . at the lower temperature , however , plaques produced by the mp virus were approximately 60 % the diameter observed at 35 ° c . plaques formed by the mp virus at 30 ° c . were 86 % the size produced at 35 ° c . plaques formed under a methylcellulose medium did not differ significantly from those produced when agarose was used in the overlay medium . the above observations are based upon the following examples with the f - 205 strain of chv , found fully virulent for newborn pups after 312 passages in dkc cultures incubated at 35 ° c . gave rise to plaques of the mp type . this type was characteristic of 12 additional chv field strains that had been passaged fewer than four times in dkc cultures . however , after fewer than 20 additional passages at low temperature ( 30 ° c . ), a stable mp variant of strain f - 205 was the principal viral type . since cloned mp virus retained the small plaque characteristic after more than 60 passages at 30 ° c ., the mp marker appears to represent a stable biological property inherent to the genome of the variant virus . the characteristics of chv ( strain f - 205 ) and methods for its cultivation and assay have been previously described ; carmichael et al , am . j . vet . res ., 26 : 802 - 814 ( 1965 ); carmichael et al , proc . soc . biol . med ., 120 : 644 - 650 ( 1966 ). additional isolates ( see table 1 , supra ) were recovered from tissue specimens submitted to the james a . baker institute for animal health , new york state college of veterinary medicine , cornell university , ithaca , new york , for viral culture or , in some instances , were obtained from other investigators . standard plaque medium for cloning was double - strength eagle minimal essential medium ( mem ; bbl ), supplemented with 10 % fetal bovine serum ( fbs ) and 0 . 5 % lactalbumin hydrolysate ( lah ) mixed with an equal volume of 2 % agarose ( sea plaque ). for plaque size determinations and viral assays , monolayer dkc cultures in 60 - mm plastic plates were inoculated with 0 . 2 ml amounts of serial 10 - fold dilutions of virus and absorbed for 1 . 5 h . at room temperature . after the absorption period , 5 - ml amounts of mem , supplemented with 5 % fbs and 0 . 5 % lah that contained 1 % methylcellulose ( hotchin , nature ( london ), 175 : 352 ( 1955 )) were added to each plate , and cultures then were incubated in a humidified 5 % co 2 atmosphere at 30 ° or 35 ° c . after 5 days of incubation , the viscous medium was decanted , and plates were fixed for 10 min . with 10 % neutral formalin , rinsed in tap water , and then stained with 1 % aqueous crystal violet solution . plaque size was measured with a calibrated ocular micrometer . at least 100 plaques were measured for each strain studied . cultures of canine spleen cells were prepared by rapidly stirring , at room temperature , minced tissue in mem without added trypsin . after 2 h ., the suspensions were filtered through six layers of sterile gauze and diluted in growth medium ( mem supplemented with 10 % fbs and lah ) to contain approximately 10 6 cells / ml . cultures were prepared in 25 - cm 2 plastic flasks , using 5 - ml amounts per flask . after 24 h ., cultures were shaken , and the adherent cells , which were more than 90 % mononuclear , were used for viral growth studies . in an attempt to select strains of reduced virulence , chv ( strain f - 205 ) was rapidly transferred ( 2 - day intervals ), for a total of 312 passages , using terminal dilutions at approximately each 10th passage . each 10th dkc passage was harvested and stored frozen at - 70 ° c . in a stabilizing menstruum for later tests of pathogenicity in newborn pups and for the study of certain biological properties , as noted later . after 312 passages at 35 ° c ., neither a reduction in virulence for newborn pups nor the emergence of plaque variants was observed . the 312th dkc passage virus , which caused fatal infections and was uniform with regard to plaque characterstics under a 1 % methylcellulose overlay , was then rapidly transferred at 30 ° c . after approximately 20 passages at 30 ° c ., subtle changes in the cytopathic effects ( cpe ) were observed in tube cultures , and the principal plaque type measured approximately one - half the diameter ( 0 . 75 mm ) of the parental strain . after plaque purification , using an agarose overlay medium , a typical mp clone was selected for further passage and study . it was designated chv - mp to distinguish it from the macroplaque ( mp ) parental virus . the mp variant has retained its unique plaque characteristics after 66 passages in dkc cultures incubated at 30 ° c . freshly harvested 24 - h . cultures of the mp and mp strains were rapidly frozen and thawed three times . cell debris was removed by centrifugation at 600 × g for 10 min . at 4 ° c ., and clarified virus in mem containing 10 % fbs was placed in a water bath at 38 ° c . aliquots were removed at intervals for infectivity titrations . virus survival was plotted versus time . antigens for immunodiffusion tests were prepared from mp and mp virus grown in 75 - cm 2 plastic flasks . when cultures had evidence of extensive cpe , the adherent cells were scraped off the flask with a rubber policeman . after centrifugation at 600 × g , the fluid portions were discarded and the cell portions were taken up in one - tenth the original volume in distilled water , frozen and thawed three times , and then clarified by low - speed centrifugation . the supernatant portions then were placed in cellophane dialysis tubing and dialyzed for 24 h . at ph 10 . 3 ( glycine - naoh buffer ) to dissociate viral subunits . after overnight dialysis against 0 . 15 m phosphate - buffered saline , ph 7 . 2 , the antigen preparation was stored frozen at - 70 ° c . tests were performed on 2 . 5 - by 7 . 5 - cm plastic immunodiffusion slides , using 0 . 6 % agarose in phosphate - buffered saline . an eight - well pattern was used to compare all permutations of mp or mp antigens with the respective hyperimmune antisera that had been prepared in specific - pathogen - free beagles . a total of 21 specific - pathogen - free beagle pups ( four litters ) from the institute &# 39 ; s disease - free colony were used . all animals were obtained from bitches without detectable chv - neutralizing antibody . serum neutralization methods have been described previously ; carmichael , j . am . vet . med . assoc ., 156 : 1714 - 1721 ( 1970 . thirteen neonatal pups ( two litters ) were given intraperitoneal or oral - nasal inoculations within the first 48 h . of birth , since pups rapidly develop resistance to generalized , usually fatal , infections after that time ; carmichael et al , j . infect . dis ., 120 : 669 - 678 ( 1970 ). to examine effects of immunosuppression on the pathogenicity of the mp variant , eight 2 - week - old pups were divided into four groups , each consisting of two pups . in one group , each pup was inoculated with 0 . 5 ml of goat anti - dog thymoctye serum ( ats ) at the time of infection with mp virus and again on post - inoculation days 2 and 4 . another group of two pups was given mp virus but no ats . the thid group was inoculated with chv - mp , and the fourth received chv - mp plus ats . viral doses were 10 5 . 2 50 % tissue culture infective doses ( tcld 50 ) ( chv - mp ) or 10 4 . 8 tcid 50 ( chv - mp ). the ats , prepared in our laboratory , had a canine lymphocyte cytotoxicity titer of 1 . 320 . after two injections ( 0 . 5 ml / kg ), there was a marked (& gt ; 80 %) diminution in the normal responses of canine peripheral blood lymphocytes to phytohemagglutinin , severe thymic atrophy , and profound alteration in the course of chv infection in normally resistant 2 - week - old pups . while primary dog kidney cell ( dicc ) cultures are the cultures of choice , the growth supporting medium employed as the tissue culture for virus passage is not unduly critical . any tissue culture medium can be employed which supports canine herpesvirus growth . a number of such media are known in the art . based upon the above and similar experimental work the following observations are made : growth characteristics of the mp and mp strains in dkc cultures incubated at 30 ° and 35 ° c . growth of the mp strain in dkc cultures at 35 ° c . was not restricted ( fig2 ). when viral inputs were approximately equal , the titers of inputs were approximately equal , the titers of chv - mp , after 24 h . of growth , were at least 0 . 8 log 10 greater than those of the mp strain . cpe of the two strains were similar , but not identical . cells infected with the mp variant generally were more swollen and refractile than those infected with mp virus , and they tended to clump around the edges of a plaque . cells infected with the mp strain were uniformly rounded , and they detached readily from the growth surface . syncytia were not observed with either virus . a consistent feature of the mp variant was the late appearance of cpe in relation to the production of infectious virus . both strains grew more slowly at 30 ° c . ; however , growth of the mp virus was somewhat more restricted at this temperature . an additional difference between the mp virus and the mp variant was the amount of infectious virus released ( table 2 ). the mp virus was significantly more cell associated throughout the growth period than was the mp variant . table 2 . ______________________________________cell - associated virus released mp and mp canineherpesvirus grown at 35 ° c . virus titer . sup . a virus releaseincubation chv - mp . sup . b chv - mp (%) time ( h ) cells fluid cells fluid mp mp______________________________________12 4 . 8 1 . 8 3 . 2 1 . 5 0 . 1 1 . 918 5 . 5 2 . 5 4 . 8 3 . 0 0 . 1 1 . 624 5 . 5 2 . 8 5 . 2 3 . 5 0 . 2 1 . 9______________________________________ . sup . a log . sub . 10 tcid . sub . 50 / 0 . 2ml . . sup . b viral inputs were 10 . sup . 6 . 3 ( mp ) and 10 . sup . 5 . 8 ( mp ) per flask culture . results ( fig3 ) suggested that the mp strain is somewhat more heat labile than the mp virus ; however , the differences were slight . antisera raised in dogs against the mp field strain ( f - 205 ) neutralized the homologous virus and the mp variant to the same extent in plaque reduction and kinetic neutralization tests . immunodiffusion analysis also failed to reveal antigenic differences , for two precipitin lines of identify were observed between the mp and mp viral antigens and the respective antisera . growth and mp and mp strains in splenic macrophage cultures at 30 ° and 37 ° c . splenic macrophage cultures maintained at 30 ° c . continued to release small amounts of infectious virus (˜ 10 plaque - forming units / 0 . 2 ml ) throughout the 70 - h . incubation period ; however , there was scant growth of either virus ( fig4 ). growth of the mp virus was restricted to a greater extent than that of the mp strain in macrophage cultures incubated at 37 ° c . at 37 ° c . the mp virus persisted intracellularly without decrease in titer for 24 h ., but infectious virus then declined . extracellular virus was not detected . by 60 h . postinfection , the mp virus no longer could be detected ; however , cell - associated mp virus still was present ( 3 × 10 3 plaque - forming units / 0 . 2 ml ) after 70 h . of incubation . at this time , cell cultures were & lt ; 90 % viable as judged by trypan blue exclusion tests . the response of pups to inoculations with mp and mp virus are summarized in table 3 . the mp strain produced generalized and fatal infections in all pups , regardless of the route of inoculation . high viral titers were found in all tissues examined . in contrast , the mp variant was markedly reduced in virulence , for none of the pups had signs of illness . nevertheless , mp virus was recovered from the nasopharynx of all pups for 4 to 6 days after inoculation . although small amounts of virus were recovered from kidney , spleen , and lung tissues of one pup ( i - 660 ) euthanized 6 days after intraperitoneal inoculation and from the liver of an additional pup ( i - 661 ), there were no macroscopic lesions . virus was not recovered from the other three pups that were infected with mp virus and then euthanized 6 or 8 days later . pups that were allowed to survive did not have signs of illness , and they all developed chv - neutralizing antibody by post - inoculation week 3 . table 3 . __________________________________________________________________________tests for virulence of mp and mp variant chv in newborn pups viral isolation ( log . sub . 10 tcid . sub . 50 / 0 . 2 g of tissue ) macro - naso - plaque viral dose inoculation survival scopic le - pharynx cerebel - pup no . type ( tcid . sub . 50 ) route . sup . a or death . sup . b sions . sup . c ( days ) kidney spleen liver lum lung__________________________________________________________________________i - 658mp 10 . sup . 4 . 8 i . p . d ( 5 ) severe +( 2 - 5 ) 4 . 5 3 . 5 4 . 5 2 . 5 3 . 5i - 659mp 10 . sup . 4 . 8 o / n d ( 6 ) severe +( 2 - 6 ) 4 . 5 3 . 0 4 . 0 2 . 5 3 . 5i - 660mp 10 . sup . 5 . 2 i . p . e ( 6 ) none +( 3 - 5 ) 1 . 0 2 . 5 & lt ; 1 & lt ; 1 2 . 5i - 661mp 10 . sup . 5 . 2 o / n e ( 6 ) none +( 2 - 6 ) & lt ; 1 & lt ; 1 1 . 0 & lt ; 1 & lt ; 1i - 662mp 10 . sup . 5 . 2 i . p . e ( 6 ) none +( 3 - 5 ) & lt ; 1 & lt ; 1 & lt ; 1 & lt ; 1 & lt ; 1ii - 644mp 10 . sup . 4 . 5 i . p . d ( 6 ) severe +( 2 - 6 ) 5 . 0 3 . 5 3 . 5 2 . 5 4 . 5ii - 645mp 10 . sup . 4 . 5 o / n d ( 8 ) severe +( 2 - 6 ) 5 . 5 4 . 5 4 . 0 3 . 0 5 . 0ii - 646mp 10 . sup . 5 . 0 i . p . e ( 6 ) none +( 2 - 6 ) & lt ; 1 & lt ; 1 & lt ; 1 & lt ; 1 & lt ; 1ii - 647mp 10 . sup . 5 . 0 i . p . s . sup . d --. sup . e +( 2 - 4 ) -- -- -- -- -- ii - 648mp 10 . sup . 5 . 0 i . p . s -- +( 2 - 5 ) -- -- -- -- -- ii - 649mp 10 . sup . 5 . 0 o / n e ( 8 ) none +( 2 - 4 ) & lt ; 1 & lt ; 1 & lt ; 1 & lt ; 1 & lt ; 1ii - 650mp 10 . sup . 5 . 0 o / n s -- +( 2 - 5 ) -- -- -- -- -- ii - 651mp 10 . sup . 5 . 0 o / n s -- +( 2 - 4 ) -- -- -- -- -- __________________________________________________________________________ chv growth was greatly restricted in the 2 - week - old pups given the mp or mp virus ( groups 1 and 2 ; table 4 ); however , the pups that received ats at the time of inoculation with mp virus ( group 4 ) dies by the 6th post - inoculation day . pups that were inoculated with mp virus and then treated with ats did not have signs of illness ; however , at necropsy , their thymus glands were approximately one - third the weight of the thymuses from the non - treated animals . none of the pups that received mp virus had prominent macroscopic lesions , although occasional small areas of focal necrosis , but no hemorrhages , were observed microscopically in the lung and liver of one pup ( no . 4 ). the lesions were similar to those seen in 2 - week - old pups that were given mp virus but not ats . in contrast , the pups ( no . 7 and 8 ) given mp virus and ats had high viral titers in several organs . remarkably high titers were found in the brain . microscopic lesions in the inoculated 2 - week - old pups that were given the mp virus but no ats were disseminated small foci of interstitial pneumonitis , necrosis of occasional hepatocytes , and minute areas of renal hemorrhage and focal interstitial necrosis , with minimal inflammatory changes . contrasting with these modest lesions were the prominent changes observed in the mp - inoculated pups treated with ats ( group 4 ). they consisted of interstitial pneumonitis , with alveolar necrosis and hemorrhages , necrotic foci throughout the liver , and multiple foci of necrosis and hemorrhages in the renal cortices , with both tubular and glomerular destruction . central nervous system changes consisted of disseminated focal enephalitis , necrosis of neuronal and astroglial cells with mononuclear cell infiltrations , and segmental leptomeningitis . other lesions typical of chv infection of neonatal puppies also were observed in these animals ; carmichael , j . am . vet . med . assoc ., 156 : 1714 - 1721 ( 1970 ); carmichael et al ., am . j . vet . res ., 26 : 802 - 814 ( 1965 ); pryde , supra . table 4 . __________________________________________________________________________response of 2 - week - old pups to chv ( mp or mp ) and effects of ats macro - viral isolation ( log . sub . 10 tcid . sub . 50 / 0 . 2g ) plaque type in - scopic le - nasol - oculated ( pup sions at ne - thymus wt pharynx cerebel - group no .) illness cropsy . sup . a ( g ) ( days ) kidney spleen liver lung lum__________________________________________________________________________1 chv - mp ( 1 ) none none 1 . 8 +( 2 - 7 ) & lt ; 1 & lt ; 1 & lt ; 1 & lt ; 1 & lt ; 1 ( 2 ) none none 2 . 0 +( 2 - 5 ) & lt ; 1 & lt ; 1 & lt ; 1 1 . 2 & lt ; 12 chv - mp ( 3 ) none none 0 . 7 +( 2 - 8 ) & lt ; 1 1 . 0 & lt ; 1 1 . 0 & lt ; 1 + ats ( 4 ) none none 0 . 5 +( 2 - 8 ) & lt ; 1 0 . 8 1 . 0 1 . 5 & lt ; 13 chv - mp ( 5 ) none mild 1 . 9 +( 2 - 8 ) 1 . 0 & lt ; 1 1 . 0 2 . 5 & lt ; 1 ( 6 ) none mild 1 . 8 +( 2 - 8 ) 0 . 8 & lt ; 1 & lt ; 1 2 . 0 & lt ; 14 chv - mp ( 7 ) died . sup . b severe 0 . 5 +( 2 - 4 ) 3 . 5 & lt ; 1 2 . 5 4 . 5 3 . 5 + ats ( 8 ) died severe 0 . 6 +( 2 - 6 ) 4 . 0 3 . 5 2 . 0 5 . 5 2 . 5__________________________________________________________________________ . sup . a see text for lesion descriptions . sup . b pups no . 7 and 8 dies on postinfection days 4 and 6 , respectively . surviving pups were euthanized at day 8 . the above studies demonstrate that the mp virus had additional biological properties that distinguished it from the parental mp virus . notwithstanding the subtle , but nevertheless distinct , differences between the mp and mp strains in the character of cytopathology , the times of onset of cpe , the rates and amounts of virus produced at 30 ° and 35 ° c ., viral persistence in canine spleen monocyte cultures , and , of lesser significance , the rates of inactivation at 38 ° c ., there were distinct differences in their virulence for pups . the mp ( wild - type ) virus was highly virulent for newborn pups and for 2 - week - old animals that had received ats at the time of infection . the mp variant , however , was clearly an attenuated strain . although the mp variant persisted for several ( 4 to 6 ) days in the nasopharynx , it rarely was recovered from other tissue sites favored by the mp virus , and then only in small amounts . association between small plaque size and reduced virulence has been reported for plaque variants or mutants of other herpesviruses [ darlington r . w . et al . &# 34 ; replication - biological aspects &# 34 ;, in the herpesvirues ( kaplan ed .) academic press inc ., new york ( 1973 ), pp 93 - 132 ; rapp , f ., et al ., proc . soc . exp . biol . med ., 116 : 361 - 365 ( 1964 )], the most noteworthy being mdhv [ biggs et al ., &# 34 ; biological properties of a number of marek &# 39 ; s disease virus isolates &# 34 ;, in oncogenesis and herpesviruses ( biggs et al ., ed .) who international agency for research on cancer , lyon ( 1972 ) pp 88 - 94 ; cho , avian dis ., 20 : 324 - 331 ( 1976 ); darlington et al . supra ]. the mp - chv , however , did not behave as an attenuated host range , temperature - sensitive mutant , as described for the large - and small - plaque mutants of hvh - 2 ; darlington et al , supra ; korment et al , supra . it did not regularly engender the formation of syncytia , as described for the chv - br strain , poste , supra , or occur as a naturally attenuated plaque variant , as described for field isolates of mdhv , but it originated after less than 20 passages in dkc cultures incubated at 30 ° c . after prolonged culture ( 312 passages ) at 35 ° c . unfortunately , the precise passage at which the mp variant emerged could not be determined , but it was the dominant type after 20 passages at 30 ° c . unlike the antigenic change ( loss of the &# 34 ; a antigen &# 34 ;) that has been associated with the attenuation of mdhv after prolonged passage in chicken renal cell cultures , purchase et al ., infect . immun ., 3 : 295 - 303 ( 1971 ), antigenic markers specific for the attenuated chv - mp strain were not detected . the following three examples demonstrate the efficacy of the mp canine herpesvirus strain in imparting resistance to wild - type virulent canine herpesvirus . because signs of illness are absent in dogs older than one week of age , ( carmichael et al , j . am . vet . med . assoc . 156 : 1714 - 1721 ( 1970 ) vaccine trials were designed so as to demonstrate efficacy based upon relative duration of viral shed ( equivalent to viral growth in host ) following challenge inoculation with virulent ( mp ) virus . littermate spf beagle dogs each were inoculated intramuscularly ( 1 m ) or oral / nasally ( o / n ) with 10 5 . 2 - 5 . 8 tissue culture infective doses ( tcd 50 ) of mp chv . blood samples were taken prior to vaccination and before challenge - inoculation with virulent ( mp , macroplaque ) chv ( strain f - 205 ), and thereafter at intervals . nasal - pharyngeal swab samples were collected for a period of 2 weeks after both vaccination and challenge - inoculation for viral isolations . results ( serologic responses , signs of illness , virus shedding ) were recorded for vaccinated and unvaccinated ( control ) animals that received challenge inoculations at the same time as the vaccinates . the first of three groups of pups was vaccinated at four months of age and challenge - inoculated three months later ( d800 through 802 ). control animals were d803 - 804 . see table 5 . vaccine virus given im did not spread to unit - contact controls over a period of three months . neither vaccinated animals nor controls had signs of illness post - vaccination or post - challenge . this was considered to be a normal response since immunity to this virus must be based on the relative restriction of viral shedding of vaccinates and controls . table 5 . ______________________________________intramuscular vaccination at 4 months of age ( chv mp ) post - vac . pre - chall . post - chall . virus shed sn antibody virus sheddog ( days ) ( 3 mo . p . v .) ( days ) conclusion______________________________________vaccinatesd800 0 1 : 16 2 ( mp ) immuned801 0 1 : 4 0 immuned802 0 1 : 4 0 immunecontrols notd803 -- & lt ; 1 : 2 8 ( mp ) immune notd804 -- & lt ; 1 : 2 11 ( mp ) immune______________________________________ the second group was vaccinated oral / nasally at three days of age and challenge - inoculated one month later , with appropriate controls ( d612 - 617 vaccinated ; d618 - 619 controls ). see table 6 . the mp ( attenuated ) virus was shed from 1 - 7 days after o / n inoculation , in contrast to the mp virus that is commonly shed in copious amounts for approximately 14 days ( 8 - 17 days in more than 30 dogs studied ). low antibody titers were generated that did not completely exclude the mp challenge virus . however , there was evidence of an anamnestic response ( results of 8 - day serology ) in vaccinated dogs , with accelerated rejection of the challenge mp virus , as compared with controls . an immune response with reduction in viral shed is clearly evident . no signs of illness were observed in vaccinated or control dogs . although not an object of this trial , it may be concluded further ( confirming published reports from this laboratory , infection and immunity 20 : 108 - 114 , april 1978 ) that mp is avirulent for neonatal pups . table 6 . __________________________________________________________________________oral - nasal vaccination at 3 days of age ( spf beagles ) post - vacc . pre - chall . post - chall . antibodydog virus shed antibody ( sn ) virus shed ( days ) 8 day post - chall . conclusion__________________________________________________________________________vaccinatesd612 1 - 5 ( days ) mp * 1 : 8 1 - 7 ( mp ) 1 : 32 immuned614 1 - 7 mp 1 : 4 2 - 7 ( mp ) 1 : 16 immuned615 1 - 7 mp 1 : 4 2 - 8 ( mp ) 1 : 12 immuned616 1 - 4 mp 1 : 8 2 - 3 ( mp ) 1 : 16 immuned617 1 - 4 mp 1 : 8 2 - 5 ( mp ) 1 : 16 immunecontrols ( non - contact ) d618 -- & lt ; 1 : 2 1 - 12 1 : 4 not immuned619 -- & lt ; 1 : 2 1 - 16 1 : 8 not immune__________________________________________________________________________ * mp = attenuated virus mp = virulent virus the third group ( d54 - 57 ) was vaccinated intramuscularly at 2 months of age . animals were challenge - inoculated one month later . 2 - months later all dogs received corticosteroid ( dexamethasone , 1 mg / day for 5 days ), a drug shown to cause recrudescence of persistent chv . swab samples were collected during and following steroid treatment for a total period of 12 days . virus recovered following steroid treatment was analyzed for plaque type ( mp = virulent , mp = vaccine strain ). see table 7 . viral shed did not occur following initial vaccination ( im route ), and there was no spread of virus to in - unit contacts . following challenge viral shed was again reduced . relative amounts of virus recovered ( generally less than 10 tcd 50 ) was significantly less in vaccinates than controls ( 100 to 10 , 000 tcd 50 ). all vaccinates developed low sn antibody titers . within 8 days following challenge , titers did not change , indicating minimal immune response to challenge virus , i . e . limited viral growth . following corticosteroid drug treatment ( dexamethasone ), there was no recrudescence of the vaccine virus . mp virus ( virulent ) was recovered , however , from the control dogs that had received challenge inoculums 2 months previous to drug treatment , and which had been negative to viral isolation attempts ( 3 × weekly ) during the period following initial viral shedding and steroid treatment . table 7 . __________________________________________________________________________host - response to mp and persistence of virus as revealed byviral recrudescence following steroid treatment post - vacc . pre - chall . post - chall . viral re - excretion virus shed sn anti - virus shed following steroiddog ( days ) body titer ( days ) immune treatment ( 2 mo . post - chall . ) __________________________________________________________________________vaccinates54 0 1 : 16 1 - 5 yes neg . 55 0 1 : 8 1 - 4 yes neg . 56 0 1 : 8 2 - 7 yes neg . 57 0 1 : 6 2 - 4 yes neg . controls58 -- & lt ; 1 : 2 1 - 16 no pos . ( 3 - 9 day ; mp virus ) 59 -- & lt ; 1 : 2 1 - 14 no pos . ( 5 - 10 day ; mp virus ) __________________________________________________________________________ the microplaque ( mp ) variant of chv ( strain f - 205 ) is available from the james a . baker institute for animal health , new york state college of veterinary medicine , cornell university , ithaca , new york 14853 , upon request .
this invention relates to a vaccine against canine herpesvirus from a newly isolated small - plaque variant of canine herpesvirus , and to a method for producing small - plaque variant reduced pathogenicity canine herpesvirus .