Duration of immunity induced by an adjuvanted and inactivated equine influenza, tetanus and equine herpesvirus 1 and 4 combination vaccine

An adjuvanted vaccine containing inactivated equine influenza, herpesvirus antigens, and tetanus toxoid was administered to young seronegative foals of 8 months of age by deep intramuscular injection in the neck (Group A). The first two vaccinations were given 4 weeks apart. The third was administered 6 months later. Another group of foals (Group B) was vaccinated according to the same scheme at the same time with monovalent equine herpes virus (EHV) vaccine (EHV1.4) vaccine. Antibody responses to the equine influenza (single radial haemolysis; SRH) and tetanus (ToBi ELISA) components of the vaccines were examined from first vaccination until 1 year after the third vaccination. The influenza components of the combination vaccine induced high antibody titres at two weeks after the second vaccination whereafter titres declined until the time of the third vaccination. After the third vaccination, the titres rose rapidly again to remain high for at least 1 year. Antibody titres against tetanus peaked only after the third vaccination but remained high enough to offer protective immunity for at least 1 year. Foals vaccinated with monovalent EHV1.4 remained seronegative for influenza and tetanus throughout the study. Four and a half months after the third vaccination of groups A and B, a third group of animals was vaccinated twice with monovalent EHV1.4 vaccine 4 weeks apart (Group C). Two weeks after the administration of the second dose in the later group, all groups (A, B, C and an unvaccinated control group D) were challenged with EHV-4. Vaccinated foals (Group A, B, C) showed a clear reduction of clinical symptoms and virus excretion after EHV-4 challenge compared with the unvaccinated control foals. No difference could be demonstrated among the vaccinated groups, suggesting that the combination vaccine protects as well as the monovalent vaccine. In EHV1.4-vaccinated foals both antigenic fractions induced clear protection up to 6 months after vaccination (9). It can therefore be anticipated that the efficacy of the combination vaccine against EHV-1 challenge is similar to the efficacy against EHV-1 induced by EHV1.4 vaccination.     

Bovine genital campylobacteriosis (vibriosis): vaccination of experimentally infected bulls

The therapeutic efficacy of a Campylobacter fetus subsp venerealis bacterin was determined in experimentally infected bulls. Ten of twelve 5-year-old Angus bulls became infected after being infused intrapreputially with C fetus subsp venerealis. Of the 10 bulls, 6 were vaccinated with 5 ml of C fetus subsp venerealis vaccine on 2 occasions 4 weeks apart. Preputial washings of the vaccinated bulls were culturally negative by the 8th week after primary vaccination. None of the 18 heifers exposed to the vaccinated bulls became infected. The 4 infected, nonvaccinated bulls remained culturally positive to C fetus (P less than 0.002), and each bull infected at least 1 heifer (P less than 0.001). Two noninfected, nonvaccinated bulls remained culturally negative and did not infect any heifer. The 4 infected, nonvaccinated bulls were then vaccinated. Two bulls remained infected 9 weeks after primary vaccination, as determined by the virgin heifer test and cultural examination of preputial washings. Serologic data from 7 sampling periods were different (P less than 0.001) for vaccinated vs nonvaccinated bulls at 4 (against K antigen) or 6 (against O antigen) weeks after primary vaccination. Vaccination was effective in eliminating the infection in most of the infected bulls, but cannot be recommended as the sole measure of control in infected herds.     

Protection of foals against experimental Rhodococcus equi pneumonia by oral immunization.

Two groups of three one to three week old foals were immunized orally on four occasions over five weeks with two strains of Rhodococcus equi, a clinical isolate from a pneumonic foal and a laboratory passaged Congo red negative variant of this strain. Three nonimmunized foals of similar age acted as controls. Three weeks after the last immunization, all foals were challenged on five occasions over seven days by aerosol infection with about 10(10) of the pneumonic foal isolate on each occasion. Control foals became seriously ill and were euthanized. Immunization with either strain protected foals equally against the challenge, and resulted in rapid lung clearance. Oral immunization can thus protect foals against severe challenge with R. equi. The proteins associated with Congo red colony staining appear not to be involved in protective immunity.     

Studies on the immunogenicity of Streptococcus equi vaccines in foals.

The ability of either formalin-treated or heat-inactivated whole Streptococcus equi cell vaccines or partially purified M-protein of S. equi to give rise to protective antibody levels was studied in Standardbred foals by serological means. Two commercial preparations, i.e. a beta-propiolactone killed whole S. equi cell bacterin and a cell-free extract of S. equi cells were included in the study. The mean passive hemagglutination antibody titers (10 X log2) in sera of foals given either four doses of formalin-treated whole cell vaccine or an initial dose of formalin-treated followed by three doses of heat-inactivated vaccine with or without levamisole were significantly higher two weeks after the final dose. These passive hemagglutination antibody titers were higher in foals given formalin-treated whole cell vaccine (6.7 +/- 1.5) than given commercial bacterin (4.5 +/- 2.1). The passive hemagglutination antibody titers in all the groups decreased at 12 to 16 weeks after fourth dose of the vaccine. Foals given a commercial cell-free extract did not show a significant increase in passive hemagglutination antibody titers even up to four weeks after third dose. A group of six pony foals immunized with partially-purified M protein showed mean passive hemagglutination antibody titers lower than those observed in foals given whole cell vaccines. In a challenge experiment with S. equi, two of six foals vaccinated with partially-purified M-protein and all three controls developed clinical disease. The passive hemagglutination antibody of vaccinated foals increased after challenge, while at 28 days postchallenge the passive hemagglutination antibody titers of vaccinates and recovered controls were similar.     

Enzyme-linked immunosorbent assay for diagnosis of Corynebacterium (Rhodococcus) equi infection in foals

An enzyme-linked immunosorbent assay (ELISA) was used to diagnose Corynebacterium (Rhodococcus) equi infection in foals. In tests done with different antigen-extraction procedures (sodium dodecyl sulfate, sodium deoxycholate, polyoxy-ethylene [9] p-tert-octylphenol, polyoxy-ethylene [9-10] p-tert-octylphenol, sonification, homogenization, and heat treatment at 121 C), Tween 20 was a satisfactory reactive antigen. Using hyperimmune rabbit sera or infected foal sera, we investigated the specificity and the sensitivity of the ELISA with the Tween 20 antigen of the different serotypes or of the isolates. Corynebacterium equi strain ATCC 6939 antigen had the best activity for detecting antibodies to C equi in foals. Sera from 218 healthy horses, 11 healthy foals, 17 healthy newborn foals, a foal with suspected C equi infection, and 5 infected foals were evaluated for antibodies to C equi, using ELISA. The optical density values of 206 healthy horses, 17 healthy newborn foals, and 9 healthy foals were less than 0.1. Infected foal sera, except from foal 3, and serum from a foal with suspected C equi infection had higher optical density values. Using ELISA, specific antibodies against C equi were detected in a naturally infected 6-week-old foal after the foal had a rapid increase in the number of bacteria in the feces and after the initial development of clinical signs of illness at 5 weeks of age. Therefore, ELISA was useful for the early diagnosis of C equi infection in foals.     

Role of antibody to extracellular proteins of Rhodococcus equi in protection against R. equi pneumonia in foals

Rhodococcus equi produces two exoenzymes (REE), a cholesterol oxidase in large amounts and a phospholipase C, which cause lysis of sheep red blood cells (SRBC) sensitized with Staphylococcus aureus beta toxin. Two immunization studies were done in foals to determine the role of antibody to REE in protection against R. equi pneumonia. In the first study, three foals (mean age 10 days) were vaccinated four times at 2-week intervals with over 1 million units of partially purified exoenzymes (PREE). In the second study, three foals (mean age 19 days) were administered plasma from an adult horse vaccinated with PREE. Relatively low titres (16-32) of neutralizing antibody were detected in the foals of the former group, and passive transfer of neutralizing antibody (titres 32-64) occurred in the latter. Following immunization, principal foals and an equal number of similarly aged nonimmunized foals were challenged by aerosol with 1 x 10(10) live R. equi per day for 5 consecutive days. No severe clinical pneumonia developed in either group and, with one exception, only minor and resolving lung abscesses developed in these foals. These studies showed that antibody response of foals to immunization with PREE was poor, antibody to PREE did not prevent foals from developing lung abscesses following experimental infection, and that foals even as young as 3 weeks of age may be largely refractory to aerosol challenge with virulent R. equi.     

Immunization by intrabronchial administration to 1-week-old foals of an unmarked double gene disruption strain of Rhodococcus equi strain 103+

Rhodococcus equi causes fatal granulomatous pneumonia in foals and immunocompromised animals and humans. However, there is no effective vaccine against this infection. In this study, the chromosomal genes isocitrate lyase (icl) and cholesterol oxidase (choE) were chosen as targets for mutation and assessment of the double mutant as an intrabronchial vaccine in 1-week-old foals. Using a modification of a suicide plasmid previously developed in this laboratory, we developed a choE-icl unmarked deletion mutant of R. equi strain 103+. Five 1-week-old foals were infected intrabronchially with the mutant and challenged intrabronchially with the parent, virulent, strain 2 weeks later. Three of the foals were protected against pneumonia caused by the virulent strain, but the other two foals developed pneumonia caused by the mutant strain during the post-challenge period. Since infection of 3-week-old foals by an icl mutant in an earlier study had shown complete attenuation of the strain, we conclude that a proportion of foals in the 1st week or so of life are predisposed to developing R. equi pneumonia because of an inability to mount an effective immune response. This has been suspected previously but this is the first time that this has been demonstrated experimentally.     

Duration of strain 2308 infection and immunogenicity of Brucella abortus lipopolysaccharide in five strains of mice

A study was conducted to compare immunogenicity of a Brucella abortus lipopolysaccharide (LPS) and the duration of infection in 5 strains of mice. Mice of strains CBA/NJ, BALB/c, CD-1, C3H/HeN, and C3H/HeJ were allotted into 2 large groups (vaccinated with proteinase K-treated LPS or nonvaccinated) and 6 subgroups based on the intervals between challenge exposure to B abortus strain 2308 and the week the response data were obtained. Criteria used in comparing responses between the various strains of mice as well as between vaccinated and nonvaccinated mice were splenomegaly, colony-forming units (CFU) from spleens, and antibody titers. Responses were evaluated at 1, 2, 3, 5, 8, and 12 weeks after challenge exposure. Results indicated that all strains of mice became infected and maintained infection throughout the 12-week period, the percentages of mice infected were significantly (P less than 0.05) less in vaccinated mice for the first 5 weeks after challenge exposure, and there were no direct correlations between increased immunoglobulins (IgM and IgG titers) and reduction in CFU. Vaccinated mice of strains BALB/c, CD-1, C3H/HeN, and C3H/HeJ had increased titers when challenge exposed and also had significantly (P less than 0.05) smaller spleens and lower CFU. Vaccinated CBA/NJ mice did not have marked antibody titers. The overall results indicated that vaccination with LPS offers some initial protection against B abortus strain 2308 infection, but this protection disappears gradually and in various degrees in the 5 strains of mice studied.     

Studies on the efficacy of intranasal vaccination for the prevention of experimentally induced parainfluenza type 3 virus pneumonia in calves.

The efficacy of intranasal vaccination in preventing or limiting disease of the lower respiratory tract induced by parainfluenza 3 (PI3) virus was evaluated under experimental conditions, using a commercially available live vaccine containing a temperature-sensitive strain of PI3 virus. In a preliminary study four colostrum-deprived calves were vaccinated intranasally at one week and again at two months of age, and two similar calves were given an intranasal placebo. After the second vaccination serum antibodies to PI3 virus were detected in all four vaccinated calves, but not in the control animals. Seventeen days after the second vaccination all six calves were challenged with virulent PI3 virus, and they were killed six days later. The clinical scores and the extent of pulmonary consolidation were reduced in the vaccinated animals; PI3 virus was detected in the upper and lower respiratory tract of the control calves but in none of the vaccinated calves. In a larger scale study with 14 colostrum-fed calves, seven were vaccinated at one week and again at five weeks of age, and seven were given an intranasal placebo. Two weeks after the second vaccination all 14 calves were challenged with virulent PI3 virus. The clinical scores and lung consolidation were significantly reduced in the vaccinated calves in comparison with the controls. Six days after infection, 10 of the 14 calves were killed; PI3 virus was detectable in the nasal secretions of all seven control calves but in only one of the vaccinated animals, and PI3 viral antigen was detected in the lungs of the control calves but not in those of the vaccinated animals. One of the vaccinated calves had developed a severe clinical response after the challenge, but it had only minor lung consolidation when killed.     

Immunogenicity and Efficacy of a Commercial Feline Leukemia Virus Vaccine

Twenty young adult specific pathogen-free cats were randomly divided into two groups of 10 animals each. One group was vaccinated with two doses of feline leukemia virus vaccine according to the manufacturer's recommendations. All 20 cats were challenge exposed oronasally (4 times over a 1-week period), beginning 3 weeks after immunization, with a virulent subgroup A strain of FeLV (CT600-FeLV). The severity of the FeLV infection was enhanced by treating the cats with methylprednisolone acetate at the time of the last FeLV exposure. Ten of 10nonvaccinated cats became persistently viremic compared with 0/10 of the vaccinates. ELISA antibodies to whole FeLV were present at high concentrations after immunization in all of the vaccinated cats, and there was no observable anamnestic antibody response after challenge exposure. ELISA antibodies to whole FeLV appeared at low concentrations in the serum of nonvaccinated cats after infection but disappeared as the viremia became permanently established. Virus neutralizing antibodies were detected in 3/10 vaccinates and 0/10 nonvaccinates immediately before FeLV challenge exposure, and in 8/10 vaccinates and 1/10 nonvaccinates 5 weeks later. Although vaccination did not consistently evoke virus neutralizing antibodies, it appeared to immunologically prime cats for a virus-neutralizing antibody response after infection. Active FeLV infection was detected in bone marrow cells taken 14 weeks after infection from 10/10 nonvaccinates and 0/10 vaccinates. Latent FeLV infection was not detected in bone marrow cells from any of the vaccinated cats 14 weeks after challenge exposure.     

Use of a toxoid vaccine to protect goats against intradermal challenge exposure to Corynebacterium pseudotuberculosis

Two groups of male, 9-week-old goats (5 goats/group) were vaccinated subcutaneously with formalized exotoxin of Corynebacterium pseudotuberculosis, with Freund's incomplete adjuvant. Each goat was given 2 vaccinations, 2 weeks apart. At each vaccination, each group 1 goat was given 0.5 ml of toxoid, and each group 2 goat was given 1 ml of toxoid. Twenty days after the 2nd vaccination, vaccinated goats and 5 nonvaccinated 12-week-old goats (controls) were inoculated intradermally (challenge exposed) with live C pseudotuberculosis, monitored for 13 weeks, and euthanatized. At necropsy, 5 of the 10 vaccinated goats did not have C pseudotuberculosis lesions, 3 had abscesses limited to the inoculation site and draining lymph node, and 2 had disseminated bacterial lesions. Of the 5 nonvaccinated controls, 4 had disseminated abscesses and 1 had a single abscess in an internal node. Serologically, 9 of the 10 vaccinated goats developed positive (greater than or equal to 1:8) antibody titers against the exotoxin within 1 week after inoculation; the 10th goat seroconverted 2 weeks after inoculation, whereas control goats required 3 weeks to develop a positive antibody response. Therefore, early during an infection with C pseudotuberculosis, antibodies against the exotoxin may protect a goat against spread of the organism. All goats were injected intradermally before challenge exposure, 10 days after challenge exposure, and at 4, 8, and 12 weeks after challenge exposure with a skin-test reagent composed of fragmented bacterial cells.(ABSTRACT TRUNCATED AT 250 WORDS)     

Vaccination Against Porcine Parvovirus Infection

Of 13 gilts 7 were vaccinated twice at an interval of 3 weeks with an inactivated vaccine against porcine parvovirus (PPV) infection, while the 6 nonvaccinated gilts served as controls. Starting after the 1st vaccination the gilts were bred and, after about 40 days of gestation, challenged intravenously with virulent PPV. The vaccinated gilts produced an antibody respons after the 1st and 2nd vaccination compatible with a primary and a secondary immune response, respectively. The nonvaccinated gilts remained low-titered or PPV antibody negative until after challenge. The gilts were killed after about 90 days of gestation, and their litters were examined. All of 53 fetuses from the vaccinated gilts were alive, and infection with PPV could not be demonstrated. Conversely, 50 of 65 fetuses from the non-vaccinated gilts were infected with PPV, and 43 were dead.In a field study comprising 2 herds, PPV seronegative or lowtitered gilts were vaccinated before mating. There were no obvious signs of reproductive disorders in the 2 herds during the vaccination trials, and the reproductive performance of vaccinated gilts did not differ significantly from that of non-vaccinated gilts.     

Field evaluation of a commercial M-protein vaccine against Streptococcus equi infection in foals

A double-blind randomized clinical trial was undertaken to determine the value of parenterally administered Streptococcus equi M-protein vaccine in foals during an epizootic of strangles. Weaned mixed-breed foals (n = 664) housed on 2 adjacent feed-lots (A and B) arrived over a 5-day period, 2 weeks before primary vaccination. Foals in lot B (n = 114) were randomly administered vaccine (n = 59) or saline solution (placebo; n = 55) on 3 occasions at biweekly intervals. Foals in lot A (n = 450) were given 1 dose of vaccine (n = 225) or placebo. The following clinical observations were scored blindly by a single observer for all foals in lot B and for 120 (randomly sampled) foals in lot A on a single day, 2 (Lot B) and 6 (lot A) weeks after final vaccination: cervical lymphadenopathy, type of bilateral nasal discharge, and palpable swelling at injection site(s). Bacteriologic culture of nasal swab specimens or lymph node aspirates from selected foals with clinical disease yielded S equi. Cervical lymphadenopathy was observed in 17 of 59 (29%) vaccinates and 39 of 55 (71%) nonvaccinated controls in lot B and in 32 of 60 (53%) vaccinates and 29 of 60 (48%) controls in lot A. Contingency chi 2 analysis confirmed significantly lower cervical lymphadenopathy rate (chi 2 = 18.5; P less than 0.001) and prevalence of mucopurulent nasal discharge (chi 2 = 11.4; P less than 0.01) for vaccinates in lot B only. Swelling(s) at the vaccine injection site were palpated in 44% of lot B and 29% of lot A vaccinates vs less than 2% of placebo controls.(ABSTRACT TRUNCATED AT 250 WORDS)     

Detection of EHV-1 and EHV-4 DNA in unweaned Thoroughbred foals from vaccinated mares on a large stud farm

REASONS FOR PERFORMING STUDY: A silent cycle of equine herpesvirus 1 infection has been described following epidemiological studies in unvaccinated mares and foals. In 1997, an inactivated whole virus EHV-1 and EHV-4 vaccine was released commercially in Australia and used on many stud farms. However, it was not known what effect vaccination might have on the cycle of infection of EHV-1. OBJECTIVE: To investigate whether EHV-1 and EHV-4 could be detected in young foals from vaccinated mares. METHODS: Nasal and blood samples were tested by PCR and ELISA after collection from 237 unvaccinated, unweaned foals and vaccinated and nonvaccinated mares during the breeding season of 2000. RESULTS: EHV-1 and EHV-4 DNA was detected in nasal swab samples from foals as young as age 11 days. CONCLUSIONS: These results confirm that EHV-1 and EHV-4 circulate in vaccinated populations of mares and their unweaned, unvaccinated foals. POTENTIAL RELEVANCE: The evidence that the cycle of EHV-1 and EHV-4 infection is continuing and that very young foals are becoming infected should assist stud farms in their management of the threat posed by these viruses.     

Experimental intramammary inoculation with Mycoplasma bovis in vaccinated and unvaccinated cows: effect on the mycoplasmal infection and cellular inflammatory response

The effect of vaccination on mycoplasmal infection and the cellular inflammatory response was evaluated in 4 vaccinated and 4 control cows experimentally challenged in 2 of 4 quarters with live Mycoplasma bovis. In unchallenged quarters during the first three weeks after experimental challenge exposure, 6 of 8 quarters on control cows, and 7 of 8 quarters on vaccinated cows became infected with low numbers (10(2)-10(4) cfu/ml) of M bovis. During the same period all challenge-infused quarters on both control and vaccinated animals became infected with high numbers (10(9) cfu/ml) of M bovis. Thereafter, all quarters on vaccinated cows became culture-negative for M bovis, while 2 of 8 unchallenged quarters, and 4 of 8 challenged quarters on 3 of 4 control cows remained infected. A cellular inflammatory response as measured by the California Mastitis Test accompanied the experimental infection in proportion to the infection level except in challenged quarters on vaccinated cows after the first three weeks post challenge in which the cellular inflammatory response remained high despite the advent of negative M bovis culture results. This study indicates that the course of experimental M bovis mastitis can be affected by vaccination, and that vaccination results in an adverse cellular inflammatory response in challenged quarters.     

Transmission of foot-and-mouth disease by vaccinated cattle following natural challenge

Cattle vaccinated with a conventional monovalent type O1 foot-and-mouth disease (FMD) vaccine were challenged between four and 21 days after vaccination by short-term exposure to homologous airborne virus produced by pigs. Transmission was then assessed by housing susceptible cattle with the vaccinated animals and testing and observing all the animals for signs of infection and clinical disease. All 18 cattle vaccinated three weeks before challenge resisted clinical disease and although four contracted subclinical infection, there was no transmission to susceptible cattle in contact. One of the two groups of cattle vaccinated two weeks previously transmitted subclinical infection, but not disease, to susceptible animals housed with them from day 0 after challenge. Subclinical infection was manifested by a transient viraemia which was not followed by a detectable circulating antibody response. Shorter periods (seven or four days) from vaccination to challenge resulted in transmission of disease from clinically normal vaccinated to in-contact animals in one of two experiments. The severe challenge presented by the diseased in-contact animals than overwhelmed the immunity of the vaccinated animals. The results indicate that during emergency vaccination programmes it is advisable to vaccinate all FMD-susceptible animals within the vaccination zone and that at the outer boundary of the zone vaccinated animals should be kept separated from unvaccinated animals for at least three weeks.     

Immune response of sows and their offspring to pseudorabies virus: serum neutralization response to vaccination and field virus challenge.

One month prior to breeding, sows were vaccinated with an attenuated pseudorabies virus vaccine or challenged with a field strain of pseudorabies virus. A third group of sows were not vaccinated or challenged before breeding. Pigs from these sows were vaccinated at 3, 6, or 12 weeks of age and challenged with virulent virus three weeks later. One pig from each litter served as an unvaccinated, unchallenged control. Serum neutralization titers of these pigs were monitored from birth until 22 weeks of age. Titers of the sows were monitored through breeding, gestation and farrowing. The maximum prefarrowing anti-pseudorabies virus titer in the field virus challenged sows occurred four weeks following challenge. A significant decline in titers occurred at farrowing. Titers rose from one week postfarrowing and then declined. Titers in the field virus infected sows were consistently two to threefold greater than those of the vaccinated sows. The maximum prefarrowing anti-pseudorabies virus titer in the vaccinated sows occurred six weeks following vaccination. The geometric mean titer in these sow's then decreased and increased for two weeks after farrowing. The results in the pigs can be summarized as follows: Pigs from control sows had a greater serological response following field virus challenge than following vaccination with a modified live virus. Pigs from control sows responded serologically to vaccination at 3, 6 and 12 weeks of age. Pigs from control sows which were challenged at 6, 9 and 15 weeks of age had similar antibody responses. Pigs from vaccinated sows had no increase in titer following vaccination at three and six weeks of age. Titers increased when these pigs were vaccinated at 12 weeks of age. There was no significant increase in mean titers of pigs from challenged sows following vaccination at 3, 6 and 12 weeks of age. Vaccinated pigs from control and vaccinated sows had a secondary response following challenge three weeks after vaccination.(ABSTRACT TRUNCATED AT 250 WORDS)     

Comparison of duration of immunity in chickens infected with a live infectious bronchitis vaccine by three different routes.

Three groups of chicks were vaccinated by aerosol, intra-ocular and drinking water routes with a live infectious bronchitis (IB) vaccine. At one, two, six, 15 and 32 weeks after vaccination five birds from each group were sampled for testing for IB haemagglutination-inhibiting (HI) antibodies and challenged. Assessment of susceptibility to infection was measured by recovery of virus from individual tracheas and from kidney and gonad pools four days after challenge. Virus was isolated from all kidney and gonad pools of birds challenged one week after vaccination, the kidney and gonad pools of the drinking water vaccinates at two weeks, the kidney pool of the intra-ocular group at 15 weeks and all organ pools except the gonads of the intra-ocular group at 32 weeks. Tracheal resistance was found in most of the birds challenged one week after vaccination and in all the birds tested at two weeks but had begun to wane by six weeks after vaccination. No correlation was found between low HI antibody titres of individual birds and their susceptibility to challenge measured by reisolation of virus from the traches, but birds with titres over log2 6 were always resistant.     

Maternal antibodies against equine influenza virus in foals and their interference with vaccination.

Foals that were born to mares vaccinated twice a year against influenza had moderate to high haemagglutination-inhibition antibody titers at 24 hours after birth. The foals were vaccinated at six and ten weeks of age, and again at three to five months after birth. Four months after the third vaccination no antibodies against A/H7N7 and A/H3N8 influenza viruses were detected in these foals. Thus, maternal antibodies probably prevented the development of antibodies against equine influenza virus after vaccination. Foals born to the same mares one year later were monitored to determine the rate of decline of maternal antibodies against influenza viruses. Antibody titers of the foals shortly after birth were similar to those of the mares at foaling. The antibodies persisted for three to six months, and their biological half-life was estimated to be approximately 38 days. Two vaccinations of foals against influenza after the maternal antibodies had virtually disappeared resulted in an antibody response in most, but still not all, foals. These findings suggest that foals should not be vaccinated against influenza until maternal antibodies have disappeared.