Virulence of low pathogenicity H7N2 avian influenza viruses from the Delmarva peninsula for broiler and leghorn chickens and turkeys.

The virulence of low pathogenicity (LP) type A H7N2 avian influenza virus (AIV) isolates recovered from chickens in Delaware and the eastern shore of Maryland in 2004 was evaluated. Three-week-old leghorn- and broiler-type chickens and turkeys were inoculated via the conjunctival sac with 10(3.5)-10(4.0) 50% embryo infections dose (EID50) of virus per bird with A/ chicken/Delaware/Viva/04, A/chicken/Delaware/Hobo/04, and A/chicken/Maryland/Minh Ma/04. In broilers, the viruses produced respiratory signs, airsacculitis, and microscopic lesions in the trachea and lung. In contrast, signs and lesions were less severe in turkeys, and they were rarely observed in specific-pathogen-free (SPF) leghorns. In broilers and SPF leghorns, AIV peaked on day 3 postinoculation (PI), based on virus isolation and real-time reverse transcription-polymerase chain reaction, and antigen capture testing. Infection in turkeys peaked on day 7 PI. Serum antibodies generally were detected earlier in broilers (day 7 PI) than in turkeys or SPF leghorns (day 14 PI) using agar gel immunodiffusion, hemagglutination-inhibition, and the enzyme-linked immunosorbent assay. A second trial was performed to further examine the disease susceptibility of the leghorn chicken given the comparatively mild responses noted in the first trial. A 10-fold higher dose of 10(4.5)-10(5.0)EID50 per chick given via the conjunctival sac was used. In addition, commercial-type leghorns were tested as were chicks from the SPF leghorn source. The higher AIV dose resulted in more rapid and consistent rates of infection and higher serum antibody responses in both types of leghorn chickens. However, as observed in the first trial, clinical signs and microscopic lesions in both types of leghorns were infrequent and very mild. These findings indicate leghorn-type chickens, which are commonly used for pathogenicity assessments because of their availability, may not be the most suitable host for evaluating the virulence potential of LP AIV.

The effects of avian leukosis virus subgroup J on broiler chicken performance and response to vaccination.

The effects of avian leukosis virus subgroup J (ALV-J) infection on meat-type chickens reared in a simulated commercial setting were evaluated. Each of three ALV-J isolates was evaluated with both simulated horizontal transmission (SHT) and simulated vertical transmission (SVT). Mortality, morbidity, disease condemnations, and feed conversions were increased and body weights at processing were decreased in ALV-J infected birds as compared to sham inoculated hatch mates. The adverse effects of ALV-J infection were more severe in birds exposed by SVT than in birds exposed by SHT. At 8 weeks of age response to vaccination for infectious bronchitis virus and Newcastle disease virus or prior exposure to a pathogenic reovirus was assessed in the ALV-J and sham inoculated broiler chickens by challenge studies. Although not statistically significant, an overall trend of decreased protection to challenge after vaccination, or prior exposure, was observed in the ALV-J inoculates as compared to sham inoculated hatch mates. Differences in vaccine response were most evident in groups inoculated with ALV-J by the SVT route.

Comparison of a putative second serotype of chicken infectious anemia virus with a prototypical isolate II. Antigenic and physicochemical characteristics.

A putative new serotype of chicken infectious anemia virus (CIAV) isolated from 17-wk-old broiler breeder pullets was compared with a known, previously characterized CIAV isolate, the Del-Ros strain. Physicochemical characteristics evaluated induded thermal stability, size, pH, and chloroform sensitivity. Physicochemically, CIAV-7 was identical to CIAV. The virus isolates were compared antigenically by enzyme-linked immunosorbent assay, virus neutralization, immunofluorescence assay, and western blot. All four serologic assays demonstrated that CIAV-7 is antigenically distinct from the Del-Ros strain of CIAV. Additionally, polymerase chain reaction (PCR) and Southern blot were used to determine if there were similarities in genome sequence between the two viruses. CIAV-7 could not be detected with CIAV-specific PCR primers or a with CIAV-specific probe by Southern hybridization.

Comparison of a putative second serotype of chicken infectious anemia virus with a prototypical isolate I. Pathogenesis.

CIAV-7 is a virus with similar pathogenic and physicochemical characteristics to, but antigenically distinct from, chicken infectious anemia virus (CIAV). The pathogenesis of CIAV-7 was evaluated in a comparative study with a representative isolate of CIAV, the Del-Ros strain. The pathogenesis of CIAV-7 was similar to Del-Ros on the basis of the clinical disease induced and gross and microscopic lesions, although CIAV-7 produced fewer and less severe lesions overall. A second comparative pathogenesis study was performed with Del-Ros and CIAV-7, both alone and in combination with infectious bursal disease virus (IBDV). In this study, the pathogenesis of CIAV-7 was similar to Del-Ros in clinical, gross, and microscopic lesions in the bone marrow. However, thymic lesions were less severe in CIAV-7-inoculated birds. The interaction between Del-Ros and IBDV was synergistic, whereas there was no observed potentiation of CIAV-7-induced disease by IBDV. Progeny from breeder flocks from several geographic locations in the eastern United States were challenged with CIAV-7 or Del-Ros to assess protection by maternal antibodies. Some progeny from all flocks had protection against CIAV-7 challenge, providing evidence for the presence of CIAV-7 in the field. Additionally, the number of birds protected against CIAV-7 or Del-Ros challenge varied within flocks, demonstrating that the agents are serologically distinct.

Phenotypic variation among three broiler pure lines for Marek's disease, coccidiosis, and antibody response to sheep red blood cells.

To identify candidate genes, chicken lines with the most divergent phenotypes are usually crossed to generate resource mapping populations, for example, either backcrossed or F2 populations. Linkage between the genetic marker and the phenotypic trait locus is then tested in the mapping population. As an initial step in the development of a mapping population from commercial broilers, the goal of the current research was to evaluate the phenotypic variation among three pure lines for antibody response to SRBC and in resistance to two economically important poultry diseases, Marek's disease (MD) and coccidiosis (Eimeria acervulina). Chicks from each line were received and separated into three experimental studies to evaluate each of their responses. In summary, broiler Line 3 had significantly lower antibody responses to SRBC immunizations compared to the other two lines, and nonvaccinated birds from Line 3 were also more susceptible to MD. With coccidiosis, the response was complex, and ranking of the lines was dependent on the age of infection, and whether it was a first or second challenge. With the first challenge, Line 1 was most susceptible at the younger age (Day 30), whereas Line 3 was susceptible at the older age (Day 58). Upon the second challenge, broiler Line 1 remained susceptible at the younger age, but Line 2 was more susceptible at the older age. Line 3 was completely resistant to the second challenge at the older age. Thus, although the broiler lines have been intensively selected for productivity and general livability, this study also demonstrates that the lines differ for immune response and disease resistance. Based on the phenotypic differences between Lines 1 and 3, they were chosen to establish a mapping population for identifying candidate genes that affect MD and coccidiosis in commercial broiler chickens.

Genetic variation in susceptibility to Marek's disease in a commercial broiler population.

Two commercial broiler pure lines that were previously identified to differ in their susceptibility to Marek's disease (MD) were line-crossed to generate an F1 population. Eight F1 males were randomly mated to four or five F1 females to produce an F2 test population that would be segregating for genes affecting MD. All F2 progeny (four hatches) were pedigreed at hatch and placed in colony houses as nonvaccinated. At 5 days of age, they were challenged intraabdominally with MD virus RB1B. Clinical signs, mortality, and gross and microscopic lesions were recorded during the MD challenge. At 8 wk postchallenge, all remaining birds were euthanatized and necropsied. During the MD challenge of the first two hatches, we observed that several severely stunted broilers originated from certain families and the differences in body weight among birds appeared as early as 3 wk postchallenge. To confirm this observation, body weight at 6 wk postchallenge was determined for all surviving birds in hatches 3 and 4 (n = 242). Genetic variation in body weight among broiler sire families was apparent; the average body weight for males at this time was 2.07 kg, whereas with females, it was 1.87 kg. At least 12.2% of the broilers, including both sexes, weighed less than 1 kg ("severely stunted") at this time. The incidence of these growth-stunted birds within each broiler sire family ranged from 0 to 26% and for dam families, 0 to 60%. Correlation analyses between stunting and other MD-associated traits revealed that the incidence of stunting had a significant and positive association with paralysis (r = 0.50). Therefore, the data suggest that there may be a genetic component affecting body weight loss during MD infection. The genetic component is speculated to affect susceptibility to MD paralysis with an indirect effect on the body weight of birds. The significance of this finding is best exemplified by the identification of a broiler sire family with over 26% of its progeny affected by this MD-associated trait.

Comparison of two serotype 1 MDV isolates.

We compared the RB1B and T. King (TK) serotype 1 isolates of Marek's disease virus (MDV) in vivo. Body and organ weights, mortality, and lesions indicated that the TK inoculum established early infection more efficiently than RB1B and did greater damage to the bursa of Fabricius and thymus. Subsequent studies showed that the TK inoculum that we used contained chicken infectious anemia virus (CIAV). Therefore, pathogenicity profiles shown here should be interpreted with the presence of CIAV contamination in the TK stock in mind.

Chicken anemia virus.

Chicken anemia virus is commonly found in commercially produced chickens and has a worldwide distribution. It is difficult to inactivate thermally or with common disinfectants, which limits the utility of normal sanitization practices. The virus is important because of the disease it produces following transovarian transmission and because of its potential for inducing immunosuppression alone or in combination with other infectious agents. Control measures are directed at limiting vertical transmission and preventing coinfections with other lymphocidal agents.

Antigenic and S-1 genomic characterization of the Delaware variant serotype of infectious bronchitis virus.

A previously unrecognized infectious bronchitis virus (IBV) serotype, referred to hereafter as the Delaware variant (DE var), was isolated from commercial broiler chickens during a severe, widespread respiratory disease epornitic in the Delmarva peninsula region of the United States in January-March 1992. The DE var serotype was found to be antigenically unrelated by virus-neutralization (VN) test to nine reference IBV serotypes from North America. Additional VN tests indicated that the DE var isolates (DE/072/92, DE/121/ 92, DE/152/92, and DE/174/92) from broilers were fully or partially neutralized by monospecific antisera prepared against themselves and against two IBV field isolates (DE/492/90 and DE/903/90) recovered from a Delmarva commercial layer flock experiencing egg production losses in 1990. Antigenic relatedness values determined by VN indicated layer isolate DE/492/90 was more closely related to the broiler DE var isolates than was layer isolate DE/903/90. Cross-challenge tests performed in specific-pathogen-free chickens also demonstrated the antigenic similarity of the broiler (DE/072/92 and DE/174/92) and the layer isolates (DE/492/90 and DE/903/90), with heterologous strain protection values ranging from 55% to 100%. Protection values of DE var isolates vs. Massachusetts 41 and Arkansas DPI were considerably lower (0-60%). The S-1 gene of the US/DE/072/92 isolate of the DE var serotype was amplified by reverse transcription polymerase chain reaction, cloned, and sequenced. The DE var S-1 gene sequence was compared with the S-1 gene sequences of IBV serotypes from North America, Europe, and Australia. A dendrogram based on this analysis supported the conclusion that the DE var serotype is highly novel among IBV. A high degree of similarity (> 88%) was observed between the S-1 genes of the DE var broiler isolates (DE/072/92 and DE/174/92) and layer isolates (DE/492/90 and DE/903/90). These data, taken with the VN and cross-challenge results, establish a genetic as well as an antigenic link between the isolates from layers and broilers and indicate the DE var serotype was responsible for both infectious bronchitis outbreaks.

Restriction endonuclease analysis of Delmarva field isolates of infectious laryngotracheitis virus.

Restriction endonuclease (RE) digestion patterns of six Delmarva field isolates of infectious laryngotracheitis virus (ILTV) were compared with three standard reference strains. With one exception, all of the field isolates generated RE digestion patterns identical to an embryo-propagated vaccine strain of ILTV when the six-base-recognizing REs EcoRI, HindIII, PstI, and BamHI were used. In order to increase the sensitivity of the RE analysis technique, a more sensitive DNA fingerprinting approach using four-base-recognizing enzymes was developed. One field isolate could be differentiated from the embryo-propagated vaccine strain using all three enzymes, Sau3AI, MspI, and HinfI. A second isolate could be differentiated only by comparing HinfI digestion patterns. This work provides additional evidence that differentiable strains of ILTV exist in the United States. Furthermore, currently used RE analysis methods may not be sensitive enough to discriminate between field isolates and vaccine strains of ILTV, thus challenging the theory that vaccine strains of ILTV are responsible for field outbreaks of infectious laryngotracheitis.

Immune dysfunction following infection with chicken anemia agent and infectious bursal disease virus. I. Kinetic alterations of avian lymphocyte subpopulations.

The potential effect of chicken anemia agent (CAA) alone or in combination with infectious bursal disease virus (IBDV) on the immune system of young chickens was determined by measuring alterations in hematocrit values, lymphoid organ-to-body weight ratios and lymphoid cell concentrations at 4, 7, 10, 14, 17, 21, 28 and 42 days post-inoculation (PI). Lymphocyte subpopulations were identified and counted by flow cytometry using cell suspensions stained with monoclonal antibodies (Mabs) for panlymphocytes (K55), cytotoxic T-cells (CTLA3), T-helper cells (CT3), Ia-expressing cells (P2M11) and macrophages (P7). Chicken anemia agent induced a substantial but transient decrease in hematocrit value, thymus-to-body weight ratio and bursa-to-body weight ratio between 7 and 21 days PI corresponding to a generalized lymphocytopenia in the thymus, bursa and spleen. However, cytotoxic T-cell, T-helper cell and Ia-expressing cell concentrations increased in the bone marrow of birds inoculated with CAA alone or in combination with IBDV during the same time period. T-helper-to-cytotoxic T-cell ratios increased in the thymus and spleen during severe lymphocytopenia, indicating a selective decrease in cytotoxic T-cells. T-helper-to-cytotoxic T-cells ratios increased in the bone marrow, indicating a selective increase in T-helper cell concentrations. The increase in Ia-expressing cells in the bone marrow may be a reflection of increased number of activated T-cells which express Ia antigen. Infectious bursal disease virus alone induced a persistent depression of Ia-expressing cells in the bursa and the spleen and no measurable change in the bone marrow lymphocyte subpopulations. Chickens inoculated simultaneously with CAA and IBDV experienced clinical signs observed in chickens inoculated with each virus separately with a prolonged acute phase prior to recovery or mortality.

Immune dysfunction following infection with chicken anemia agent and infectious bursal disease virus. II. Alterations of in vitro lymphoproliferation and in vivo immune responses.

To determine the functional impact of alterations in lymphocyte concentrations and ratios following infection with chicken anemia agent (CAA) alone or in combination with infectious bursal disease virus (IBDV) on the immune system of young chickens, in vitro lymphoproliferation assays and in vivo responses to vaccination with several common viral agents were assessed at various time intervals post-inoculation (PI). Concanavalin A (Con A), phytohemagglutinin (PHA) and pokeweed mitogen (PWM) stimulation of splenic lymphocytes (SPL) collected from control birds could not be detected until 10-14 days PI. Infection with CAA was characterized by significantly higher PWM stimulation of SPL at 17 days PI and significantly lower PWM stimulation of peripheral blood lymphocytes (PBL) at 14 days PI, compared with uninfected controls. Concanavalin A and PWM stimulation of SPL was significantly increased in birds inoculated with IBDV alone. Lymphocytes harvested from birds inoculated simultaneously with CAA and IBDV had significantly lower responses. Effects on humoral and cell-mediated immunity following CAA and/or IBDV were determined by evaluating vaccination responses to Newcastle disease virus (NDV), fowl pox virus (FPV) and infectious laryngotracheitis virus (ILTV) during the acute phase of CAA infection (2 weeks PI). Vaccination of birds 2 weeks following CAA infection at 1 day of age resulted in decreased protection against NDV (85.7%) and ILTV (7.1%) challenge compared with protection rates in control birds (100% and 53.3% respectively). Infectious bursal disease virus infection was associated with decreased protection against NDV (60%) only. Concomitant infection at 1 day of age resulted in a greater reduction in NDV challenge protection (33.3%), slightly decreased FPV protection (87.5%), increased numbers of persistent FPV vaccination lesions and increased protection against ILTV challenge (71.4%). Vaccination of birds 2 weeks following CAA infection at 2 weeks of age resulted in slightly decreased NDV humoral antibody, development of persistent FPV vaccination lesions (17%) and increased immunity to ILTV challenge compared with control birds (83.3% vs. 66.7%). Chickens inoculated with IBDV alone displayed a more severe depression in NDV antibody titers and only a slight decrease in ILTV protection. Vaccination following concomitant infection at 2 weeks of age resulted in a higher percentage of FPV persistent vaccination lesions (39%) and greatly enhanced immunity to ILTV challenge (100%).

Protection of chickens from Newcastle and Marek's diseases with a recombinant herpesvirus of turkeys vaccine expressing the Newcastle disease virus fusion protein.

Recombinant strains of herpesvirus of turkeys (HVT) were constructed that contain either the fusion protein gene or the hemagglutinin-neuraminidase gene of Newcastle disease virus (NDV) inserted into a nonessential gene of HVT. Expression of the NDV antigens was regulated from a strong promoter element derived from the Rous sarcoma virus long terminal repeat. Recombinant HVT strains were stable and fully infectious in cell culture and in chickens. Chickens receiving a single intra-abdominal inoculation at 1 day of age with recombinant HVT expressing the NDV fusion protein had an immunological response and were protected (> 90%) against lethal intramuscular challenge at 28 days of age with the neurotropic velogenic NDV strain Texas GB. Recombinant HVT expressing the NDV hemagglutinin-neuraminidase provided partial protection (47%) against the same challenge. Chickens vaccinated with recombinant HVT vaccines had low levels of protection against NDV replication in the trachea when challenged ocularly. Recombinant HVT vaccines and the parent HVT strain provided similar levels of protection to chickens challenged with the very virulent RB1B strain of Marek's disease virus, indicating that insertion of foreign sequences into the HVT genome did not compromise the ability of HVT to protect against Marek's disease.

Marek's disease virus isolates with unusual tropism and virulence for ocular tissues: clinical findings, challenge studies and pathological features.

Outbreaks of Marek's disease (MD) were diagnosed in two flocks from the same company. Clinical signs, mainly blindness (>90%), but also depression, mild paralysis, and 11 to 12% mortality by 20 weeks of age were observed. MD virus, serotype 1 was isolated. The isolates were designated NC-1 (flock 1) and NC-2 (flock 2). Challenge experiments were conducted with these isolates and with two reference MD virus strains (JM/102W and Md5) in unvaccinated, turkey herpesvirus- (HVT) vaccinated and bivalent- (HVT and SB-1) vaccinated chickens. Blindness, gross ocular lesions and tumour formation were observed in a high proportion of all groups challenged with NC-1 and NC-2 when compared with chickens challenged with JM/102W and Md5. In chickens challenged with isolates NC-1 and NC-2, corneal changes included oedema, midstromal cellular infiltration consisting of macrophages, lymphocytes, plasma cells and lesser numbers of heterophils, collagen degeneration and keratic precipitates consisting primarily of macrophages covering the central endothelium. Eosinophilic intranuclear inclusion bodies were present in mononuclear cells infiltrating the cornea. Changes in the uveal tract consisted of inflammatory cell infiltrates similar to those present in the cornea. Retinal lesions included disruption of the retinal pigmented epithelium, inflammatory cell infiltration in the subretinal space, photoreceptor degeneration and in severely affected eyes, necrosis of retinal cellular elements. Pecten changes consisted of necrosis and mononuclear cell infiltration. Intranuclear inclusion bodies were abundantly present in cells of the retina's ganglion and inner nuclear cell layers. The unusual clinical manifestation of MD, the unusual tropism and virulence of NC-1 and NC-2 for ocular tissues and the incomplete protection afforded by conventional vaccination suggest that these isolates may be new pathotypes.

The isolation and characterization of chicken anemia agent (CAA) from broilers in the United States.

A chicken anemia agent (CAA) isolated from commercial broilers in the United States was characterized in vivo and in vitro. When inoculated into susceptible 1-day-old chickens, the agent induced a severe bone marrow aplasia, thymic atrophy, multiple subcutaneous and intramuscular hemorrhages, and anemia, as evidenced by reduced hematocrits. Chickens derived from different breeder flocks and inoculated in ovo or at 1 day of age varied in their susceptibility to the CAA, with some flocks being highly susceptible, while others were almost totally resistant. This was true for both specific-pathogen-free and commercial chickens. The isolate was able to pass through a 50-nm-pore-size filter and was resistant to inactivation at 56 C for 30 minutes. It failed to agglutinate avian and mammalian erythrocytes and could not be propagated in conventional cell cultures. The physical and biological characteristics of the agent and the disease it induces indicate that it is similar to the CAA found in Japan and Europe.

The effects of age, route of exposure, and coinfection with infectious bursal disease virus on the pathogenicity and transmissibility of chicken anemia agent (CAA).

Specific-pathogen-free (SPF) chickens were inoculated with several different concentrations of chicken anemia agent (CAA) by the intra-abdominal, intratracheal, or oral routes. Based on lowered hematocrit values, the birds were most susceptible to CAA introduced by the intra-abdominal route. When SPF chickens were infected with infectious bursal disease virus (IBDV) at 1 day of age, they remained susceptible to CAA up to at least 21 days, whereas birds inoculated with CAA alone were susceptible only at 1 day of age. Infectious bursal disease virus introduced at 1 day of age also increased the susceptibility of birds to contact infection with CAA and resulted in increased mortality rates in CAA inoculates. The response of SPF birds to CAA infection varied following exposure at 1 day of age to two different strains of IBDV (STC and Variant-E). Chicken anemia agent contacts and inoculates infected with the Variant-E strain were affected 1 week earlier by CAA than by STC inoculates, as evidenced by depressed hematocrits. However, the total number of birds affected was similar for both the Variant-E and STC-inoculated chickens. Commercial broiler chickens inoculated at 1, 7, 10, and 14 days of age by non-parenteral routes with CAA or a combination of CAA and IBDV had mean hematocrits that were lower than controls. Several CAA-inoculated birds were considered anemic, with hematocrit values of 25 or less, while uninoculated birds remained within normal ranges.

Protection afforded infectious bronchitis virus-vaccinated sentinel chickens raised in a commercial environment.

The protective efficacy of three infectious bronchitis virus (IBV) vaccines for sentinel chickens raised with commercial Delmarva broiler chickens was evaluated during winter 1987. Specific-pathogen-free leghorn sentinel chickens were vaccinated with Massachusetts (Mass) alone, Mass and JMK, or Mass and Arkansas (Ark) combination live vaccines, or they remained unvaccinated. Four weeks post-vaccination, sentinels were placed on broiler farms at weekly intervals for 3 weeks corresponding to weeks 4, 5, and 6 of the broiler growing cycle. Vaccine efficacy was evaluated based on IBV reisolation attempts from tracheal swabbings following a 1-week field exposure period. Sentinel chickens vaccinated with Mass and Ark combination vaccine were best protected against IBV field challenge. Only four IBV isolations were made out of a 3-week total of 36 attempts, for an 11% isolation rate. IBV vaccines containing either Mass alone or Mass and JMK offered much lower levels of protection.

In vitro and in vivo characterization of avian reoviruses. I. Pathogenicity and antigenic relatedness of several avian reovirus isolates.

Pathogenicity, pathogenesis, and antigenic relatedness of four avian reovirus isolates obtained from commercially reared broilers were investigated. Chickens of various ages were inoculated both orally and intratracheally with reovirus. Based on disease signs, mortality, weight depression, tissue lesions, invasiveness, and viral persistence in chickens inoculated at 1 day of age, the isolates were classified as being of low, intermediate, or high pathogenicity. The low-pathogenicity isolate (2177) did not cause mortality, weight depression, or clinical disease. The isolate of intermediate pathogenicity (2035) produced low mortality rates (8%), some weight reduction by 7 weeks postinoculation, and microscopic lesions in the intestine and gastrocnemius tendons. The pathogenic isolates, 2408 and 1733, caused severe clinical disease characterized by stunting, feathering abnormalities, mortality as high as 84%, and microscopic lesions in the liver, intestine, pancreas, and/or gastrocnemius tendon. Highly pathogenic isolates also persisted longer in tissues of infected birds and elicited a more prompt and prolonged antibody response. Birds inoculated at 1 day or 1 week of age were more susceptible to reovirus-induced disease than birds inoculated at 2 weeks, suggesting an age-associated resistance. All isolates produced mortality with equal frequency in embryos. The isolates characterized were found to be antigenically similar based on cross-neutralization and cross-protection studies.

In vitro and in vivo characterization of avian reoviruses. II. Clinical evaluation of chickens infected with two avian reovirus pathotypes.

The effect of two avian reovirus isolates (2408 and 1733) on digestion and nutrient metabolism in infected chickens was assessed by an in vitro absorption assay and clinical blood chemistry analysis. Birds of various ages were inoculated orally and intratracheally with reovirus and sampled periodically for the respective assays. Transitory malabsorption was observed in the duodenum of birds infected with reovirus 2408. Conversely, increased absorption was detected in the ileum of these same birds. Clinical blood chemistry analyses of birds infected with both isolates revealed that severely affected birds had abnormally elevated plasma total protein, plasma albumin, and calcium levels. Decreases were found in percent bone ash and, due to abnormally high globulin levels, in albumin:globulin (A:G) ratios. A significant (P less than 0.05) correlation between body weights and total protein, albumin, A:G ratio, and bone ash was found in infected birds. The most pronounced metabolic and physiologic changes occurred in the severely affected birds, and, in general, pathogenicity of the isolates was reflected by the degree of metabolic change.