Antimicrobial Sensitivity Profile in Psittacine Birds at an Avian Teaching Hospital: A Retrospective Study, 2015-2022.

Veterinary hospitals house patient populations with diverse infectious statuses, microbiota, and histories of prior antibiotic therapy. Choanal swabs are commonly used for assessing the upper respiratory tract of birds for bacterial disease, with the samples submitted for cytologic testing and/or culture and antimicrobial sensitivity testing. The aim of this retrospective study was to identify and quantify bacteria isolated from choanal swabs collected from psittacine patients at a veterinary teaching hospital in Mexico City, Mexico. Data regarding bacterial isolates from choanal swabs were obtained from the medical records of companion psittacines suspected of upper respiratory bacterial disease that presented between November 2015 and December 2022. A total of 47.8% (175 of 366) of the bacterial isolates were from specimens obtained from red-lored Amazons (Amazona autumnalis). Gram-negative bacteria predominated, with 27 different genera identified. Klebsiella, Staphylococcus, and Escherichia were the most frequently isolated genera. A total of 90.4% (331 of 366) of the isolates were resistant to at least 1 antibiotic tested in the sensitivity panel, and a single Klebsiella isolate was resistant to 13 different antibiotics. Gentamicin had a high percentage of efficacy (79.5%; 182 of 229) against the bacterial isolates, whereas isolates tested against sulfonamide-trimethoprim (46.7%, 98 of 210), streptomycin (43.8%; 88 of 201), and clindamycin (12.9%; 15 of 116) had susceptibilities <50%. This is the first study to report common bacterial isolates and their antimicrobial susceptibility patterns from choanal swab samples collected from companion psittacines suspected of upper respiratory disease in Mexico. Clinicians can use the information presented in this study as a guide for therapeutic decision-making when managing upper respiratory bacterial infections in companion psittacine patients.

Swine virome on rural backyard farms in Mexico: communities with different abundances of animal viruses and phages.

Domestic swine have been introduced by humans into a wide diversity of environments and have been bred in different production systems. This has resulted in an increased risk for the occurrence and spread of diseases. Although viromes of swine in intensive farms have been described, little is known about the virus communities in backyard production systems around the world. The aim of this study was to describe the viral diversity of 23 healthy domestic swine maintained in rural backyards in Morelos, Mexico, through collection and analysis of nasal and rectal samples. Next-generation sequencing was used to identify viruses that are present in swine. Through homology search and bioinformatic analysis of reads and their assemblies, we found that rural backyard swine have a high degree of viral diversity, different from those reported in intensive production systems or under experimental conditions. There was a higher frequency of bacteriophages and lower diversity of animal viruses than reported previously. In addition, sapoviruses, bocaparvoviruses, and mamastroviruses that had not been reported previously in our country were identified. These findings were correlated with the health status of animals, their social interactions, and the breeding/rearing environment (which differed from intensive systems), providing baseline information about viral communities in backyard swine.

Mutations in the VP2 gene of rotavirus associated with benzimidazole sensitivity.

Rotavirus species A (RVA) is the etiological agent of acute gastroenteritis in young individuals of various animal species, including humans. Vaccination has helped to reduce the impact of these viruses on humans and some species of domestic mammals, but they do not confer complete immunity, so antirotavirus agents are another important control option. In this study, millimolar concentrations of benzimidazole inhibited the replication of the Rhesus rotavirus (RRV) strain of RVA. Two mutants partially resistant to the inhibitory effect of benzimidazole were independently selected, and their genomes and those of their parental strains were fully sequenced. Most (7/11) mutations occurred in the gene that encodes the VP2 protein, and similarly most of the missense mutations (5/9), including the only one shared by the two mutants (G2,414 → R[G/A], D800 N), occurred in the VP2 gene. Our results identify the VP2 gene as the primary target affected by benzimidazole.

Characterization of commercial poultry farms in Mexico: Towards a better understanding of biosecurity practices and antibiotic usage patterns.

Mexico is one of the world's major poultry producing countries. Two significant challenges currently facing the poultry industry are the responsible and judicious use of antimicrobials, and the potential occurrence of infectious disease outbreaks. For example, repeated outbreaks of highly pathogenic avian influenza virus subtype H7N3 have occurred in poultry since its first detection in Mexico in 2012. Both of these challenges can be addressed through good husbandry practices and the application of on-farm biosecurity measures. The aims of this study were: (i) to assess the biosecurity measures practiced across different types of poultry farms in Mexico, and (ii) to collect information regarding antimicrobial usage. A cross-sectional study was carried out through on-farm interviews on 43 poultry farms. A multiple correspondence analysis was performed to characterize the farms based on their pattern of biosecurity practices and antimicrobial usage. Three clusters of farms were identified using an agglomerative hierarchical cluster analysis. In each cluster, a specific farm type was predominant. The biosecurity measures that significantly differentiated the visited farms, thus allowing their clusterization, were: the use of personal protective equipment (e.g. face masks, hair caps, and eye protection), the requirement for a hygiene protocol before and after entering the farm, the use of exclusive working clothes by staff and visitors, footbath presence at the barn entrance, and the mortality disposal strategy. The more stringent the biosecurity measures on farms within a cluster, the fewer the farms that used antimicrobials. Farms with more biosecurity breaches used antimicrobials considered critically important for public health. These findings could be helpful to understand how to guide strategies to reinforce compliance with biosecurity practices identified as critical according to the farm type. We conclude by providing certain recommendations to improve on-farm biosecurity measures.

Phylogenetic characterization of a reassortant H5N2 influenza A virus from a resident Mexican duck (Anas diazi).

Congregation of different migratory and resident bird species on aquatic ecosystems during winter migration increases contact rates and enhances influenza A virus (IAV) transmission. However, scarce research has been focused on the resident bird's contribution to the viral ecology at a local scale. The Mexican duck (Anas diazi) is an endemic endangered anatid from Mexico. This resident species shares aquatic habitats with migratory birds in the wetlands of Central Mexico. Therefore, here we describe the phylogenetic analysis of an IAV (A/Mexican duck/EstadodeMexico; Lerma/UIFMVZ377/2016(H5N2)) isolated in this species, during spatiotemporal concurrence with migratory anatids in the winter season. All eight gene sequences were obtained by nextgeneration sequencing. Maximum Likelihood trees were constructed using MEGA-X, with General Time Reversible + Invariant (GTR+I), Subtree Pruning and Regrafting (SPR) heuristic method, and 1000 bootstrap replicates. Similarities with six different IAV subtypes were observed through a BLAST search: H6N5, H7N7, H5N2, H4N6, H9N2, and H11N9, detected in wild ducks during 2015 in the Pacific, Central and Mississippi flyways stop sites across the United States of America and Canada. The molecular identification of this reassortant H5N2 IAV highlights the importance of resident species as a reservoir host and its potential participation in the maintenance and transmission of IAV in wetlands surrounded by rural areas.

Cross-sectional study on Chlamydiaceae prevalence and associated risk factors on commercial and backyard poultry farms in Mexico.

Chlamydiaceae infections in poultry are mainly due to Chlamydia psittaci and Chlamydia gallinacea. While C. psittaci has long been known to affect birds and to have zoonotic potential, C. gallinacea is a newly described species that has been found to be widespread in chickens. As no data were available regarding the presence of Chlamydiaceae in Mexican poultry, a cross-sectional survey to detect the presence of Chlamydiaceae on commercial and backyard farms was carried out in eight federal states of Mexico with a high poultry density. Individual cloacal swabs were collected on 14 large-scale commercial poultry farms with controlled environment houses, 23 large-scale commercial poultry farms with open-sided houses, and 16 backyard farms. Samples were tested using a specific Chlamydiaceae real-time PCR technique. Chlamydial species were subsequently identified by a species-specific real-time PCR method. Information on potential risk factors was collected through a questionnaire. Logistic regression was performed to identify risk factors associated with Chlamydiaceae-positive results at the farm level on commercial farms. For backyard farms, a mixed-effect logistic regression model was used to consider information collected either at the animal or at the farm level. Overall, 7.1 % (n = 1/14) of controlled environment commercial farms, 26.1 % (n = 6/23) of open-sided commercial farms, and 75.0 % (n = 12/16) of backyard farms were Chlamydiaceae-positive. Apparent prevalence increased inversely to the level of confinement (controlled environment vs open-sided poultry houses vs backyards). Chlamydia gallinacea was the only chlamydial species detected. On commercial farms, egg-laying hen flocks had 6.7 times higher odds of being Chlamydiaceae-infected than broilers flocks (OR = 6.7, 95 % CI: 1.1-44.3, p = 0.04). On backyard farms, two variables were significantly associated with Chlamydiaceae infection: the lack of antibiotic use (OR = 8.4, 95 % CI: 1.84-38.49, p = 0.006), and an impaired health status (OR=8.8, 95 % CI: 1.9-38.9, p = 0.004). Further studies should be carried out to investigate the impact of C. gallinacea infection on egg quality and production performance in egg-laying hen flocks.

Virulence of infectious pancreatic necrosis virus (IPNV) isolates from Mexico.

Infectious pancreatic necrosis virus (IPNV) causes economic losses in Mexican rainbow trout industry. In this study, virulence and genetic fingerprints of Mexican IPNV isolates was investigated for the first time. Two Mexican IPNV isolates were analyzed in rainbow trout fry and the Sp strain was included as high virulence. One of the Mexican IPNV isolate was obtained from diseased fish and the other from fish without clinical signs. The infection was performed using a standardized immersion. Clinical signs were observed at 4 days post infection in fry group infected with strain Sp, two days earlier than in trout infected with IPNV isolates Mexican. Severe lesions were found in 100% of the individuals of Sp group, but only in 25% of each isolated Mexican group. Results suggest that Mexican IPNV isolates are pathogenic, but less virulent than strain Sp. The amino acid motif residues of both Mexican isolates, corresponded to a subclinical disease. Nevertheless, the accumulated motility observed in the field, suggest that other factors play a role in the virulence of the disease.

Fecal virome composition of migratory wild duck species.

The fecal virome comprises a complex diversity of eukaryotic viruses, phages and viruses that infect the host. However, little is known about the intestinal community of viruses that is present in wild waterfowl, and the structure of this community in wild ducks has not yet been studied. The fecal virome compositions of six species of wild dabbling ducks and one species of wild diving duck were thus analyzed. Fecal samples were collected directly from the rectums of 60 ducks donated by hunters. DNA and RNA virus particles were purified and sequenced using the MiSeq Illumina platform. The reads obtained from the sequencing were analyzed and compared with sequences in the GenBank database. Viral-related sequences from the Herpesviridae, Alloherpesviridae, Adenoviridae, Retroviridae and Myoviridae viral families showed the highest overall abundances in the samples. The virome analysis identified viruses that had not been found in wild duck feces and revealed distinct virome profiles between different species and between samples of the same species. This study increases our understanding of viruses in wild ducks as possible viral reservoirs and provides a basis for further studying and monitoring the transmission of viruses from wild animals to humans and disease outbreaks in domestic animals.

Molecular characterization of an influenza A virus (H4N2) isolated from waterfowl habitats in the State of Mexico.

Wild waterfowl and their habitats are the main reservoirs of influenza A virus (IAV) mainly during the breeding season and prior to migration. This study describes the molecular characterization of an IAV isolated from 240 water samples of a small wetland during non-breeding season of migratory wild ducks in the State of Mexico, Mexico. The results showed that the virus belongs to the H4N2 subtype and each of its eight segments of the viral genome has similarity to IAV isolated from ducks in North America. This study suggests that IAV can be isolated from small wetland during non-breeding season of migrating waterfowl.

Molecular characterization of the VP2 gene of infectious pancreatic necrosis virus (IPNV) isolates from Mexico.

Infectious pancreatic necrosis virus (IPNV) is one of the most important viruses in the Pacific salmon Oncorhynchus spp., Atlantic Salmon Salmo salar, and Rainbow Trout O. mykiss industry. This virus has been shown to produce high mortality among salmonid fry and juveniles, and survivors might become carriers. Since 2000, IPNV has affected Mexican Rainbow Trout culture, resulting in considerable economic losses. In the current study, molecular characterization of the VP2 gene of a number of Mexican IPNV isolates was done and the virus's phylogenetic relationships to IPNV reference strains were investigated. The phylogenetic analysis indicated that Mexican IPNV isolates are closely related to strains from the United States and Canada and that all Mexican IPNV isolates belong to genogroup 1. Furthermore, low genetic diversity was found between the Mexican isolates (identity, 95.8-99.8% nucleotides and 95.8-99.6% amino acids). The result of the analysis of the amino acid residues found at positions 217, 221, and 247 (alanine, threonine, and glutamic acid, respectively) could be associated with virulence, although the expression of virulence factors is more complex and may be influenced by the agent and host factors. The high percentage of identity among the VP2 genes from geographically distant IPNV isolates and the evidence of wide distribution in the country might have been facilitated by carrier trout. This hypothesis is supported by the identification of the amino acid threonine at position 221 in all Mexican isolates, a factor related to the carrier state for IPNV, as reported by other studies.

Prolonged excretion of a low-pathogenicity H5N2 avian influenza virus strain in the Pekin duck.

H5N2 strains of low-pathogenicity avian influenza virus (LPAIV) have been circulating for at least 17 years in some Mexican chicken farms. We measured the rate and duration of viral excretion from Pekin ducks that were experimentally inoculated with an H5N2 LPAIV that causes death in embryonated chicken eggs (A/chicken/Mexico/2007). Leghorn chickens were used as susceptible host controls. The degree of viral excretion was evaluated with real-time reverse transcriptase-polymerase chain reaction (RRT-PCR) using samples from oropharyngeal and cloacal swabs. We observed prolonged excretion from both species of birds lasting for at least 21 days. Prolonged excretion of LPAIV A/chicken/ Mexico/2007 is atypical.

Viral vaccines for bony fish: past, present and future.

Since 1970, aquaculture production has grown. In 2010, it had an annual average rate of 6.3% with 59.9 million tons of product and soon could exceed capture fisheries as a source of fishery products. However, the occurrence of viral diseases continues to be a significant limiting factor and its control is important for the development of this sector. In aquaculture farms, fish are reared under intensive culture conditions, and the use of viral vaccines has enabled an increase in production. Several types of vaccines and strategies of vaccination have been developed; however, this approach has not reached the expected goals in the most susceptible stage (fingerlings). Currently, there are inactivated and recombinant commercial vaccines, mainly for salmonids and cyprinids. In addition, updated genomic and proteomic technology has expedited the research and expansion of new vaccine models, such as those comprised of subunits or DNA. The objective of this review is to cover the various types of viral vaccines that have been developed and are available for bony fishes, as well as the advantages and challenges that DNA vaccines present for massive administration in a growing aquaculture, possible risks for the environment, the controversy regarding genetically modified organisms and possible acceptance by consumers.

Adenocarcinoma tiroideo en un faisán versicolor (Phaisanus versicolor): Informe de caso / Thyroid adenocarcinoma in a versicolor pheasant (Phaisanus versicolor): Case report

Different kinds of adenocarcinoma have been described in a variety of species; nevertheless, thyroid adenocarcinomas are relatively rare in birds. In this study, pathological findings of a thyroid adenocarcinoma in a pheasant versicolor (Phasianus versicolor) are described in order to be considered in the differential diagnosis of chronic diseases in wild birds of the Galliform order. The anatomicopathological findings belonged to a six year old male pheasant, which showed respiratory difficulty without secretion 24 h before its death. At necropsy, a non-encapsulated soft tissue multinodular mass was found at tracheal bifurcation, extending to the cranial area of lungs and left side costal bones. According to the histological features and its location, the diagnosis is consistent with thyroid follicular adenocarcinoma. The present study will be useful for the differential diagnosis of chronic diseases in the pheasant versicolor species.

Diversos tipos de adenocarcinomas se han descrito en diferentes especies, pero los adenocarcinomas tiroideos son raros en aves. Aquí se describen hallazgos patológicos de un adenocarcinoma tiroideo en faisán versicolor (Phaisanus versicolor), para considerarlo en el diagnóstico diferencial de enfermedades crónicas en aves silvestres del orden Galliforme. El hallazgo anatomopatológico se describe en un faisán macho, de seis años de edad, que presentó dificultad respiratoria sin secreción 24 horas antes de morir. A la necropsia se halló masa multinodular no encapsulada de consistencia suave, desde la bifurcación de la tráquea, que se extiende hacia la parte craneal de los pulmones, hasta la cara interna de los huesos costales del lado izquierdo. Las características histológicas y localización de la neoplasia establecieron el diagnóstico de adenocarcinoma tiroideo con patrón folicular. Este trabajo deja el precedente de una lesión que puede incluirse en el diagnóstico diferencial de enfermedades crónicas para la especie faisán versicolor.

Differential cytoplasmic and whole cellular expression of histone H1 within the avian bursa of Fabricius.

Bursal anti-steroidogenic peptide (BASP), purified from the chicken bursa of Fabricius (BF), has been previously demonstrated to be a potent and efficacious inhibitor of steroid hormone biosynthesis from chicken ovarian, and both mammalian and avian adrenal cells in vitro. Other studies have demonstrated that BASP can markedly reduce avian and mammalian mitogen-stimulated lymphocyte proliferation. Recent studies have indicated that BASP has a structural and functional relationship with histone H1. Immunohistochemical studies using a monoclonal antibody, which is known to recognize a common histone H1 epitope from several plant and animal species identified the protein within the cytoplasm and nucleus of distinct cells within both the cortex and medulla of all BF follicles. Additionally, epithelial cells within the BF expressed the protein strongly in the cytoplasm with reduced nuclear staining. In contrast, the same antibody did not recognize the protein in thymus of the same animals. The differential expression of histone H1 immunoreactivity within selected cells of the BF may support a previous proposed role of histone H1 in extranuclear and extracellular signaling in chickens and possibly other species.

Relationship of bursal anti-steroidogenic peptide (BASP) and histone H1.

Previous in vitro research from our laboratory has demonstrated the existence of a protein purified from the chicken bursa of Fabricius, with potent antisteroidogenic and antiproliferative action on granulose cells and lymphocytes, respectively called Bursal anti-steroidogenic peptide (BASP). This protein is heat-labile, basic, and amino- and carboxy-terminus blocked. In highly purified form, the protein presents as a doublet on SDS-PAGE electrophoresis with an apparent MW of approximately 29 and approximately 32 kDa. Recently, Nanoflow Q-TOF Mass Spectrometry amino acid sequencing allowed determination of a convincing partial amino acid sequence, strongly suggesting a probable relationship of BASP with histone H1. Bursal cDNA expression library screening, using an antibody produced against BASP, also identified a clone with a sequence matching histone H1. Presently, we have demonstrated that SDS-PAGE electrophoresis of highly purified and bioactive BASP, and commercially-available calf thymus derived histone H1, produced similar doublets at approximately the same apparent MW, and that the electrophoretic profile of these 2 preparations were strikingly similar following 2 dimensional gel electrophoresis. The BASP doublet produced on SDS-PAGE was recognized by a commercially available monoclonal antibody recognizing a highly conserved region of histone H1. Furthermore, calf thymus histone H1 was found to suppress mitogen-stimulated chicken B-cell proliferation in a concentration-related manner, similar to the action of BASP. These data indicate that BASP shares substantial structural homology with, and may be identical to, histone H1.

Modelo para la inducción de granulocitopenia en pollos de engorda mediante la administración de 5-fluorouracilo / A polymorphonuclear leukocyte depletion model in 5-fluorouracil treatment chickens

Se utilizaron 50 pollitos de engorda de un día de edad que fueron distribuidos aleatoriamente en 3 grupos con 15 aves cada uno, más 5 para muestreo bacteriológico. A los 17 días de edad, se inoculó en promedio 2 ml de solución salina fosfatada estéril, por vía endovenosa a las aves del grupo 1, mientras que en los grupos 2 y 3 se les administró 5-fluorouracilo (5-FU) en solución a dosis de 200 y 300 mg/kg de peso, respectivamente. A 10 aves de cada grupo se les tomó una muestra sanguínea durante los días 1, 3, 5, 6, 7, 8, 9, 10, 12 y 15 postratamiento (PT) para realizar un conteo total y diferencial de leucocitos; posteriormente se obtuvo la proporción de leucocitos polimorfonucleares/leucocitos totales (PMN/L T). Los resultados obtenidos en este estudio mostraron que la proporción de PMN/L T en los grupos tratados con 5-FU disminuyó a partir del día 1 hasta el día 9 PT, los valores más bajos se presentaron al día 9 PT y los valores normales se recuperaron hasta el día 15 PT. Se encontraron diferencias altamente significativas (P< 0.001) entre grupos, entre los días de muestreo, así como en la interacción de la dosis de 5-FU y el tiempo PT en relación con los valores de las proporciones de PMN/L T. Asimismo, se observaron diferencias (P< 0.05) entre las proporciones de PMN/L T de las aves tratadas con 5-FU en comparación con las aves del grupo testigo, así como entre los días de muestreo en los grupos tratados. Las aves tratadas con 300 mg de 5.FU/kg de peso presentaron signos de toxicidad

Penetración de Salmonella enteriditis a través de huevos fértiles e infértiles sometidos a diferentes estadios de incubación / Salmonella enteritidis penetration in fertile and infertile chicken eggs at progressive stages of incubation

Se evaluó la penetración de Salmonella enteritidis fagotipo 13, realizada mediante la inoculación experimental en la cutícula de huevos fértiles e infértiles provenientes de gallinas Babcock B-300 de 59 semanas de edad y libres de Salmonella sp. El estudio fue realizado a los 8, 12 y 16 días de incubación. Se estudió la penetración de la bacteria dentro de las tres áreas del cascarón después de 24 h de incubación a 37.7ºC. Hacia los 8 días de incubación, no se encontraron diferencias estadísticas significativas entre los huevos fértiles e infértiles en todas las áreas estudiadas (P> 0.05); sin embargo, así hubo diferencias en los huevos fértiles que fueron penetrados (P< 0.05) a los 12 y 16 días de incubación, lo cual indica que el desarrollo embrionario retarda considerablemente la susceptibilidad a la invasión por Salmonella. La penetración a través de la cutícula fue similar en ambos huevos, en contraste con la marcada diferencia observada en las estructuras más internas. Los resultados indican que los hevos incubables no fértiles pueden ser considerados más susceptibles a la penetración por Salmonella que en aquellas que están en desarrollo embrionario

Evaluación de la actividad fagocitaria y bactericida de los monocitos y heterófilos aviares contra Samonella gallinarum / Evaluation of phagocytic and bactericidal activities of avian monocytes and heterophils against Salmonella gallinarum in vitro

Se determinó, mediante ensayos in vitro, la capacidad fagocítica y bactericida de los deterófilos y de los monocitos aviares contra Salmonella gallinarum, en presencia y en ausencia de 10 por ciento de suero hiperinmune. En los ensayos de fagocitosis se observó que los hetrófilos fagocitaron al 28 ñ 4.7 por ciento de las bacterias sin opsonizar y al 45 ñ 9.9 por ciento de las bacterias opsonizadas, obteniéndose diferencias significativas (P< 0.05). En contraste, los monocitos sólo fagocitaron un 10.3 ñ 4.2 por ciento y un 11.7 ñ 3.8 por ciento de bacterias opsonizadas y sin opsonizar respectivamente (P> 0.05). En los ensayos bactericidas se observó que los heterófilos destruyeron al 90.46 ñ 3.3 por ciento de la bacteria sin opsonizar y 90 ñ 2.3 por ciento de la bacteria opsonizada (P> 0.05); sin embargo, en los monocitos se obtuvo un 10.5 ñ 6.6 por ciento y un 84.74 ñ 5 por ciento respectivamante, obteniéndose diferencias significativas (P< 0.05). Los resultados del presente estudio indican que la fagocitosis de los heterófilos fue significativamente incrementada por la opsonización; en el caso de los monocitos, no hubo un efecto significativamente mayor. Aproximadamente el 90 por ciento de las bacterias fagocitadas por los heterófilos fueron destruidos, como se determinó en el ensayo. La opsonización no incrementó significativamante el porcentaje de bacteria destruida por parte de los heterófilos, sin embargo, la psonización de Salmonella gallinarum sí favoreció la capacidad bactericida de los monocitos

Exclusión competitiva entre Salmonella enteriditis y Salmonella gallinarum en pollito de 1 día de edad infectados simultáneamente o consecutivamente / Competitive exclusion between Salmonella enteritidis and Salmonella gallinarium in one day-old broiler chicken challenged consecutive-or simultaneously

Se evaluó la inducción a la resistencia de desafíos consecutivos o simultáneos entre Salmonella enteritidis (SE) y Salmonella gallinarum (SG). En los experimentos con desafíos consecutivos, siempre se observó que la primera variedad de Salmonella administrada, predominó casi exclusivamante en la invasión del hígado, bazo y tonsilas cecales. Cuando se administraron simultáneamente ya sea al día uno o dos, ambas variedades fueron recuperadas del hígado, bazo y tonsilas cecales. Estos resultados indican que aun cuando las aves son susceptibles a ambas infecciones de Salmonella sp (24 horas), éstas pueden ser refractarias a un segundo desafío con otra variedad, y que si ocurriera una exposición simultánea, son posibles las infecciones mixtas. Los resultados obtenidos en el presente estudio confirman y extienden informes previos de la rápida inducción a la resistencia a desafíos consecutivos con diferentes variedades de Salmonella