Within-herd prevalence threshold for the detection of Mycobacterium avium ssp. paratuberculosis antibody-positive dairy herds using pooled milk samples: A field study.

Herd-level diagnosis of paratuberculosis using a pool-milk ELISA (pool size: n ≤ 50) is a novel, economical, and convenient method to identify blood serological Mycobacterium avium ssp. paratuberculosis (MAP) antibody-positive herds. To date, the diagnostic performance of the pool-milk ELISA has been described only under laboratory conditions where herd prevalence was simulated by the preparation of milk pools consisting of milk samples of cows with a known MAP status determined by fecal culture. In our observational study, test performance under field conditions was studied using pooled milk and individual blood samples. A total of 486 herds within the MAP prevalence reduction program of Lower Saxony, from which pooled milk and individual blood ELISA results were available, were assigned to this study. Data were analyzed for the period between January 1 and December 31, 2018, the first year after herd testing became obligatory in this federal state of Germany. To evaluate whether pooled milk samples reliably distinguish between herds with a MAP-apparent blood serological within-herd prevalence (MAP-Ab-WHPapp) ≥5% and herds with a MAP-Ab-WHPapp <5%, the distribution of the MAP-Ab-WHPapp was compared between pool-positive and pool-negative herds. The MAP-Ab-WHPapp was 3.4% (median; 95% confidence interval = 0-11.4%) in pool-positive herds and 1.2% (median; 95% confidence interval = 0-6.4%) in pool-negative herds. Only 10.8% (n = 12) of the pool sample-negative herds had a MAP-Ab-WHPapp ≥5% and were therefore false negatives, given the aims of the MAP prevalence reduction program. Hence, the pool-milk sampling strategy seems well suited to distinguish between herds with a MAP-Ab-WHPapp ≥ 5% and herds with a MAP-Ab-WHPapp <5% since only 10% of serum MAP-ELISA positive herds were missed. Employing a logistic regression model, we estimated that the minimum blood serological MAP-Ab-WHPapp to detect a pool-positive herd with a probability of 95% was 8%, which fits well with the aim of the MAP prevalence reduction program to focus on herds with a MAP-Ab-WHPapp of ≥5%. Despite the limitations of the control approach, which include milk pool sample collection and a low sensitivity of the ELISA used in milk pools and serum samples, the aims of the MAP prevalence reduction program can be achieved. The results of these field data support that pool-milk sample ELISA is a useful, economical, and low labor-intensive tool to identify herds seropositive for MAP in a MAP prevalence reduction program.

Reduction of viable Mycobacterium avium ssp. paratuberculosis in slurry subjected to anaerobic digestion in biogas plants.

Cattle infected with Mycobacterium avium ssp. paratuberculosis (MAP) shed the bacterium in their feces. This may lead to considerable concentrations of MAP in slurry, which has been postulated to contribute to MAP transmission when this slurry is used as fertilizer. For other bacterial species, anaerobic digestion has been shown to reduce bacterial load and to increase the safety of organic waste. Therefore, the objective of this study was to investigate the effects of anaerobic digestion in biogas plants on MAP survival in slurry from 16 dairy farms with a history of MAP infection. Presence of MAP was determined using MAP culture and a commercial MAP IS900 quantitative PCR (qPCR) applied on untreated slurry samples, slurry samples after primary fermentation, and digestate. Unfermented slurry samples from most enrolled farms tested positive for MAP, via both culture and qPCR. After the fermentation process, MAP could no longer be cultured in most samples, with the exception of 2 samples from farms where high numbers of MAP-shedding cows were kept at the time of sampling. A Bayesian binomial model predicted a probability of 93% for a MAP-negative culture result after fermentation. In most samples, MAP DNA was still detectable when using the IS900 qPCR. The probability of a negative result in qPCR was estimated to be 27%. Results of this study indicate that subjecting MAP-positive slurry to anaerobic digestion in biogas plants leads to a reduction of viable MAP below the detection limit; however, MAP DNA remained detectable. It remains undetermined whether MAP DNA detected in fermentation products is a residue of MAP degradation or belongs to viable MAP below the detection limit or in a dormant state. In conclusion, subjecting MAP-positive slurry to anaerobic mesophilic digestion reduces viable MAP concentration below the detection limit. The use of digestion products as fertilizer on pasture and agricultural soils instead of untreated slurry may therefore reduce the risk of MAP transmission.

Effect of dietary phosphorus deprivation on leukocyte function in transition cows.

Phosphorus depletion and hypophosphatemia have been described to hamper immune function in different species, an effect barely studied in dairy cows commonly developing hypophosphatemia in early lactation. Dietary P deprivation in mid lactating dairy cows was associated with a decline of the number of granulocytes and impaired granulocyte survival, whereas the phagocytic activity remained unaffected. The objective of the study reported here was to determine the effect of P deprivation on the leukocyte function of periparturient dairy cows. Eighteen multiparous and late pregnant dairy cows were randomly assigned to either a treatment group that was offered a markedly P-deficient diet or a control group receiving the same ration with adequate P content. The study consisted of a 2-wk acclimation period that was followed by a P deprivation period extending from 4 wk before to 4 wk after parturition and a P repletion period of 2 wk thereafter. Blood samples for leukocyte counts and leukocyte function analysis were obtained at the end of the acclimation period, after 2 wk of P deprivation, within the first week of lactation, at the end of the P depletion period and after 2 wk of dietary P supplementation. Blood samples for biochemical analysis were obtained weekly. Immune function was assessed by means of a phagocytosis assay and a lymphocyte stimulation test. Dietary P deprivation resulted in pronounced and sustained hypophosphatemia. Time effects were observed on the counts of different leukocyte fractions, the relative number of phagocytic granulocytes, the degree of phagocytosis, and the lymphocyte proliferation. Differences between P-deprived and control cows were only identified for the degree of phagocytosis that was lower in P-deprived cows compared with control cows. The correlation and regression analyses, however, revealed positive associations of the plasma phosphate concentration and the granulocyte count, the relative number of phagocytic granulocytes, and the degree of phagocytosis at the end of the dietary P deprivation when P depletion was most severe. The results of the study reported here indicate a mild negative effect of pronounced and sustained hypophosphatemia on the granulocyte count and the phagocytic activity of granulocytes in transition dairy cows. The clinical relevance of this effect for health and productivity of dairy cows remains to be determined.

Diurnal differences in milk composition and its influence on in vitro growth of Staphylococcus aureus and Escherichia coli in bovine quarter milk.

In experimental intramammary inoculation studies, it has been observed that mastitis susceptibility is influenced, among others, by cow factors. To identify milk characteristics leading to these differences, quarter milk samples of morning and evening milk were collected and analyzed for their composition (protein, fat, lactose, urea, lactoferrin, lactoperoxidase, and ß-lactoglobulin concentrations), somatic cell count, and antibodies against Staphylococcus aureus. Furthermore, in vitro growth of S. aureus and Escherichia coli in fresh quarter milk samples was determined. All measured parameters differed significantly between quarters and also between morning and evening milk with the exception of lactose levels. In addition, quantitative growth of S. aureus and E. coli was significantly different in morning milk compared with evening milk. Mixed model analysis revealed that replication of S. aureus was negatively associated with the presence of fat, S. aureus-specific IgG1 antibodies, contamination of the milk sample and morning milk. Replication of E. coli was negatively associated with fat concentrations, and positively associated with morning milk. The significant difference between morning and evening milk supports the theory that changes in milk composition influence bacterial growth. Although all determined milk components differed significantly between quarters and in time no significant association with bacterial growth could be identified with the exception of fat for both studied species and IgG1 titers for S. aureus. The negative association of fat with bacterial growth was assumed to occur due to activation of lipolysis by milk handling and can most likely be neglected for in vivo relevance. The fact that S. aureus-specific IgG1 titers were negatively associated with S. aureus growth in vitro encourages the ongoing effort to develop a vaccine against S. aureus-induced mastitis.

Reisolation of Staphylococcus aureus from bovine milk following experimental inoculation is influenced by fat percentage and specific immunoglobulin G1 titer in milk.

The associations of management parameters, herd characteristics, and individual cow factors with bovine mastitis have been subject of many studies. The present study aimed to evaluate the association between milk composition parameters, including fat, protein, lactose, urea, and specific immunoglobulin levels, at the time of experimental bacterial inoculation of the mammary gland and subsequent shedding dynamics of Staphylococcus aureus. Sixty-eight cows were experimentally infected with S. aureus and closely monitored for 3 wk. Mixed model analyses were used to determine the influence of management and herd characteristics (farm and experimental group), individual cow factors (days in milk, milk yield, and quarter position), and a challenge-related parameter (inoculation dose) in combination with either the milk components fat, protein, lactose and urea, or the S. aureus-specific antibody isotype titers at the time of bacterial inoculation, on the number of S. aureus reisolated from milk after inoculation. A positive association was observed between the milk fat percentage and the number of S. aureus reisolated from quarter milk, and a negative relationship between the S. aureus-specific IgG1 titer in milk and the number of S. aureus. These findings should be considered in the development of a vaccine against S. aureus-induced bovine mastitis.

A longitudinal study of factors influencing the result of a Mycobacterium avium ssp. paratuberculosis antibody ELISA in milk of dairy cows.

The influence of milk yield and milk composition on the diagnosis of Mycobacterium avium ssp. paratuberculosis (MAP) by milk ELISA in the context of the total IgG secretion patterns in milk throughout lactation and serum concentrations were investigated. A 2-yr trial was performed in which 1,410 dairy cows were sampled monthly and MAP milk ELISA status and milk yield and composition were determined. Data were analyzed by mixed model analysis. Milk yield was found to significantly influence ELISA results expressed as sample-to-positive (S/P) ratios. For each 5-kg increase in milk, the S/P ratio has to be multiplied by 0.89; therefore, high milk yield can change the MAP milk ELISA outcome of a cow in early infection from positive to negative. Parity influenced ELISA outcome significantly, indicating that cows with a parity >1 are more likely to be identified by milk testing. Also, herd was an important predictor, showing that herd prevalence influences the milk ELISA strongly. Other factors influencing the S/P ratios were protein concentration, somatic cell count, and days in milk. The IgG concentration and mass excreted per day were determined longitudinally in a subset of 41 cows of which samples and data of a complete lactation were available. Again, the IgG concentration in milk was mainly influenced by milk yield. The total IgG mass secreted per day in milk was found to be relatively constant, with a mean of 8.70 ± 5.38 g despite an increasing IgG concentration in serum at the same time. The variation of IgG concentration in milk can be mainly attributed to dilution through changes in milk yield. This supports the assumption that concentrations of MAP-specific antibodies are influenced by changes in milk yield similarly. In conclusion, we confirmed that antibody concentrations, and therefore MAP ELISA outcome, were influenced by milk yield, herd, and parity. To enhance performance, milk ELISA tests should either be performed in early or late lactation, when milk yield is low. From a management perspective, sampling should be done during early lactation before cows are bred again. Based on the slow progressive infection dynamics, only first-parity cows should be preferentially tested at the end of their first lactation to avoid false-negative results.

Effect of continuous milking on immunoglobulin concentrations in bovine colostrum.

UNLABELLED: Continuous milking is defined as a dairy cattle management system without a planned dry period for cows in late gestation. Continuous milking has been described to reduce health problems common in periparturient cattle, but may affect colostrum immunoglobulin (Ig) concentration and subsequently calf health. This study reports the influence of continuous milking on Ig concentrations of bovine colostrum in commercial dairy farms. Colostrum Ig concentrations of 227 cows from 13 herds were quantified with a quantitative ELISA for IgG, IgG1, IgG2, IgA and IgM. Colostrum samples of continuous milked (CM) cows (n=38) were compared with colostrum samples of cows (n=189) after a traditional dry period (DP) of at least 42 days. RESULTS: indicated that colostrum Ig concentration was significantly lower in continuous milking systems where IgG, IgG1, IgG2, IgA and IgM concentrations were reduced by half compared with cows that had a planned dry period. When relating the results from this study to recommendations for colostrum management it can be concluded that although colostrum Ig concentrations are significantly lower in a continuous milking management system an adequate passive immune transfer can still be achieved based on colostrum quality provided colostrum feeding management is optimal.

Influence of feeding a low-phosphorus diet on leucocyte function in dairy cows.

Phosphorus depletion and hypophosphatemia have been described to interfere with immune function in rats and humans. In dairy cows, hypophosphatemia has been associated with muscle weakness and recumbency as well as with intravascular hemolysis resulting from increased osmotic fragility of erythrocytes, but so far, the influence of P depletion and hypophosphatemia on immune function has not been studied. Therefore, the aim of this study was to investigate whether P depletion and ensuing hypophosphatemia are associated with impaired granulocyte and lymphocyte function. Eight mid-lactation dairy cows were fed a P-deficient ration (0.2% P/kg of DM) for a period of 4wk. The depletion phase was preceded by a 2-wk acclimatization period and followed by a 2-wk repletion phase, during which the same ration was supplemented with P to meet or exceed daily requirements. Blood samples were collected at the end of the acclimatization period, after 2 and 4wk of P depletion, and at the end of the repletion phase. Plasma phosphate concentrations ([Pi]) were determined and white blood cells were counted and isolated. General immune function was investigated by performing a phagocytosis assay with Staphylococcus aureus and a lymphocyte stimulation test (LST) with concanavalin A and pokeweed mitogen. The plasma [Pi] decreased significantly, with the lowest values (mean 0.7±0.2mmol/L) occurring after 2wk of depletion, although depletion was continued for another 2wk. During repletion, plasma [Pi] increased above baseline concentrations. Granulocyte counts changed in parallel with plasma [Pi] over time, decreasing significantly at 2wk after P depletion and increasing again thereafter. Granulocyte survival after phagocytosis was lowest after 4wk of P depletion. Phagocytosis activity of surviving granulocytes determined by mean fluorescence intensity was higher, indicating that phagocytosis was not negatively influenced by P depletion. Lymphocyte stimulation showed a similar trend, with a decreasing stimulation index at the end of P depletion, but differences were not statistically significant. Data presented in this study indicate that hypophosphatemia leads to a decrease in granulocyte counts. Chronic P depletion impairs granulocyte survival during phagocytosis but not phagocytosis activity. Lymphocyte function is not influenced by P depletion.

Milk and rice.

The following short article is an account of Israeli nurses caring for Syrian wounded. These wounded are shuffled across the Syrian border into Israeli hospitals. Until today and including today, we are considered 'enemy countries' with no diplomatic relations and fire arms pointing at each other. Six months ago when the Syrian wounded started trickling into our hospitals, the nurses did not know how to react and stood on shaky ground. The casualties were admitted directly into the intensive care units and emergency rooms without knowledge of mechanism of injury, date or circumstances of injury, and alone with no family support. We were told not to communicate with them. However, that request was quickly overlooked and relationships developed. The following report is that of one of the bedside nurses in an Israeli border hospital and her experience of caring for a Syrian casualty.

Environmental contamination with Mycobacterium avium subspecies paratuberculosis within and around a dairy barn under experimental conditions.

To establish environmental contamination in and around a dairy barn, cows shedding Mycobacterium avium ssp. paratuberculosis (MAP) were housed in a freestall barn. Fecal samples were collected 15 times at 3-wk intervals, and samples of all animals were cultured by using the Trek Diagnostic Systems culture system (Cleveland, OH) to quantify levels of MAP shedding. In parallel, air and floor dust samples were collected inside and outside the experimental farm and analyzed by IS900 real-time PCR for the presence of MAP DNA. Inside the barn, MAP was detected with equal frequency in samples directly contaminated with feces compared with air dust samples above animal level and in dust samples of the corridor. Dust samples collected within the barn were positive more frequently than outside samples, with exception of the outside sample from the farmer's doormat. The risk of MAP exposure was distributed evenly within the dairy barn. Additionally, footwear should be considered as a high-risk fomite for dispersion of dust-related MAP outside the barn. Prevention of MAP exposure in youngstock may require housing of youngstock in separate barns as an additional management measure.

Mycobacterium avium subspecies paratuberculosis in bioaerosols after depopulation and cleaning of two cattle barns.

Settled dust samples were collected on a commercial dairy farm in the Netherlands with a high prevalence of Mycobacterium avium subspecies paratuberculosis (MAP) (barn A) and on a Dutch experimental cattle farm (barn B) stocked with cattle confirmed to be MAP shedders. Barns were sampled while animals were present, after both barns were destocked and cleaned by cold high-pressure cleaning, and after being kept empty for two weeks (barn A) or after additional disinfection (barn B). MAP DNA was detected by IS900 real-time PCR and viable MAP were detected by liquid culture. MAP DNA was detected in 78 per cent of samples from barn A and 86 per cent of samples from barn B collected while animals were still present. Viable MAP was detected in six of nine samples from barn A and in three of seven samples from barn B. After cold high-pressure cleaning, viable MAP could be detected in only two samples from each barn. After leaving barn A empty for two weeks, and following additional disinfection of barn B, no viable MAP could be detected in any settled dust sample.

Detection of spatial and temporal spread of Mycobacterium avium subsp. paratuberculosis in the environment of a cattle farm through bio-aerosols.

Environmental samples were collected to investigate the spatial and temporal spread of Mycobacterium avium subsp. paratuberculosis (MAP) in a dairy cattle barn before and after the introduction of two groups of MAP-shedding animals. Samples collected off the floor of the barn reflected the moment of sampling whereas samples collected by microfiber wipes at a minimal of 3m height contained the accumulated settled dust over a 3-week period. Samples were analysed by IS900 qPCR for the presence of MAP DNA and by culture for viable MAP bacteria. MAP DNA was detected in a large number of sites both before and after introduction cattle. MAP DNA was detected inside the barn in floor and dust samples from cubicles and slatted floors and in settled dust samples located above the slatted floors and in the ventilation ridge opening. Outside the barn MAP DNA was detected by PCR in samples reflecting the walking path of the farmer despite hygiene measures. No viable MAP was detected before the introduction of shedder cattle. Three weeks later viable MAP was found inside the barn at 7/49 locations but not outside. Fifteen weeks later viable MAP was also detected in environmental samples outside the barn. In conclusion, introduction of MAP shedding cattle lead to widespread contamination of the internal and external environment of a dairy barn, including the presence of viable MAP in settled dust particles suggesting potential transmission of MAP infection through bio-aerosols.

Evidence of Mycobacterium avium subsp. paratuberculosis infection in Dutch farmed red deer.

Paratuberculosis is a chronic disease in ruminants caused by Mycobacterium avium subsp. paratuberculosis (MAP). Most economic losses due to MAP occur in the dairy industry. However, the infection is not restricted to cattle, but also occurs in other ruminants, such as sheep, goat, and deer. Although deer are of minimal economic importance in The Netherlands, they may constitute a source of infection for the dairy industry. This pilot study was conducted to estimate the prevalence of Johne's disease in farmed red deer in The Netherlands. Serum and faecal samples were collected from 140 animals, originating from 8 different farms. Four of the farms had animals that tested positive for Johne's disease. The within-herd MAP seroprevalence varied between 4.8% and 21.2%. In conclusion, this pilot study provides evidence of MAP infection in the Dutch farmed deer population, and thus there might be a risk of MAP transmission between farmed red deer and dairy cattle.

Influence of 17beta-oestradiol, nortestosterone and dexamethasone on the adaptive immune response in veal calves.

In veal calf production androgens, estrogens and glucocorticoids are used to stimulate growth. However, sexhormones and glucocorticoids also influence the function of the immune system. From studies in humans and mice, androgens are known as immunosuppressive, while estrogens stimulate the production of antibodies and glucocorticoids also enhance the T-helper 2 response. To investigate whether the adaptive immune system is influenced by hormone administration, calves were treated with a hormone cocktail containing androgens, estrogens and glucocorticoids and vaccinated against Mycobacterium avium spp. paratuberculosis. The activity of the adaptive immune system was measured by using an antigen specific elispot assay (ES), lymphocyte stimulation test (LST) and an enzyme-linked immuno sorbent assay (ELISA). The results showed that the hormone treatment did not lead to significant differences in the function of the adaptive immune system between the hormone treated and the not hormone treated group while growth was stimulated in the hormone treated group.

Detection of circovirus with a polymerase chain reaction in the ostrich (Struthio camelus) on a farm in The Netherlands.

This study describes for the first time the presence of circoviruses in ostrich tissue including embryos. A polymerase chain reaction (PCR) was used for the detection of the virus in liver samples. The use of a polymerase for low copy detection significantly increased the sensitivity of the test as well as a Southern blot. Viral DNA could be detected in chicks and eggs that did not hatch. For localisation of the virus in the liver in situ hybridisation was performed on a selection of positive liver tissues.

A single nucleotide polymorphism in the RAD51 gene modifies cancer risk in BRCA2 but not BRCA1 carriers.

BRCA1 and BRCA2 carriers are at increased risk for both breast and ovarian cancer, but estimates of lifetime risk vary widely, suggesting their penetrance is modified by other genetic and/or environmental factors. The BRCA1 and BRCA2 proteins function in DNA repair in conjunction with RAD51. A preliminary report suggested that a single nucleotide polymorphism in the 5' untranslated region of RAD51 (135C/G) increases breast cancer risk in BRCA1 and BRCA2 carriers. To investigate this effect we studied 257 female Ashkenazi Jewish carriers of one of the common BRCA1 (185delAG, 5382insC) or BRCA2 (6174delT) mutations. Of this group, 164 were affected with breast and/or ovarian cancer and 93 were unaffected. RAD51 genotyping was performed on all subjects. Among BRCA1 carriers, RAD51-135C frequency was similar in healthy and affected women [6.1% (3 of 49) and 9.9% (12 of 121), respectively], and RAD-135C did not influence age of cancer diagnosis [Hazard ratio (HR) = 1.18 for disease in RAD51-135C heterozygotes, not significant]. However, in BRCA2 carriers, RAD51-135C heterozygote frequency in affected women was 17.4% (8 of 46) compared with 4.9% (2 of 41) in unaffected women (P = 0.07). Survival analysis in BRCA2 carriers showed RAD51-135C increased risk of breast and/or ovarian cancer with an HR of 4.0 [95% confidence interval 1.6-9.8, P = 0.003]. This effect was largely due to increased breast cancer risk with an HR of 3.46 (95% confidence interval 1.3-9.2, P = 0.01) for breast cancer in BRCA2 carriers who were RAD51-135C heterozygotes. RAD51 status did not affect ovarian cancer risk. These results show RAD51-135C is a clinically significant modifier of BRCA2 penetrance, specifically in raising breast cancer risk at younger ages.

Regulation of an endogenous locus using a panel of designed zinc finger proteins targeted to accessible chromatin regions. Activation of vascular endothelial growth factor A.

We have mapped conserved regions of enhanced DNase I accessibility within the endogenous chromosomal locus of vascular endothelial growth factor A (VEGF-A). Synthetic zinc finger protein (ZFP) transcription factors were designed to target DNA sequences contained within the DNase I-hypersensitive regions. These ZFPs, when fused to either VP16 or p65 transcriptional activation domains, were able to activate expression of the VEGF-A gene as assayed by mRNA accumulation and VEGF-A protein secretion through a range exceeding that induced by hypoxic stress. Importantly, multiple splice variants of VEGF-A mRNA with defined physiological functions were induced by a single engineered ZFP transcription factor. We present evidence for an enhanced activation of VEGF-A gene transcription by ZFP transcription factors fused to VP16 and p65 targeted to two distinct chromosomal sites >500 base pairs upstream or downstream of the transcription start site. Our strategy provides a novel approach for dissecting the requirements for gene regulation at a distance without altering the DNA sequence of the endogenous target locus.

Electroporation-mediated gene transfer in free-swimming embryonic Xenopus laevis.

Xenopus laevis are a rich resource for vertebrate embryology and cell biology. Transplantation and transgenesis have provided much information about the developmental mechanisms of embryogenesis and molecule function, however existing methods have faced limitations regarding either the precise localization of gene expression or flexibility in the timing of gene transfer. Here we have found that electroporation of tailbud (stage 29/30) embryos is a rapid and efficient method of combining cell-specific expression with variation in temporal delivery. At the low voltages required for electroporation, embryos resumed normal swimming behavior and development. We conclude that electroporation has wide experimental application to Xenopus developmental and cell biology.

A microstructural model of elastostatic properties of articular cartilage in confined compression.

A microstructural model of cartilage was developed to investigate the relative contribution of tissue matrix components to its elastostatic properties. Cartilage was depicted as a tensed collagen lattice pressurized by the Donnan osmotic swelling pressure of proteoglycans. As a first step in modeling the collagen lattice, two-dimensional networks of tensed, elastic, interconnected cables were studied as conceptual models. The models were subjected to the boundary conditions of confined compression and stress-strain curves and elastic moduli were obtained as a function of a two-dimensional equivalent of swelling pressure. Model predictions were compared to equilibrium confined compression moduli of calf cartilage obtained at different bath concentrations ranging from 0.01 to 0.50 M NaCl. It was found that a triangular cable network provided the most consistent correspondence to the experimental data. The model showed that the cartilage collagen network remained tensed under large confined compression strains and could therefore support shear stress. The model also predicted that the elastic moduli increased with increasing swelling pressure in a manner qualitatively similar to experimental observations. Although the model did not preclude potential contributions of other tissue components and mechanisms, the consistency of model predictions with experimental observations suggests that the cartilage collagen network, prestressed by proteoglycan swelling pressure, plays an important role in supporting compression.