The interaction of the animal, the environment and the pathogen (in the case of infectious diseases) in determining the health status of an animal or group of animals can be schematically represented as in Figure 1. There are normally many different factors that determine the health status of an animal - it is the balance between the 'protective' and the 'harmful' factors that determines whether or not disease occurs, or at what level the animal is affected. By bolstering the protective factors and minimizing the harmful factors an animal may be made more resistant to developing disease. This is the challenge facing animal managers in today's dairy herds.
Figure 1. Interaction of the animal, environment and pathogen in determining the health status of an individual or group.
This model can be used to explore the complex set of interacting factors that influences the level of lameness and foot health in a cow. In the case of non-infectious lameness conditions, the interaction of factors internal ("Animal") and external ("Environment") to the animal determines the health of the foot. In the case of infectious conditions, the impact of the pathogen on the animal is modulated by animal and environmental factors.
The investigation of lameness in dairy herds presents an interesting and complex challenge. Herds are obviously comprised of individuals and groups, each of which may have a different balance of 'protective' and 'harmful' factors. Therefore, the investigation should determine the relative level of foot health in different groups of animals, and should also systematically evaluate the factors that have the greatest impact in the animal-environment-pathogen interaction. In this way the veterinarian should be able to make meaningful recommendations to reduce the level of lameness in a specific herd.
A number of animal factors have been determined to be associated with foot health. Rarely will a single harmful factor be sufficient to cause hoof disease on its own - rather two or more factors can often be found to be working in concert to compromise hoof health. The condition of the claw capsule is a significant factor in determining the health of the foot. A healthy claw is able to withstand greater environmental and pathogen challenges than a weak or compromised claw capsule. The claw capsule also provides a history of the health of the corium, which is responsible for producing all of the capsular components, including the hoof wall, sole, white line, heel and coronary band. It is important to note that many of the lesions that are observed on the outer aspect of the claw reflect pathological changes that have occurred approximately two to three months previously.
Historical hoof trimming reports, if kept with sufficient detail and accuracy on an adequate number of animals, can provide the basis for an investigation of the health of the claw capsule. These records may provide information about the distribution of claw lesions and disease across different groups of animals and across different times of the year. Direct examination of a number of cows' feet in the course of an investigation will provide current information on the health status of the claws, and may be the only available information in herds without adequate records. Ideally, a number of lame and non-lame animals, with at least some of the animals being in early to peak lactation should be examined in the trimming chute. This examination can be carried out by the veterinarian if he/she is adequately experienced in hoof trimming or can be done in conjunction with a professional hoof trimmer. Abnormalities should be noted using the recording system discussed in the previous article ( Part I - Defining and Monitoring the Situation ).
The following lesions may be due to pathological changes in the corium:
- The presence of blood (hemorrhage) and serum in the sole.
- A 'double sole' (or under-run sole) or sole abscesses if bacteria and foreign matter become entrapped in a layer of poor quality sole tissue.
- A sole ulcer -- a full-thickness defect of the sole resulting in exposure of the corium.
- White line separation and abscessation secondary to changes in the laminar corium.
- "Hardship grooves" on the hoof wall.
- Erosion of the heel horn.
The condition of the interdigital space and the bulbs of the heels is also important in determining the susceptibility of these areas to the development of lesions. The following conditions may be due, at least in part, to pathologies of these tissues:
- Interdigital dermatitis
- Interdigital hyperplasia ("corns")
- Digital dermatitis ("hairy heel warts")
- Interdigital phlegmon ("Foot rot")
Rumen acidosis is a condition that is associated with the production of an inferior claw capsule by the corium. This is thought to be due to circulatory changes that occur in the claw secondary to the acidosis, although the exact mechanism of these changes have not yet been clearly elucidated. Various vasoactive substances such as histamine have been implicated. Numerous factors must be investigated if rumen acidosis is suspected, including the formulation of the ration and feeding management. Excessive levels of rumen fermentable starch, inadequate levels of effective fiber, slug feeding, and feed sorting are examples of factors to be evaluated.
Weakening of the suspensory mechanism, which suspends the pedal (P3) bone within the claw capsule, may occur under the influence of hormones such as relaxin which are present in higher concentrations around parturition. The activity of other enzymes such as metalloproteinases may also be increased in response to rumen acidosis or other undefined factors. This increased proteinase activity can degrade the attachment of the corium to the bone resulting in shifting of P3 within the claw capsule. Instability of the P3 bone may cause abnormal pressure on the solar and laminar corium causing pathology of the sole and white line.
Severe disease, such as metritis or mastitis, may initiate circulatory changes in the corium which can result in pathologies as discussed above.
The conformation of the foot affects the surface area and the weight distribution across the plantar surface. If the dorsal hoof wall is too long and/or the angle between the dorsal hoof wall and sole is too acute, more weight will be shifted to the posterior aspect of the sole as well as the heel. Under these circumstances the heels will also be closer to the walking surface. Soles trimmed to be concave have dramatically less weight-bearing surface when cows stand or ambulate on unyielding surfaces, as well as concentrating the walking forces in the region of the white line (Figure 2). Concave soles may also cause some instability and splaying of the digits on unyielding surfaces, with subsequent irritation of the interdigital skin and heels. There is a genetic component to conformation as well (such as heel depth and hock angle), although these are normally difficult to change in the near-term!
Figure 2. Weight bearing surface area of a bovine foot before trimming (~60cm²) and after trimming (~108cm²), using the "Dutch" hoof trimming method.
Various trace minerals (eg. zinc, copper, manganese) and vitamins (eg. A, D, E, biotin) have been shown to affect the incidence and severity of various claw capsule lesions by impacting the quality of the tissue produced by the corium.
As described above, a number of environmental factors interact with the animal and pathogen factors to determine the foot health status of a dairy cow.
Pressure on the sole that is persistent can cause changes in the perfusion of the corium as well as exacerbating coincident harmful 'animal factors'. Stall usage and management practices (eg. holding pen time, stocking density) in confined housing situations can play a critical role in minimizing the unnecessary damage to the corium brought about by excessive weight-bearing. Poor (eg. rough) walking surfaces can lead to excessive point loading of the sole and bruising within the corium.
Walking surfaces that provide poor traction (eg. smooth, wet concrete) may aggravate splaying of the claws, as well as causing an increase in the incidence of traumatic upper limb injuries. Conversely, rough, abrasive walking surfaces (often seen in new facilities) may lead to excessive hoof wear and thinning of the soles, especially in heifers. Travel lanes and paddocks with hard projections may cause traumatic damage and irritation to the interdigital space and heels.
Poor hygiene in the cow housing and travel areas can result in manure and slurry accumulation on the feet and lower limbs. This increases the likelihood that a moist, anaerobic environment will exist at the skin, favoring the survival of pathogens. At the same time, the feet will be exposed to destructive chemicals and infectious pathogens present in manure slurry.
See part 1:
Lameness in Dairy Herds - Part 1: Defining and Monitoring the Situation