The Cambridge AV (Fig. 5.2) is essentially similar to the Colorado or Lane model (Lane Manufacturing Co., Denver, Colorado) and the CSU model
(Animal Reproductive Systems, Chiro, California). The Cambridge model is primarily used in the UK and the rest of Europe, whereas the Colorado or Lane model and the CSU AV find greater popularity in the USA.
The Cambridge model is constructed of a stiff outer casing (of varying lengths and normally made of plastic, though metal may be used) which is enclosed firstly in an outer rubber lining (Fig. 5.3) to provide protection. This outer lining is tightly clamped to the plastic tube by means of two large jubilee clips attached at either end. Secondly, there is an inner rubber lining (Fig. 5.3), which is slightly longer than the outer lining but smaller in diameter. The inner liner is placed inside the lumen of the AV and stretched over both ends of the casing and outer liner, and also secured by, the two jubilee clips. The inner liner is a tight fit and stretching it is an art. It is advisable to secure one end and then stretch the other end over the casing, trying to ensure that the inner liner is smooth throughout the length of the AV. Any wrinkles in the liner may cause irritation and the AV may therefore be rejected by a fussy stallion, with low libido, especially during the transition period between the breeding and non-breeding season. This second liner and the plastic casing provide the inner and outer parts of the water jacket that enable the internal temperature of the AV to be controlled. It is normal practice to use a third sterile plastic or polyethylene disposable inner liner, which is the one that comes into direct contact with the stallion's penis and into which ejaculation occurs. These liners tend to be much looser in their fitting and do sometimes cause irritation to stallions; in such circumstances, ejaculation directly into the smooth inner rubber liner may be used, omitting the use of the disposable third inner liner.
The Colorado or Lane model bears a significant resemblance to the Cambridge model, the major differences being that the Colorado model (which is made of plastic) and the Lane model (which is made of aluminium) are designed to be used primarily, but not exclusively, without the use of the additional plastic disposable inner liner. These models also tend to be longer than the Cambridge, which is available in varying lengths for different sized stallions. Some variations on this model include the slight flaring of the AV casing, at the end into which the stallion thrusts. The CSU model is largely a development of the Colorado model, the major modification being a reduction in its total weight when ready for use - a significant advantage over the other two models, which tend to be heavy when filled with water and ready for use. The CSU also has specially designed disposable liners.
All three of these models have the advantage of good heat-retaining capabilities over relatively long periods and they find their greatest popularity in colder climates. Heat retention is conferred by the large volumes of water they contain, but such volumes of water have the major disadvantage of adding considerable weight to the AV when ready for use. A fully assembled Cambridge AV ready for use may weigh up to 10 kg. This weight means that they are heavy to hold and cumbersome to manoeuvre. A further disadvantage is that some variation in the lumen temperature may occur with the movement of the water within the water jacket as the AV is manoeuvred. The tendency is for the AV to be held at a slight angle during collection, with the distal (collecting vessel) end lower, this tendency being further exacerbated by the weight of the AV. As a result the semen tends to be ejaculated into and pass over the warmer end of the AV prior to its collection in the collecting vessel. If the temperature of the water in the AV is too high, there is a greater risk of heat damage to the spermatozoa as they pass over this area on their way to the collecting bottle. These models also have the disadvantage of being a similar size at the distal and proximal ends. This does not give the resistance at the distal end required by some stallions - a requirement that is reported to be particularly evident in Thoroughbreds (J.M. Parlevliet, The Netherlands, 1998, personal communication).
With all these models, the collecting vessel is attached to the tapered end of the inner liner, be it plastic or rubber. The collecting vessel may have a built-in nylon filter, in the shape of a pocket or bag, that fits over the neck of the bottle before it is attached to the inner liner and protrudes into the collecting vessel. A simple piece of sterile gauze may be used in a similar fashion to provide a cheaper alternative to the nylon filters. Either of these filters allows the semen gel fraction, plus any detritus, to be filtered off during the collection process. This eliminates any detrimental effects that such detritus may have on the semen sample, and reduces the time in the laboratory, as removal of the gel fraction is required before evaluation can take place.
The collecting vessel needs to be kept at a constant 38°C in order to protect the spermatozoa from cold shock. This can be achieved by a simple insulating sleeve or cone placed over the collecting bottle and the distal end of the AV. The insulating cone should be pre-heated to 38°C. This can achieved by simply placing it in an incubator, or more precise methods of heating the cone using a rheostat may be used (Pickett, 1993a).
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