drinking water should take account of the species specific daily water consumption.
In addition, within a given species, many biological (body weight, age, gender),
environmental (lighting period, environmental temperature) and managerial factors
(flock size, composition of the diet) can influence individual animal water intake.
For example, the external temperature may influence drug exposure as water
consumption is increased by approximately 7% for every 1C above 21C. More-
over, birds do not drink in dark periods so the light period may be manipulated to
increase drug exposure especially for drugs having a short half-life.
The alternative to the drinking water is the administration of a drug through the
foodviapre-mix formulations. In contrast to water that is offeredad libitum, food
may be given and is ingested in a restricted way and competition exists between
birds. Therefore, the pecking order that influences food intake will modulate drug
exposure and lead unavoidably to differences between individuals. Parenteral
administration is seldom used in poultry but is frequent in pet birds.
Birds have no teeth (no chewing is possible) but a beak that is often trimmed at
hatching and re-trimmed later to prevent behavioural problems (feather pecking).
It was shown that for all diets including medicated feed, birds with short upper
beaks consumed significantly less than birds with long upper beaks. In many bird
species (scavenging birds, etc.), the cervical esophagus is expanded to form a crop
allowing the storage of food before digestion occurs. Drugs are not absorbed in the
crop, which has a keratinised epithelium. The pH of the crop is about 6 and some
drugs ingested in solution in drinking water can precipitate in the crop, resulting in
delayed transit and poor absorption, as in the case of tetracyclines. The presence of
alactobacillusflora in the crop can inactivate antimicrobial drugs of the macrolide
group. Drinking water (and drug in solution) passes directly through the crop, but
solid or pasty feed (possibly containing medication) may reside for a long time in
the crop with an emptying time in broilers ranging between 3 and 20 h. This is the
case when direct administration of food into the crop is carried out at the fattening
stage for fattened duck or goose. The food passes from the crop to the stomach. The
stomach consists of two parts: the proventriculus that is the glandular portion of the
stomach secreting acidic digestive juices, followed by the muscular gizzard that
contains gravel (or grit) which works together with the muscles to grind up food.
Most drugs used in poultry (antibiotics, coccidiostats, etc.) are weak organic bases
and are not absorbed by the proventriculus. The gizzard is a powerful triturating
apparatus (replacing the teeth), and in birds any solid dosage forms are rapidly
disintegrated to release active ingredients. Most drugs ingested as a solution pass
rapidly along the crop and the two stomach chambers to arrive within a few minutes
in the intestine. The alkaline pancreatic juice neutralises the acidic contents leaving
the gizzard and absorption in birds occurs, as in mammals, in the duodenum and
upper jejunum. This rapid transit to the small intestine and the limited development
of the distal part of the digestive tract (related to adaptation to flight) explain the
very rapid overall transit time of about 5–6 h in broilers of those drugs that are not
entrapped in the crop with food.
Force feeding in ducks or geese produces a liver that is six to ten times its
ordinary size. The storage of fat in the liver produces steatosis of the liver cells. This
42 P.-L. Toutain et al.