particles that require cellulose breakdown and the delay will be longer as the
turnover time for the solid phase is approximately 50–60 h.
A vast reticulo-rumen functionally isolated from the most distal part of the
digestive tract by the ROO offers the unique therapeutic opportunity to administer
a large depot of drug using a delivery device to provide prolonged and sustained
release of appropriate amounts of drugs (mainly anthelmintics), trace elements,
anti-foaming agents, etc. Such drug delivery systems are designed to remain lodged
in the reticulo-rumen for several days, weeks, or even months. Such an approach is
not possible in monogastric species, due to the gastric emptying that occurs within
the 12 h following administration. In ruminants, regurgitation of the device is
prevented by an appropriate geometric design, for example, expanding plastic
wings. Thereafter, the device should reside permanently in the reticulum or the
cranial sac of the rumen to release drug close to the ROO and in the fluid phase of
the RR contents and not in the distal part of the rumen, where there is the possibility
of being entrapped in the fibrous raft that floats above the ventral ruminal fluid. The
density of the device should suffice to avoid floating in the ruminal liquids.
A density of 2.25–3.5 g/cm^3 is required in grazing animals, but a lower density
(1.8 g/cm^3 ) may be sufficient in animals fed a cereal-based food. For a review on
ruminal drug delivery systems, see Vandamme and Ellis ( 2004 ) and also the
chapter, “Drug Delivery Systems in Domestic Animal Species” of this text.
The forestomach precedes the true secretory stomach or abomasum. In the
newborn ruminant, the reticulo-rumen is not developed. From a digestive physiology
point of view, the calf is essentially a monogastric animal and the RR should be
bypassed when the animal is fed with milk. This is possibly due to the presence of a
reticular groove. The reticular groove is formed by muscular folds, able to form a
closed channel that extends from the cardia to the abomasum. Reticular groove
closure occurs reflexively in pre-ruminant animals in response to different stimuli
including suckling, which allows ingesta (milk with drug in solution) to bypass the
rumen to gain access directly to the abomasum.
In adults, the closure reflex of the reticular groove no longer operates but it may
be facilitated by some practises like yarding in small ruminants. Yarding consists of
holding sheep in a paddock with little or no food for 12–24 h. Yarding (but not
withdrawal of water) for 24 h before drenching has been shown to stimulate RR
bypass in approximately 35% of a group of 9-month-old lambs (Sargison et al.
1999 ). Yarding (1224 h before drenching) has been proposed as a method to
improve the efficacy of anthelmintics (Prichard and Hennessy 1981 ).
The horse and rabbit are hindgut fermenters and the caecum and colon are major
sites of microbial digestion of feed for these species. This makes horses and rabbits
particularly prone to antimicrobial drug-induced enterocolitis, secondary to disrup-
tion of their normal microflora leading to anovergrowth of pathogenic microorganisms
likeClostridiumssp. In the rabbit,C. spiriformehas been implicated as the primary
causative agent producing iota toxin and causing enterotoxaemia and death.
It should be stressed that the delay between the end of antibiotic administration
and these catastrophic events may be up to 10 days, making it sometimes difficult to
identify the origin of an enterotoxaemia. For this reason, antibiotics administered
36 P.-L. Toutain et al.