The uterus is a hollow muscle, like the heart or the intestines . The pattern of muscle contraction has to be rhythmic in order to expel the babies, blood or feces. Basically, this is something akin to peristalsis in the intestine, the rhythmic contraction, like a worm moving. Several things can cause this to be disrupted, such as fatigue, low blood sugar, malpositioning, low calcium or low oxytocin levels, and things don't flow.
Let's take the instance of a sow during delivery, who comes down with a uterine infection after she delivers her babies. The muscle in front and behind may try to keep contraction going but there is a section of the uterine muscle that can't complete the rhythm (imagine a wave at a stadium). If the whole uterus is flaccid, or limp, and not contracting during delivery, babies are not expelled. Hopefully the body compensates by bringing in stores from other organs, tissues or reserves.
Calcium is pulled from the other tissues and oxytocin stimulated by nursing previously expelled young are all sources of minerals necessary to induce more contractions. If any contaminate was available it could conceivably be transported up the vagina as the babies are pushed out, then slightly pulled in as she contracts again, and during the contractions those small pieces (bedding, food, hay) cling to the irritated and moist surfaces. Couple this with lowered resistance due to the physiologically challenged post partum condition allows these and normally present opportunistic bacteria to proliferate abundantly at will. Now we have vaginitis by fluid gradient. When the contaminate moves on to adjacent tissues now we have metritis, infected wall of the uterus, also called pyometria. At some point the degree of infection becomes more than that little stressed body can cope with now we have septicemia, or literally, the body going septic. When the infection becomes blood borne, all the organs are exposed to heavy amounts of bacteria and decomposition begins to take place, and now we have toxemia . The toxins produced by the tissue death overwhelm the kidney, degrade the heart valves and saturate the liver and spleen. At some point, it overwhelms this small closed system, and the sow can't flush out the toxins at the same rate they are building up. At this point she becomes poisoned to death on the toxins.
Dextrose (sugar), calcium and oxytocin are all factors to consider in dystocia, or the technical term for a difficult labor. When a sow weakens after trying to push out a suck or malpositioned baby, this is when a buildup of bacteria can begin. Also, she wears out her system from calcium (used during contractions) and has no more resources, causing other muscles to tremor, weaken, and lose the ability to contract and expand. She then cannot deliver the babies without outside assistance.
Provided you get a sow that does not deliver, yet has a uterine infection, how do you suppose she caught it? Again, during intercourse is one way for bacteria to be introduced into the reproductive cavity, as is marking in heat (when the third membrane shrinks back to open that aperature into the vagina), and also, an infection becoming systemic, or not localized any more.
Prophylactic antibiotic are some what controversial when she's pregnant, but are sure appropriate to stem the tide of organisms, and are necessary to combat this infection. Some folks suggest soaking the sow in a bin of warm water with Epsom salts. Epsom salt works by concentration gradient, and osmosis. The dense solution of Epson salt draws fluid from the tissues. The body is tricked into producing more fluid (she has to be drinking to flush this out) and this toxic and infected fluid is replaced by more (clean) body fluid and the solution to pollution is dilution.
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