(From the ‘what were you thinking' files ...)
Every year in the Manawatu a few mobs of artificially-reared calves start scouring, or coughing (maybe at about 5-6 weeks of age), and a ‘bush diagnosis' - something along the lines of "Arrrr that be worrrms" - is made by someone with years of experience. So the calves are duly treated for worms; occasionally they may improve, most often nothing happens, and sometimes they drop dead! What the?
Let's get something straight right at the start: artificially-reared calves living outdoors almost never need treatment for worms (gastrointestinal or lungworm) before they are weaned.
Why? There are lots of good reasons for this which we'll get to in a minute. The other point that is really important is that a number of the commonly used drench products are quite toxic in pre-ruminant calves.
Why don't young calves need worm treatment?
- Worms can't actually establish in the gut until calves have a fully functioning rumen; the process of worm larvae developing through to the adult stage requires rumen fluid as a trigger
- Milk has some anthelmintic activity
- Whilst calves are eating meal and drinking milk they are not likely to be grazing down low enough into the pasture to pick up a production-limiting worm burden
- The ill effects we see from gut worms (scouring and weight loss) are largely a consequence of the calf's own immune response to the ‘invading' worms, and very young calves lack the capacity to mount this response; it kicks in later
- Lungworm has a much longer lifecycle than the gut worms and even if a lungworm challenge is present it will rarely have risen to levels that require treatment in the period before weaning - unless weaning is later than normal
And why might a worm drench be toxic?
- Two of the common components of calf drench products, abamectin and levamisole (think most of the cheaper injectables and the oral combo drenches), have quite a narrow safety margin in young calves
- Overestimation of calf liveweight can be enough to cause toxicity especially in individuals that are already ill-thrifty or sick with something else
- Giving the poorer ones ‘a bit extra because they look like they need it' - more is not always better!
- Using an oral drench (which is designed to go into the rumen) in animals that don't yet have a functioning rumen - in this case the drug goes straight into the abomasum where it may be absorbed much faster and rise to toxic levels in the blood
- Putting the drench in the milk - yes we know it goes on (!) - process is the same as above plus in the ‘towed feeder' situation you'll have some individuals drinking more and potentially overdosing on the worm drench
So the take-home message is: ill-thrift, scouring or coughing in artificially-reared calves pre-weaning is probably not worms - get a better diagnosis so you can give the right treatment!
