Information and advice from the RVC Farm Animal Clinical Centre on the importance of accurate weight estimation for drug dosing in alpacas.

It is vital that each individual animal has a correct weight estimated or known prior to calculating how much of a drug to give it, whether this be a wormer drug, antibiotic or any other.

This is because under dosing a drug, especially a wormer or antibiotic can rapidly lead to resistance to the drug, both in this individual and in the remainder of the herd, as well as the animal being treated ineffectively.

Over dosing a drug can lead to potential drug toxicity issues with harmful effects to the patient.

Drug resistance

Drug resistance is a major problem worldwide and is becoming increasingly common and difficult for vets and owners to deal with.

Resistance of parasites to wormer drugs, including some of the more common worms, such as Haemonchus contortus, and resistance of bacteria to antibiotic drugs are a major threat to both animal and human health.

A major route to resistance is administering drugs at suboptimal doses. This occurs for a variety of reasons, including a visual underestimation of body weight and therefore calculating too low a dose; poor storage of drugs or use of drugs beyond their expiry date; sick animals not absorbing the drug; individual animals being missed from dosing when the rest of the group is treated.

Resistance to the drug develops because of the ‘survival of the fittest’ rule. When a drug is given at too low a dose some worms, or bacteria, survive, these survivors then multiply setting up a population of resistant worms or bacteria which will no longer be susceptible to that treatment.

A good example of an emerging drug resistant major threat to camelid health is Haemonchus contortus, the barber’s pole worm which causes profound anaemia, weight loss and can lead to death of an animal without treatment. This worm is a massive egg layer and can go from being an egg to being an adult egg layer in less than 3 weeks. This rapid life cycle enables genetic adaptation to ensure survival, so it can very rapidly acquire new resistant genes, and surviving worms following treatment can rapidly multiply, setting up a resistant population.

This adaptability means this worm has a vast ability to develop resistance to de-worming drugs, especially if they are used inappropriately, such as under dosing or over use. Every time an animal is treated with a de-wormer the resident worms are subjected to selective pressure. Treatment eliminates susceptible worms leaving the resistant worms to repopulate.

This is why it is vital to perform appropriate faecal egg count tests and treat based on these and use these to check the treatment has worked; your veterinarian should be able to advise you further on this. It is also a reason why removing contaminated faeces from the pasture is so important, not just to reduce the overall worm burden.

Visual weight estimation

It has been well documented in other species, such as cattle and sheep that visual weight estimation leads to significant under-dosing by farmers who are much more likely to underestimate weight compared to vets.

The heavier the animal, the greater the degree of under-dosing was found. In one study of sheep farmers, 86% significantly underestimated the weight of their animals. Sometimes inexperienced owners struggle to accurately guess the weight of their animals through visual estimation alone.

With some drugs, over estimation can also be a problem as the correct dosage of the drug is very close to the toxic levels of the drug causing harm to the patient, examples include the wormer levamisole, certain antibiotics and anti-inflammatory drugs. Therefore, it is important to have a way of correctly estimating the weight of the animals.

In horses, weigh tapes exist which use girth measurements to calculate the animal’s weight; however, attempts to devise such a tape for alpacas has so far proven to be difficult and too inaccurate for use in this species.

Better weight estimation

So, what is the best method of knowing the weight of an alpaca?

Undoubtedly, the best method is a weigh scale or weigh bridge which will give you an accurate weight for each animal and allow regular accurate monitoring of weight gain, or loss, as well as accurate weights for calculating drug dosages.

Walk on weigh scales for alpacas can be purchased for around £200-400, and if stored and kept properly will last. Although, to weigh the animal it needs to be on a hard level surface, which is not always very practical.

Failing this, visual estimation by an experienced person, taking into account body condition score, fleece depth and density, and stage of pregnancy if applicable is next best.

It is not possible to accurately estimate the weight of an alpaca by only looking at it, hands on body condition score should help guide the weight estimation, as well as the height of an animal. A heavily fleeced alpaca in poor body condition is more likely to be overestimated, whilst a shorn animal of a high body condition, or a pregnant animal, is more likely to be underestimated.

If you are not sure what your alpacas weigh, next time the vet is out or a more experienced keeper, ask them to check with you what you estimate the weights to be.

Correct dosage

When treating an alpaca or giving a de-worming drug, it is important to have an accurate weight of the animal, ensure the drug is in date and has been stored appropriately, and if giving an oral drug especially, make sure the animal safely swallows the amount intended by appropriate restraint and administration techniques.


Dr Alex McSloy wrote an article on this subject for Alpaca Magazine, which is produced by the British Alpaca Society. This Fact File includes the information submitted to the magazine.

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