Roundworm Resistance to Anthelmintics in Angora Goats

Roundworm resistance to Anthelmintics in Angora Goats

Dr Mackie Hobson

Angora Goats metabolise anthelminthics at a much faster rate than sheep and so roundworms develop resistance to the drugs at a more rapid rate than in sheep.

Resistance by roundworms to an anthelminthic drug is the ability of roundworms to survive a dose of that drug. It is a worldwide problem which dominated the recent International Sheep Veterinary Congress held in New Zealand in February 2013. Roundworms have a significant effect on production and are one of the more common causes of death in Angora goats found by vets when doing post mortems.

In Australia studies indicate that on 98% of farms there is resistance to drug groups  Benzamidazoles (‘witmiddles’) and Levamizoles (‘Koeldrankmiddles’) with 87% of farms having resistance to Macrolytic lactones (Ivermectin family) and 28% to triple combinations.  Resistance appears to be following a similar trend in South Africa.

New active ingredients for doses are unlikely to be developed in the near future and due to the quicker development of resistance in goats, drug companies are not likely to register any new doses for goats. A number of the companies are advocating dosing goats at 1.5 times the sheep dose (off license) due to the sorter half-life of anthelminthics in goats.

Implications of Roundworm Infestation

Female wireworm ‘haarwurm’ can produce up to 10 000 eggs a day. Under optimal conditions these may develop into the infective L3 larval stage within a week and anywhere between 5-40% of the L3 stage larvae can establish as adults depending on the immunity of the goat at the time of ingestion of the larvae.  As a basic indication of the impact on the goat a faecal egg count (FEC) of 1000 epg (eggs per gram) would roughly equate to 200 adult worms being present in the abomasum and result in roughly 10ml blood loss a day. In the same way an egg count of 5000 epg would result in an estimated blood loss of 50ml per day.

Severe anaemia in an Angora goat

Determine the Resistance Status on Your Farm

The first step in combatting the problem is to determine the current status on a farm. Every farm is different and resistance levels will depend on previous management strategies. Due to the economic implications of roundworms, it is best to find out in as much detail as possible what the situation is on individual farms specifically with regard to the status of roundworm resistance.

To check for dose resistance in a flock, do a Faecal Egg Count Reduction Test (FECRT).

• Take a pooled faecal sample from about 5 marked goats. Keep the faecal sample in the fridge and get your vet do a FEC (faecal egg count) within 48 hrs. 
• Dose with an anthelminthic if indicated, recording the product used.
• Take another sample from the same marked goats 10-14 days later and have a FEC done.

Any roundworm eggs that are found in this second sample must have come from worms that survived the anthelminthic dose as it takes at least 21 days for ingested infective L3 stage larvae to start producing eggs.

Causes of Resistance Development and How These Can be Avoided

• Excessive dosing.  Avoid this by doing regular FEC and dosing only when indicated.
• Repeated use of the same active ingredients. There are 12 different drug groups with variations within each group. To simplify matters for the farmer, the different drug groups have been given numbers which are displayed on the container.  A different drug trade name does not always mean the actives are different so check these numbers to ensure you do not repeatedly use the same drugs.

• Under dosing.  Dose according to the heaviest animal in the group. Check with your vet or drug company representative vet whether the off-license dose for goats is higher than that for sheep.  Off licence – Most doses must be given at 1.5 times the dose that is recommended for sheep due to the difference in the metabolism of the goat. Check with your vet or drug company.
• Quarantine newly purchased stock and dose all animals with an effective “clean-out “dose. Do a FECRT 10-14 days after the quarantine dosing to ensure you are not introducing some other farm's ‘super- worms’ onto your farm.
• Combination drugs have been proven to be slower in developing anthelmintic resistance than single drugs.
• Long acting drug formulations. These will cause rapid development of resistance in the same way that repeated dosing with the same active ingredient will do.
• Dosing during winter. Few eggs and larvae survive cold dry winters outside the goat. This means that any eggs passed by the flock after a winter dose are from resistant worms which have survived dosing and this increases the resistant proportion of the worm population on the pasture
• Dosing the whole flock and moving them onto a new pasture. This has the same effect of increasing the resistant proportion of the worm population as with dosing in winter and should therefore naturally be avoided.

Further Management Strategies- “Refugia”

 “Refugia” has proven to slow down the development of anthelminthic resistance by maintaining an anthelminthic-susceptible worm population on the veld/pasture. Studies indicate that there needs to be at least a 10 fold dilution of resistant eggs to maintain the effectiveness of a dose in a flock.

If an anthelminthic dose is 99 % effective then 10% of flock needs to be left untreated. This rises to 34% if a dose is only 95 % effective, in order to maintain a 1:10 ratio. This can be achieved by TST (Target selected Treatments) where not all animals in the flock are dosed.

  (i) The FAMACHA dosing system is a good way of implementing this and maintaining “refugia” and a means of identifying and culling animals that have a poor worm resistance.

(ii) Under certain circumstances such as kids on pastures where a farmer feels the need to dose all the animals, the dilution of resistant worms is maintained by running untreated adult goats with the kids. Adults are considered worm neutral when running with kids as they remove larvae and deposit eggs at an equal rate.

Improving  the  immunity of the Angora Goat

Immunity to roundworms varies between individuals so only a small part of the flock is often infested with most of the worms.

Innate immunity in an individual animal is the inherent ability of that animal to resist infection with worms and is genetically predetermined and inherited.

Acquired immunity develops after goats have been exposed to worms. Adult goats therefore have a better acquired immunity whereas kids until 6-8 months old have very poor acquired immunity and are reliant on their genetic status.

Immunity to worms affects worm burdens in 3 stages

1.                     I.                        Fewer incoming worm larvae establish and become adults
2.                   II.                        Established female worms produce fewer eggs
3.                 III.                        Established adult worms are rejected by the goat.

The immunity of individual animals can be influenced by multiple factors. Normally about 5% of infective L3 stage larvae establish as adult worms but this increases to 30-40% during late pregnancy and lactation when ewes are under stress and their natural immunity declines.

• Genetics: In Australia the focus has long been on breeding livestock for resistance and on merino stud farms where selection criteria are used for worm resistance a 6.8 fold improvement in worm resistance has been achieved since 1998. This has also reflected in the immunity of the lambs developing by 3 months rather than 6-8 months of age. Rams which are shown to have a higher immune status are also fetching a premium price at sales.

Selection and breeding for wireworm resistance on a Dohne merino stud farm in Stutterheim district has resulted in in the lambs from selected sires having FEC of 500 epg where the average of the group was 11 000 epg.

• Nutritional plane: Nutrition is vital in maintaining a good immune system and many studies demonstrate the much better immune response by heavier lambs to worms.
• Selection for certain production traits: This may have inadvertently led to decreased immunity in some animals. It is interesting to note that fibre diameter (fineness) has been correlated to higher worm burdens (poorer worm immunity) in sheep so we would expect the same would be likely to apply in goats.
• SAMGA ‘s Involvement:  SAMGA have done a trial to evaluate  the feasibility of implementing an index system to aid selection for worm resistance in Angora goat veld rams. The index is based on on FEC and FAMACHA scores..
• Vaccine: A future possibility for improving immunity is the development of vaccines. This has occurred in Australia where a registered vaccine against Heamonchus (wireworm) is available. The vaccine has been effective in reducing egg counts by 95% but the down side is that the vaccine has to be made from wireworm harvested antigens rather than manufactured antigens, so mass production is a problem. The other issue is that the vaccine has to be repeated every 5 weeks during summer.

• Fungal Control: Grootfontein is also investigating fungi as a biological control against the free living stages of wireworm.

Conclusion

There are no new drugs scheduled for introduction to the market in the short term. Due to the economic impact of roundworms this means the focus must be on management strategies of avoiding drug resistance and improving the immunity of the Angora goat..