July seasonal focus

Clean pastures. Keeping sheep, goats and cattle healthy relies on minimising the amount of worm larvae they pick up from the grass. At this time of year, it is a good idea to look at the mechanisms in place to reduce pasture larval contamination (PLC) to ensure that the most susceptible animals are given priority pick. This includes pregnant ewes and does, from the few weeks before giving birth through the first twelve weeks of lactation as well as young stock, particularly weaner calves, lambs and kids. 

Creating ‘safe’ paddocks for lambing/kidding and weaners relies on knowing or estimating the pasture larval contamination (PLC). This is one aspect of livestock management for which we don’t have adequate diagnostic tools.

1. The most accurate technique to determine PLC is called ‘pasture larval counts’. Multiple samples of grass, cut to ground level, are taken from a variety of places around the pasture. These are then taken to the lab, washed, filtered and processed so that the larvae are concentrated in a collecting flask and then counted. The grass is then weighed, dried, weighed again to calculate moisture percentage and the number of larvae per kilogram of dry matter of pasture (L3/kgDM) is calculated. Several parasitology laboratories have conducted this process for research, but due to the expense, none in Australia currently offer it for commercial clients.
2. PLC can be estimated using the known worm egg counts (WEC) of stock that were previously in the paddock. For example, a sheep with a WEC of 100 eggs per gram (epg) produces an estimated 1kg of dung per day, so its total egg production will be 100,000 eggs per day. Assuming 50% of these eggs are successful in completing development to the infective (L3) stage, each day’s output equates to 50,000 new larvae on the pasture. A mob of 100 sheep will create 5 million fresh larvae per day, which distributed over a 10 hectare pasture that has 1,000 kg dry matter per hectare, means a PLC of 500 L3/kgDM.
3. If the mob graze the paddock for 30 days, they’ll create 150 million infective larvae which will add to the PLC. However, the numbers of larvae will decline at a rate that is proportional to temperature. Therefore, the actual PLC at any given time depends on the WEC and the time the mob spends in the paddock, versus the time and conditions that control decline (temperature, ground cover, moisture) in the days and weeks since they started grazing then were moved from the paddock.
4. Computer modelling has been conducted to attempt to gain an accurate measure of the PLC. This is important for farm economics, because as well as affecting the health of sheep and lambs, growth rates of weaners are affected by the PLC. A recent open access journal article by Filipe et al. (2023), in International Journal for Parasitology showed the complexity of interactions that leads to these estimates. 
5. Any estimates of PLC relies on how well the tests describe flock WEC (e.g. WEC may fluctuate between sheep and over the period of time sheep are in the paddock), carry-over contamination from previous grazing, the amount of infective worm larvae that develop from eggs and conditions that affect larval survival on the grass.

Weaner calf management. As well as providing low-risk pasture with low PLC, it is important to protect young cattle from worms until they develop their own immunity (resistance). This ‘acquired immunity’ develops at a different rate for the various worms. For example, after 9 months of constant exposure to small intestinal worms (Cooperia), calves develop immunity that limits worm development. However, this comes at a cost and the immune stimulation caused by the worms themselves may restrict growth rates. On the other hand, immunity against small brown stomach worms (Ostertagia) is very slow and disease due to this worm may be seen even in adult cattle.

Choosing a weaner drench is critical as weaners already have to deal with the social stress of weaning, nutritional stress from dietary changes and often have inappetence and lose weight in the process. The weaner drench needs to be effective against the worms that the weaners have picked up to clean them out, allow their intestinal lining to recover and ease the pressure from inflammation of the gut wall that accompanies worm infections.

Be sure to consider drench resistance when choosing a weaner drench. If you don’t know how well the drench works, do a DrenchCheck to find out.

Cattle ticks. Prepare for the spring rise in cattle tick numbers by planning ahead. The spring rise in ticks causes heavy contamination of pastures that may force treatments to be done to keep cattle healthy.

As well as using genetics and paddock management, the spring rise can be beaten with two approaches to strategic treatment.

• Long-acting treatments given at the start of the tick season, usually sometime in September for southern Queensland. This can be an injectable moxidectin product, or a tick development inhibitor (TDI) such as fluazuron, both preferably given with a knockdown treatment.
• Repeat short-acting treatments given 21 days apart.

Either of these regimes will break the tick life cycle and prevent build-up of large numbers of tick larvae on pasture, meaning the second half of the tick season should have much lower tick pressure.  Knockdown treatment. Make sure you consider the impact of a treatment on non-target parasites when applying a tick treatment. For example, if using a long-acting moxidectin injection to control ticks on cattle, this will also expose worms in the gastrointestinal tract to the chemical. Best practice is to use an effective treatment with a different class of active ingredient for worms, e.g levamisole or benzimidazole at the same time as the injectable to complement the moxidectin and minimise selection for resistant worms.

Similarly, fluazuron treatments on their own may not be effective against ticks in all areas. Using a knockdown treatment e.g. amitraz dip as a knockdown, then applying fluazuron to dry cattle later will have a better long-term outcome compared to just using fluazuron alone.

Cattle tick resistance testing. Many areas in Queensland already have deeply established resistant ticks, with synthetic pyrethroids, organophosphates, amitraz and fluazuron resistance being regularly detected. However, the only way to know which treatments still work on your property is to do a tick resistance test, available through the Department of Agriculture and Fisheries of the Queensland Government.

This simply involves collecting 30-60 mid-size adult ticks from the cattle, (avoiding the fully-engorged ones as they will  start to lay eggs almost immediately after collection). Place them in a plastic container e.g. a takeaway food container then place a few blades of grass inside. Cover and submit to the laboratory. Your District Veterinarian or Biosecurity Officer will assist with all details of the test including submission.

Liver fluke. August is the traditional time of year to give a strategic second treatment of flukicide to stock (cattle, sheep or goats), or to give an adulticide if only one treatment is planned for the year. This is because the liver fluke life cycle relies on freshwater snails. The snails are very sensitive to temperature changes and will not breed or be metabolically active when temperatures are below 10°C or above 26ºC. This means that two of the major ‘multipliers’ of liver flukes are not active during mid-summer (too hot) or winter (too cold).

The major multipliers of liver fluke on a property are:

1. Snail reproduction– under ideal conditions of 22-26ºC, a single snail can lay 3,000 eggs per month.
2. Fluke ‘cloning’ (asexual reproduction in the intermediate host)- a single liver fluke intermediate stage, known as the ‘miracidia’, can enter the skin of the snail and invade the tissues, then effectively ‘clone’ itself up to 3,000 times (the ‘redia’ stage), again when temperatures are warm enough (22-26ºC ) for the snail to be metabolically active.    
3. Adult liver flukes are hermaphroditic and can have long lives, up to several years, in the liver of the host. A single fluke in a sheep can lay 20,000 to 50,000 fertile eggs per day.

Because 2 of the 3 ‘multipliers’ do not take place in winter, stock at the end of winter usually only harbour adult fluke and not immature fluke.

If you are considering a liver fluke treatment, be aware that resistance to the common flukicides is common. Check that your fluke treatment has worked by conducting a second fluke test 30 days after treatment. Winter is a good time of year to test your fluke treatments as there are few immature fluke that can confound testing results in other seasons.

For a summary of available liver fluke treatments for Australian livestock, see the table in the May 2024 Boss Bulletin

Resistance mechanisms in liver fluke Fasciola hepatica

1. Albendazole (ALB) at 7.5mg/kg in artificially-infected lambs was shown to work at 94% efficacy against Triclabendazole (TCBZ)-Resistant Fasciola hepatica  (Coles and Stafford 2001). Nitroxynil (a component of Nitrofluke along with clorsulon, 100%) and oxyclozanide (the flukicide component of Nilzan, 99.6%) were also shown to work against 12-week old (adult) fluke. In this trial TCBZ had 0%, clorsulon alone 73.2%.
2. Resistance to TCBZ in liver fluke suggested to take 3 forms (Fairweather et al. 2020):
   • Changed beta-tubulin structure (altered drug receptor)
   • Reduced drug uptake
   • Drug metabolism- pgp-linked drug efflux pumps
3. TCBZ Drug absorption thought to be mainly across integument rather than ingestion. Clear differences in uptake of TCBZ between Resistant and Susceptible populations of Fasciola, but ALB had same absorption in both. R flukes had equal fitness (PPP, egg-laying) as S isolates (Brennan et al. 2007).
4. Combination products suggested as likely means of overcoming resistant fluke.
5. Fasciola hepatica with resistance to ALB but susceptibility to TCBZ has been described.
6. Dawbuts (Camden, NSW)  lab results show sheep farm with liver fluke resistant to both albendazole and triclabendazole (unpublished results). Producer feedback also indicates resistance to closantel is common in NSW (unpublished results).

References

Brennan, G. P., I. Fairweather, A. Trudgett, E. Hoey, McCoy, M. McConville, M. Meaney, M. Robinson, N. McFerran, L. Ryan, C. Lanusse, L. Mottier, L. Alvarez, H. Solana, G. Virkel and P. M. Brophy (2007). “Understanding triclabendazole resistance.” Exp Mol Pathol 82(2): 104-109.

Coles, G. C. and K. A. Stafford (2001). “Activity of oxyclozanide, nitroxynil, clorsulon and albendazole against adult triclabendazole-resistant Fasciola hepatica.” Vet Rec 148(23): 723-724.

Fairweather, I., G. P. Brennan, R. E. B. Hanna, M. W. Robinson and P. J. Skuce (2020). “Drug resistance in liver flukes.” Int J Parasitol Drugs Drug Resist 12: 39-59.

Photos are courtesy of Dawbuts Pty Ltd.