Animals have had to coexist with parasites since they first walked the face of the earth; under natural conditions parasitic burdens are low. However, since domestication modern animals which are now farmed intensively particularly on small farms suffer from increased parasitic burdens. Concern has increased over the regularity and severity of worm infections, resulting in large agricultural production and economic loses. In more recent times, worm and drench resistance has become a major issue facing small farmers.

A background to worms in farming

What worms do cattle, sheep and goats suffer from?

The vast majority of worms are species specific to the animals they inhabit. Ruminants comprise the vast majority of farmed grazing animals and therefore suffer from the greatest number of worms. These include; 1) Nematodes (roundworms) 2) Tapeworms and 3) Liver fluke.
 
1) Nematodes

Nematodes (roundworms) are, by far, the most significant parasite infecting ruminants, and can cause considerable economic losses. Examples of nematodes in cattle, sheep and goats include roundworm (Ostertagia sp.), barbers pole worm (Haemonchus sp.) and black scour worm (Trichostrongylus sp.).

It is important to understand how the lifecycle of the worm works before looking at which management strategies are best to control it. Figure 1 illustrates the lifecycle of a nematode. 

Figure 1: Lifecycle of a Nematode1

The male and female worms live inside the stomach or intestine of the animal. Here the female produces a large number of eggs which are then passed out in the faeces. These eggs, under the right environmental conditions, then develop into infective larvae on the pasture. Infective larvae are then eaten by the grazing animals and once ingested develop into adults in the stomach or intestines.

What environmental conditions favour nematodes?

The development of infective larvae on pasture depends largely on environmental conditions. Larvae require adequate oxygen, water (moisture) and temperature. Table 1outlines those conditions which are most favourable for the development of eggs into infective larvae.

Table 1: Environmental conditions which favour development of eggs into infective larvae.

With an understanding of the lifecycle and environmental conditions favouring worm development, management practices can be implemented to reduce these burdens.

How can I manage nematodes?

Pasture management 

• Light or lax grazing results in lower larval intake and reduced faecal contamination of pasture. Intensive grazing will increase larval intake and increase pasture contamination.
• Most larvae live deep in a pasture, so graze animals on taller pastures to reduce larvae intake.
•  Irrigation will increase moisture levels in the pasture aiding in larval development.
•  Harrowing pastures in wet conditions speeds the spread of larvae and in dry conditions aids in desiccation and destruction of larvae.
•  Rest pasture over summer to allow time for larvae to desiccate and die. Over winter larvae survive for longer, therefore reducing the effectiveness of resting pasture at this time of year.
• Make hay and silage - this exposes pasture to dryer conditions, killing residual larvae.
•  Use crop and pasture rotations - the use of cultivation and resting paddocks for long periods of time are highly effective in reducing larvae numbers.
• Graze pastures with adult animals; this is effective as adult animals have a greater resistance to nematodes.

Animal choice

• Graze pasture in a rotation between cattle and sheep (not goats and sheep) - this will reduce larvae on pastures as nematodes are species specific.
• Each animal develops its own host resistance to parasites once it reaches six months of age.
• Young animals: lambs, calves and kids are more susceptible to worm burdens.
• Certain breeds of animals have more resistance to parasites than others, e.g. Bos-Indicus cattle and Dorper sheep.
• Fe  male animals are highly susceptible to worm burdens in late pregnancy and early lactation.

Drenching

• Before drenching for worms you should conduct a WormTest to identify the number and type of worm eggs present. Sample kits are available from most rural stores, veterinarians and the Department of Primary Industries (DPI).  Follow the WormTest link for more information on sampling and a guide to worm egg counts.
• Drenching is the use of drugs or anthelmintics to remove adult worms from the host animal.
  Examples of drenches include;
  1) Benzimidazoles, e.g. fenbendazole
  2) Imidothiazoles, e.g. Levamisole
  3) Avermectins/milbemycins, e.g. Ivermectin.
  
  Most drenches are short acting, therefore if after drenching the animal remains on infected pastures it will start  to be reinfected with larvae; subsequently increasing its worm burden. If drenching was the only tool used to control worms, animals would need to be drenched every two weeks. However, because most animals are drenched and then moved to clean pastures, drenching is only necessary at strategic times. For example:
  
  1) Pre lambing: Ewes due to lamb are drenched and then moved to a clean paddock for lambing.
  2) Weaning: Lambs or calves are drenched and then moved to a clean, well rested paddock. A further drench and move to another ‘safe’ pasture may be required in late summer.
   
• For regional drenching programs, click on the link to Wormkill (Northern NSW), Westworm (Western NSW), Drenchplan (Southern and Central NSW), and FarWestWorm (Far Western NSW).

2) Tapeworms
 
Tapeworms are regularly found in ruminants, but unlike nematodes rarely do they cause production losses. However, many tapeworms can cause cysts in an animal’s muscles which results in carcase condemnation. To complete their lifecycle, tapeworms require another animal as an intermediate host. There are several types of tapeworms which may infect ruminants, such as, Taenia saginata, Moniezia sp., Taenia hydatigenia and Taenia ovis.

The tapeworm Echinococcus granulosus is of particular consequence to both animals and humans as it can cause a disease called hydatidosis. This disease is caused by the tapeworm infecting the small intestine of dogs and the process initially requires the development of cysts in the body of sheep and cattle. Cysts can also develop in humans if eggs from infected dogs are accidentally ingested.
The lifecycle of the ‘hydatid’ tapeworm can be seen in Figure 3.

Figure 3: Hydatid Lifecycle2

How can I manage tapeworms? 

• Interrupt the hydatid lifecycle by with regularly worming dogs with ‘praziquantal’, which is available in most wormers.
• Cook all meat thoroughly.
• Avoid feeding pets or working dogs, sheep or cattle offal.
• Prevent dogs from scavenging on dead animal carcases.
  
  3) Liver fluke

Liver fluke (Fasciola hepatica) is an important parasite impacting on ruminants and other mammals in Australia. It primarily infects the liver of affected animals by living and developing in the bile ducts. Liver fluke requires fresh water snails as an intermediate host to complete its lifecycle (Figure 4). Due to this host requirement, the disease is only seen in animals which graze near swampy or wet areas.

Figure 4: Liver fluke lifecycle3

How can I manage liver fluke? 

•  Pasture management - fencing off swampy areas of the farm and supplying water via troughs.
• Using drenches - these are readily available (benzimidazoles and imidothiazoles).
• Drenching and moving to cleaner pastures, thereby reducing prospective infection.

Where to from here?

Visit the link Pathology and Internal Diagnosis of Internal Parasites in Ruminants.
 

Author: David Woodward BVSc. (honsII). MRCVS (Veterinarian)

Notes
1. Collins, Dr. Henry (2001) Veterinary Parasitology 4
2. Collins, Dr. Henry (1999) Veterinary Parasitology 1
3. Collins, Dr. Henry (2001) Veterinary Parasitology 4

Glossary

•  Ruminant: Cud chewing animal with four stomachs.
• Nematode: Internal parasite that lives in the intestine and stomach causing production losses such as reduced weight gain and infertility.
• Tapeworm: Internal parasite which causes cysts resulting in carcase condemnation.
• Liver fluke: Internal parasite which affects the liver. Common in wet areas.
• Drenching: Oral application of drugs to remove adult worms.