When should you deworm cattle and buffalo?

The short answer, from Thailand's National Institute of Animal Health (NIAH) at the Department of Livestock Development (DLD): deworm calves at three weeks of age and again at six weeks, then deworm animals six months and older twice a year. Where liver fluke is a problem, give a fluke treatment in August–September and again in March–April.

It's early March, which is the second of those fluke rounds. Below is the schedule age by age, what each parasite does, and why deworming more often than that can make the drugs stop working.

LIVER FLUKE CYCLE                                               
                                                                
cow/buffalo                       eggs in dung                  
             ●──────────────────────────────┐                   
flukes in    ·                              │                   
bile ducts   │                              │ eggs hatch        
             │                              │ in water          
             │                              │    ~ ~ ~ ~ ~      
eaten        │                              │                   
with grass   │                              │                   
             └──────────────────────────────┘ inside a          
               young flukes on wet grass      pond snail        
               |   /   /   /   /   |   |   |    @_              
                                                                
fluke areas: dose months                                        
       J   F   M   A   M   J   J   A   S   O   N   D            
               ▲   ▲               ▲   ▲                        

Which internal parasites are common in cattle and buffalo?

Two groups matter most: roundworms in the gut, and flukes (liver fluke and rumen fluke). NIAH says the important parasites of cattle and buffalo are mostly roundworms.

Roundworms

• Ascarids in young calves. NIAH says these are passed from the dam through the placenta and the milk, and cause high mortality. A February 2025 study in Veterinary World on Toxocara vitulorum in buffalo describes dormant larvae in the dam reaching the calf through colostrum and milk, with infection peaking in the first two days after birth.
• Nodular worms, hookworms and stomach worms, such as Oesophagostomum, Bunostomum, Haemonchus and Mecistocirrus. These feed on blood and are most dangerous in cattle six months and older. Animals pick them up by eating infective larvae on grass.

Housed animals are not safe from them either. NIAH explains that in the morning, before the sun gets strong, larvae climb to the tips of the grass, and animals that never go to pasture can still be infected through cut grass carried to the pen.

Flukes

• Liver fluke (Fasciola gigantica). The adult is 30–55 mm long and lives in the gall bladder and bile ducts. Its eggs leave in the dung, hatch in water, develop inside a pond snail (Lymnaea rubiginosa), then leave the snail and settle on grass and water weeds, where the next animal eats them.
• Rumen fluke. A DLD survey in the Pak Phanang river basin, Nakhon Si Thammarat, from 2003 to 2005 examined 2,898 cattle dung samples and found rumen fluke in 91.75% and liver fluke in 8.18%, with liver fluke present in every district surveyed.

What are the signs of worms in cattle and buffalo?

Loss of condition, slow growth, pale membranes and long-running diarrhoea. For gut roundworms NIAH lists weight loss, anaemia, chronic diarrhoea, swelling under the jaw and slow growth.

Liver fluke takes two forms:

None of these signs is specific. A thin animal may be short of feed, have bad teeth or another disease. That's why I don't think "she looks thin, drench her" is a good rule; check the dung first (last section).

When should calves first be dewormed?

At three weeks, then again at six weeks. NIAH recommends dosing every calf at three weeks of age and repeating at six weeks.

The reason is the ascarids from the dam. Waiting until the calf is a few months old and dosing it with the herd may be too late. The Veterinary World study describes buffalo calves of untreated dams with severe diarrhoea, colic and emaciation, and notes that in earlier observations on the same farm some calves had severe diarrhoea and died.

Step by step for calves

1. Write down each calf's birth date and work out the dates at three and six weeks.
2. Weigh the calf before dosing. Every drug in the DLD's table is dosed by body weight.
3. Use the product your vet or district livestock office recommends for calves. I've left doses out of this post on purpose; follow the label of the product you actually use.
4. Record the drug, the date and the weight in that calf's history.

How often should older cattle and buffalo be dewormed?

Twice a year from six months of age. NIAH gives the same advice on its gut roundworm page and its rainy-season disease page. Dairy cows should be dosed during the dry period.

The DLD puts parasites first on its list of diseases seen at the start of the rainy season. If I were choosing the months myself, I'd put one round just before the rains. That's my reading of that list, not a month the DLD sets.

When should you treat for liver fluke?

In areas with liver fluke, treat animals eight months and older twice a year: August–September, then March–April the following year, according to NIAH.

The two rounds do different jobs. The August–September round is there to reduce infection in the snails, by cutting the fluke eggs going out in the dung while water and snails are plentiful. The March–April round lets the animals recover and regain condition.

Not every dewormer kills liver fluke. The NIAH page keeps a separate list of drugs that act on liver fluke, such as triclabendazole and closantel, and warns that these drugs must be given at the set dose or they may harm the animal. Tell the vet plainly that you want a fluke treatment when you buy.

Cutting fluke without drugs

From the control section of the same page:

• Keep animals off grass and water plants in places known for liver fluke.
• Drain pastures so water doesn't stand for long; standing water is where the snails live.
• Check dung samples at least once a year.

Does deworming too often make the drugs stop working?

Yes. When the same drugs are used often, on every animal at once, the worms that survive treatment make up more and more of the population until the drug no longer works. This is anthelmintic resistance.

In Thailand it is already well documented in goats. A 2022 study in Veterinary World tested 29 meat goat herds in In Buri district, Sing Buri, and found every herd resistant to albendazole and ivermectin, with levamisole resistance in two herds. The researchers also found that about 36% of farmers gave ivermectin above the label dose. That is goat data, not cattle or buffalo, but the drugs are the same families used in cattle.

A 2001 review by J.A. van Wyk in the Onderstepoort Journal of Veterinary Research argues that refugia matters more than reducing drenching frequency or avoiding under-dosing. Refugia means the part of the worm population that is not exposed to the drug in a given treatment: worms in untreated animals and larvae out on pasture. Because those worms haven't been selected by the drug, they keep resistant worms from taking over too quickly. The same review warns of too much complacency about resistance spreading to cattle.

What I take from this:

1. Don't deworm more often than the DLD schedule unless a dung test says so.
2. Dose by actual weight. Don't guess, and don't add "a bit extra".
3. Ask the vet whether the whole herd needs treating, or only the animals whose tests or condition show a heavy burden.

What should you check before deworming?

Test the dung to find out which parasites are there, and weigh the animals so the dose is right. The DLD finds roundworm eggs by salt flotation and liver fluke eggs by sedimentation, so a result tells you which kind of drug to use.

Checklist before deworming day

• Collect fresh dung from a few animals and ask the district livestock office where to send it.
• Weigh every animal that will be dosed.
• Ask the vet which drug acts on what the test found, especially if it found liver fluke.
• Plan dairy cows' treatment for the dry period, because the drugs can leave residues in milk.
• Record the drug, date and weight for each animal, so you can see which drug has been used how many times.
• If an animal is still thin, pale or scouring after treatment, test again instead of dosing again straight away.

The schedule on one page

Sources

• NIAH, DLD: Important animal diseases in the rainy season (Thai)
• NIAH, DLD: Important gastro-intestinal nematodes (Thai)
• NIAH, DLD: Fasciolosis (Thai)
• Ruchirat Worasing (2007): Gastrointestinal and blood parasites of cattle in the Pak Phanang river basin, Nakhon Si Thammarat (Thai), Thai-NIAH eJournal 2(1)
• Veterinary World (February 2025): Targeted pre-partum strategies to suppress Toxocara vitulorum hypobiotic larvae
• Ratanapob N, Thuamsuwan N, Thongyuan S. (2022): Anthelmintic resistance status of goat gastrointestinal nematodes in Sing Buri Province, Thailand, Veterinary World 15(1)
• van Wyk J.A. (2001): Refugia: overlooked as perhaps the most potent factor concerning the development of anthelmintic resistance, Onderstepoort Journal of Veterinary Research