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5 Reasons To Approach Worm Prevention the Natural Way: A Vet’s Perspective

Nobody wants their dog or cat to be riddled with gut worms. It’s yucky; it can cause weight loss and pain and can lead to gut damage and even death.

Dr Nick Thompson

Author: Dr Nick Thompson
BSc (Vet Sci) Hons, BVM&S, VetMFHom, MRCVS

5 Reasons To Approach Worm Prevention the Natural Way: A Vet’s Perspective

The conventional approach is straightforward and, on the face of it, logical and practical. Drops on the scruff or tablets to keep the dog or cat worm-free are ubiquitous. Join the local vet practice ‘health plan’, and they’ll give you the products at discount prices every 1-3 months. What’s wrong with that? 

Nothing. But is it the optimal way to ensure vitality and a long, healthy life? Answer: no one knows. I know of no studies where a population of, say, dogs on a life-long regime of wormers has been compared to another comparable group eating the same food, with similar lifestyles and other factors (vaccines, antibiotic use and neutering status, for example) but using minimal worm products. 

So, I ask you to follow me on a journey of logic and reason to see if we can determine the best approach to healthy, gut-clean pets. 

I favour a minimum pharmaceutical system. Note – this is not a zero-drug approach. Sometimes, in certain individuals, we use the fantastic technology that represents modern wormers. But in my practice, these times are rare. Here are five reasons why I take this line of treatment:

Microbiome Disruption

All drugs disrupt the gut microbiome. It used to be called the gut flora, but that’s now out of favour because ‘flora’ generally refers to plant life. The bugs in your gut are not plants, so ‘microbiome’ is now considered a more accurate description of the gut’s bacteria, protozoa, archaea, yeasts, fungi and microscopic worms known as helminths.

It’s generally agreed that all the bugs above work together, like an orchestra, to provide nutrients, help balance gut health and modulate the pet’s immune system. 

In humans, one study showed that patients infected with helminths experienced fewer symptoms of multiple sclerosis (MS) than those not. Other results suggest that certain parasitic species could reduce the symptoms of Irritable Bowel Syndrome (IBS), Crohn’s disease, Type 1 diabetes and even arthritis.

It stands to reason that anything that disrupts the balance of all these different bugs could have negative health impacts, even if long-term and subtle. 

Antibiotics disrupt all the various bacteria. They can also prejudice the health of the other small non-bacterial elements of the gut microbiome, like yeast and fungi. 

But wormers are not generally classed as antibiotics. They are, however, ‘antimicrobials’. Antimicrobial drugs include antiseptics, antibiotics, antivirals, antifungals and antiparasitics. All can disrupt the gut microbiome. 

Studies in dogs reveal that using wormers, even those dropped onto the pet’s scruff, significantly impacts the microbiome. Most dogs and cats can cope with one or two insults like this, but if the manufacturers recommend using these tablet or drop-on products every one to three months for life, it can be safely assumed that this will significantly disrupt gut function. 


All drugs are toxic. All vets and doctors accept this. We use them where the benefits generally outweigh the risks. Taking paracetamol for a headache is not very risky, but everyone has some risk. Chemotherapy for cancer, on the other hand, is at the other end of the scale, where oncologists have to carefully manage the toxic effects of these drugs against any therapeutic benefits.  

Most wormers are considered relatively safe at normal dose ranges. 

Bravecto, Nexgard, and Simparica are UK-brand examples of the isoxazoline class of antiparasitic antimicrobials. They are in tablet form. Bravecto is given every 8-12 weeks. Nexgard and Simparica have lower concentrations of active drugs and are recommended monthly for the treatment of fleas, ticks, worms and mites.

In 2019, the FDA (Federal Drug Administration) in the USA singled out isoxazoline drugs as neurotoxins in an online warning announcement. They cautioned, ‘Isoxazoline products have been associated with neurologic adverse reactions, including muscle tremors, ataxia [incoordination], and seizures [fits] in some dogs and cats’. 

My approach to using ANY pharmaceutical wormers is one of caution. I will use them (not the isoxazolines) only as a last resort or where pet or human welfare may be compromised.

Good Medicine

I always tell my clients that ‘the best vet is the vet you don’t need to see’. Equally, the best medicine is the medicine you don’t need to take. 

It stands to reason that you wouldn’t give an antibiotic to a perfectly healthy pet. And yet, we are encouraged by big pharma companies to use their antiparasitics repeatedly, whether the patient needs it or not. This, to me, is ridiculous, dangerous and incurs unnecessary costs. 

In equine medicine, we’ve used faecal worm counts for 50 years to assess which horses need worming and which don’t. Anthelmintic resistance is a big deal in the horse world. It’s considered less so in pets, but all world governments consider the march of antimicrobial resistance to be a significant threat to human and animal health in the future. 

I recommend that my clients do regular (every 1-3 months) faecal worm counts on their pets, especially cats that hunt. At my practice, we send samples to or feclab and get results by email within days. Having done this testing regularly on dogs for a decade, I conclude that 95% of adult dogs have no detectable gut or lungworms. 

Yet, in conventional practice, most pets get wormers when only about five per cent might need them. Can you see how topsy-turvy this is? Worm counts are very economical and are paid for by the pharmaceutical wormers you can avoid. 

Not only this, but you’re saving the pet’s gut and gut biome from regular life-long insults. What’s not to like?


Dog Ecology

In his 2022 antimicrobial review [1], Patrick Di Martino states, ‘Every year, several thousand tonnes of antimicrobials and their by-products are released into the environment, particularly into the aquatic environment. 

‘This type of xenobiotic [a substance, typically a synthetic chemical, that is foreign to the body or an ecological system] has ecological consequences in the natural environment but also in technological environments such as wastewater treatment plants and methane fermentation sewage sludge treatment plants. 

‘The constant exposure of microbial communities to high and sub-inhibitory concentrations of antibiotics is a key element in the development of antibiotic resistance in aquatic environments and soils. 

‘The future of antimicrobials lies in the development of bio-sourced or bioinspired molecules. The observation and deciphering of interactions between living organisms is the key to this development’.

In simple terms, this means the more antimicrobials we pump into waterways across the globe (bathing a treated dog, for example), the more we disrupt the ecology of the land, water and sea and promote the potentially deadly consequences of a future without antibiotics and wormers for the human and animal world.

Idiosyncratic Adverse Reactions to Worming Tablets

Charlie, a five-year-old Vizsla, an entire male, was brought to see me a few months ago. He was ill (vomiting, regurgitation, and loose stool) with every worming product they tried: Milbemax, Drontal Plus, Droncit, Bravecto, and Nexgard. The owners and even the vet were at their wit’s end as to what to do. 

Charlie’s situation is pretty unusual- or is it? Several pets feel ill after their monthly worming dose or tablet. Owners are either unaware of this side effect or accept that it’s the price they must pay to keep the dogs clean and healthy. 

I explained my approach to Charlie’s owners and the reasoning behind a minimum pharmaceutical, maximum vigilance approach, and they were delighted. We’ve been worm-counting Charlie for the last two years. In that time, he’s never yet had a positive worm result. His gut is stable, and he’s been at the peak of good health ever since. We’re all chuffed!

Learn more about taking a natural approach to worm prevention in ProDog’s expert guide.


When treating pets for internal parasites, less is most definitely more. We have a wealth of technically beneficial compounds to keep our animals worm-free. But if we concentrate on general and microbiome health, we may be able to avoid them all. 

Sidestepping these antiparasitics benefits the world, you and your family, your pet’s microbiome, and most of all, your dogs and cats.


1. Di Martino P.(Jan 2022) Antimicrobial agents and microbial ecology. AIMS Microbiol. 6;8(1):1-4. PMCID: PMC8995183.

Image credit Jamie Street on Unsplash

Image credit  Ivan Louis on Unsplash

Dr Nick Thompson

BSc (Hons) Path Sci., BVM&S, VetMFHom, MRCVS. Founding President of the Raw Feeding Veterinary Society. Petplan Vet of the Year Nominee 2009, 2015, 2017, 2018 & 2020. The practice of the Year Nominee 2018.


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