There is a wide range of helminths (parasitic worms) including roundworms (nematodes), tapeworms (cestodes) and even flukes (trematodes) that can infect dogs and cats in Europe. Commonly we hear about intestinal worms such as Ascarids (roundworms), Tapeworms, Hookworms and Whipworm.
Then we also have non-intestinal worms. And in the UK the most talked about is Lungworms, or French heartworm (Angiostrongylus vasorum). So there are lots of them out there – but are we over-treating?
Table of contents
Meet the suspects…
Roundworm
The common roundworm Toxocara canis is a large, intestinal nematode. The adults can measure as much as 15cm in length that can cause clinical disease in young dogs. Similarly, Toxocara cati, an intestinal nematode with adults measuring up to 10 cm in length, can cause disease in young cats. Toxocara infection can occur in puppies and kittens but also in older dogs and cats. Infection of humans can occur as a result of accidentally ingesting infective eggs or eating undercooked meat containing larvae.
Tapeworms
Echinococcus granulosus (dog tapeworm) is a small cestode that inhabits the small intestine of dogs and some other canids, excluding foxes.
Dipylidium caninum is a tapeworm of dogs and cats and is common throughout Europe. The intermediate hosts are the flea or louse; dogs and cats become infected when they ingest these infected hosts. The adult tapeworm develops within the dog or cat in the small intestine, thankfully infection with D. caninum is rarely associated with clinical signs in dogs and cats. Occasionally, the mature segments leaving the anus (yuck) may result in anal irritation (pruritus) causing an animal to rub its bottom along the ground!
Taenia are a family of tapeworms that can infect dogs, cats and foxes by the ingestion of intermediate hosts; they are common throughout Europe. Dogs and cats become infected when they eat the tissue or viscera of infected intermediate hosts. The intermediate hosts are varied and, depending on the precise Taenia species, range from sheep and cattle (Taenia multiceps) to rabbits (Taenia serialis, Taenia pisiformis), rodents (Taenia taeniaeformis), ruminants and pigs (Taenia hydatigena) and sheep and goats (Taenia ovis). Again, the Taenia infections are rarely associated with clinical signs in dogs or cats but may result in an itchy bottom! As owners you should try and prevent dogs and cats having access to the various intermediate hosts; the feeding of raw meat and viscera should be discouraged.
Lungworm
One worm that has been talked about more in recent years is Angiostrongylus vasorum or ‘lungworm’. Angiostrongylus has spread rapidly over the past decade from endemic foci in Wales, and the south-west and south-east of England across the whole of the UK. It is a nematode that resides, not in the lungs, but as the adult stage in the pulmonary arteries and the right side of the heart in dogs and other carnivores (excluding cats).
Dogs infected may become clinically unwell, but prevalence seems to vary geographically. The most common clinical presentation in dogs is mild to moderate lung problems; such as coughs (either productive or unproductive) and difficulty breathing (dyspnoea), with or without fast breathing (tachypnoea), some may experience coagulopathies (bleeding disorders).
De-wormer resistance in horses and livestock – will it happen in dogs and cats?
We may have heard that it is not advisable to routinely worm horses due to resistance issues. And wonder if the same can be said for cats and dogs, will deworming our pets too much cause resistance?
In horses, as well as in animals such as livestock, horses usually have access to pasture(s) that are, in the grand scheme of things, fairly limited areas. These areas are naturally infected with a small number of worms that are not a threat to the animal’s health. And the small worm burden and low level of infection is unlikely to cause issues.
Every time we treat a horse with an anthelmintic (deworming) medicine, some of these worms may develop resistance. The more we worm, statistically, the more likely it is that we may develop resistance issues. These resistant worms then reproduce and ultimately the worms in the population become resistant; so that the drugs no longer work.
So what about dogs and cats?
This is not seen as much in cats and dogs, owing to the fact that they are not grazed on pasture. And the lifecycle of worms that impact them are likely to occur from a much more varied source geographically and by the host, than in horses. That’s not to say resistance will not occur. One paper noted that anthelmintic resistance has thus far only rarely been reported (mainly only in rare cases in the USA) for intestinal helminths of dogs and cats, in contrast to parasites of livestock and horses.
Anthelmintic (dewormer) effectiveness field surveys in dogs and cats are sparse. However there have been two, one in Germany and one in Spain. And neither showed evidence of anthelmintic resistance concerning benzimidazole drugs against the present intestinal nematodes (roundworm), or for praziquantel against cestodes (tapeworm).
Overall, resistance has probably not been seen yet due to a number of factors including major epidemiological and biological differences; significantly different husbandry settings such as individual rather than herd keepings and better hygiene options; overall lower anthelmintic treatment frequency in most circumstances (apart from in high risk areas;, plus a smaller parasite population size leading to a comparatively low genetic diversity in the parasite populations.
Should we deworm our pets, so we don’t catch worms too?
You may think that the only reason to deworm your pet is to keep them healthy. But since some dog and cat parasites can also potentially cause infection in humans, veterinary professionals have an additional responsibility for human/public health too.
A particular zoonotic risk comes from the widely present Toxocara roundworms. After oral ingestion of infective eggs, the larvae can perform a somatic migration (larva migrans complex) where they burrow into human tissues. If larvae become blocked in the human eye, nerve tract and/or brain during migration, serious health problems can occur. This is larva migrans syndrome.
Echinococcus granulosus can form large cysts (properly, extra-intestinal metacestode stages) in intermediate hosts and are a rare zoonotic concern. In humans, infections result in the formation of cysts, most commonly in the liver (E. multilocularis, E. granulosus) or in the lung (E. granulosus). These occur following the oral ingestion of eggs or proglottids excreted in the faeces of the definitive hosts. It is suggested that high risk animals, those in areas where E. granulosus and related species are endemic, care should be taken to prevent dogs having access to raw offal and carcasses. Where dogs may have access to carcasses or raw viscera especially from sheep, pigs, cattle or horses (depending on the Echinococcus genotypes present locally) they should be treated, according to ESCCAP, at least every six weeks with an effective anthelmintic containing praziquantel or epsiprantel.
Faecal examination
One alternative could be that you do regular faecal testing. Although this is not the perfect solution, as with everything there are pros and cons.
Patent (egg-laying) infections of worms can be identified by faecal examination. The analysis of faecal samples collected over different days increases the sensitivity of the employed methods.
However, it should be noted that for some roundworms such as Toxocara, a negative correlation between fecundity (ability to produce a number an abundance of offspring) per worm and number of adult worms has been reported. This means that the egg count doesn’t give you a good measure of how many worms are present. Furthermore, there is poor correlation between taeniid tapeworm infection and the detection of eggs in faeces.
In relation to lungworms, who of course do not reside in the gastrointestinal tract, where larvae (L1) are produced, faecal samples should be examined using a different method called the Baermann technique.
So, are we deworming too much?
The European Scientific Council on Companion Animal Parasites, ESCCAP, suggest treating depending on risk factors, and based on faecal analysis. ESCCAP state that indoor animals with no access to other cats/dogs, parks, sandpits, playgrounds, snails and slugs, raw meat or prey animals may only be advised to be wormed for roundworm 1-2 times a year, or after faecal examination. Whereas those who eats prey animals and/or goes outdoors to hunt without supervision may be suggested to treat monthly against tapeworms; 4–12 times a year against roundworms depending on risk analysis.
Martin Whitehouse, veterinary surgeon, stated that for Toxocara, in adult dogs who have not been recently wormed, in the northwest Europe, and in countries like the UK, the prevalence of Toxocara is often about 2 to 4 cent. Martin states that you have to treat about 25-50 adult dogs to reduce shedding in 1 dog.
So looking at the risk – to us, to our pet or of simply getting infected, we can see that the risk analysis for deworming relies on working on a very individual basis. Are they indoor? Outdoor? Do they eat raw meat or hunt? Do they have other ectoparasites? Or do they live rurally or in urban areas? And so on.
It is likewise not advisable to routinely worm rabbits, and only suggested where treatment is indicated.
So maybe, if we put all animals on the same protocol, we may be treating pets when they have no worm burden, or no significant worm burden or have a burden which is not clinically relevant or going to make our pets sick, or us sick.
Are wormers safe?
De-worming products on the whole in the UK, whether prescription or some over the counter products, are generally extremely safe with low risk for toxicity, so are unlikely to cause your pets any harm. But if you have a very very low risk animal it may be worth doing a risk assessment. This is to decide if you need to treat at very regular intervals. We are also not currently seeing resistance, but could that change in future?
That risk assessment should include factors such as who your pet comes into contact with. And if that is children, pregnant women, or those who are immunocompromised, then you may want to reassess the protocol.
As with all things, our pets are individual and the way they interact in our lives and who they interact with are so unique. We should ensure we have discussions on a case-by-case basis.