13 Jun 2022
Laura Lacey
Job Title
Sadly, worms are commonly encountered unwanted passengers that affect most animals at some point within their lifetime. They are unpleasant, yet efficient, parasites that can cause variable symptoms depending on the species we are dealing with and any other concurrent concerns the animal may be challenged with at that time. Younger animals are more commonly affected and the damage they cause can be life-threatening.
It is essential that we help support owners by educating them about life cycles, treatment options and any other ways we can reduce the prevalence.
Three main types of endoparasite exist that form the helminth group we commonly encounter in the veterinary field: trematodes, nematodes and cestodes.
Trematodes are usually referred to as flukes – they commonly affect cattle and sheep, and occasionally horses, but vary rarely cats and dogs in the UK. The nematode group includes roundworms, hookworms, whipworms, heartworms and lungworms. Cestodes are tapeworms. These latter two are the main groups we will focus on in this article.
Roundworms are one of the most common intestinal parasites and are a particular concern due to their zoonotic nature. Roundworms are so called due to the fact their bodies are cylindrical, often referred to as looking like spaghetti (Figure 1).
When we discuss roundworms with clients we are usually talking about the ascarid group of nematodes, which includes Toxocara canis in dogs and Toxocara cati in cats, as well as the less commonly encountered Toxascaris leonina.
The life cycle of roundworms can be completed in around two months. Roundworm eggs are shed by the adult female worm and passed in the faeces of infected dogs.
Millions of unembryogenated eggs may be produced daily and they are capable of withstanding extreme environmental conditions for many years. Eggs embryonate over a period of one to four weeks in the environment. They are considered infective once they contain third-stage (L3) larvae. These may be ingested by a new host, which may be due to obvious coprophagia or more subtly from an animal walking through an area after faeces have been cleared, then simply cleaning its own feet later.
After ingestion, T canis and T cati larvae migrate within the host through the tissues of the intestines into the blood vessels and liver. In younger animals the larvae migrate through the lungs, bronchial tree and oesophagus. The larvae will irritate the tissues causing the animal to cough occasionally. These larvae are then swallowed and end up back in the intestines.
Older animals may not complete the process of hepato-tracheal migration and instead the larvae form cysts within the organs. Some of these will remain dormant; some will break out and settle in the small intestines as adult worms. Once there they will feed on intestinal contents and epithelial cells, and the females will begin to produce eggs, perpetuating the cycle.
Infection of the young can also occur transplacentally in puppies, and transmammary in puppies and kittens. This occurs as some larvae cease migration and development, and remain in a larval state in tissues or cysts. It helps to account for the abnormally high incidence of infection in younger animals.
If left untreated, large numbers of worms can cause blockages and intussusception. Even small numbers can lead to malnutrition, underdevelopment and failure to thrive. This is why we need to consider worming protocols at an early stage, which we will look at later in the article.
If the transport host is a mouse or a rabbit, for example, they may be ingested by the host where the cysts containing larvae will be present in the tissues. The larval developmental stage is where zoonotic transmission can occur also. If an animal other than the target host ingests eggs or larvae then they can develop in various organs, commonly the eye in humans, which is known as visceral larva migrans.
Hookworms and whipworms are less prevalent in the UK than roundworms. Hookworms can be fatal to young animals as they strip nutrients from the body as they suck blood from the intestinal wall.
Whipworms cause fewer problems as they require fewer nutrients from the animal. They rarely cause symptoms unless the load is great and they burrow into the intestinal wall, causing cellular damage.
The worm we call a lungworm is often associated with Angiostrongylus vasorum and, despite its common name, it is actually a type of heartworm.
The adult worm, which is only short, resides within the heart and pulmonary artery. It reproduces by producing eggs, which then hatch into larvae. Larvae and eggs are coughed up, swallowed and then pass out in faeces. The coughing and associated respiratory signs are where the common name was derived.
The eggs and larvae that have been passed are picked up by slugs and snails, and continue to mature. As the slugs and snails move about, they shed mature larvae in their slimy deposits. These larvae can survive off the slugs and snails for a short time, so if they have traversed toys or bowls, and a suitable host interacts with that object, then they can be ingested. They then migrate to the heart and vessels to start the cycle again as adults.
A vasorum causes variable clinical signs – from a mild cough or diarrhoea to clotting abnormalities – and can at times be difficult to diagnose. It can be fatal, with up to nine per cent of infected dogs dying (Vets4Pets, 2022). It is considered an emerging disease in the UK as its prevalence is increasing. It is not zoonotic, but is now becoming endemic in the fox population in the UK, which is causing the incidence to increase.
Oslerus osleri is a true lungworm that is endemic within the UK and causes respiratory problems. The adult worm resides in the bronchi and trachea, and can be passed directly from dog to dog, commonly being passed vertically from bitch to puppies.
Tapeworms are classed as cestodes and rarely cause a significant clinical problem in their healthy definitive hosts. That is what makes them such efficient parasites.
The clinical signs can vary – from general malaise and poor coat to diarrhoea and even seizures if the animal is compromised in other ways. However, their presence often causes offence to the animal’s owners, who may witness the small egg packets, or proglottids, located around the pets anus that identify them as having worms.
The life cycle of tapeworms is very interesting as they always require an intermediate host. The most common tapeworm in domestic dogs and cats is Dipylidium caninum, whose intermediate host is the flea (Ctenocephalides species). Animals that hunt or eat raw meat may also be exposed to Taenia species.
Tapeworms are made up of three distinguishable portions: the scolex or head, which allows them to attach to the intestinal mucosa using the hooks in the rostellum; the neck; and the rest of the worm is made up of the segments known as strobilla. The more distal ones – the proglottids – are the ones that contain the eggs.
Eggs that are passed out in the proglottids fall to the ground and may be ingested by an intermediate host. They then hatch out as larvae and develop, moving through the circulatory system, and finally forming cysts within the muscles and organs. If this animal is then eaten by the definitive host, such as a cat or dog, as raw meat then the parasites are released from the cysts and will develop into adult tapeworms in the intestines.
When fleas are the intermediate host, their larvae ingest the egg packets from within the proglottids. As the flea larvae mature they develop infective cysticercoids. Adult fleas containing the cysticercoids are then swallowed by the cat or dog as they groom to remove the irritation. The adult worm then develops in the intestines and the cycle continues.
Some tapeworms have significant zoonotic potential and, as such, we need to be very aware of prevention and the treatment options.
Echinococcus species can cause hydatid cysts in humans and livestock as a result of the ingestion of the metacestodes. The presence of any metacestode in livestock will render that carcase unsaleable; therefore, tapeworms in pets can be seen to affect the economic climate, as well as the risk they pose to public health.
Obviously public education about the risks associated with dog faeces not being picked up is essential to reducing the incidence. With the knowledge we have, we can help to explain why restrictions are in place regarding dogs in certain parks or areas where children play.
It can be very challenging at times to completely clear up all faecal matter from dogs, and outdoor cats will commonly choose to defecate in gardens that are not their own. This reinforces why we should recommend regular “de-worming” of all pets.
Outdoor cats that are known to hunt and scavenging dogs are at higher risk of ingesting intermediate hosts of some species of helminths. Some dogs can be trained to wear muzzles to reduce the incidence when off lead and cat owners may try to ensure their cats wear safety collars that have bells on to make them less successful hunters.
Of course, good personal hygiene will also help to reduce the incidence of ingestion of eggs. Children in particular should be encouraged to wash their hands after playing outside and handling any animals, and before eating.
It can be difficult to prevent worm infections from occurring. No repellents exist, but regular prophylactic treatment can prevent them from causing too much damage to the individual.
As some worm life cycles can be completed in as little as three weeks, we should risk assess individual situations and make specific recommendations.
Worm prevention should start with the pregnant bitch or the young kitten. As previously mentioned, earlier roundworms can be transmitted transplacentally in dogs, so breeders should perform an appropriate schedule for de-worming during the pregnancy as advised by the veterinary team. This should then continue with the bitch and puppies during the lactation period.
Roundworm transmission is only transmammary in cats, so no need exists to address this as vigorously in the queen, and deworming generally starts once kittens are a few weeks old. However, to ensure the queen is in the best shape possible, it is always advisable to ensure she is fully de-wormed prior to the mating at least.
Deworming protocols in young animals under six months of age are generally of a high intensity to ensure they do not succumb to the effects of a worm burden that they may have acquired early on. We know that younger animals are more susceptible to the damage that may occur.
After an animal reaches six months of age we should consider the individual, the family unit and the lifestyle they have. The protocols may differ if young children are involved, or if the animal lives in a rural or an urban environment.
The choice of product might be made based on ease of use, dosing frequency and the prevalence of certain worms in the area. Local knowledge is invaluable in making these decisions, but also conversing with owners is important. A client may live in a big city, but regularly spends time in the country, so the dog may be exposed to a higher risk of ingestion of Echinococcus species.
It is also essential we consider any overseas travel plans as several endoparasites are prevalent in Europe, but not in the UK. The extent of this information is outside the scope of this article, but detailed information can be found on the European Scientific Counsel Companion Animal Parasites (ESCCAP) website (ESCCAP, 2022).
Staying on top of ectoparasites such as fleas is also a consideration due to their involvement in the life cycle of tapeworms.
Parasites can be very distressing for the owner and as a minimum they are irritating to the pet. The anal pruritus alone can be distracting.
When endoparasites are left untreated, the effects can be devastating. Animals can, in some instances, live their lives with manageable parasite burdens as it is not in the best interest of the parasite to significantly harm the host. During times of stress or disease these burdens may no longer be manageable. Weak or compromised animals may succumb to parasite burdens much more easily.
The impact for human health and financial damage should also be considered. So, bearing all of this in mind, we need to ensure we are offering all of the most current and best advice to our clients that help protect their pet, their family and the wider population.
