BSAVA review of worm control in cats
Prepared by Maggie Fisher (Petsavers Grant Recipient)

Introduction
This document examines the background to worming: considering the worms that are endemic in the UK, and those that may be introduced by cats entering the UK from Europe, the US and elsewhere. It examines the risks to the feline host and other animals, including humans, presented by these worms. It is generally recommended that cats in the UK are wormed at repeated intervals, the rationale for repeated treatments will be examined and recommendations for the retreatment interval considered, together with a review of the anthelmintics currently available.

What worms?
Cats may be infected with a variety of worms, and in many cases the adult worms are found in the gastrointestinal tract. It is these nematodes and cestodes located in the intestinal tract that have been the traditional target of worming (Table 1), however worms that may be present in other organs of the cat will also be considered. In addition to endemic cestodes and nematodes, non-endemic species are also shown in Table 2. There is an increased risk with the increase in the numbers of animals entering the UK under the Pets Travel Scheme (PETs), that these infections may be introduced. Once present, whether or not they might establish depends on a variety of factors relating to the biology of the individual worm species.

Worm lifecycles and epidemiology, in brief

Toxocara cati
Toxocara cati is the most common of our endemic worms. There is a lack of good data on prevalence, particularly since the introduction of the variety of modern anthelmintics now available. Kittens receive their first infection through the queen’s milk, and may have a patent infection by about six weeks of age.  Like other ascarids, the female worms are prolific egg-layers. Little is known about the factors that determine whether cats continue to harbour T.cati through adulthood, and indeed whether the same cats are always infected. Surveys have detected that a proportion of adult cats, perhaps 10%, continue to pass eggs in their faeces, though prevalence in feral cats can be substantially higher. Evaluations of other factors such as the effect of anthelmintic treatment and other aspects of husbandry and breed have not been made.

Eggs passed into the environment will embryonate after a period of development that depends on temperature. Eggs are extremely long-lived in the environment, often surviving for more than a year. Extremes of temperature, desiccation and UV light can kill the eggs. Cats are infected through ingestion of embryonated eggs, eating prey or uncooked meat containing somatic larvae.

Toxascaris leonina
Toxascaris leonina infection is acquired through ingestion of paratenic hosts or embryonated eggs, thus cats are normally weaned or older before they acquire infection. Development occurs in the small intestine and adult worms are found in the small intestine. Infection is not commonly seen in pet cats in the UK.

Hookworms
Hookworms are uncommon in cats in the UK. Moreover, it is unclear which hookworm, U.stenocephala or Ancylostoma tubaeforme, if either, occurs in the UK. The life cycle is direct with larvae hatching from eggs passed in the faeces and developing into third stage larvae in the environment. The larvae are susceptible to desiccation and extremes of temperature as they are free-living, and can only survive in the environment for a limited time. The hookworm A.tubaeforme is present in cats in Belgium and elsewhere in Europe so may be imported in cats entering the UK even if it is not currently present.

Lungworms
The lungworm Aelurostrongylus abstrusus has been reported in cats in the UK, though it is unclear how commonly infection occurs.

Heartworm
Cats are not such good hosts for heartworm as dogs, and prevalence in cats is often around 10% of the level in dogs, with dogs providing the main reservoir of infection.

Tapeworms
There are a number of species of tapeworm that occur in cats in the UK (Table 1), infection depends on cats ingesting  intermediate hosts that are carrying infection. In general, prevalence data on tapeworm infections is lacking as egg-counts are unreliable for detecting tapeworm infections and post mortem prevalence studies are lacking. Once ingested, the immature tapeworm attaches to the wall of the small intestine using hooks and/or suckers and begins to produce proglottids containing eggs, with the prepatent period being the interval between infection occurring and the first proglottids being passed.

Echinococcus multilocularis does not occur in the UK, but is endemic across a large area of central Europe, from eastern France through Germany, Poland and down through Switzerland. The intermediate hosts are small rodents and the “normal” final host is the fox. Although dogs and cats are both susceptible to infection, cats appear to be less successful hosts, generally carrying fewer worms and allowing fewer to reach patency. As with other cestodes, there is no impact on the final host, but alveolar echinococcosis can develop in humans that become accidentally infected. This tapeworm is the reason that the Pets Travel Scheme (PETS) requires praziquantel treatment of all cats prior to their entry into the UK.

Risks associated with infection to the feline host
Clinical signs that may been seen as a result of worm infection in cats are shown in Table 3. Toxocara cati infects kittens when they are slightly older than pups, so though the worms can cause signs similar to those seen in dogs they are perhaps less commonly seen in kittens than pups. The hookworm, Ancylostoma tubaeforme, can cause anaemia if present in sufficient numbers. Tapeworms are well tolerated by the final host, though Dipylidium caninum or Taenia taeniaeformis segments passed through the anus may cause transient irritation and owner repugnance.

Lungworm infection caused by Aelurostrongylus abstrusus may go unnoticed or may cause a persistent cough in a heavier infection. The presence of adult heartworm may be without clinical signs in cats but acute respiratory episodes, often coinciding with the death of the worms does occur. In some cases nothing is seen before the cat dies suddenly. 

Risks associated with infection in other hosts, including man
Few of the worms that infect cats have never been identified in man, some, however, are more significant zoonoses than others. All three ascarid species T.canis, T.cati and T.leonina are zoonotic, causing several syndromes in man including visceral larva migrans and ocular larva migrans. In the past T.canis had acquired the worst reputation, but recent work suggests that 1/3 of the cases of Toxocara-related ocular and visceral larva migrans are caused by T.cati. In the UK visceral larva migrans is rare but a number of cases of ocular larva migrans occur each year, with children most likely to be affected. Dermal penetration of Ancylostoma spp. or U.stenocephala can cause cutaneous larva migrans.

Tapeworms generally cause more serious pathology in their intermediate hosts, so a variety of clinical signs may be seen in these hosts (Table 4). Infection of humans with eggs of E.multilocularis may result in the development of a multilobulated invasive cyst, which if left untreated results in death of the affected individual. Individuals that are suffering from echinococcosis report pain and reduced ability to fully participate in individual and group functions. 
 

Background to current worming practices
Worming cats has progressed, as understanding of epidemiology, pathology and zoonotic potential of the worms has increased, away from low efficacy treatments administered to reduce the risk of clinical disease in the host to high efficacy treatments each of which exhibits an individual spectrum of activity (Fig.1). Most wormers remove those worms present at the time of treatment but do not possess any residual activity. The situation is complicated a little as there may be some anthelmintics amongst the avermectin and milbemycin groups that show a useful prolongation of activity, as these products may possess an extended presence in the host. Cats can reacquire infections as soon as worming activity has passed so, in the case of most anthelmintics, there is immediate potential for the animal to become re-infected. Some cats may indeed be re-exposed to infection immediately after treatment, others may not reacquire infection for some time after treatment, but as it is likely that any infection will be unapparent, infected animals will be indistinguishable from uninfected animals.

Newly patent infections may be identified by sampling animals and carrying out faecal egg counts, which is a practice that has been more widely adopted in the US than the UK. Alternatively treatment can be re-administered at intervals, irrespective of whether the animal has a patent infection to remove any new infections present, and it is this method that has been widely adopted in the UK. The challenge is then to determine appropriate re-treatment intervals. The commonly recommended period has reduced over the past twenty or so years from six to three months, possibly motivated by the recognition that cats can have patent infections within three or four weeks of treatment, and because some of the infections are significant zoonoses. Scientific determination of re-treatment intervals has not been extensively conducted, except in the case of heartworm prophylaxis, where it has been established that monthly treatment with any one of the heartworm prophylactics will totally prevent the development of adult heartworms. Whilst re-treatment intervals have not been established by appropriate studies, there are a number of empirical considerations that can be made in order to assess the likelihood and importance of reinfection, and thus to decide whether frequent or less frequent re-treatment should be recommended:

1) There is a spectrum of how likely an animal is to reacquire infection, for example some cats, whether in the top floor of a block of flats or in a pathogen free colony may never be exposed to infection, whilst it is recognised that colonies of feral cats may become reinfected almost immediately.
2) The likelihood of any infection developing is related to a number of factors including age, thus the younger the animal the more likely that many nematode infections will develop.
3) The importance of any infection for the animal must also be considered: in the case of heartworm this evaluation has been conducted and the conclusion is that even one heartworm can be so risky for the animal that the prophylactic treatments and their retreatment intervals should be determined to totally prevent adult heartworm developing.
4) The impact of egg elimination into the environment as a source of infection for humans and other animals must be considered, thus the aim in hydatid endemic areas is to ensure that dogs are prevented from passing E.granulosus eggs. A similar assessment could be made for T.cati, particularly in households where there are children present.
5) The prepatent period is the minimum interval before eggs can be shed again after treatment.
6) With other farmed host species there is a trend to increase the retreatment interval in order to minimise treatment and thus the risk of resistance development. Resistance has not been a development within canine and feline anthelmintics and the zoonotic risk of infections, absent from considerations about other farmed species, mean that treatments should be spaced to minimise this risk, though avoiding unnecessarily excessive treatments.
7) When travelling or living in heartworm endemic areas heartworm prophylaxis is recommended throughout the mosquito season using a recommended heartworm prophylactic at monthly intervals.
8) When travelling or living in E.multilocularis endemic areas, monthly treatment with an effective cestocide will prevent the risk of cats passing eggs in their faeces.
9) Removal of a flea infestation is important in the control of Dipylidium caninum.

Worming recommendations
Where a cat has clinical signs relating to or evidence of a specific infection then the infection should be treated with an appropriate anthelmintic and an appropriate control strategy including management and repeat treatment, as appropriate, to prevent recurrence of infection.

Treatment of kittens may start later than treatment of pups as infections are acquired postnatally, possibly treatments beginning at three weeks of age will prevent patency, though more work is needed to determine the pattern of infection and egg-excretion in kittens. Treatment of kittens should be repeated at the recommended retreatment intervals.

There is no one-recommendation-fits-all for adult cats, or all situations. In some cases the decision may be made to go for maximum control, or this may result from the instigation of a monthly programme for control of another parasite such as fleas. In other cases, the risk of reinfection may be low so a longer inter-treatment interval may be logical. Figure 2 has been devised to assist in decision-making about appropriate re-treatment intervals.

Choice of anthelmintic is based on preferences for formulation, presentation, range of activity, price and so on. The range of activity is shown in Figure 1, which demonstrates the choice of treatment spectrum afforded by different products. It is now necessary to decide which parasites are going to be controlled together: for example, flea control and worm control may be offered in combination or separately. Once a decision has been made about products to offer it is necessary to ensure that no aspects of worm control, such as tapeworms, have been left unconsidered.

References

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www.esccap.org website of ESCCAP a European group whose vision is that parasites of pets are no longer an issue for pets or humans across Europe

Acknowledgements

Veterinary Review and Novartis are thanked for allowing reproduction of Figs 1 and 2, respectively