This article seeks to provide a ‘helicopter view’ or overview of the impact of companion animal parasitic zoonoses as causes of human disease, and demonstrates that several of the infections contribute substantially to the burden of human disease globally. Responsibility for reducing as far as possible the risk to humans, and to pets where they too are clinically affected by the infections, is sometimes undertaken by governmental organizations but often falls to individuals. Hence an individual pet owner’s decisions can have an impact, however small, on the global burden of human and animal disease.
Calculating the ‘cost’ of disease
The impact of companion animal parasitic zoonoses can be evaluated in a number of ways, one of which is to assess the cost of infection and to compare this with the costs and feasibility of prevention in both the human and the animal host. In humans the cost of infection can be formally described in ‘disability adjusted life years’ (DALYs), defined as the sum of years of potential life lost due to premature mortality and the years of productive life lost due to disability (Murray et al, 1994). This system has been used by the World Health Organisation (WHO) to allow assessment of the impact of an infection and comparison of one infection with another. It has been used in a wide range of infections including, for example, malaria, to rank the relative importance of infections by creating an annual global DALYs burden for each disease (World Health Organisation, 2012).
Problems with assessments
Assessment of the number of affected humans is confused where the quality of the information is poor. This occurs when, for example, there is no mandatory reporting for an infection, as is the case with toxocarosis in humans. Without mandatory reporting the numbers reported are likely to be an under estimate. Human infection with Toxocara spp. can be difficult to diagnose as many toxocarosis syndromes are not associated with pathognomic clinical signs or biochemical changes (Smith and Noordin, 2006). Moreover, since a considerable proportion of the population is seropositive, simply finding positive serology does not confirm cause of the observed clinical syndrome (Smith and Noordin, 2006). Individuals affected by the association of Toxocara infection with asthma are similarly difficult to enumerate (Torgerson, 2012a). Even retinal damage caused by Toxocara is likely to be under recorded as the damage caused by the migrating larva does not normally result in full loss of vision and normally affects only one eye, thus excluding it from blindness as defined by level of vision of 6/60 or less in the better eye (Taylor, 2006).
Compiling numbers also relies on accurate diagnosis, which may be absent for some conditions; once again Toxocara infection provides a good example. There is evidence that both T. canis and T. cati are zoonotic but there is no widely accepted test that allows differentiation of human infection caused by one species or the other (Smith and Noordin, 2006). There is thus no clarity or agreement about the proportion of cases caused by the dog-related worm or the cat-related worm.
Burdens of parasitic zoonoses
The total global burden has been estimated for some zoonotic infections. The global burden for cystic echinococcosis (caused by Echinococcus granulosus) is estimated to be 1 x 106 DALYs per year and for alveolar echinococcosis (caused by E. multilocularis) 600 000 DALYs per year with the major proportion of cases in China (Torgerson and Macpherson, 2011). The burden of congenital toxoplasmosis is estimated to be 1.5 million DALYs per year, although the source of the infection may be undercooked meat containing Toxoplasma cysts as well as soil-transmitted infection from locations where cats have defaecated (Torgerson, 2012a). Canine leishmaniosis (Figure 1) caused by the protozoa Leishmania spp. can present as cutaneous, mucocutaneous or visceral disease. The infection accounts for approximately 2 million DALYs per year globally, but this has contribution from human to human transmitted infection as well as dog to human infection (Torgerson, 2012a). Globally, compared with the ‘big three’, malaria, HIV and tuberculosis, these zoonotic infections do not compare, however they may cause an annual global burden akin to that of tooth decay, intestinal helminths or tetanus in humans (Torgerson, 2012a).
The impact of infection on individuals will vary, for example, infection with Leishmania in an immunocompetent individual may result in a skin lesion. However, the same infection in an individual with HIV or immunodeficiency, caused for example by transplantation therapy, can be fatal, causing systemic, visceral infection.
Prevention and intervention
While prevention may be desirable where the impact in terms of DALYS on affected individuals is known to be high, the feasibility and economic cost of prevention (which may be compared with the annual cost of treatment of affected individuals) may be prohibitive. The highly successful rabies eradication campaign conducted across Europe between 1998 and 2008 included vaccination of foxes using baits and the compulsory vaccination of pet dogs in continental Europe. A widespread attempt to control the tapeworm E. multilocularis in Europe, in the absence of a vaccine, would require regular cestocidal treatment of foxes (the main definitive host) using a bait approach, in addition to regular treatment of domestic dogs residing in the endemic area with praziquantel (Gemmell et al, 2001).
The evidence base for intervention and its likely impact is often surprisingly poor, so picking up fresh dog faeces before Toxocara eggs have had a chance to embryonate and regularly worming dogs and cats at suitable intervals should work to reduce the likelihood of Toxocara infection in children, but formal assessment of which interventions are the most beneficial may be lacking. Hence the debate continues as to whether more frequent anthelmintic treatment to prevent Toxocara patency is useful or overuse. Of course the situation is confused by reallife-life complexity where, for example dogs living in close proximity with children present a different risk scenario to those living in a kennel situation. Current evidence suggests that treatment at least four times per year is beneficial in reducing egg output (Morgan, personal communication,, 2012) — whether that translates to a reduction in the risk of children being infected is assumed.
Public versus private responsibility
Public versus private responsibility for control is a relatively undefined area. The two may be interlinked, for example when the hydatid control programme in Wales was scaled down during the 1990s, an education arm continued. One veterinary surgeon was responsible for travelling around schools and markets in Wales educattion children and farmers about the need to ensure that dogs were regularly wormed, to prevent re-emergence of the Echinococcus life cycle (Walters, personal communication, 2005). Re-emergence of the infection in sheep suggests that this approach alone was insufficient. More recently the Welsh Assembly has mounted a publicly funded campaign with regular dog worming and sampling in an effort to prevent human infections.
There are examples of radical public interventions that did not result in effective control. One such is the policy in some non-European countries of killing dogs infected with leishmaniosis. This is reported to have not resulted in a reduction of the level of infection (ESCCAP, 2009).
Since January 2012, with the withdrawal of mandatory tick treatment before entry into the UK (where treatment and certification was privately funded), there is no longer any legal requirement to treat dogs for ticks prior to entry into the UK (DEFRA, 2012); this change was brought about by the EU authorities being insufficiently convinced to maintain the treatment despite consensus amongst a number of UK authorities that it remains desirable. It therefore becomes the responsibility of non-governmental organizations such as the British Veterinary Associations to provide education to veterinary surgeons and veterinary nurses and through them to the general public to prevent the introduction of ticks and tick-borne infections.
Impact of diseases in companion animals
So what of pets? Despite recognition that disease may have an impact on both the quality and duration of life in animals in a similar way, there is no system to calculate and then compare the impact of diseases similar to DALYs on animals (Torgerson, personal communication (2012b)). The impact on animals of some of these infections is negligible; a dog infected with E. granulosus is unlikely to show any clinical signs, while other infections such as Leishmania spp. may cause severe disease. Leishmaniosis is complicated by the lack of a treatment capable of causing a parasitological cure, hence treatment of affected dogs may have to continue regularly or intermittently for the remainder of the animals life. It has been estimated that the cost of treatment in Portugal for a 20 kg dog is between 300 and 600 Euros per year (Madeira de Carvalho, 2012). The situation for a dog that has been rescued from a Mediterranean country and rehomed in the UK is likely to be similar, with the additional cost of veterinary bills and a specialist diet not considered in the figure above. This may be compared with the cost of vaccination, an approach that has recently become commercially available.
Conclusion
There is evidence that, despite the difficulty of obtaining good quality information, the burden of disease from companion animal zoonoses is substantial in humans and in animals. There is research work to be done to produce estimates for ‘neglected’ infections where WHO does not compile data, and to provide estimates for the burden of disease in pets. There is an important and ongoing educational task since the prevention of many of these infections lies in the hands of the individual pet owner. Methods to prevent and manage them are available through various sources including www.esccap.org and www.esccapuk.org.uk.