The raw end of the deal: controlling parasites in raw pet food

While the benefits of a raw pet food diet are much deliberated, one thing is for certain – raw feeding, by intention or by scavenging, can present a parasitic risk. However, by providing clear information and following best practice, the risk posed by parasites can be minimised. As raw diets continue to increase in popularity, the use of rapid parasite screening measures to ensure pet health and safety is more important than ever before.

From pet owners and veterinary professionals to pet food manufacturers and research scientists, feeding companion animals raw food is still a widely debated topic. While raw feeding may present a polarising topic, a unifying goal is to maximise the health and safety of our animals and the people around them. Arguably, one of the biggest risks of raw feeding is the spread of pathogenic and parasitic microorganisms to household pets, owners and beyond. However, understanding and continually reviewing the potential risks posed by parasites in raw food will positively impact health and parasite management.

Is raw feeding contributing to the spread of parasites, how is it affecting pets, people and businesses, and what can we do to mitigate parasite risk?

In a recent free CPD webinar hosted by the veterinary team at HORIBA UK, parasitology specialist Ian Wright, MRCVS, head of the European Scientific Counsel for Companion Animal Parasites (ESCCAP) UK and Ireland, provided expert insight on the topic of the role of testing in controlling parasites associated with raw feeding (Wright, 2022). This article provides an overview of the webinar and subsequent discussion, highlighting the advice which helps educate and support veterinary professionals on the importance of identifying and mitigating parasitic risk from raw foodstuffs.

Raw feeding and exposure to parasites

The term “raw feeding” can encompass planned raw pet food diets as well as food scavenged or hunted by pets. In particular, dogs in rural areas often seek out and eat dead farm animals, while cats actively hunt creatures such as birds and rodents. Each of these raw feeding routes presents a different level and type of parasitic risk.

The zoonotic risk posed by Toxocara is of greatest concern in cats and dogs that hunt paratenic hosts such as birds and rodents

While there are a huge number of different pathogens that could be transmitted through raw feeding, most parasite species pose a relatively low risk; however, there are several species where raw food represents a major route of transmission. In the UK, the main parasites that can cause issues in relation to raw feeding are tapeworms and protozoa. But the zoonotic risk posed by Toxocara is of greatest concern in cats and dogs that hunt paratenic hosts such as birds and rodents.

Tapeworms

Tapeworms, specifically Echinococcus granulosus (Figure 1) and Taenia spp., are common in dogs with access to raw carcasses. Dogs acquire these tapeworms by ingesting cysts in infected farm animal carcasses, particularly sheep. The cysts migrate to the dog’s intestines, where they develop into mature tapeworms, giving rise to eggs. Eggs found in a dog’s stool can, in turn, contaminate pasture and then infect grazing livestock (Figure 2).

The first and most obvious risk of these parasites is the direct infection of domestic animals or livestock, resulting in ill health. If left untreated, tapeworms may cause weight loss and potentially serious discomfort in dogs, although in most cases, both dogs and sheep are tolerant to E. granulosus and Taenia spp. Despite this tolerance, there is still potential for huge economic loss to farmers – the Food Standards Agency recorded 66,500 lamb carcasses rejected in 2012 due to Taenia spp., equating to a cost of £5 million. In the case of E. granulosus, the most significant risk to humans is hydatidosis, which can result in the development of cysts in several organs, including the lungs, liver and brain. These cysts gradually enlarge, inducing a range of symptoms, including nausea, vomiting and pathological fractures, and in rare cases, the rupture of a cyst can result in allergic reactions or even death.

Protozoa

Protozoa are microscopic, single-celled organisms that are either free-living or parasitic. Neospora caninum, Sarcocystis spp. and Toxoplasma gondii represent the three primary protozoan parasites associated with raw feeding in the UK. N. caninum and Sarcocystis spp. are a particular problem in dogs with off-lead access to pastures and hunt dogs fed directly off-farm. Dogs become infected with N. caninum through ingestion of contaminated cattle meat and, in rare cases, can become sick with potentially fatal creeping paralysis. A major agricultural concern with N. caninum infection is its ability to induce abortions in cows. If infective N. caninum oocysts present in the stool of canines transmit to cattle, “abortion storms” can ensue – a spate of abortions in a herd or geographically isolated area. Observing good management of dogs and their waste on pastureland can avoid these highly damaging events.

If infective N. caninum oocysts present in the stool of canines transmit to cattle, “abortion storms” can ensue – a spate of abortions in a herd or geographically isolated area

It is not just dogs who can be hosts to parasites of concern: by hunting and eating birds and small mammals, cats are known to acquire the protozoan parasite Toxoplasma gondii. In humans, infection can induce miscarriage in pregnant women and can be life-threatening for the immune-suppressed. T. gondii has also more recently been linked to several mental health conditions, including bipolar disorder, depression and suicidal tendencies. 

The risk of a raw diet

Historically, these parasites have only been a problem in isolated groups and geographical regions. In fact, they are mostly acquired by unsupervised feeding on pasture or active hunting. So, in terms of feeding a raw diet to the general pet population, what’s the risk?

Currently, there is no definitive answer because, in most cases, there is a considerable lack of testing. This lack of clarity regarding the risk of raw pet food diets is a particular cause for concern when considering E. granulosus. This parasite has a huge incubation period before it develops hydatid disease in humans – typically 7 to 10 years, but potentially as many as 25 years. This means we cannot see the effects of any spread due to raw feeding for approximately a decade. Worryingly, we have very little information on the distribution of E. granulosus outside of Powys, Wales, where its control has been an ongoing challenge since the 1970s.

It is important to note that the risk of parasites in raw food is not theoretical. One study demonstrated that 11 percent of raw pet food, composed of human-grade meat, contained Sarcocystis cruzi, and 6 percent contained T. gondii (van Bree et al., 2018). Thus, the rise in raw pet food diets has the potential to increase parasitic transmission opportunities and widen the risk. 

Mitigating parasite risk with best practices

ESCCAP has compiled several guidelines (ESCCAP, 2018) that are specific to parasite type and give clear advice on how to minimise risk. Universally, keeping dogs on leads around pasture, cleaning up their faeces and encouraging farmers to be vigilant about removing carcasses from their grounds can help curb parasite spread. When choosing to use raw foods, the most effective way of preventing parasitic problems is to ensure that they are correctly prepared. Studies have shown that adequate freezing at the right temperature and for the right amount of time can remove most parasitic risks. Iansuggests that the ideal freezing time is a minimum of −18°C for 10 days (Wright, 2022).

For some parasite species, there are highly effective treatment measures. For example, regular administration of praziquantel will eliminate the risk of a tapeworm infection; however, identifying at-risk dogs requires lifestyle assessment and effective testing and screening strategies.

Parasite testing

The importance of parasite testing

Testing for prevalent parasites offers several important benefits. Primarily, testing provides a means to diagnose household pets, pinpoint the source of infection in agricultural settings and survey their incidence rate and geographical spread. Collectively, this provides a quantitative and robust indication of risk. Understanding the risk severity allows for informed precautions to be taken at household, local and national scales. For instance, diagnosis of a house cat with T. gondii will be used to advise pet owners that extra precautions should be taken while keeping the risks in perspective. This would include increased hygiene and extra vigilance with children, immunocompromised and/or pregnant individuals. In another scenario, mapping the source of an infection to a particular farm dog can guide isolation and help contain any further spread. On a larger scale, understanding the distribution of E. granulosus and any increase in its spread could help guide mitigating strategies and avoid potential future catastrophes.

Testing provides a means to diagnose household pets, pinpoint the source of infection in agricultural settings and survey their incidence rate and geographical spread

What are the options?

Parasite diagnosis is a significant challenge in veterinary practices, not least because many approaches are low throughput and time-consuming, and may be ineffective. Helminths and protozoan parasites can be identified by using microscopy. However, though pooled faecal samples are easy to collect, individual organisms can be missed on examination, especially tapeworm eggs, which float poorly in standard flotation solutions. In the case of T. gondii, cysts are also rarely shed and, even when present, can be difficult to observe and often evade detection (Figure 3).

Helminths and protozoan parasites can be identified by using microscopy. However, though pooled faecal samples are easy to collect, individual organisms can be missed on examination

An alternative approach is serological testing, which measures parasite-specific antibodies. However, while they remain an important technique, serological tests only indicate exposure and not active infection, as detectable levels of antibodies can remain long after infection. Another option is PCR: a useful diagnostic tool that allows for the diagnosis of many active parasitic infections and can detect the presence of the organism in pets and other samples. As a highly sensitive and specific technique, it is sensitive to very low levels of pathogens. The use of PCR to screen for parasites to gather epidemiological data and, in veterinary practices, to confirm infections represents a powerful approach to identifying risk and informing risk-mitigation strategies.

Practical PCR solutions

There is now the opportunity to utilise PCR as a fast and simple diagnostic tool within a veterinary practice. HORIBA’s POCKIT Central benchtop PCR instrument (Figure 4) is an easy-to-use, sample-in-answer-out in-house PCR solution providing rapid results for over 190 assays and is able to simultaneously screen up to eight samples. The test range includes commonly seen canine and feline pathogens, specialist assays for equine, farm and aquaculture, as well as common parasites that represent a potential raw-feeding risk, such as Echinococcus spp. (including E. granulosus), Neospora and Toxoplasma gondii.

The user simply needs to enter the sample identification data and load the sample and reagent. From nucleic acid extraction and amplification to detection, the entire PCR workflow takes just 85 minutes to complete and is integrated into one benchtop machine, making it ideal for use in veterinary practices of all sizes. The intuitive user interface and straightforward test process, coupled with full staff training and support from HORIBA’s veterinary product specialists, means PCR testing is accessible to all practices and staff members.

Conclusions

Parasites pose a significant threat not only to our pets, but also to human health. In addition, outbreaks can result in considerable impacts to agriculture and losses to farmers, and can even put small independent farms out of business. With accessible sensitive diagnostics and rapid turnaround times veterinary staff have the opportunity to proactively engage and educate pet owners, animal carers and farmers in order to enable swift intervention. Moreover, as raw food diets increase in popularity, screening methods allow us to effectively monitor parasite distribution and continually reassess any associated risk. Ultimately, this facilitates informed decision making and helps manage the potentially devastating consequences of parasites, while supporting the choice of pet owners to feed raw.