14 Mar 2016
Ian Wright offers advice for vets for cat owners on parasite Toxoplasma gondii and its transmission potential to people, while preventing human-cat bond erosion.
Ian Wright
Job Title
Figure 2. Mixture of unsporulated and sporulated Toxoplasma oocysts. Image: © Pedro Serra, NationWide Laboratory Services.
Cats carry a number of different endoparasites of clinical and zoonotic significance, a few of which are immediately recognised by the public due to concerns surrounding their zoonotic potential. Toxoplasma gondii is one of these and has a justified reputation for being a risk factor for abortion and congenital abnormalities if contracted by people during pregnancy. It also carries other significant zoonotic risks, as well as potentially causing disease in infected felines.
Control of human toxoplasmosis is centred around sound hygiene and food preparation practice, and pet owners need to be made aware of the necessity of these precautions, while not causing unnecessary alarm. Veterinary professionals are at the forefront of dispensing this advice and need to be prepared to give accurate information and put any concerns into perspective.
This article considers the transmission of T gondii, the potential for it to cause zoonotic disease and strategies for minimising the risk of human infection.
Names of only a few parasites are recognised by the public, with some known and feared due to their zoonotic potential.
Toxoplasma gondii is one of these parasites and carries a number of significant zoonotic risks, the most well known of which is the risk to pregnant women, though immune-compromised individuals are also at particular risk. The possible effects of T gondii on the human brain have also come to light and received much attention in the press.
Treatment is difficult and no effective chemoprophylaxis exists, making hygiene and careful food preparation the basis of disease control. Veterinary professionals are regularly asked questions about these zoonotic risks and, as a result, play a vital role in dispensing accurate control advice, as well as keeping concerns in perspective and allaying public fears.
This is particularly true of clients who own cats and may be worried about the risk their pet represents. Questions may also be asked by clients working in areas of increased risk of infection, such as catteries, sheep farms and abattoirs.
This article considers the zoonotic disease risks posed by T gondii, its transmission and practical advice people working in the veterinary profession can give to help keep the public safe.
Evidence suggests T gondii can infect all mammals, but only felids can act as definitive hosts, producing unsporulated oocysts (Figure 1) in the intestine that are then passed in the faeces (Boothroyd, 2009). Oocysts then sporulate in the environment (Figure 2), becoming infective.
Although cats can be infected through ingestion of oocysts in the environment, infection is most commonly acquired through ingestion of tissue cysts. This occurs through predation of intermediate hosts, such as rodents and birds, by feeding raw or undercooked meat from infected livestock or though access to aborted ovine material.
Intermediate hosts, including humans, are infected either transplacentally or through consumption of oocysts and tissue cysts. Oocysts are easily disseminated from faeces into surface water, where they can survive for several months. This makes open water sources, such as ponds and rivers – as well as humid soil – prime sources of infection.
Feedstuffs may also be contaminated, with cat faeces and oocysts subsequently ingested. Infective tissue cysts may be consumed in raw or undercooked meat and this is a significant source of human infection.
Healthy adults have a low risk of developing significant signs from acute toxoplasmosis if infected; 10% to 20% of otherwise healthy individuals may develop mild transient clinical signs, including pyrexia, muscle fatigue, lymphadenopathy and nausea.
Immunocompromised individuals, however, and children infected in utero, can suffer from severe ocular and cerebral signs that can lead to blindness or death. Consequences from chronic infection are also emerging that, until recently, had been unrecognised.
Zoonotic disease from T gondii infection can be broadly grouped into three different categories.
Acquired toxoplasmosis occurs worldwide through consumption of oocysts and tissue cysts. The seroprevalence of T gondii in the human population has decreased significantly in Europe over the past 20 years, thought to be due to changes in eating habits, such as reduced foraging for wild foodstuffs including berries and decreased consumption of raw or undercooked meat.
Water-borne infection and contamination of fruit and vegetables from kitchen gardens and allotments with cat faeces continues to be a significant source of infection (Boothroyd, 2009). Strains of T gondii in Europe and North America appear to be less virulent than those found in South America (Ajzenburg, 2012) and many cases are subclinical.
Veterinary professionals should, therefore, keep clients’ concerns about adult infection in perspective. This is particularly important as a number of press reports have been published about the role of T gondii infection in suicide risk (McDermott, 2012; The Telegraph, 2012).
Reports have identified infection as a risk factor for schizophrenia, bipolar disorders, epilepsy and migraine (Flegr, 2013), but it is recognised these illnesses have complex aetiologies, with often multiple causes.
Links to these factors, however, and the high seroprevalence worldwide (up to one third of the world population may have been exposed to infection), continue to make toxoplasmosis one of the most common and significant adult zoonoses in the developed world (Ajzenburg, 2012).
If women are infected while pregnant, transplacental infection can occur. The consequences of infection while pregnant are potentially serious and children infected in utero can suffer from severe, sometimes fatal, toxoplasmosis. This may be local, with ocular or cerebral forms, or generalised, with multiple organs being affected.
Infections acquired during the first trimester usually result in miscarriage or abortion. Women infected during the second trimester may have children that survive birth, but with severe, sometimes life-threatening, defects. Children infected in the third trimester tend to be less severe, with ocular or cerebral defects developing later on in life.
These complications have a high level of recognition by the public and, although the complications of toxoplasmosis in pregnancy have long been established, news reports continue to highlight, and sometimes sensationalise, the risk (Roberts, 2012).
Immunocompromised patients – such as those suffering from HIV, undergoing chemotherapy or transplant patients – are at particularly high risk from life-threatening complications associated with T gondii infection.
The greatest risk comes from acquiring infection, while the individual is immunocompromised, but reactivation of previous chronic infection can also occur.
CNS, ocular or multiple organ signs can develop, leading to permanent organ damage or death.
The significant disease risks and high prevalence of T gondii infection will inevitably lead to veterinary professionals being asked questions about toxoplasmosis by concerned owners – especially pregnant women. Owners should be reassured infection poses minimal risk to immunocompetent adults or to pregnant women that were seropositive prior to pregnancy.
The potential consequences of infection to at-risk groups and the emerging consequences of chronic disease, however, means infection risk needs to be taken seriously and can be minimised by simple control measures.
A number of misconceptions need to be addressed before practical control measures can be addressed.
No specific need exists for cats to be removed from households with a pregnant or immunosuppressed family member. Infected cats can shed oocysts in large numbers, but this is a rare event and only occurs for a short period of time. As long as good hygiene is maintained then the risk an individual cat poses is minimal, compared with the risk of contaminated foodstuffs or those working with large numbers of cats.
Feline serology is a poor indicator of active infection and risk. Not all cats with subclinical infections will display antibody titres and no correlation exists between antibody titres and oocyst shedding. In clinical feline cases, treatment with clindamycin (used under the cascade) can be useful at 12.5mg per kg twice daily for up to four weeks.
However, no evidence exists that treatment has any effect on oocyst shedding, so use of antibiotics is contraindicated in cats not exhibiting clinical signs, irrelevant of their serological status.
Oocysts are shed by infected cats for a short period of time and repeat shedding is infrequent. This, in combination with the small size of the oocysts (typically 12.5µm × 10.5µm, thin-shelled; Figure 3), makes detection by faecal examination difficult.
Investigating whether cats in contact with at-risk groups of people are infected with T gondii is, therefore, of very limited use. However, simple and highly effective control measures exist that will limit human exposure to T gondii infection.
Daily cleaning will ensure any oocysts shed do not have time to sporulate and become infective. This should not, ideally, be carried out by pregnant women or immunocompromised individuals and, if no alternative is available, strict hand hygiene must be maintained.
Covering sandpits and vegetable and fruit patches can be carried out to avoid faecal contamination from cats.
Good hand hygiene will help reduce the risk of oocyst ingestion. Pregnant women and immunosuppressed individuals should, ideally, wear gloves when working with soil and handling sand from sandpits where cats frequently defecate and bury their faeces.
They should also not handle lambs or kids due to risk of hand-to-mouth contamination by contact with recently infected dams during delivery. If they must handle them, strict hygiene must be maintained.
People working in the meat industry are also over-represented for infection, demonstrating the need for strict hygiene in these environments.
Thoroughly washing fruit and vegetables will help reduce the risk of oocyst contamination. Cats are frequent visitors to allotments and kitchen gardens, so particular attention should be paid to fruit and vegetables from these sites intended for raw consumption.
Cooking meat thoroughly will help destroy tissue cysts before consumption. The US Centers for Disease Control and Prevention (CDC) advises for at-risk groups, whole meat cuts are cooked to at least 63°C, ground meat to at least 71°C and poultry to at least 73°C. It also advises meat is rested after cooking for at least three minutes before consumption (CDC, 2013). If intended for raw or rare consumption, meat should be frozen first at -20°C for at least two days.
The serological status of pregnant women is important as seroconversion during pregnancy is a significant risk. This must be monitored through pregnancy if women are initially seronegative as the stage of pregnancy that infection occurs is also important in terms of disease outcomes (Robert-Gangneux and Darde, 2012).
While it is important not to panic clients and keep health concerns in perspective, impressing on owners the importance of good hygiene control measures will help minimise the risk of not only T gondii infection, but also other faecal-borne pathogens, such as Toxocara, Giardia and Campylobacter.
Toxoplasmosis is a disease very much at the forefront of public awareness. This is due historically to its well-known links to complications during pregnancy and, more recently, news reports highlighting links to psychiatric disorders and suicidal tendencies. This will continue to lead concerned clients to consult veterinary professionals about the risks – especially those clients who own cats.
T gondii poses a significant zoonotic disease risk, so clients need to be accurately advised on best practice to minimise risks of infection. These risks, however, should also be kept in perspective so the human-cat bond is not eroded.
