Zoonoses - Animal diseases that may also affect humans
There are many disease agents that can cause disease in multiple species of animals including humans. These diseases are called zoonoses. People are exposed to the bacteria, protozoa, fungi, viruses and parasites that cause zoonoses in a number of ways and therefore anyone working with or handling animals needs to know about zoonoses and the precautions they must take to minimise their risk of infection.
People who have close contact with large numbers of animals such as farmers, abattoir workers, shearers, knackery workers and veterinarians are at a higher risk of contracting a zoonotic disease. Members of the wider community are also at risk from those zoonoses that can be transmitted by family pets. Occasionally infection can also occur through indirect contact with other animals such as Listeriosis from drinking un-pasteurised milk or Leptospirosis from contact with infected urine that has contaminated streams or ponds. Some people are more susceptible to contracting a zoonotic disease due to their immune status, for example those people who are on immunosuppressive treatment, pregnant women, alcoholics and diabetics.
Fortunately the occurrence of zoonotic disease is uncommon and contact with zoonotic disease agents is preventable by taking a number of precautions including:
- practising good personal hygiene;
- providing prompt and effective first aid treatment to cuts and scratches;
- using personal protective equipment eg overalls, gloves, boots, goggles, aprons;
- cleaning and disinfecting work spaces and equipment;
- vaccinating pets and livestock;
- worming pets;
- controlling rodents;
- isolating and treating sick animals.
It is important to realise that zoonoses may be contracted from both ill and apparently healthy animals.
Diseases caused by Bacteria
Bacterial infection from an animal bite in particular involving cats and dogs is the most commonly suffered zoonosis in Australia. The mouths of dogs and cats contain huge numbers of dangerous species of bacteria including Pasteurella multocida. A penetrating bite pushes these bacteria deep into the skin and underlying tissues causing infection and resulting in pain, inflammation and swelling. All bites should be treated as serious, and if the skin is broken the bitten person is advised to attend a medical practitioner.
Anthrax is an acute bacterial disease causes by Bacillus anthracis that is a serious zoonotic disease in parts of Australia including Victoria. Almost all cases of human anthrax can be directly linked to contact with infected animals, particularly cattle, or indirectly through contact with heavily contaminated soil. Most at risk are people working with carcases especially animals that died suddenly - for example knackery workers, farmers and veterinarians. Occupational exposure can also occur via animal products such as contaminated wool or hides. Human anthrax acquired in this way is confined to the skin in 95% of cases. The lesion is unique initially presenting as an itchy painless papule which ulcerates and progresses to a black necrotic "eschar". Rarely the lungs or gastrointestinal tract are involved and these cases are fair more serious and very often fatal.
Bovine brucellosis is a serious zoonotic disease caused by Brucella abortus that has been eradicated from Australia via the Brucellosis and Tuberculosis Eradication Campaign (BTEC). Brucellosis in humans can also be caused by another species Brucelloa suis, which is transmitted from pigs, often feral pigs, and causes a severe "flu like" disease.
Cat Scratch Fever
Cat Scratch Fever is a clinical syndrome that has been reported in humans for nearly a hundred years however the aetiological agent Bartonella henslae was only identified in 1992. Several other Bartonella sp. are also implicated in the same or similar disease syndromes. Disease results from a cat scratch (or bite) and transmission of the bacteria. Papules and pustules initially form around the site of infection. The disease progresses with failure of the wound healing to regional lymph node swelling and abscessation. Those people with close contact with large numbers of cats such as veterinarians are most at risk.
Escherichia coli O157 (E coli O157)
E coli O157 is a bacterium that is part of the normal gut flora of cattle, sheep, goats, pets and wild birds. These animals carry it without causing disease however when humans are infected the toxins that the bacteria produce can cause serious illness. This can range from diarrhoea to kidney failure and fatal cases have been reported. The practice of personal hygiene particularly after contact with animal faeces is critical, as very few organisms are required to infect a human.
Leptospirosis is another bacterial disease spread by the inhalation of organisms in aerosolised urine droplets, or by direct exposure to the organism from the urine of infected animals. The people at highest risk for this disease include dairy farmers, piggery workers and stock transporters, but any person handling livestock or native wildlife is at risk. An effective vaccine is available for dairy cows, and should be administered to all milking cattle to protect both the cattle and farm workers. This has lessened the incidence of this disease markedly in Victoria over the last few years. Affected people generally suffer an acute onset of headache, fever and occasionally conjunctivitis, vomiting or abdominal pain. 249 cases of human leptospirosis were reported Australia wide 2001, 40 of those in Victoria. Affected animals may be asymptomatic carriers, or show signs of clinical disease including blood tinged urine, jaundice and eventual death.
Listeria monocytogenes is most commonly associated with clinical disease in ruminants including encephalitis, abortion, septicaemia and mastitis. It is transmitted through the ingestion of contaminated feed, often silage. It can also be transmitted through the upper respiratory tract mucosa, conjunctiva and wounds. Listeriosis in humans is predominantly a food borne disease that is associated with soft cheeses, vegetables, meats and milk. Listeria monocytogenes is cold tolerant and can survive in extended periods of cold chain storage. The clinical disease in humans can be mild and "flu like" or more serious causing abortion, disease in neonates infected prepartum, meningoencephalitis and death.
Mycobacterium infectious (non-tuberculosis)
Rarely Mycobacterium sp. that do not cause Tuberculosis can also cause zoonotic disease and a number of these are present in Australia. Mycobacterium marinum is the cause of "tank granuloma" in aquarium fish and it can be spread to humans usually causing a nodular skin lesion that often ulcerates. It usually occurs on the extremities and is acquired through direct contact with the infected tank environment and fish.
Mycobacterium avium-intracellular (MAC) causes granulomatous disease in birds and occasionally pigs, however transmission to humans is rare. Infection can cause swollen gland disease in children and disseminated disease in people with AIDS. A separate subspecies of Mycobacterium avium called Mycobacterium avium subspecies paratuberculosis is the bacterial cause of Johnes Disease in cattle and sheep. This subspecies has been implicated as a possible cause of Crohn's disease in humans however to date extensive research has not been able to conclusively demonstrate this.
Psittacosis is a disease caused by a bacterium Chlamydophila psittaci and is transmitted to humans from birds. 161 cases were diagnosed in Australia in 2005, of which 34 cases originated in Victoria. This disease is usually contracted by inhalation of dust containing faecal matter from infected birds. Clinical signs are most severe in immunosuppressed individuals, and birds carrying this disease range from being clinically normal, to very ill. All ill birds should be handled carefully, and never allow any bird to place their head near your mouth. Good personal hygiene, such as washing hands with soap after handling your bird and dampening down the floor of your bird's cage before cleaning to prevent aerosolisation of dusty faecal matter, will help prevent transmission of this disease to humans.
Q-fever is a disease caused by an organism named Coxiella burnetii. The clinical signs of this disease in humans range from no noticeable signs, to a severe flu like syndrome that may last for months. It is spread by inhalation of the organism from the placental fluids and urine of sheep, goats, cattle and native animals (i.e. bandicoots, wallabies etc.). Affected animals appear normal. The people most at risk of contracting this disease are abattoir workers (particularly those dealing with foetuses), veterinarians, shearers and farm workers. An effective vaccine is available for people at risk, but it cannot be administered without first testing for previous exposure. If an already immune person is vaccinated, a severe local reaction at the injection site will occur. In 2005, 355 cases of Q-fever were diagnosed Australia-wide.
Salmonella sp. are bacteria that live in the intestinal tract of carrier animals of many species including livestock, poultry and reptiles. Infective numbers of the bacteria are shed into the faeces of these animals particularly during periods of stress such as being yarded and transported. Other animals and humans can ingest the salmonella bacteria through direct or indirect contact with faecal material and infection then produces gastroenteritis.
Streptococcus suis is a bacterial infection carried by pigs that may be apparently healthy. Humans are infected most often through skin wounds or rarely by inhalation. The disease caused can include meningitis and be fatal. Good stock management to eliminate the agent in the herd and comprehensive personal hygiene are very important in preventing this disease.
Bovine tuberculosis has been eradicated in Australia through the Brucellosis and Tuberculosis Eradication Campaign (BTEC) however it is worthy to remember that this very serious zoonosis is still present in many overseas countries. Most people are immunised in childhood with the BCG immunisation, which provides substantial but not absolute protection. Before travelling to any foreign destination where Bovine Tuberculosis is still common it is important to have a current vaccination status and to practice good personal hygiene and caution when handling potentially infectious cattle. This includes not drinking raw (un-pasteurised) milk.
Yersiniosis (Yersinia enterocolitica)
Yersinia enterocolitica is a bacterium that can replicate at refrigerator temperatures and exists as a number of different subgroups called serotypes. Serotype 3 and 9 cause most of the disease in humans and are zoonotic agents that are common in dogs, cats and pigs. Transmission to humans occurs through contact with a household pet that is shedding the bacterium in their faeces, by similar direct contact with pigs or by consumption of undercooked pork.
Several other Yersinia sp are zoonotic agents including Yersinia pseudotuberculosis, which causes gastrointestinal disease in humans and systemic granulomatous disease in cats.
Diseases caused by Protozoa
Cryptosporidiosis is caused by a protozoan that is carried in the gut of a number of livestock species including calves, lambs, goats and deer. It is shed in faeces and transmitted to humans either through direct contact with dung or via contaminated drinking water. Swimming in contaminated water can also result in infection. Children and immuno-compromised people are most at risk of contracting Cryptosporidiosis, for example the elderly or AIDS patients. In immune competent individuals the disease is usually restricted to a "flu-like" illness that lasts up to six weeks and is accompanied by diarrhoea and abdominal pain however in immune deficient people the disease can be fatal.
Giardia intestinalis/duodenalis is a flagellate protozoan that is present in the intestinal tract of many domestic and wild animal species and causes a diarrhoeal disease. Infection in humans and animals occurs via ingestion of contaminated water and it is necessary to understand that such water may appear to be pristine. Giardia sp. is cold tolerant and can survive some chlorination systems. Giardia can also be spread through direct contact, and infected cats and dogs are a high risk zoonotic risk to humans.
Toxoplasmosis is a parasitic protozoal disease affecting many animal species. Humans are most likely to become affected by contact with domestic cat faeces. Cats are infected after eating birds, rodents or raw meat containing the intermediate stage of this parasite. The cat then excretes an infective stage in its faeces. Toxoplasma gondii most often causes clinical disease in immunosuppressed people (including the elderly, those on immunosuppressive drugs, people with AIDS) and in women during the initial 4 months of pregnancy. It can then affect the foetus, and may cause deformities. Women who are planning a pregnancy may decide to have a blood test, to assess whether they have already been exposed to this organism. Between 25 and 50% of women of child bearing age have been exposed, and are therefore immune. However, everyone should take a few simple precautions to minimise their chance of infection.
- cook all meat well – to 65°C –this kills the organisms (they are too small to see) that may be present in meat and meat products;
- clean cat litter trays frequently - every 12 hours is recommended, or get someone else to do it for you! The organism within the faeces is not infectious until 24 hours after it is excreted from the cat;
- wear gloves when gardening to prevent exposure to old cat faeces that may be present in soil;
- don't allow your cat to catch or eat mice, birds, or uncooked meat;
Young cats are most likely to be shedding infectious stages of this parasite soon after they themselves become infected. At any one time less than 1% of mature cats will be shedding organisms in their faeces.
Diseases caused by Fungi
Figure 1. Ringworm lesions on a human arm.
Ringworm is a generic term used to describe a number of fungal skin infections in animals and man. They are called "ringworm" due to the characteristic red ringed appearance of the infection in humans and not because they are caused by worms!
Children are most commonly affected, although people of any age may develop these characteristic lesions. This range of fungal diseases is spread to people by contact with the fungal organisms. However, it is not necessary to have direct contact with an infected animal, as the fungal spores can live for months on bedding, brushes, leads etc.
Prevention of infection can be achieved by careful handling of any animals with skin lesions, and good personal hygiene. It should also be remembered that infected people might spread ringworm to their animals! Treatment for both you and your animal is simple and can be obtained from your medical practitioner (for yourself) and your veterinarian (for your animal).
Diseases caused by Parasites
Hydatid disease is caused by small tapeworms (Echinococcus granulosis) that live in the intestine of dogs, dingoes and foxes. This worm is spread from dog to dog ONLY via an intermediate host – usually sheep, wallabies or kangaroos. The sheep eat the worm eggs from pasture contaminated with dog faeces, which hatch inside them, forming cysts. These cysts are usually in the offal (particularly the liver and lung), and when a dog then eats them the life cycle is complete. Hydatid disease in humans is seen as large cysts in various organs, particularly affecting the liver.
Humans become infected by ingesting the eggs from the dog faeces, and take the place in the life cycle of the sheep, wallaby or kangaroo. Again, basic hygiene, such as washing hands with soap after gardening or touching the dog and washing vegetables that may have been contaminated by dog faeces, is important in prevention of this disease. Also, not allowing your dog access to raw offal from infected sheep or kangaroos will prevent the life cycle continuing. Treating your dogs with a de-worming tablet that contains praziquantel (a de-worming preparation specifically for tapeworm) every six weeks in rotation with a broad-spectrum de-worming preparation, will break the life cycle of the parasite and the disease in dogs will be prevented. In 1999, of the 27 cases of hydatid disease diagnosed nationally in humans, 17 were from Victoria.
Visceral larval migrans
Visceral larval migrans is a parasitic disease that primarily affects children. It is caused by a species of nematode worm (Toxocara canis or T. cati) that lives in the intestine of dogs and cats. It is spread to children when they inadvertently eat worm eggs contained in dog or cat faeces. When the animal is patted, worm eggs that are on the pet's coat may be transferred onto the hands of the child, and then into the mouth. These eggs hatch in the child's intestine and migrate around the body causing internal lesions. In the worst cases, the worms can migrate into and block the artery that supplies blood to the retina of the eye, and in some cases may cause temporary or permanent blindness.
Basic hygiene - washing hands with soap after gardening and touching the dog or cat, covering sand-pits to prevent pets defecating in them and most importantly, treating your dog and cat with a de-worming tablet recommended by your veterinarian every three months, will prevent this disease from occurring.
Diseases caused by Viruses
Hendra, Nipah and Menangle viruses
All three of these viruses are recently discovered paramyxoviruses that have Flying Foxes (fruit bats) as their reservoir host. Nipah has not been reported in Australia however it has caused illness in pigs and from pigs to humans in Malaysia. The disease syndrome in humans is a highly fatal encephalitis.
Menangle virus also causes disease in pigs and it has occurred in Australia however it has to date only caused subclinical disease in a small number of piggery workers.
Unlike Nipah and Menangle, Hendra is associated with a highly fatal respiratory disease in horses and has been diagnosed in that species on a number of occasions since it first appeared in 1994. The virus transmits from horse to human during close contact with the infected animal and two people have died, one of a respiratory syndrome and the other with encephalitis. Although no cases of Hendra have been diagnosed in either horses or humans in Victoria the possibility should always be considered because of the potential incubation period and the known presence of the reservoir species in this state.
Lyssa virus and Rabies
Australian bat lyssa virus is a virus from the same group of viruses as rabies and it is present in the Australian bat population. It was first diagnosed in 1996 in a Black Flying Fox from New South Wales. It is now known that both large bats (flying foxes) and very small bats (insect eating bats) carry the virus and can transmit it to humans. Human contact with bats occurs rarely however the serious nature of the disease means that all people who handle or may come in contact with bats should be vaccinated for this disease. Bats carrying the virus may appear healthy.
Rabies occurs in many countries and on many continents throughout the world. Currently Japan, New Zealand, Great Britain, a number of island nations and Australia are the only countries considered to be free of the virus. Only two cases of human rabies have ever been reported in Australia. The first in 1867 was a poorly documented case that occurred in Tasmania involving the death of a child and a dog. The second occurred when a ten-year-old boy died in 1987 after being bitten on the finger by a monkey in India sixteen months previously. Traditionally, in Europe and America, dogs have been associated with rabies because they can act as the carrier of the disease to man. At present however, the main reservoir of infection in these areas is wildlife. Foxes, wolves, mongooses, skunks and vampire bats have all been implicated as carriers of the disease. Normally, transmission occurs as a result of a bite from an infected animal.
Orf (scabby mouth, contagious ecthyma)
Figure 2. A scabby sore on a human hand caused by orf infection
Orf (scabby mouth, contagious ecthyma) is caused by a poxvirus and is a common disease of sheep in Australia. It is effectively prevented by vaccination of sheep with a live virus into the bare inguinal area. Lesions in people typically occur on the hands, when the virus from an infected sheep scab or the vaccine enters a cut or scratch, but lesions can then spread to the face and body if other areas are scratched and exposed to the virus. The lesion appears as an ugly, raised, scabby sore that does not heal for up to six weeks (see figure 2).