Zoonotic Diseases: Definition, Causes, Impact, and Control

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Public Health Impact of Animal Diseases (Zoonoses)

In the last 100 years, scientific and technological progress in the field of veterinary public health has shed more light on the epizootiology of livestock diseases, which are transmissible to humans. These diseases of livestock and those of other lower vertebrate animals like monkeys, dogs, cats, rabbits, etc. are commonly referred to as zoonoses, or zoonotic diseases.

Some diseases, which were not thought to be very important in the past, are now emerging as serious zoonotic diseases. For example, in 1988, a 7-year-old girl was reported to have ascending motor (neurological) paralysis, lysis, and speech difficulties. She was diagnosed as having tick paralysis. The tick, which was found to have transmitted this disease to the girl, is not a vertebrate animal but a parasite of dogs, cats, cattle, and other livestock from which the tick itself was infected before biting and transmitting the disease to the girl. Also in 2006, a woman was reported to have died of Avian Influenza in Nigeria.

What are Zoonotic Diseases (Zoonoses)?

According to FAO/WHO (Food and Agricultural Organization/World Health Organization), "zoonoses are defined as those diseases and infections that are naturally transmitted between vertebrate animals and humans.".

From the above example, zoonoses has been made to include not only infections and diseases acquired by humans from lower animals (whether vertebrate or non-vertebrate), but also diseases induced by non-infective agents like toxins and poisons from foods of animal origin, e.g., clostridium or salmonella poison from eggs and meat. Thus, tick paralysis is one of the emerging zoonosis.

Over 80% of human infections are suspected to be zoonotic, and many more zoonoses are still being discovered as emerging and re-emerging zoonoses.

Classification of Zoonoses

Zoonoses could be classified using the following criteria:

  • Aetiology: viral, bacterial, parasitic, rickettsal, and fungal
  • Reservoir Host: Anthropozoonoses, Zooanthroponoses. Amphixenoses
  • Infectious Cycle and Prospects for Control
    • Direct Zoonoses: Transmission's by contact or vehicle and biologically requires a single vertebrate reservoir species to maintain the infectious cycle, e.g., rabies, brucellosis, and trichinosis.
    • Cyclozoonoses: biologically require at least two species of vertebrate animals to complete their infectious cycles, e.g., taeniases and hydatid diseases.
    • Metazoonoses: transmitted to vertebrate hosts by invertebrates and dependent upon invertebrate vector intermediate hosts for the completion of their infectious cycles, e.g., arborviral infections, fasioliasis, plague, etc.
    • Saprozoonoses: They require organic or other inanimate matter as a reservoir, e.g., cutaneous larvae.

Why Do We Study Zoonoses?

We study zoonoses for various reasons; among these are:

  1. Man and animals are in direct competition for food and water, e.g., rodents;
  2. Man consumes vertebrate hosts, and several bacterial diseases are spread via this process.
  3. Man keeps vertebrate hosts as pets, e.g., rabies;
  4. As a common ancestor, we all evolved from a single-cell organism about 3 billion years ago.
  5. Primarily to improve public health.

Major Bacterial Zoonoses

1. Brucellosis

Synonyms: undulant fever, Gillbrata fever, Bang's disease (in cattle), contagious abortion (in cattle), Mediterranean fever (in men), Malta fever.

Etiology: Brucella abortus (from cattle) Brucella melitensis (from goat).

Brucellosis is a worldwide systemic disease of cattle, goats, sheep, swine, etc. that causes a storm of abortions, particularly in cattle. It is also a systemic infection (involving the blood system) in man, which is characterized by fever of variable duration, headache, weakness, profuse sweating, chills, and general body aches. It may also cause non-purulent meningitis and penumonitis in men.

The disease may last from several weeks to several years in a patient who is not diagnosed correctly and treated in time. In chronic sufferers, the patient, if male, shows swollen, inflamed testicles (orchitis) and bone disease problems (osteomyelitis). The reservoirs of B. abortus are cattle and other wild ruminants, while the reservoirs of B. melitensis are goats. It is from these sources that human beings get infected.

Infections can occur by drinking the milk of infected cows or goats that is not properly pasteurized (boiled). Humans can also be infected by contact with the tissues, blood, urine, and vaginal discharges of aborted animals that are also infected.

Control is achieved by ensuring good hygiene practices at dairy farms. Boil the fresh milk of cattle, if not pasteurized, before drinking it. The veterinaries vaccinate animals against this disease and control its spread. No immunization of man is necessary, and the disease is curable if a diagnosis is correctly made.

2. Anthrax

Synonyms: malignant pustule, malignant edema, Woolsorters' disease, Charbon.

Etiology: Bacillus anthracis

Anthrax is primarily a bacterial disease of livestock, cattle, sheep, and goats in particular. It also occurs in wild and zoo animals. The disease is infective to human beings who work, particularly with cattle and in establishments where wool and hides are stored. It occurs among industrial workers in factories that process wool, goatskins, and other hides.

In man, there are three clinical forms of anthrax, namely: the cutaneous (skin form), the pulmonary or respiratory (affecting the lungs), and the gastro-intestinal (affecting the stomach and intestines). Of these, the cutaneous form is the most common and accounts for more than 90% of human anthrax. The initial lesion is a vesicle (a small blister), which develops into a black, painless, or slightly painful swelling. About 25–20% of people die from untreated cutaneous anthrax.

The pulmonary form of anthrax is contacted by the inhalation of spores of B. anthracis. At the onset of the disease, the patient suffers mild symptoms of common upper respiratory tract infections. Some 3-5 days later, symptoms become more acute, with fever, shock, and death, with the mortality rate in this form of anthrax being very high. The gastrointestinal anthrax is contracted when contaminated meat is eaten. This is followed by severe gastro-enteritis with vomiting and blood in the stools. Mortality is also very high.

Anthrax is found worldwide, and there is no true reservoir host for this bacterium. Animals that contact the disease before they infect people do so by consuming garbage or drinking water that has been contaminated with spores of B. anthracis, especially in places where anthrax-infected animals have died. Anthrax is treatable in hospitals, provided an early diagnosis is made.

The veterinarians who will routinely vaccinate susceptible cows in endemic areas or use drugs to treat clinically ill animals also control it. Veterinary public health doctors also prevent innocent members of the public from being infected by condemning anthrax-suspect animals at the abattoirs.

3. Tuberculosis (TB)

This is a worldwide disease of man caused by Mycobacterium tuberculosis. Tuberculosis can also be caused by M. bovis (TB of cows) and M. avium (TB of birds). Human beings contact the disease either by drinking the organism from contaminated milk, eating infected meat that has not been properly cooked, or from a sufferer.

In the past, people thought that the bovine (cattle) type of tuberculosis (M. bovis) was extra-pulmonary (i.e., found outside the lungs), but today it is known that it can also cause generalized tuberculosis. Again, people thought that avian tuberculosis could not affect human beings. But now it is also known that it does affect them. The only difference is that they do not occur at the same frequency as the human type of tuberculosis caused by M. tuberculosis.

In countries where raw and unpasteurized milk is drunk and people have extra-pulmonary tuberculosis, such clinical signs as cervical adentities (inflammation of the backbone) do occur. This may lead to bending of the backbone hump. There may also be an infection of the intestine and bladder, leading to difficulty in digestion and urinating.

In order to control tuberculosis, the sufferer should go to the hospital. Veterinary public health officers also prevent human infections by condemning tuberculosis in cows. There are many other bacterial zoonoses, like tularemia caused by Francisella (Pasteruella), tularensis, shigellosis caused by Shigella, dysenteric, and salmonellosis (enteric) caused by Salmonella.

4. Botulism

Synonyms: A Hantiasis, Limbemeck

Etiology: Clostridiun botulinum

Botulism is a toxin-producing bacterial organism that rarely affects humans, but when it does, death is often the result. The toxin (not an infection) produced by microorganisms is a serious food poison that,  when eaten by humans, is often characterized by extreme dryness of the mouth, weakness, and other serious nervous system problems. Vision is blurred or one sees double images of an object; vomiting and diarrhea may follow constipation.

One-third of patients who are poisoned by this toxin usually die within 3 to 7 days as a result of respiratory or other nervous system failure. The intestinal tracts of domestic animals, including fish, are the reservoir areas of Clostridium species. They are also found in soil, water, and canned food products. Human beings contract the disease when they ingest the toxins by eating contaminated foods, vegetables, fruits, sardines, meatballs, jams, and corn beef.

Control is the careful examination of what one eats. Watch the tin foods bought. For example, sardine tins that are swollen up may be a result of gas produced by the bacteria. This may be poisonous if eaten. Adequate cooking or boiling destroys many pathogens and toxins; hence, people must be educated on the processing and preparation of commercial canned and preserved foods.

5. Campylobacteriosis

This is an enteric bacterial infection in man caused by Campylobacter jejune or C. fetus. It causes a severe abdominal or stomach ache with diarrhea, fever-like feelings, vomiting, and weakness. It is easily mistaken for typhoid fever disease.

The disease is found worldwide because cattle, sheep, pigs, cats, dogs, and even chickens can be reservoirs of this disease. A human being contacts it through improperly cooked food or food contaminated after cooking and through unpasteurized milk.

If livestock are sick and they are not well taken care of by qualified veterinary doctors, such livestock become chronic carriers of the disease. They then remain the primary source of human infections. The disease in livestock is, therefore, best controlled and eradicated by veterinarians. Human beings that are sick should go to the hospital for medical care.

Viral Zoonoses

Some examples of viral zoonoses are cow pox, monkey pox, Western and Eastern equine encephalitis, foot and mouth disease, non-human primate hepatitis (contacted from monkeys), influenza, Lassa fever, yellow fever, Rift Valley fever, rabies, etc. Some of them are discussed below.

1. Rabies (Hydrophobia Lyssa)

Rabies is primarily a disease of warm-blooded animals caused by the rubies virus, a rhadovirus of the genus Lyssavirus. The organism is bullet-shaped. Rabies-related viruses that exist in Africa (Mokola and Duvenhage) have been associated rarely with fatal rabies like human illness.

Infected dogs are the most commonly known animals that transmit rabies to humans through biting. However, infected cats, cows, donkeys, and mules also infect men, usually by biting. The disease occurs worldwide, with an estimated 30,000 deaths a year, almost all in developing countries.

The only areas free of rabies in the animal population at present include Australia, New Zealand, New Guinea, Japan, Hawaii, Taiwan, the Pacific Islands, the UK, the Irish mainland, Norway, Sweden, Portugal, and some of the West Indies and Atlantic Islands. Dogs transmit urban (or canine) rabies, whereas sylvatic rabies is a disease of wild carnivores and bats (vampire, frugivorous, and insectivorous), with sporadic spillover to dogs, cats, and livestock.

Rabies transmission occurs when the virus-laden saliva of a rabid animal is introduced by a bite or scratch (or, very rarely, into a fresh break in the skin or through intact mucous membranes). The incubation period is usually 2 to 8 weeks, occasionally as short as 5 days, or as long as a year or more. The incubation period depends on the severity of the wound, the site of the wound in relation to the richness of the nerve supply and its distance from the brain, the amount of virus introduced, protection provided by clothing, and other factors.

Period of Communicability: in dogs and cats, for 3 to 10 days clinical signs for (rarely over 3 days) and before onset of clinical throughout the course of the disease.

Methods of Rabies Control

(A) Preventive Measures

  • Register, license, and vaccinate all dogs; collect and destroy ownerless animals and stray animals as Vaccinate all cats as a preventive measure. Educate pet owners and the public.
  • Detain and clinically observe for 10 days any healthy-appearing dog or cat known to have bitten a person. Dogs and cats showing suspicious signs of rabies should be sacrificed and tested for rabies.
  • Destroy immediately unvaccinated dogs and cats bitten by known rabid animals; if detention is elected, hold the animal in an approved pound or kennel for at least 6 months under veterinary supervision and vaccinate against rabies 30 days before release. Individuals at high risk should receive pre-exposure immunization.
  • The prevention of rabies after animal bites ("post-exposure prophylaxis") consists of all;
    • Physical removal of the virus by proper cleansing of the bite wound and
    • Specific immunologic protection.

(B) Control of Patient Contacts and the Immediate Environment

  • Report to a local health authority;
  • Isolation: contact isolation for respiratory secretions for the duration of the illness;
  • Concurrent disinfection of saliva and articles soiled therewith;
  • Immunization of Contact: contacts that have an open wound or mucous membrane exposure to the patient's saliva should receive anti-rabies-specific treatment;
  • Investigation of contacts and source of infection;
  • Specific treatment for clinical rabies, intensive supportive medical care.

(C) Epizootic Measures

It is applicable only to animals. A sporadic disease in men.

(D) Disaster Implications

A potential problem if the disease is freshly introduced or enzootic in an area where there are many stray dogs or wild reservoir animals.

(E) International Measures

  • Strict compliance by common carriers and travelers with national law and regulations requiring quarantine, vaccination of animals, certificates of health and origin, etc.
  • W.H.O. collaboration centers.

2. Yellow Fever

There are two types of yellow fever. One is called the Cosmopolitan Yellow Fever (CYF), and the other is the Jungle Yellow Fever (JYF). The first one (CYF) is relatively harmless, but the JYF is very dangerous. It is the type that has been causing epidemics and deaths in Nigeria.

An RNA virus causes the Jungle Yellow Fever, which is transmitted by the bite of an infected mosquito. This virus exists naturally in monkeys and circulates in the jungle between monkeys and mosquitoes. It is when an unvaccinated man who goes into the jungle is bitten by an infected mosquito that he becomes infected. When this man returns to the village or town and he is bitten by another mosquito, that mosquito now becomes infected, and it bites other people, who then may come down with yellow fever. The disease starts by causing colds and chills. It later damages the liver, which leads to jaundice (yellow eyeballs, pale hands and faces), weakness, and death. Some patients may be paralyzed for some time, but later they get well.

Control is achieved by people having a protective vaccination. The prevention of mosquito bites is very essential. Breeding water containers for mosquitoes, like sardine tins, broken pots, used cans, useless plastic bowls, etc., should be destroyed. Insecticides can also be used.

3. Lassa Fever

This is another deadly viral zoonosis that was first discovered in Nigeria. Lassa is the name of a village in one of the northern Nigerian states. The disease, also caused by an RNA virus, is characterized by serious fever-like symptoms. The disease is also found in Liberia, Sierra Leone, Ivory Coast, Mali, the Central African Republic, and Senegal.

Human beings come into contact through the contamination of the environment by the reservoir host, a multimamate rat, which is found in houses and fields. The infected rats excrete the virus through their faces and urine. If food is so contaminated and eaten, then lassa fever may occur. Control is achieved by controlling rats in homes, keeping foods covered, and cleaning drinking cups and eating plates before use.

4. Monkey Pox and Cow Pox

Both cow pox and monkey pox cause a disease similar to small pox in human beings. There is also rabbit pox and mouse pox (ectromelia) that could also affect men. However, all these poxes cause mild infections, which do not usually result in death. However, they can predispose an infected person to other secondary bacterial infections.

The mode of transmission for humans is direct contact with infected animals. The disease affects all age groups, but children under 10 years are at the highest risk. However, transmission from man to man is rare.

Control is achieved by using a small pox vaccination, which is protective against some of these pox.

Parasitic Zoonoses

Some of the parasitic zoonoses of importance in our environment are: trypanosomiasis, babesiosis, malaria of non-human primates, toxoplasmosis, cutaneous and visceral ueshmaniasis.

1. African Trypanosomiasis

This disease is called sleeping sickness in men. It is caused by Trypanosoma gambiesnse in West African regions, while sleeping sickness in the East African region is caused by T. rhodesiens. They are both transmitted to humans by the bite of an infected fly called Glossina.

The mechanism of a fly bite is simple. When a fly lands on the skin, it pierces the skin with its sharp mouth parts. It then deposits saliva into the surrounding area, waits for some time, and then suckers it back. In this process, parasites of the trypanosomes resident in the gut of the fly go into the blood stream. The bitten man may then become infected.

Flies get the infection from infected cows and other wild ruminants. The disease can be transmitted to many people at the same time, so we have an epidemic outbreak. The disease manifests itself in man first by causing sleeplessness (insomnia), difficulty breathing, altered vision, anemia, muscular pain, joint pain, and abdominal pain.

Later, it progresses to weakness and sleeping sickness, which, if not cured in time, will lead to coma and death.

Control: controlling flies, protecting oneself when one goes to the bush. Sick people should go to the hospital in time.

2. Malaria of non-human Primates

Malaria is a common blood parasite that people are familiar with. It is caused by Plasmodium vivax, P. malaria, or P. falciparum. Mosquitoes transmit this disease to humans. However, non-human primate malarias are those that have monkeys, apes, baboons, etc. as their hosts. There are about 20 species of this type of plasmodium, and they can infect humans as well. They are transmitted by vector mosquitoes called anopheles.

Many of the non-human primate malarias cause low-grade fevers. However, much work is still required on the pathogenesis of this type of human malaria and its vectors. Control is similar to the control of human malaria and its vectors.

3. Babesiosis

This disease is primarily a blood parasite of ruminants, e.g., cows, caused by Babesia bigemina or 16 other species. They infect the red blood cells of the ruminants, inside which they divide by binary fission, i.e., divide into two parts, each of which becomes an adult.

When ticks bite to suck blood from an infected cow, the tick also becomes infected. It is the infected tick that transmits the disease to man by bite, too. The spleen easily protects most infected humans. However, any person who has had his spleen removed because of one problem or another stands a nearly 100% chance of death if infected by babesiosis. The disease is treatable if diagnosed in time. Control is achieved by preventing tick bites.

4. Toxoplasmosis

This is another dangerous parasitic zoonosis that can affect human beings through the ingestion of eggs from contaminated food. Cats, in particular, when infected, serve as the final or definitive host. The adult worm called Toxoplasma gondii develops to maturity in cats, and then the eggs are shed in the cat's feces to the outside environment.

Infection is very common in men, but infected men rarely show clinical disease before the eggs are destroyed. However, in a pregnant woman, the eggs can develop into larval stages, which can migrate (larval migrants) to the fetus and cause abortion. It can also cause deformities like blindness and deafness in newborn babies.

Control is achieved by taking pet cats to veterinary clinics for periodic checkups. If one prevented cats from going into the bush to kill rats, the cat would not have it. Also, preventing food from being contaminated with cats' feces is a key control measure. Pigs can also have toxoplasmosis, so good cooking is required before eating infected pork.

Other Zoonoses

There are other zoonoses like tapeworms, hookworms, chlamydial agents like psittacosis and omtithosis, Q-fever, etc. All of which can infect human beings.

Man can be infected with the following by eating raw or undercooked fish: Clonorchis sinensis (Chinese liver fluke), Heterophyes sp. Paragonismus westermani, Diphyllobothrium latum, and Angiostrongus cantonensis. Education and prevention are the best ways to control.


Livestock and domestic animals provide food, animal products, services, and companionship to humans. This results in continuous contact and interaction between these animals and humans. It is therefore pertinent that extra care must be taken when handling, processing, preserving, consuming, and interacting with these animals and their products. This is because zoonotic diseases have direct and indirect impacts on human health, well-being, and the socioeconomic development of societies.

The various pathogens associated with livestock and domestic animals also pose a serious threat to public health and could lead to epidemics of global proportion. Hence, livestock producers, pet owners, veterinarians, processors, retailers, and consumers require continuous education and enlightenment, while the sector must be regulated at local, regional, national, and global levels.

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