Zoonotic diseases are infectious diseases that are naturally transmitted between humans and animals. They can be transmitted by bacteria, fungi, viruses, or parasites. Bacteria are chiefly responsible for propagating bacterial zoonoses.
Some of them can cause serious diseases in people while others will not show any symptom. Since zoonotic diseases amount to over 200, diagnosis is important to identify the specific disease to ensure administration of correct treatment.
Effects of zoonotic diseases in animals and humans
Some affect man and animals and can cause severe disease, even death e.g. anthrax. Others have little effect in animals but cause severe effects in man e.g. Q-fever and brucellosis.
Others can cause severe disease in animals but almost invisible effects in man e.g. Newcastle and FMD
Transmission to man
Transmission majorly occurs during the clinical stage of the disease. The risk increases when the host animal is a career i.e. does not show symptoms.
Everybody is at risk of contracting zoonotic diseases. However, those who directly deal with animals and animal products are at an increased risk.
Other people who come into contact with materials that are easily contaminated by the animal products such as soil and water also have a higher risk of infections.
Farmers, anglers, sewage treatment workers, and veterinary officers must take extra care and use the personal protective gear while executing their duties.
Classification of zoonotic diseases
We classify zoonotic diseases according to the degree of host specificity. They include:
Arthropozoonosis – diseases transmitted from animals to man.
Zooarthropozoonosis – transmitted from man to animals.
Ampixenosis – transmitted either way (from man to animals and vice versa).
Classification according to the mode of transmission
a) Direct zoonosis
These are transmissions perpetrated from one host to the other by either direct or indirect contact. These include diseases/infections like rabies.
These diseases require an intermediate vertebrate host between man and animals e.g. taeniasis
These diseases require an intermediate invertebrate host so that they become transmitters e.g. yellow fever and rift valley fever
Part of the developmental cycle of the disease-causing agent require inanimate reservoir to complete the metamorphosis e.g. hystaplasmosis and most fungal infections.
Classification according to causative agent
Here, there are either bacterial or viral zoonoses.
Bacterial zoonoses are as a result of a bacterial infection while viruses cause viral zoonoses.
Controlling zoonotic diseases
- Mass education in handling of infected animals is usually the most effective control measure. The public should be enlightened to minimize contact with animals and animal products and to use protective gear while handling animals, their products and byproducts.
- There should be strict rodent and vector control
- Public health officials should properly check meat meant for public consumption
- Proper cooking of all animal products especially meat from unfamiliar sources
- In the processing industry, there should be strict processing and hygiene procedures
- Avoid animal diseases at all costs
These are zoonoses attributed to bacteria. They include:
Anthrax is a very critical infection that cause severe effects in both man and animals. It progresses rapidly and often leads to deaths of the victims.
The bacterium Bacillus anthracis is the causative agent. It is a gram positive, non-motile, and spore forming bacterium.
Soprulation occurs upon exposure of infected material to air. It is the bacteria’s form of defensive mechanism to avoid death. The spores are resistant to heightened salting, extremes of temperature and most disinfectants.
In the presence of organic matter, the bacteria get into the material or undrained alkaline soils. They thrive in warm climates for up to 60 years. Acidic soils reduce survival rates of the spores.
Anthrax in animals
Anthrax affects all domestic animals as well as some wild animals. Ruminants are most susceptible, followed by the equine species.
The porcine (pigs) species are the least affected.
When outbreaks occur, they are usually associated with soil borne infection after a major climatic change such as a heavy downpour after a prolonged drought.
Since the bacteria sporulate when exposed to air, it is advisable to never open up a carcass. Putrefaction usually destroys the bacteria.
However, there is still some level of risk, which the animals grazing in the fields assume.
Transmission occurs through many routes including:
i) Ingesting contaminated feeds or water or soil.
Animals directly pick the spores from the mentioned sources. Water contamination is very common especially from the tannery effluent and contamination with infected carcass.
Though a minor route, the animals can still get anthrax by inhaling spores in dusty air.
iii) Skin penetration
Needles used for treating animals can be a major route of transmission if it is used on multiple animals. Vectors can also transmit the disease through bite marks on the skin.
Clinical signs can be described in two forms:
a. Pre-acute form (1 – 2 hours)
Observable symptoms include:
- Sudden death (within one and two hours)
- Very high fever (42°C)
- Muscle tremors
- Severe dyspnea (difficulty in breathing)
- Congested mucus membranes
- Animal collapses and convulses before death
- Immediately after death, the animal exudes a black tarry unclotting blood through the body orifices.
b. Acute form (about 48 hours)
- High fever (42°C)
- Severe depression
- Congested mucus membranes
- Animals completely lose appetite
- Abortion in incalf animals
- Drastic reduction in milk production (milk may have blood stains or a yellow taint)
- Local oedema (usually of the tongue, throat, and the lower abdominal region)
- Animal collapses, convulses and dies
- After death, the animal exudes a tarry blood from the body orifices. The carcass lacks rigor mortis.
Post mortem lesions
Never perform post-mortem on the carcass.
You will know it is anthrax when the carcass exudes tarry blood and it lacks rigor mortis.
There is a rapid decomposition taking place in the carcass and the abdomen swells as a result. However, the limbs remain flaccid, i.e. “saw horse”.
If by any chance the carcass is opened up, you will observe echmotic haemorrhage and gross enlargement of the liver and the spleen (may double their usual sizes).
Anthrax in man
Humans also get affected through the same three routes. They include:
i. Skin penetration
When infection occurs through skin penetration, the victim will get cutaneous anthrax. It is usually a professional hazard.
Lesions occur within two to three days after contact. You will first observe pimples (pastules) that collect fluid and becomes a vesicle surrounded by a zone of hyperaemia (reddening). The fluids will change into a bloody substance, which bursts to form blisters.
With progression, the area becomes blue-black and can easily peel off. You may also notice some fever and other general symptoms of disease.
People can easily recover from this form with or without treatment provided the bacteria does not get into the circulatory system.
Results into pneumonic or pulmonary anthrax.
It is common for people who work in dusty environment, especially in the wool industry. This is the reason it is called “wool sorters’ disease”.
It results into a typical pneumonia. The victim shows the rapid onset of disease, high fever, dyspnea, and chest pains.
Bacteria in the blood travel to the heart and causes cardiac failure. Bio-terrorists have used anthrax in the past in different parts of the world.
iii. Ingestion route
Causes intestinal anthrax.
It occurs after consumption of meat from infected animals. Signs include:
- Acute diarrhea
- Severe abdominal pain
- Death can easily occur
Diagnosis is done the same way as in animals. To confirm the bacteria, isolate and identify it (gram positive and non-motile). You can also do cultural isolation of the bacteria.
- Seek medical help as soon as possible.
- Use streptomycin and penicillin in high doses.
- Use antitoxins to neutralize the effects of the toxins.
Controlling Anthrax in animals
- Prevent occurrence
- Vaccinate animals annually and bi-annually
- During outbreak, issue a notification and impose quarantine then isolate and treat all suspect cases during quarantine. Carefully handle all infected material (burry the carcasses without opening them up).
- Add quick lime on the graves to avoid sporulation
- When dealing with infected hides and skins, dispose them unless they are irradiated.
- Use a very strong disinfectant (like formalin or Lysol) if you suspect sporulation.
Controlling Anthrax in Humans
- Minimize contact with animals and use protective clothing if you have to contact them.
- Proper meat inspection is critical.
This is a very important disease in both man and animals because it affects all animal species and all the corresponding bacteria will cause disease in humans.
Brucellosis in Cattle / Bovine Brucellosis
It is one of the easiest diseases to detect because it causes abortion in all affected pregnant females. In bovine, Brucellla abortus is the causative organism. They are gram-negative coco-bacilli bacteria.
It is a fairly sensitive bacterium that will be eliminated by ordinary environmental conditions such as sunlight and heat.
The bacteria can survive in the aborted material for long periods if kept under refrigeration.
The disease has a global presence and majorly affects adult pregnant females. However, it is not limited to these potential victims alone.
The bacteria will be shed in milk, vaginal and uterine discharges, and in aborted material. The bacteria are also present in the semen of infected males.
Transmission occurs through the following routes:
When animals drop their aborted material on the pastures, they cause contamination and become a risk to the rest of the herd that uses the same pastures for feeding. Calves are contaminated through the milk as well.
ii) Normal mating / coitus
The bacteria from the male get into the female through the semen.
iii) Artificial insemination
Artificial insemination can lead to transmission if contaminated equipment / semen are used. This route of transmission can be very catastrophic if undetected early enough.
Pathogenesis of Brucella abortus
It will move from the mucus membrane of the genital system and move into uterus via the circulatory system. In the uterus, the bacteria attack the endometrial cells and begin to multiply.
Once they have reached the attacking threshold, they move into the foetus through the umbilical cord. The effect on the endometrial cells will be so severe that the foetus will not be able to survive.
The bacteria will also lodge in other areas like the mammary glands leading to chronic mastitis. It can also get into the lymphatic system and cause inflammation of the lymph vessels.
In males, the bacteria will majorly affect the testis. The epididymis will be inflamed leading to blockage and such a male will be rendered infertile.
- Storm abortions in the third stage of the pregnancy. After one or two abortions, the animals get self-cure.
- Males get sterility due to infertile ejaculations. In some instances, they may have reduced libido.
Diagnosis for Brucellosis
Tentative diagnosis is based on the clinical signs
Bacterial isolation can also work. Take samples from any part of the animal e.g. vaginal swabs, or semen. The most commonly used sample is the foetal material after abortion.
Treatment and control measures in animals
Treatment is usually not necessary in animals due to self-cure. However, you should control the disease to prevent any incidence of an outbreak.
Vaccination is very effective. The vaccine uses strain 1a. Vaccinate the heifers before serving them. Ensure the artificial insemination process is aseptic and proper.
Improve and maintain hygiene to prevent occurrence through ingestion. Properly dispose any aborted material that may contaminate the pastures.
Other species affected by brucellosis include:
Clinical signs / zoonotic importance
Males will have painful epididymis and otitis or atrophy of the testis, which may also reduce the functionality of the organ
Females abort. Causes severe brucellosis in man
Females will have either abortions or reduced litter. Males will have otitis of the testis. The bacteria localize in the lymph nodes and muscles. Pork has very high levels of contamination of the bacteria. Contamination spreads during slaughter and it is impossible to identify it during meat inspection.
Brucellosis in Humans
Brucellosis in humans is also called ardulent fever or Malta fever. This is because brucellosis was first discovered in Malta, an island in the Mediterranean Sea. It is also called ardulent because it comes and goes.
All animal species are potential pathogens for man. People can get the disease via the following routes:
Consumption of raw / underpasteurized milk and milk products can lead to infection. Contaminated pork and other animal products are a viable route as well.
b) Skin penetration
Usually a professional hazard for those dealing with sick animals and aborted material
When you accidentally touch your eyes with your fingers after touching the contaminated material, there is a likelihood that the bacteria will get into your system.
Symptoms of brucellosis in man
This disease is usually confused with malaria due to the similarity in their symptoms. For this reason, many people tend to assume it leading to advanced infection.
It takes up to two weeks to manifest its symptoms after entering the host. The observable symptoms include:
- Fever, which persists for many days. This is usually accompanied by body pains, which go away only to relapse after a few days.
- Constipation may occur
- Dry cough due to infection of the bronchus
- Characteristic night sweating
- If the disease progresses without treatment, the organism will move to the joints and cause arthritis. The disease will not go away for more than 12 months.
- There will be extreme loss of body weight due to improper feeding.
- The fatality rates are rare.
Treatment for Brucellosis in man
Seek medical attention at the nearest hospital. It may require prolonged periods of treatment and the patient may be required to take medication for several months.
Controlling brucellosis in humans
Controlling this disease is most important. You can ensure the following:
- Proper handling of animals: – use protective clothing and avoid contact if you have an open wound
- In case of an abortion, properly dispose the aborted material as well as the placenta.
- Handle milk and milk products with a lot of care. Ensure the milk is properly pasteurized before consumption. Avoid raw milk and milk products.
- Ensure there is sufficient hygiene in milking parlours, processing plants, and even abattoir.
- Constantly test for the disease.
Occurs in almost all animals in mild form and produces catastrophic consequences in humans. Tuberculosis simply means the formation of holes in the lungs. When the spaces break up and lodge in the capillaries, the situation can be fatal.
Mycobacterium tuberculosis is the causative microorganism that causes tuberculosis in cattle, sheep, and goats. All the species that affect the animals also affect humans. Mycobacteria are acid fast and resistant to a number of conditions.
The disease is inapparent in animals. It is a big public health concern in humans because the bacteria are present in the urine, saliva, faeces, and uterine discharges. Since it is shed in milk, people may not be aware of its presence since the animals do not show any symptoms.
Ingestion is the major route of transfer. Animals can also get it through coitus / during mating.
Urine and uterine discharges contaminate the pastures
The major route of transmission is through inhalation, especially in a dusty environment. Consumption of contaminated milk from inapparent animal carriers is a huge risk since people will assume that the milk is not contaminated. The disease produces exact symptoms in man as in animals.
- The bacteria are highly concentrated in the sputum.
- Transmission is ampixenotic (i.e. animals transmit to man and vice versa as well as man to man and animal to animal). Transmission between man and dog is widespread due to their close association.
- Spread between humans is very rapid.
- This disease is prevalent among people living with HIV due to their compromised immunity.
Clinical signs in animals and man
The primary site of infection is in the lungs no matter what the route of transmission is. It produces pneumonia in the lungs with the following observable symptoms:
- Constant coughing
- Chest pains
- Other signs are secondary to the infection of the lungs. The bacteria can localize in other body parts leading to other special diseases e.g.
Endometritis – when the bacteria attacks the uterus (the uterus will produce pus)
Mastitis – when the bacteria attacks the mammary gland. The bacteria will be continuously shed in milk during milking.
- During coughing, people may spit blood due to rupturing of the blood vessels. The victim gets severe septicaemia and death follows.
Diagnosis of tuberculosis
Tentative diagnosis is based on the clinical signs.
For animals, post-mortem lesions from the carcasses can be useful for comprehensive diagnosis.
Confirmation of tuberculosis infection
Conduct tuberculin test – commonly done in animals to check even the carrier state. Get an extract. An interdermal injection will elicit a series of reactions leading to swelling that will be typical for a positive case.
The most common test for humans is cultural isolation of the sputum to confirm the causative bacteria.
Treatment of tuberculosis
The drugs used for treatment are exactly the same for humans and animals.
Treating animals is discouraged as the drug residues will find their way into humans leading to resistance. The resistance to drugs will make it practically impossible to treat sick people.
Isoniazid drugs are commonly used for the treatment. They are very strong and must be used over prolonged periods to eliminate the bacteria from the system.
- Regular testing of animals using tuberculin test. Cull all positive animals; do not treat them.
- You can vaccinate the animals to prevent occurrence.
- Notify the public and quarantine in case of an outbreak. Ensure that products from these animals do not get to humans. Withhold milk and meat until there is clearance of drugs if any drug was administered.
- In humans, BCG vaccination is the most common method. BCG is usually an attenuated form of Mycobacterium bovis, which gives a lifelong protection.
[D] Bubonic Plague / Tauni (Black Death)
This is a very serious disease in humans even though it is not of much concern in domestic animals. However, rodents suffer severe disease when infected.
It is a bacterial zoonosis caused by Pasteurella pestis / Pasteurella yersinia. It is a gram negative rod-shaped coco-bacillus bacterium. These bacteria are quite prevalent in animals though they may not show the symptoms.
It is a widely distributed disease that cause acute and fatal infections in humans and rodents. It exhibits sporadic occurrences. In Kenya, it has been reported in Machakos, Nairobi, and Mombasa.
Humans usually get it from rodents. In animals, dogs and cats are the most affected and are the ones associated with infections in humans. People get bitten by the fleas from these animals and rodents and transmit the disease to them in the process.
Domestic animals usually manifest inapparent form of the disease (no observable symptoms). If they show any sign, it is very mild e.g. mild fever, low toxaemia, depression, and anorexia.
In rodents, deaths are common due to massive toxaemia.
Humans usually start to show signs one week after the bite. Observable symptoms include:
- High fever
- Swelling of lymph nodes, especially the inguinal lymph (in the thigh / groin area), that become painful after multiplication of the bacteria.
- Toxaemia will set in followed by severe weakness.
- Pneumonia will occur if the bacteria are localized in the lungs. This is considered secondary pneumonic plague, which is highly fatal and is transmissible between people. The primary disease comes from animals and is not transmissible between people.
Suspect the disease from clinical signs
Confirm disease from bacterial isolation
Blood or sputum may be used if it has the pathogen
- Aim to treat man because they exhibit visible symptoms. Seek hospitalization for the patient.
- Administer antibiotics such as streptomycin or tetracycline as advised by the medical officer.
Control measures for bubonic plague
- Avoid unnecessary contact with animals
- Eradicate fleas in animals
- Eradicate rodents from human habitation
- Vaccinate in prevalent cases
Bacterial zoonoses are hard to contain. They can wreak havoc and lead to massive losses of both income and lives.
Always seek professional advice when you suspect that your animal is suffering from any of these bacterial zoonoses.
Life is irreplaceable, seek medical attention if you spot any of the listed symptoms. It is always better to be safe than sorry.