African tick bite fever is often asymptomatic or mild in clinical presentation and complications are rare. The onset of illness is typically 5–7 days after the tick bite, although in some cases it may take up to 10 days for symptoms to occur. Symptoms can persist for several days to up to three weeks. Common presenting symptoms include:

- Fever

- Headache

- Muscle aches

- Inoculation eschar, which is dead, often black, tissue around a bite site (see photo above)

- Eschars may or may not be present. "Amblyomma" ticks actively attack cattle or humans and can bite more than once. In African tick bite fever, unlike what is typically seen with other Rickettsial spotted fevers when only one eschar is identified, multiple eschars may be seen and are considered pathognomonic.

- Swollen lymph nodes near the site of the bite

- Maculopapular and/or vesicular rash

Spotted fever can be very difficult to diagnose in its early stages, and even experienced doctors who are familiar with the disease find it hard to detect.

People infected with "R. rickettsii" usually notice symptoms following an incubation period of one to two weeks after a tick bite. The early clinical presentation of Rocky Mountain spotted fever is nonspecific and may resemble a variety of other infectious and non-infectious diseases.

Initial symptoms:

- Fever

- Nausea

- Emesis (vomiting)

- Severe headache

- Muscle pain

- Lack of appetite

- Parotitis in some cases (somewhat rare)

Later signs and symptoms:

- Maculopapular rash

- Petechial rash

- Abdominal pain

- Joint pain

- Conjunctivitis

- Forgetfulness

The classic triad of findings for this disease are fever, rash, and history of tick bite. However, this combination is often not identified when the patient initially presents for care. The rash has a centripetal, or "inward" pattern of spread, meaning it begins at the extremities and courses towards the trunk.

The rash first appears two to five days after the onset of fever, and it is often quite subtle. Younger patients usually develop the rash earlier than older patients. Most often the rash begins as small, flat, pink, non-itchy spots (macules) on the wrists, forearms, and ankles. These spots turn pale when pressure is applied and eventually become raised on the skin. The characteristic red, spotted (petechial) rash of Rocky Mountain spotted fever is usually not seen until the sixth day or later after onset of symptoms, but this type of rash occurs in only 35 to 60% of patients with Rocky Mountain spotted fever. The rash involves the palms or soles in as many as 80% of the patients. However, this distribution may not occur until later on in the course of the disease. As many as 15 percent of patients may never develop a rash.

Complications are rare and are not life-threatening. No deaths due to African tick bite fever have been reported. Reported complications include:

- Prolonged fever > 3 weeks in duration

- Reactive arthritis

- Moderate to severe headache

Rocky Mountain spotted fever (RMSF), also known as blue disease, is the most lethal and most frequently reported rickettsial illness in the United States. It has been diagnosed throughout the Americas. Some synonyms for Rocky Mountain spotted fever in other countries include “tick typhus,” “Tobia fever” (Colombia), “São Paulo fever” or “"febre maculosa"” (Brazil), and “"fiebre manchada"” (Mexico). It is distinct from the viral tick-borne infection, Colorado tick fever. The disease is caused by "Rickettsia rickettsii", a species of bacterium that is spread to humans by "Dermacentor" ticks. Initial signs and symptoms of the disease include sudden onset of fever, headache, and muscle pain, followed by development of rash. The disease can be difficult to diagnose in the early stages, and without prompt and appropriate treatment it can be fatal.

The name “Rocky Mountain spotted fever” is something of a misnomer. The disease was first identified in the Rocky Mountain region, but beginning in the 1930s, medical researchers realized that it occurred in many other areas of the United States. It is now recognized that the disease is broadly distributed throughout the contiguous United States and occurs as far north as Canada and as far south as Central America and parts of South America. Between 1981 and 1996, the disease was reported from every state of the United States except for Hawaii, Vermont, Maine, and Alaska.

Rocky Mountain spotted fever remains a serious and potentially life-threatening infectious disease. Despite the availability of effective treatment and advances in medical care, approximately three to five percent of patients who become ill with Rocky Mountain spotted fever die from the infection. However, effective antibiotic therapy has dramatically reduced the number of deaths caused by Rocky Mountain spotted fever. Before the discovery of tetracycline and chloramphenicol during the latter 1940s, as many as 30% of those infected with "R. rickettsii" died.

Leptospiral infection in humans causes a range of symptoms, and some infected persons may have no symptoms at all. Leptospirosis is a biphasic disease that begins suddenly with fever accompanied by chills, intense headache, severe myalgia (muscle ache), abdominal pain, conjunctival suffusion (red eye), and occasionally a skin rash. The symptoms appear after an incubation period of 7–12 days. The first phase (acute or septic phase) ends after 3–7 days of illness. The disappearance of symptoms coincides with the appearance of antibodies against "Leptospira" and the disappearance of all the bacteria from the bloodstream. The patient is asymptomatic for 3–4 days until the second phase begins with another episode of fever. The hallmark of the second phase is meningitis (inflammation of the membranes covering the brain).

Ninety percent of cases of the disease are mild leptospirosis. The rest experience severe disease, which develops during the second stage or occurs as a single progressive illness. The classic form of severe leptospirosis is known as Weil's disease, which is characterized by liver damage (causing jaundice), kidney failure, and bleeding. Additionally, the heart and brain can be affected, meningitis of the outer layer of the brain, encephalitis of brain tissue with same signs and symptoms; and lung affected as the most serious and life-threatening of all leptospirosis complications. The infection is often incorrectly diagnosed due to the nonspecific symptoms.

Other severe manifestations include extreme fatigue, hearing loss, respiratory distress, and azotemia.

The acute stage of the disease, occurring most often in the spring and summer, begins one to three weeks after infection and lasts for two to four weeks. Clinical signs include a fever, petechiae, bleeding disorders, vasculitis, lymphadenopathy, discharge from the nose and eyes, and edema of the legs and scrotum. There are no outward signs of the subclinical phase. Clinical signs of the chronic phase include weight loss, pale gums due to anemia, bleeding due to thrombocytopenia, vasculitis, lymphadenopathy, dyspnea, coughing, polyuria, polydipsia, lameness, ophthalmic diseases such as retinal hemorrhage and anterior uveitis, and neurological disease. Dogs that are severely affected can die from this disease.

Although people can get ehrlichiosis, dogs do not transmit the bacteria to humans; rather, ticks pass on the "ehrlichia" organism. Clinical signs of human ehrlichiosis include fever, headache, eye pain, and gastrointestinal upset. It is quite similar to Rocky Mountain spotted fever, but rash is not seen in patients.

East Coast fever (theileriosis) is an animal disease in Africa caused by the protozoan parasite "Theileria parva". It excludes diseases caused by other "Theileria"

Signs and symptoms include fever, headache, muscle pain, cough, and gastrointestinal symptoms. More virulent strains of "O. tsutsugamushi" can cause hemorrhaging and intravascular coagulation. Morbilliform rash, eschar, splenomegaly, and lymphadenopathies are typical signs. Leukopenia and abnormal liver function tests are commonly seen in the early phase of the illness. Pneumonitis, encephalitis, and myocarditis occur in the late phase of illness.

Acute scrub typhus appears to improve viral loads in patients with HIV. This interaction is challenged by an "in vitro" study.

Depending on the site of infection, tularemia has six characteristic clinical variants: ulceroglandular (the most common type representing 75% of all forms), glandular, oropharyngeal, pneumonic, oculoglandular, and typhoidal.

The incubation period for tularemia is one to 14 days; most human infections become apparent after three to five days. In most susceptible mammals, the clinical signs include fever, lethargy, loss of appetite, signs of sepsis, and possibly death. Nonhuman mammals rarely develop the skin lesions seen in people. Subclinical infections are common, and animals often develop specific antibodies to the organism. Fever is moderate or very high, and tularemia bacilli can be isolated from blood cultures at this stage. The face and eyes redden and become inflamed. Inflammation spreads to the lymph nodes, which enlarge and may (mimicking bubonic plague). Lymph node involvement is accompanied by a high fever.

Horses are the most susceptible host with close to 90%

mortality of those affected, followed by mules (50%) and donkeys (10%). African donkeys and zebras very rarely display clinical symptoms, despite high virus titres in blood, and are thought to be the natural reservoir of the virus. AHS manifests itself in four different forms: the pulmonary form, the cardiac form, a mild (horse sickness fever) form, and a mixed form.

Pulmonary form

The peracute form of the disease is characterized by high fever, depression, and respiratory symptoms. The clinically affected animal has trouble breathing, starts coughing frothy fluid from nostril and mouth, and shows signs of pulmonary edema within four days. Serious lung congestion causes respiratory failure and results in death in under 24 hours. This form of the disease has the highest mortality rate.

Cardiac form

This subacute form of the disease has an incubation period longer than that of the pulmonary form. Signs of disease start at day 7–12 after infection. High fever is a common symptom. The disease also manifests as conjunctivitis, with abdominal pain and progressive dyspnea. Additionally, edema is presented under the skin of the head and neck, most notably in swelling of the supra-orbital fossae, palpebral conjunctiva, and intermandibular space. Mortality rate is between 50–70% and survivors recover in 7 days.

Mild or horse sickness fever form

Mild to subclinical disease is seen in zebras and African donkeys. Infected animals may have low grade fever and congested mucous membrane. The survival rate is 100%.

Mixed form

Diagnosis is made at necropsy. Affected horses show signs of both the pulmonary and cardiac forms of AHS.

African horse sickness (AHS) is a highly infectious and deadly disease. It commonly affects horses, mules, and donkeys. It is caused by a virus of the genus Orbivirus belonging to the family Reoviridae. This disease can be caused by any of the nine serotypes of this virus. AHS is not directly contagious, but is known to be spread by insect vectors.

Scrub typhus or bush typhus is a form of typhus caused by the intracellular parasite "Orientia tsutsugamushi", a Gram-negative α-proteobacterium of family Rickettsiaceae first isolated and identified in 1930 in Japan.

Although the disease is similar in presentation to other forms of typhus, its pathogen is no longer included in genus "Rickettsia" with the typhus bacteria proper, but in "Orientia". The disease is thus frequently classified separately from the other typhi.

Ehrlichiosis (; also known as canine rickettsiosis, canine hemorrhagic fever, canine typhus, tracker dog disease, and tropical canine pancytopenia is a tick-borne disease of dogs usually caused by the organism "Ehrlichia canis". "Ehrlichia canis" is the pathogen of animals. Humans can become infected by "E. canis" and other species after tick exposure. German Shepherd Dogs are thought to be susceptible to a particularly severe form of the disease, other breeds generally have milder clinical signs. Cats can also be infected.

Anaemia may be severe and result in cardiovascular changes such as an increase in heart rate. Blood in the urine may occur due to the lysis of red blood cells. General systemic signs such as diarrhea, anorexia and weight loss may also be present. A blood smear stained with Giemsa should be observed for identification of infected red blood cells and will allow definitive diagnosis.

Anaplasmosis is a disease caused by a rickettsial parasite of ruminants, "Anaplasma" spp. The microorganism is gram-negative, and infects red blood cells. It is transmitted by natural means through a number of haematophagous species of ticks. Anaplasmosis can also be transmitted by the use of surgical, dehorning, castration, and tattoo instruments and hypodermic needles that are not disinfected between uses.

Tularemia, also known as rabbit fever, is an infectious disease caused by the bacterium "Francisella tularensis". Symptoms may include fever, skin ulcer, and enlarged lymph nodes. Occasionally a form that results in pneumonia or a threat infection may occur.

The bacterium is typically spread by ticks, deer flies, or contact with infected animals. It may also be spread by drinking contaminated water or breathing in contaminated dust. It does not spread directly between people. Diagnosis is by blood tests or cultures of the infected site.

Prevention is by using insect repellent, wearing long pants, rapidly removing ticks, and not disturbing dead animals. Treatment is typically with the antibiotic streptomycin. Gentamicin, doxycycline, or ciprofloxacin may also be used.

Between the 1970s and 2015 around 200 cases are reported in the United States a year. Males are affected more often than females. It occurs most frequently in the young and the middle aged. In the United States most cases occur in the summer. The disease is named after Tulare County, California were the disease was discovered in 1911. A number of other animals, such as rabbits, may also be infected.

In the subgroup of patients where an inoculating event was noted, the mean incubation period of acute melioidosis was 9 days (range 1–21 days). Patients with latent melioidosis may be symptom-free for decades; the longest period between presumed exposure and clinical presentation is 62 years. The potential for prolonged incubation was recognized in US servicemen involved in the Vietnam War, and was referred to as the "Vietnam time-bomb". A wide spectrum of severity exists; in chronic presentations, symptoms may last months, but fulminant infection, particularly associated with near-drowning, may present with severe symptoms over hours.

Symptoms usually appear 2 to 4 weeks after exposure. There are four general types of infection including localized, pulmonary, blood-borne, or disseminated throughout the body. The type and location of the infection usually determines which symptoms appear first as well as which symptoms are more prominent.

Patients with melioidosis usually present with fever. Pain or other symptoms may be suggestive of a clinical focus, which is found in around 75% of patients. Such symptoms include cough or pleuritic chest pain suggestive of pneumonia, bone or joint pain suggestive of osteomyelitis or septic arthritis, or cellulitis. Intra-abdominal infection (including liver and/or splenic abscesses, or prostatic abscesses) do not usually present with focal pain, and imaging of these organs using ultrasound or computed tomography should be performed routinely. In one series of 214 patients, 27.6% had abscesses in the liver or spleen (95% confidence interval, 22.0% to 33.9%). "B. pseudomallei" abscesses may have a characteristic "honeycomb" or "swiss cheese" architecture (hypoechoic, multiseptate, multiloculate) on CT.

Regional variations in disease presentation are seen: parotid abscesses characteristically occur in Thai children, but this presentation has been described only once in Australia. Conversely, prostatic abscesses are found in up to 20% of Australian males, but are rarely described elsewhere. An encephalomyelitis syndrome is recognised in northern Australia.

Patients with melioidosis usually have risk factors for disease, such as diabetes, thalassemia, hazardous alcohol use, or renal disease, and frequently give a history of occupational or recreational exposure to mud or pooled surface water. However, otherwise healthy patients, including children, may also get melioidosis.

In up to 25% of patients, no focus of infection is found and the diagnosis is usually made on blood cultures or throat swab. Melioidosis is said to be able to affect any organ in the body except the heart valves (endocarditis). Although meningitis has been described secondary to ruptured brain abscesses, primary meningitis has not been described. Less common manifestations include intravascular infection, lymph node abscesses (1.2–2.2%), pyopericardium and myocarditis, mediastinal infection, and thyroid and scrotal abscesses and ocular infection.

Mortality can be up to 100%, with death occurring around 18–30 days after the initial attachment of infected ticks, because the incubation required is around 10–25 days, and the parasite spreads quickly and is rather aggressive.

Clinical signs for diagnosis include, but are not limited to, fever and enlarged lymph nodes near the tick bite(s). Smears and stains can also be done to check for the parasite. Schizonts (meronts, or segmentors) can be found in infected lymphocytes. Pathology includes anorexia, dyspnea, corneal opacity, nasal discharge, frothy nasal discharge, diarrhea, pulmonary edema, leukopenia, and anemia. Endemic cattle given medication sometimes recover to varying degrees, or death follows due to blocked capillaries and parasites infecting the central nervous system. Cattle that are endemic and manage to survive, tend to be carriers.

A form of East Coast fever called corridor disease is observed when the organism is transmitted from the African buffalo to cattle. Another form, called January disease, only occurs over the winter months in Zimbabwe due to the tick lifecycle.

For diagnosis, "post mortem" findings are characteristic and mainly include damage to the lymphoid and respiratory systems.

In humans, the virus can cause several syndromes. Usually, sufferers have either no symptoms or only a mild illness with fever, headache, muscle pains, and liver abnormalities. In a small percentage of cases (< 2%), the illness can progress to hemorrhagic fever syndrome, meningoencephalitis (inflammation of the brain and tissues lining the brain), or affect the eye. Patients who become ill usually experience fever, generalised weakness, back pain, dizziness, and weight loss at the onset of the illness. Typically, people recover within two to seven days after onset.

About 1% of people with the disease die of it. In livestock, the fatality level is significantly higher. Pregnant livestock infected with RVF abort virtually 100% of foetuses. An epizootic (animal disease epidemic) of RVF is usually first indicated by a wave of unexplained abortions.

Other signs in livestock include vomiting and diarrhoea, respiratory disease, fever, lethargy, anorexia and sudden death in young animals.

Leptospirosis is an infection caused by corkscrew-shaped bacteria called "Leptospira". Signs and symptoms can range from none to mild such as headaches, muscle pains, and fevers; to severe with bleeding from the lungs or meningitis. If the infection causes the person to turn yellow, have kidney failure and bleeding, it is then known as Weil's disease. If it also causes bleeding into the lungs then it is known as severe pulmonary hemorrhage syndrome.

Up to 13 different genetic types of "Leptospira" may cause disease in humans. It is transmitted by both wild and domestic animals. The most common animals that spread the disease are rodents. It is often transmitted by animal urine or by water or soil containing animal urine coming into contact with breaks in the skin, eyes, mouth, or nose. In the developing world the disease most commonly occurs in farmers and poor people who live in cities. In the developed world it most commonly occurs in those involved in outdoor activities in warm and wet areas of the world. Diagnosis is typically by looking for antibodies against the bacterium or finding its DNA in the blood.

Efforts to prevent the disease include protective equipment to prevent contact when working with potentially infected animals, washing after this contact, and reducing rodents in areas people live and work. The antibiotic doxycycline, when used in an effort to prevent infection among travellers, is of unclear benefit. Vaccines for animals exist for certain type of "Leptospira" which may decrease the risk of spread to humans. Treatment if infected is with antibiotics such as: doxycycline, penicillin, or ceftriaxone. Weil's disease and severe pulmonary haemorrhage syndrome result in death rates greater than 10% and 50%, respectively, even with treatment.

It is estimated that seven to ten million people are infected by leptospirosis per year. The number of deaths this causes is not clear. The disease is most common in tropical areas of the world but may occur anywhere. Outbreaks may occur in slums of the developing world. The disease was first described by physician Adolf Weil in 1886 in Germany. Animals which are infected may have no symptoms, mild symptoms, or severe symptoms. Symptoms may vary by the type of animal. In some animals "Leptospira" live in the reproductive tract, leading to transmission during mating.

Melioidosis is an infectious disease caused by a Gram-negative bacterium, "Burkholderia pseudomallei", found in soil and water. It is of public health importance in endemic areas, particularly in northeast Thailand, Vietnam, and northern Australia. It exists in acute and chronic forms. Signs and symptoms may include pain in chest, bones, or joints; cough; skin infections, lung nodules, and pneumonia.

"B. pseudomallei" was previously classed as part of the "Pseudomonas" genus; until 1992, it was known as "Pseudomonas pseudomallei". It is phylogenetically related closely to "Burkholderia mallei" which causes glanders, an infection primarily of horses, donkeys, and mules. The name melioidosis is derived from the Greek "melis" (μηλις) meaning "a distemper of asses" with the suffixes -oid meaning "similar to" and -osis meaning "a condition", that is, a condition similar to glanders.

The incubation period of the chikungunya virus ranges from one to twelve days, and is most typically three to seven. The disease may be asymptomatic, but generally is not, as 72% to 97% of those infected will develop symptoms. Characteristic symptoms include sudden onset with high fever, joint pain, and rash. Other symptoms may occur, including headache, fatigue, digestive complaints, and conjunctivitis.

Information gained during recent epidemics suggests that chikungunya fever may result in a chronic phase as well as the phase of acute illness. Within the acute phase, two stages have been identified: a viral stage during the first five to seven days, during which viremia occurs, followed by a convalescent stage lasting approximately ten days, during which symptoms improve and the virus cannot be detected in the blood. Typically, the disease begins with a sudden high fever that lasts from a few days to a week, and sometimes up to ten days. The fever is usually above and sometimes reaching and may be biphasic—lasting several days, breaking, and then returning. Fever occurs with the onset of viremia, and the level of virus in the blood correlates with the intensity of symptoms in the acute phase. When IgM, an antibody that is a response to the initial exposure to an antigen, appears in the blood, viremia begins to diminish. However, headache, insomnia and an extreme degree of exhaustion remain, usually about five to seven days.

Following the fever, strong joint pain or stiffness occurs; it usually lasts weeks or months, but may last for years. The joint pain can be debilitating, often resulting in near immobility of the affected joints. Joint pain is reported in 87–98% of cases, and nearly always occurs in more than one joint, though joint swelling is uncommon. Typically the affected joints are located in both arms and legs, and are affected symmetrically. Joints are more likely to be affected if they have previously been damaged by disorders such as arthritis. Pain most commonly occurs in peripheral joints, such as the wrists, ankles, and joints of the hands and feet as well as some of the larger joints, typically the shoulders, elbows and knees. Pain may also occur in the muscles or ligaments.

Rash occurs in 40–50% of cases, generally as a maculopapular rash occurring two to five days after onset of symptoms. Digestive symptoms, including abdominal pain, nausea, vomiting or diarrhea, may also occur. In more than half of cases, normal activity is limited by significant fatigue and pain. Infrequently, inflammation of the eyes may occur in the form of iridocyclitis, or uveitis, and retinal lesions may occur.

Temporary damage to the liver may occur.

Rarely, neurological disorders have been reported in association with chikungunya virus, including Guillain–Barré syndrome, palsies, meningoencephalitis, flaccid paralysis and neuropathy. In contrast to dengue fever, Chikungunya fever very rarely causes hemorrhagic complications. Symptoms of bleeding should lead to consideration of alternative diagnoses or co-infection with dengue fever or coexisting congestive hepatopathy.

In the acute form of the disease caused by highly virulent strains, pigs may develop a high fever, but show no other noticeable symptoms for the first few days. They then gradually lose their appetites and become depressed. In white-skinned pigs, the extremities turn blueish-purple and hemorrhages become apparent on the ears and abdomen. Groups of infected pigs lie huddled together shivering, breathing abnormally, and sometimes coughing. If forced to stand, they appear unsteady on their legs. Within a few days of infection, they enter a comatose state and then die. In pregnant sows, spontaneous abortions occur. In milder infections, affected pigs lose weight, becoming thin, and develop signs of pneumonia, skin ulcers, and swollen joints.

Symptoms include severe headache, a sustained high fever, cough, rash, severe muscle pain, chills, falling blood pressure, stupor, sensitivity to light, delirium and death. A rash begins on the chest about five days after the fever appears, and spreads to the trunk and extremities. A symptom common to all forms of typhus is a fever which may reach 39 °C (102 °F).

Brill-Zinsser disease, first described by Nathan Brill in 1913 at Mount Sinai Hospital in New York City, is a mild form of epidemic typhus which recurs in someone after a long period of latency (similar to the relationship between chickenpox and shingles). This recurrence often occurs in times of relative immunosuppression, which is often in the context of malnutrition and other illnesses. In combination with poor sanitation and hygiene which leads to a greater density of lice, this reactivation is why typhus forms epidemics in times of social chaos and upheaval.

Acute: The acute form is a sudden onset of the disease at full-force. Symptoms include high fever, anemia (due to the breakdown of red blood cells), weakness, swelling of the lower abdomen and legs, weak pulse, and irregular heartbeat. The horse may die suddenly.

Subacute: A slower, less severe progression of the disease. Symptoms include recurrent fever, weight loss, an enlarged spleen (felt during a rectal examination), anemia, and swelling of the lower chest, abdominal wall, penile sheath, scrotum, and legs.

Chronic: The horse tires easily and is unsuitable for work. The horse may have a recurrent fever and anemia, and may relapse to the subacute or acute form even several years after the original attack.

A horse may also not appear to have any symptoms, yet still tests positive for EIA antibodies. Such a horse can still pass on the disease. According to most veterinarians, horses diagnosed EIA positive usually do not show any sign of sickness or disease.

EIA may cause abortion in pregnant mares. This may occur at any time during the pregnancy if there is a relapse when the virus enters the blood. Most infected mares will abort, however some give birth to healthy foals. Foals are not necessarily infected.

Studies indicate that there are breeds with a tolerance to EIA.

Recent studies in Brazil on living wild horses have shown that in the Pantanal, about 30% of domesticated and about 5.5% of the wild horses are chronically infected with EIA.