In endemic areas, a high index of suspicion is warranted, especially with a known exposure to ticks. The diagnosis can be confirmed by using PCR. A peripheral blood smear can also be examined for intracytoplasmic inclusions called morulae.

Diagnosis is achieved most commonly by serologic testing of the blood for the presence of antibodies against the ehrlichia organism. Many veterinarians routinely test for the disease, especially in enzootic areas. During the acute phase of infection, the test can be falsely negative because the body will not have had time to make antibodies to the infection. As such, the test should be repeated. A PCR (polymerase chain reaction) test can be performed during this stage to detect genetic material of the bacteria. The PCR test is more likely to yield a negative result during the subclinical and chronic disease phases. In addition, blood tests may show abnormalities in the numbers of red blood cells, white blood cells, and most commonly platelets, if the disease is present. Uncommonly, a diagnosis can be made by looking under a microscope at a blood smear for the presence of the "ehrlichia" morulae, which sometimes can be seen as intracytoplasmic inclusion bodies within a white blood cell.

The prognosis is good for dogs with acute ehrlichiosis. For dogs that have reached the chronic stage of the disease, the prognosis is guarded. When bone marrow suppression occurs and there are low levels of blood cells, the animal may not respond to treatment.

Doxycycline and minocycline are the medications of choice. For people allergic to antibiotics of the tetracycline class, rifampin is an alternative. Early clinical experience suggested that chloramphenicol may also be effective, however, in vitro susceptibility testing revealed resistance.

Clinically, HGA is essentially indistinguishable from human monocytic ehrlichiosis, the infection caused by "Ehrlichia chaffeensis", and other tick-borne illnesses such as Lyme disease may be suspected. As Ehrlichia serologies can be negative in the acute period, PCR is very useful for diagnosis.

No human vaccine is available for ehrlichiosis. Tick control is the main preventive measure against the disease. However, in late 2012 a breakthrough in the prevention of CME (canine monocytic ehrlichiosis) was announced when a vaccine was accidentally discovered by Prof. Shimon Harrus, Dean of the Hebrew University of Jerusalem's Koret School of Veterinary Medicine.

Tick exposure is often overlooked. For patients living in high-prevalence areas who spend time outdoors, a high degree of clinical suspicion should be employed.

Ehrlichia serologies can be negative in the acute period. PCR is therefore the laboratory diagnostic tool of choice.

Diagnosis of PHF is accomplished by measuring antibody titers or PCR testing to look for the bacterium in the blood and feces. However, most veterinarians opt to initiate treatment right away, as the disease can progress quite quickly. Veterinarians may also run complete blood counts and chemistry and electrolyte panels to determine the course of care. Radiographs may be taken to track the progress of laminitic horses.

Abnormal laboratory findings seen in patients with Rocky Mountain spotted fever may include a low platelet count, low blood sodium concentration, or elevated liver enzyme levels. Serology testing and skin biopsy are considered to be the best methods of diagnosis. Although immunofluorescent antibody assays are considered some of the best serology tests available, most antibodies that fight against "R. rickettsii" are undetectable on serology tests the first seven days after infection.

Differential diagnosis includes dengue, leptospirosis, and, most recently, chikungunya and Zika virus infections.

Currently, there is no vaccine against human granulocytic anaplasmosis, so antibiotics are the only form of treatment. The best way to prevent HGA is to prevent getting tick bites.

If ehrlichiosis is suspected, treatment should not be delayed while waiting for a definitive laboratory confirmation, as prompt doxycycline therapy has been associated with improved outcomes. Doxycycline is the treatment of choice.

Presentation during early pregnancy can complicate treatment.Rifampin has been used in pregnancy and in patients allergic to doxycycline.

While a vaccine is available for PHF, it does not cover all strains of the bacterium, and recent vaccine failures seem to be on the rise. Additionally, the vaccine usually produces a very weak immune response, which may only lessen the severity of the disease rather than prevent it. The vaccine is administered twice a year, in early spring and in early summer, with the first one inoculation given before the mayflies emerge and the second administered as a booster.

Some veterinarians have started making recommendations for farm management to try to prevent this disease:

- Maintaining riparian barriers along bodies of water may encourage aquatic insects to stay near their places of origin

- Turning off outside lights around the barn will prevent insects from being attracted

- Cleaning water buckets and feed areas frequently and keeping food covered will reduce the chance that the horse will accidentally ingest infected insects

Humans contract the disease after a bite by an infected tick of the species "Amblyomma americanum".

Those with an underlying immunodeficiency (such as HIV) appear to be at greater risk of contracting the disease. Compared to HME, ewingii ehrlichiosis has a decreased incidence of complications.

Like "Anaplasma phagocytophilum", the causative agent of human granulocytic ehrlichiosis, Ehrlichia ewingii infects neutrophils. Infection with "E. ewingii" may delay neutrophil apoptosis.

Several forms of laboratory testing for Lyme disease are available, some of which have not been adequately validated. The most widely used tests are serologies, which measure levels of specific antibodies in a patient's blood. These tests may be negative in early infection as the body may not have produced a significant quantity of antibodies, but they are considered a reliable aid in the diagnosis of later stages of Lyme disease. Serologic tests for Lyme disease are of limited use in people lacking objective signs of Lyme disease because of false positive results and cost.

The serological laboratory tests most widely available and employed are the Western blot and ELISA. A two-tiered protocol is recommended by the Centers for Disease Control and Prevention: the sensitive ELISA test is performed first, and if it is positive or equivocal, then the more specific Western blot is run. The reliability of testing in diagnosis remains controversial. Studies show the Western blot IgM has a specificity of 94–96% for people with clinical symptoms of early Lyme disease. The initial ELISA test has a sensitivity of about 70%, and in two-tiered testing, the overall sensitivity is only 64%, although this rises to 100% in the subset of people with disseminated symptoms, such as arthritis.

Erroneous test results have been widely reported in both early and late stages of the disease, and can be caused by several factors, including antibody cross-reactions from other infections, including Epstein–Barr virus and cytomegalovirus, as well as herpes simplex virus. The overall rate of false positives is low, only about 1 to 3%, in comparison to a false-negative rate of up to 36% in the early stages of infection using two-tiered testing.

Polymerase chain reaction (PCR) tests for Lyme disease have also been developed to detect the genetic material (DNA) of the Lyme disease spirochete. PCR tests are susceptible to false positive results from poor laboratory technique. Even when properly performed, PCR often shows false negative results with blood and cerebrospinal fluid specimens. Hence, PCR is not widely performed for diagnosis of Lyme disease, but it may have a role in the diagnosis of Lyme arthritis because it is a highly sensitive way of detecting "ospA" DNA in synovial fluid.

Culture or PCR are the current means for detecting the presence of the organism, as serologic studies only test for antibodies of "Borrelia". OspA antigens, shedded by live Borrelia bacteria into urine, are a promising technique being studied. The use of nanotrap particles for their detection is being looked at and the OspA has been linked to active symptoms of Lyme. High titers of either immunoglobulin G (IgG) or immunoglobulin M (IgM) antibodies to "Borrelia" antigens indicate disease, but lower titers can be misleading, because the IgM antibodies may remain after the initial infection, and IgG antibodies may remain for years.

Western blot, ELISA, and PCR can be performed by either blood test via venipuncture or cerebrospinal fluid (CSF) via lumbar puncture. Though lumbar puncture is more definitive of diagnosis, antigen capture in the CSF is much more elusive; reportedly, CSF yields positive results in only 10–30% of affected individuals cultured. The diagnosis of neurologic infection by "Borrelia" should not be excluded solely on the basis of normal routine CSF or negative CSF antibody analyses.

New techniques for clinical testing of "Borrelia" infection have been developed, such as LTT-MELISA, although the results of studies are contradictory. The first peer reviewed study assessing the diagnostic sensitivity and specificity of the test was presented in 2012 and demonstrated potential for LTT to become a supportive diagnostic tool. In 2014, research of LTT-MELISA concluded that it is "sensible" to include the LTT test in the diagnostic protocol for putative European-acquired Lyme borreliosis infections. Other diagnostic techniques, such as focus floating microscopy, are under investigation. New research indicates chemokine CXCL13 may also be a possible marker for neuroborreliosis.

Some laboratories offer Lyme disease testing using assays whose accuracy and clinical usefulness have not been adequately established. These tests include urine antigen tests, PCR tests on urine, immunofluorescent staining for cell-wall-deficient forms of "B. burgdorferi", and lymphocyte transformation tests. The CDC does not recommend these tests, and stated their use is "of great concern and is strongly discouraged".

Rocky Mountain spotted fever can be a very severe illness and patients often require hospitalization. Because "R. rickettsii" infects the cells lining blood vessels throughout the body, severe manifestations of this disease may involve the respiratory system, central nervous system, gastrointestinal system, or kidneys.

Long-term health problems following acute Rocky Mountain spotted fever infection include partial paralysis of the lower extremities, gangrene requiring amputation of fingers, toes, or arms or legs, hearing loss, loss of bowel or bladder control, movement disorders, and language disorders. These complications are most frequent in persons recovering from severe, life-threatening disease, often following lengthy hospitalizations

Neuroimaging is controversial in whether it provides specific patterns unique to neuroborreliosis, but may aid in differential diagnosis and in understanding the pathophysiology of the disease. Though controversial, some evidence shows certain neuroimaging tests can provide data that are helpful in the diagnosis of a patient. Magnetic resonance imaging (MRI) and single-photon emission computed tomography (SPECT) are two of the tests that can identify abnormalities in the brain of a patient affected with this disease. Neuroimaging findings in an MRI include lesions in the periventricular white matter, as well as enlarged ventricles and cortical atrophy. The findings are considered somewhat unexceptional because the lesions have been found to be reversible following antibiotic treatment. Images produced using SPECT show numerous areas where an insufficient amount of blood is being delivered the cortex and subcortical white matter. However, SPECT images are known to be nonspecific because they show a heterogeneous pattern in the imaging. The abnormalities seen in the SPECT images are very similar to those seen in people with cerebral vacuities and Creutzfeldt–Jakob disease, which makes them questionable.

Removal of the embedded tick usually results in resolution of symptoms within several hours to days. If the tick is not removed, the toxin can be fatal, with reported mortality rates of 10–12 percent, usually due to respiratory paralysis. The tick is best removed by grasping the tick as close to the skin as possible and pulling in a firm steady manner.

Unlike the other species of ticks, the toxin of Ixodes holocyclus (Australian Paralysis Tick) will not resolve itself and will be fatal if medical assistance is not immediately sought after pulling the tick off of the animal. Contrary to popular belief, if the head detaches from the body while being pulled off, leaving the head will not inject more venom. The head may cause a skin irritation but it will not inject any more venom. Once the tick is removed, place it in a clear bag [preferably ziplock] so the vet can identify it.

Water and food can worsen the results of the animal as the venom can prevent the animal from swallowing properly. If you find an Australian Paralysis Tick on your animal, immediately remove the tick and seek veterinary assistance even if you do not think the tick has been on the animal long enough to inject venom.

Diagnosis is based on symptoms and upon finding an embedded tick, usually on the scalp.

In the absence of a tick, the differential diagnosis includes Guillain–Barré syndrome. Early signs of tick poisoning could be a change of an animals' ‘voice’, weakness in the back legs or vomiting.

Methicillin-resistant Staphylococcus aureus (MRSA) evolved from Methicillin-susceptible Staphylococcus aureus (MSSA) otherwise known as common "S. aureus". Many people are natural carriers of "S. aureus", without being affected in any way. MSSA was treatable with the antibiotic methicillin until it acquired the gene for antibiotic resistance. Though genetic mapping of various strains of MRSA, scientists have found that MSSA acquired the mecA gene in the 1960s, which accounts for its pathogenicity, before this it had a predominantly commensal relationship with humans. It is theorized that when this "S. aureus" strain that had acquired the mecA gene was introduced into hospitals, it came into contact with other hospital bacteria that had already been exposed to high levels of antibiotics. When exposed to such high levels of antibiotics, the hospital bacteria suddenly found themselves in an environment that had a high level of selection for antibiotic resistance, and thus resistance to multiple antibiotics formed within these hospital populations. When "S. aureus" came into contact with these populations, the multiple genes that code for antibiotic resistance to different drugs were then acquired by MRSA, making it nearly impossible to control. It is thought that MSSA acquired the resistance gene through the horizontal gene transfer, a method in which genetic information can be passed within a generation, and spread rapidly through its own population as was illustrated in multiple studies. Horizontal gene transfer speeds the process of genetic transfer since there is no need to wait an entire generation time for gene to be passed on. Since most antibiotics do not work on MRSA, physicians have to turn to alternative methods based in Darwinian medicine. However prevention is the most preferred method of avoiding antibiotic resistance. By reducing unnecessary antibiotic use in human and animal populations, antibiotics resistance can be slowed.

Serological testing is typically used to obtain a definitive diagnosis. Most serological tests would succeed only after a certain period of time past the symptom onset (usually a week). The differential diagnosis list includes typhus, ehrlichiosis, leptospirosis, Lyme disease and virus-caused exanthema (measles or rubella).

An emerging infectious disease (EID) is an infectious disease whose incidence has increased in the past 20 years and could increase in the near future. Emerging infections account for at least 12% of all human pathogens. EIDs are caused by newly identified species or strains (e.g. Severe acute respiratory syndrome, HIV/AIDS) that may have evolved from a known infection (e.g. influenza) or spread to a new population (e.g. West Nile fever) or to an area undergoing ecologic transformation (e.g. Lyme disease), or be "reemerging" infections, like drug resistant tuberculosis. Nosocomial (hospital-acquired) infections, such as methicillin-resistant Staphylococcus aureus are emerging in hospitals, and extremely problematic in that they are resistant to many antibiotics. Of growing concern are adverse synergistic interactions between emerging diseases and other infectious and non-infectious conditions leading to the development of novel syndemics. Many emerging diseases are zoonotic - an animal reservoir incubates the organism, with only occasional transmission into human populations.

Tetracycline-group antibiotics (doxycycline, tetracycline) are commonly used. Chloramphenicol is an alternative medication recommended under circumstances that render use of tetracycline derivates undesirable, such as severe liver malfunction, kidney deficiency, in children under nine years and in pregnant women. The drug is administered for seven to ten days.

The treatment for bacillary angiomatosis is erythromycin given for three to four months.

A canine vector-borne disease (CVBD) is one of "a group of globally distributed and rapidly spreading illnesses that are caused by a range of pathogens transmitted by arthropods including ticks, fleas, mosquitoes and phlebotomine sandflies." CVBDs are important in the fields of veterinary medicine, animal welfare, and public health. Some CVBDs are of zoonotic concern.

Many CVBD infect humans as well as companion animals. Some CVBD are fatal; most can only be controlled, not cured. Therefore, infection should be avoided by preventing arthropod vectors from feeding on the blood of their preferred hosts. While it is well known that arthropods transmit bacteria and protozoa during blood feeds, viruses are also becoming recognized as another group of transmitted pathogens of both animals and humans.

Some "canine vector-borne pathogens of major zoonotic concern" are distributed worldwide, while others are localized by continent. Listed by vector, some such pathogens and their associated diseases are the following:

- Phlebotomine sandflies (Psychodidae): "Leishmania amazonensis", "L. colombiensis", and "L. infantum" cause visceral leishmaniasis (see also canine leishmaniasis). "L. braziliensis" causes mucocutaneous leishmaniasis. "L. tropica" causes cutaneous leishmaniasis. "L. peruviana" and "L. major" cause localized cutaneous leishmaniasis.

- Triatomine bugs (Reduviidae): "Trypanosoma cruzi" causes trypanosomiasis (Chagas disease).

- Ticks (Ixodidae): "Babesia canis" subspecies ("Babesia canis canis", "B. canis vogeli", "B. canis rossi", and "B. canis gibsoni" cause babesiosis. "Ehrlichia canis" and "E. chaffeensis" cause monocytic ehrlichiosis. "Anaplasma phagocytophilum" causes granulocytic anaplasmosis. "Borrelia burgdorferi" causes Lyme disease. "Rickettsia rickettsii" causes Rocky Mountain spotted fever. "Rickettsia conorii" causes Mediterranean spotted fever.

- Mosquitoes (Culicidae): "Dirofilaria immitis" and "D. repens" cause dirofilariasis.

Feline infectious anemia (FIA) is an infectious disease found in felines, causing anemia and other symptoms. The disease is caused by a variety of infectious agents, most commonly "Mycoplasma haemofelis" (which used to be called "Haemobartonella"). "Haemobartonella" and "Eperythrozoon" species were reclassified as mycoplasmas. Coinfection often occurs with other infectious agents, including: feline leukemia virus (FeLV), feline immunodeficiency virus (FIV), "Ehrlichia" species, "Anaplasma phagocytophilum", and Candidatus "Mycoplasma haemominutum".

Chronic Lyme disease is a generally unrecognised diagnosis that encompasses "a broad array of illnesses or symptom complexes for which there is no reproducible or convincing scientific evidence of any relationship to "B. burgdorferi" infection." There is no clinical evidence that "chronic" Lyme disease is caused by a persistent infection. It is distinct from post-treatment Lyme disease syndrome, a set of lingering symptoms which may persist after successful treatment of infection with Lyme spirochetes. The symptoms of "chronic Lyme" are generic and non-specific "symptoms of life".

A number of alternative treatments are promoted for "chronic Lyme disease", of which possibly the most controversial and harmful is long-term antibiotic therapy, particularly intravenous antibiotics. Most medical authorities advise against long-term antibiotic treatment for Lyme disease, though they agree that some patients do experience lingering symptoms. Following disciplinary proceedings by State medical licensing boards in the United States, a subculture of "Lyme literate" physicians has successfully lobbied for specific legal protections, exempting them from the standard of care and Infectious Diseases Society of America treatment guidelines. This "troubling" political interference in medical care has been criticised as an example of "legislative alchemy", the process whereby pseudomedicine is legislated into practice.