Host Susceptibility

Fungal infections, with the exception of dermatophytosis, are generally rare in equine species, but they still represent important pathogens. Disease caused by fungal infection depends on many factors, primarily host susceptibility, pathogenicity of the fungal species, and dose of fungal organisms. Fungal infections such as candidiasis can be observed in neonatal foals that are often immunocompromised through failure of passive transfer, have increased susceptibility because of alterations in normal flora from antibiotic use that decreases colonization resistance, and may have poor physical defenses resulting from indwelling medical devices and wounds or abrasions. These foals illustrate the major host factors contributing to fungal infection.

Sources of Fungal Pathogens

Most fungal infections are opportunistic and originate from the environment, transient contamination, or normal flora. Fungi that survive and reproduce in the environment are termed saprophytes; they can be opportunistic pathogens. Some fungi utilize host substances without causing harm; these are commensal fungi. Many saprophytic fungi can be found on body surfaces without causing damage or utilizing host substances; these are termed transients. Some fungi require an animal host for survival, cause host damage, and are considered obligate pathogens; for example, the zoophilic dermatophytes require an animal host but are transmissible from animal to animal and to humans.

Knowledge of the source of agents can be important for interpretation of fungal culture results. Because most fungal infections arise from the environment, they can be a frequent contaminant in samples collected for fungal culture. Samples such as skin can have a large number of transient fungal species that can be detected in the laboratory. Transient fungal organisms are also common in the upper respiratory tract, where spores are filtered from inspired air by nasal turbinates and mucus. Feeds such as hay and alfalfa will often have high numbers of molds, which can easily contaminate the nasal passages and upper trachea. Many saprophytes will grow on conventional agar used to isolate and identify fungi, leading to false-positive results on fungal culture.

Laboratory Safety

Although safety is an important concern when working with any infectious agent, fungal pathogens require special consideration, especially in the laboratory. Filamentous fungi cultured in the laboratory produce millions of conidia, that are easily aerosolized when a culture plate is opened. This leads to widespread contamination of the laboratory that can be introduced into other cultures and expose laboratory personnel to large numbers of infectious fungi. Even saprophytic fungi can cause disease in healthy adults if sufficient numbers of conidia are present. The systemic dimorphic fungi such as Coccidioides immitis are generally not infectious from the patient but are highly infectious when they are in mold form in the laboratory (see Chapter 51). C. immitis is one of the most common and dangerous of laboratory-acquired infections. Consequently, manipulation of all fungal cultures should be carried out in a biosafety hood, and plates should be sealed to prevent aerosolization of hyphae and conidia. The clinician should always inform the laboratory when a fungal infection is suspected, especially the dimorphic fungi.

Skin

The skin has many fungi on its surface, and cleaning with 70% ethanol before sample collection can help to remove these superficial contaminants (see Chapters 7 and 54). Hair and scales from a suspected ringworm lesion should be collected using forceps or by skin scraping. The leading edge of the lesion is best for culture because it will contain the most fungal elements. Collected specimens should be placed in a dry container for transport to the laboratory. It is not necessary to store skin scrapings and hair at cool temperatures as long as the sample can be inoculated on fungal culture media within 72 hours. If a deeper infection is considered likely, a punch biopsy is a preferred sample.

Eye

Fungal infections of corneal tissue are common, particularly in neonatal foals or as a complication of corneal ulceration (see Chapter 10). As for bacterial culture, the cornea should be scraped gently. Scrapings should be placed in a sterile container and can be left on the instrument used for scraping. A sample should be placed immediately on a slide for cytologic examination.

Blood

The lysis-centrifugation method or broth medium for blood culture can be used for fungal culture (see Chapter 27). The pellet resulting from centrifugation or media from the blood culture broth should be inoculated to the appropriate fungal culture medium.

Fluids

Fluids collected by aspiration are appropriate for fungal isolation and identification. The volume collected should be enough to represent the lesion present and should allow for inoculation of multiple media and for cytologic examination. If the sample also contains bacteria, such as a transtracheal aspirate, it should be kept cold (4° C) and shipped on wet ice to minimize overgrowth of the bacteria.

Tissue

Ideally, a biopsy of affected tissue should be collected for fungal culture and cytologic examination. Swabs may be used for diagnosis of oral lesions of suspected candidiasis (see Chapter 53).