Bovine dermatology: How to approach skin diseases in this species

Frequently underestimated in the daily activities of food animal practitioners, bovine dermatology is nevertheless omnipresent. Who has never observed hair loss or scabs in cattle? In 2019, a study conducted over 1 y at the Faculty of Veterinary Medicine in St-Hyacinthe, Université de Montréal, (Quebec, Canada) revealed the presence of skin lesions in 91% of admitted dairy cattle (1). However, unlike small animals, in which skin disorders are the second most frequent reason for consultation (2), dairy and/or beef producers rarely consult a veterinarian for such reasons. In this study, only 13% of dairy farmers surveyed were concerned about dermatological conditions (1,3).

Because bovine dermatology is not limited to ringworm, chorioptic mange, or lice infestation, cattle presenting with skin lesions may be puzzling for the veterinarian. Indeed, the veterinarian may wonder whether to suspect external parasitosis (which could have an impact on the entire herd), to use a topical or systemic antibiotic (which could require withdrawal periods for milk and meat), to anticipate herd-wide complications, or to collect costly diagnostic samples.

The practice of farm animal dermatology is simpler and less frustrating when a complete history can be taken, lesions are well-described, and quick and easy basic skin samples are collected. One purpose of this article is to provide some tips on collecting a useful dermatological history. The identification of common skin lesions in cattle is also important, so another objective is to discuss skin lesions and guide the veterinary practitioner in collection of basic skin samples. In fact, many inexpensive and minimally invasive skin samples can be collected on-farm and provide important clues to ongoing skin disease.

First step, the history

Reviewing the patient’s history is not a waste of time, and is one of the most important steps when approaching a dermatology case. Indeed, details of the patient’s history support the practice of dermatology. Although some information may seem obvious to obtain, such as sex, age, breed, or coat color, other information may be more challenging to obtain (Table 1). This requires time and attention but will give directions to the diagnostic and/or treatment plans. A good way to start the discussion with the farm owner or manager is to ask an open-ended question. Two examples include: “What did the skin lesions look like initially and how the problem has evolved since you noticed them?” or “What are your concerns about your recent findings on the skin of this cow or these cattle?” Questions can then be refined and directed to obtain more details and a better picture of the situation. Many questions can be answered in an open discussion with the farmer (4,5).

Table 1.

List of the most useful questions in bovine dermatology cases.

Signalment	History
Species	Origin (born on the farm or purchased)
Breed	Travel history
Color	Management (type of stall and access to the outside)
Sex	Contact with other animal species
Age	Preventative care (vaccinations and antiparasitics)
Date of last calving	Medical history and treatments administered
Dermatological history

Cutaneous lesions (primary versus secondary lesions, when noticed, areas affected)

Evolution of the skin lesions (localization, descriptions, and duration)

Presence or absence of pruritus (scratching, rubbing, licking) or signs of cutaneous pain

Other animals or humans affected (especially with pruritic dermatosis)

Environmental factors with a potential impact on the dermatosis (e.g., season, feed, topicals, soil type)

Treatments tried specifically for the skin issue and response

There is some basic information that the veterinarian needs to know. Farm management is an essential piece of information. Knowing that the animals have access to the outdoors is only part of the history. It is important to ask questions about other aspects of management, such as exposure to parasites (e.g., forest edges, standing water), possible contact with other animal species (e.g., horses, sheep, deer), or the presence of photosensitizing forages (e.g., St. John’s wort, buckwheat, or perennial ryegrass) (6). Similarly, introduction of new animals into the herd can potentially introduce new pathogens, and therefore the origin of the individuals and their health status, including possible contacts with other sick cattle (e.g., previous farm, transportation, or auction), must be assessed. Any medication administered to the animals may also affect skin health. For example, sulfonamides and tetracycline are recognized sources of photosensitization. Some drugs (penicillin, streptomycin, tetracycline, or sulfonamides) can also trigger cutaneous adverse drug reactions (4). Moreover, the diagnostic plan can be influenced by some treatments, such as broad-spectrum antiparasitic treatments having an activity against both endoparasites and ectoparasites. Regarding antiparasitic treatment, it is important to have details about the active ingredient, the dosage, the date of last administration, and details on which animals were treated.

Finally, the history should be concluded by a more specific discussion on the dermatological condition (appearance and location of primary lesions or initial lesions observed, progression of lesions, potential triggering external factors or contagious diseases, treatments initiated or tried). Let’s be realistic — it is likely that much information will be missing! Observing skin lesions in a free-range beef herd is much more complicated for a producer than seeing lesions in tethered dairy cows. However, it is useful to have information about the chronicity of the skin lesions and the nature of the treatments initiated, as these factors may influence the lesions observed on the day of examination and the differential diagnoses (4).

Physical examination and identification of skin lesions

Part of the diagnosis is based on identification of skin lesions. However, before performing the dermatological examination, it is recommended that the animal be observed from a distance. A remote examination will facilitate recognition of lesional patterns that could be associated with specific dermatoses (4). For example, lesions located mainly on the back line could be suggestive of Hypoderma bovis infestation or lice; mucosal involvement could suggest a viral disease such as bovine viral diarrhea (BVD); and involvement of only white areas could suggest a photosensitization reaction. A complete physical examination is also important, especially for identifying clinical signs that could be suggestive of a multisystemic disease. For example, skin ulcers and mucosal lesions associated with emaciation, diarrhea, or excessive salivation could indicate a viral infection. The differential diagnosis will be influenced by these dermatological and clinical findings, with various concerns at the herd level (e.g., BVD, infectious bovine rhinotracheitis, or malignant catarrhal fever).

The second step is identification of the skin lesions. For novices, dermatological semiology may seem complex, even superfluous. However, differentiating a pustule from a vesicle (Figure 1) by the nature of their contents (pus versus serous fluid, respectively) can be relevant diagnostic information. For example, finding pustules is often indicative of a staphylococcal infection, whereas vesicles and bullae are common primary lesions in viral infections (5). Another example is differentiating erythema from purpura/petechiae by its capacity to fade under in-vitro pressure (diascopy). In-vitro pressure is a simple test performed by applying pressure with a glass slide on the erythematous area and observing whether or not the lesion whitens. Because erythema is secondary to dilation of blood vessels, the lesion will whiten under in-vitro pressure. However, purpuric lesions resulting from extravasation of erythrocytes do not whiten under in-vitro pressure (Figure 2). Differentiating between primary and secondary lesions will provide a better understanding of the evolution of the dermatosis. The primary lesions reflecting the initial insult to the skin (the most characteristic of the dermatosis) are clues to the diagnosis. The type of skin lesions will also help for selecting appropriate diagnostic tests (skin scraping, cytology, bacterial culture and antimicrobial sensitivity, or skin biopsies).

Figure 1.

Pustules (white arrows), ruptured vesicle (yellow arrowhead), and vesicle (yellow arrow) in 2 Holstein cows.

Figure 2.

Purpuric lesions on the skin. Note the persistence of redness of the skin under in-vitro pressure, revealing small skin hemorrhages.

One of the pitfalls of the dermatological examination in cattle is omission of some areas that are difficult to approach (e.g., mouth, udder, interdigital spaces). Omitting some lesions localized in these areas can lead to an incorrect diagnosis or make the diagnosis more challenging. For example, the triad of lesions associating ulcers in the mouth, udder, and interdigital spaces should lead to a clinical suspicion of foot-and-mouth disease, whereas observation of ulcers localized only to the teats is more likely to be associated with herpetic thelitis caused by Bovine herpesvirus-2 & 4 (5). Finally, it should be noted that a long coat (due to breed or season) can mask certain skin lesions. Therefore, it is recommended that the skin surface of cattle be palpated to search for lesions and/or skin changes (e.g., thickness, deformities), or even sometimes hair clipped for the area to be thoroughly examined.

Diagnostic tools

In cattle, the clinical diagnosis of a skin condition is usually based on history and clinical findings. A probabilistic approach easily allows for first-line treatment. Nevertheless, in the case of an atypical lesion distribution or a dermatosis that does not respond adequately to initial treatment, the veterinary practitioner may perform several complementary examinations, such as skin scrapings, trichography (i.e., a hair pluck examined under the microscope), cytologies, skin biopsies, or microbiological analyses. The main “disadvantage” of basic diagnostic tools such as cytology or skin scraping is the need for a microscope. It is not always practical to travel from farm to farm with a microscope, but most samples can be collected and subsequently examined at the clinic. Slides for cytology can be kept in a microscope slide box and stained at the clinic. Skin scrapings are less convenient to transport, we recommend checking them at the farm, or keeping them on a foam tray that can be stabilized horizontally for travel.

Simple and useful complementary tests for ectoparasites may include the flea comb, scotch test, curved spatula (“scoopula”) or skin scrapings. Both the flea comb and scotch tests are used to detect superficial parasites such as lice or more rarely Dermanyssus spp. (poultry mites), Trombicula spp. (chiggers or harvest mites), or fleas. The dermal spatula and skin scraping (dulled no. 21 or 22 scalpel blade) are used if deeper parasites such as Chorioptes bovis, Psoroptes ovis, Sarcoptes scabiei var bovis, or Demodex bovis are suspected.

To perform skin scrapings, a few drops of mineral oil are applied to the skin (if hairs are dense, shave or trim the area to be scraped) before scraping the skin perpendicularly with the dulled scalpel blade in the direction of hair growth. On stretched skin, a moderate pressure should be applied with the dulled scalpel blade and the skin scraped until erythema (or even oozing blood for deep parasites, e.g., Demodex mites) is observed (Figure 3). The debris obtained is transferred to a microscope slide, mineral oil is added, and the collected material is observed between the slide and a coverslip under a microscope at low magnification. Not all ectoparasites are obvious at first sight. Adult parasites, as well as immature forms and eggs can be present. These tests will allow precise identification of the ectoparasites involved. For example, some parasites, such as Demodex bovis, are not contagious and do not require treatment of the entire herd. The differentiation between sucking and chewing lice can help to choose the route or the specific antiparasitic treatment. In the case of scabies with an atypical lesion distribution, skin scrapings are essential for differentiating among chorioptic, sarcoptic, or psoroptic mange. These are just a few examples of the usefulness of skin scrapings.

Figure 3.

Skin scraping technique. a — Apply a few drops of mineral oil on the skin. b — Scrape the skin perpendicularly with a dulled scalpel blade in the direction of hair growth. c — Scoop the debris and oil from the skin. d — Transfer the material collected to a microscope slide.

Trichography is a simple and inexpensive test that can be used to guide the differential diagnosis of hair loss, provide clues on whether the hair loss is primary or secondary to pruritus, evaluate hair abnormalities that may result in alopecia, and even to find the rare parasite Demodex bovis. Ideally, the hairs are pulled out with hemostatic forceps, and laid on a microscope slide with a few drops of mineral oil before a coverslip is applied. Light microscopy is used to evaluate the integrity of the hair tips (broken end when pruritus) and the growth phase of the hair thanks to the appearance of the hair bulb (golf club — anagen phase versus spearhead — telogen phase). Examination of the hair shaft can also reveal sign of dermatophytosis (ringworm), as ectothrix spores can be visualized in some cases.

Cytologies are of particular diagnostic interest in the presence of crusts, pustules, vesicles, and masses (6). Whether performed by swabbing, direct impression smear, or fine-needle aspiration, cytology stained with the modified Wright stain (or Diff-Quik staining) can confirm the presence of microorganisms (bacteria and yeasts) as well as identify inflammatory and neoplastic cells (Figure 4) (4). Swabbing and direct impression smear techniques are useful for moist, oozing, or greasy lesions (e.g., pustules, draining tracts, erosions, ulcers) (Figure 5). If crusts are observed, the edge of the crusts can be lifted with the corner of the microscope slide, and the moist lesion beneath the crust sampled by applying the slide directly to the surface (Figure 6). For pustular or vesicular dermatosis, after delicately puncturing the top of lesion with a small needle, an impression of the content on a microscope slide is recommended. The main differential diagnoses in presence of a pustular dermatitis are impetigo (staphylococcal infection involving superficial layers of the skin) and superficial bacterial folliculitis (staphylococcal infection targeting the hair follicles) (5). The judicious use of antimicrobials is mandatory to prevent antimicrobial resistance in both animals and humans. First, it is recommended that a bacterial skin infection be confirmed by cytology before prescribing an antimicrobial treatment. The judicious use of antimicrobials in livestock has been proven to reduce antimicrobial resistance and may represent an important aspect of antimicrobial stewardship (7,8). Unfortunately, cytology does not allow precise bacterial identification. However, bacterial morphology (bacilli or cocci) is often enough to initiate appropriate antibiotic therapy, even in the absence of a bacterial culture (4). In cases in which a bacterial culture is performed, the veterinary practitioner should use a sterile swab with a suitable culture medium (aerobic versus anaerobic). The same swab can be used to take samples of several lesions, and thus improve the representativeness of the results obtained.

Figure 4.

Skin cytologies (Diff-Quik staining). a — Neutrophils and cocci-shaped bacteria (400× magnification). b — Neutrophils DNA strains and rod-shaped bacteria (500× magnification). c — Budding yeast (arrows) and non-budding yeast (arrowheads) (400× magnification). Note the melanin granules inside the keratinocyte cytoplasm (small dark dots).

Figure 5.

Skin cytology — swabbing. a — Moisten the tip of a clean cotton swab. b — Rub the skin vigorously. c — Roll the tip of the cotton swab on a microscope slide. Let air dry then stain the material collected with the modified Wright stain (Diff-Quik staining).

Figure 6.

Skin cytology – impression smear. a — The edge of the crusts is lifted with the corner of the microscope slide. b — The lesion beneath the crust is sampled by applying the slide directly to the surface of the skin.

Skin biopsy remains one of the most useful diagnostic tools in dermatology when performed appropriately. It requires collaboration between the veterinary practitioner who has examined the case, the farmer who has provided a detailed history, and the pathologist who has examined the histopathology. The histopathology request should be as detailed as possible. Photographs of the skin lesions can also be included in the laboratory request. It is imperative that the skin lesions be sampled correctly to obtain the most useful and representative results. Among the common errors, the veterinary practitioner should avoid clipping and surgically preparing the sites to be sampled to avoid damaging the superficial layer of the skin epithelium (Figure 7) (6). Skin biopsies must be representative of the lesional pattern; it is recommended that 2 or 3 primary lesions be taken from the entire thickness of the skin. A sample of healthy skin is rarely required, but if it is (e.g., at the request of the pathologist, an area of the skin rarely biopsied, an uncommon cattle breed), the sample of healthy skin should be taken separately and placed in a well-marked jar of formalin (6). A tutorial on the ideal procedure for taking a skin biopsy to maximize the information that can be obtained is available to CAVD members (https://www.cavd.ca/diagnostic-methods).

Figure 7.

Skin biopsy in a dog (same technique in a cow). a — Use a 6- or 8-mm disposable biopsy punch. After local anesthesia, center the biopsy punch on the lesion and turn clockwise until reaching the hypodermis. b — To avoid damaging the superficial layers of the skin, the specimen is grabbed underneath with Adson forceps. If the specimen has remained attached to the subcutis tissue, the adipose tissue is cut with an iris scissor. c — The skin specimen collected is placed in a well-marked jar of formalin. Each site of biopsy can be sutured or left to heal by secondary intention.

In conclusion, the history is an important step of the dermatologic consultation. The dermatological examination must be as complete as possible; a complete physical examination should be performed, as well as a list of all lesions localized on the skin and mucous membranes. Performing basic skin samples requires some practice, but the veterinarian will rapidly become experienced and then these simple tests can be completed quickly. With minimal time invested, the veterinary practitioner will appreciate the benefits of searching for parasites and performing cytologies. Results of these tests will help obtain a diagnosis and properly treat the animal at the first visit, rather than treating symptomatically. Overall, the cattle farmer also benefits from these tests, both for the health of the herd and financially.

From the initial consultation to follow-up visits, managing a bovine dermatology case takes time. Remember to keep notes of successive visits to document clinical progress and therapeutic response. Furthermore, by improving the practice of bovine dermatology, therapeutics can be used more judiciously to improve herd performance and profitability.