One vs two negative cultures to confirm mycological cure in shelter cats treated for Microsporum canis dermatophytosis: a retrospective study

Stuntebeck RL, Moriello KA. One vs two negative fungal cultures to confirm mycological cure in shelter cats treated for Microsporum canis dermatophytosis: a retrospective study. J Feline Med Surg. 2019 Jul 3;:1098612X19858791. doi: 10.1177/1098612X19858791

Dermatophytosis (ringworm) is the most common fungal infection in cats. While generally not life threatening, it is notoriously difficult to definitively cure in many situations. Treatment generally consists of a combination of topical and systemic therapy, continued until two consecutive negative fungal cultures two weeks apart are confirmed.  However, this standard of care was defined in the 1950s, and modern treatment methods are very different. Two negative cultures requires significant time (cultures can take up to 21 days) and cost to owners, and so a single negative culture as indication of cure would be beneficial. The purpose of this study was to determine if the first negative culture was reflective of the results of two consecutive cultures in cats treated for dermatophytosis.

This study was designed as a retrospective observational study on cats in a dermatophyte treatment program in a shelter from 2006-2016. All cats were diagnosed with microsporum canis dermatophytosis and received at least 21 consecutive days of oral itraconazole and twice weekly lime sulfur rinses. Cats had weekly fungal cultures collected after resolution of gross lesions and were considered finalized at 21 days post inoculation. Cultures were collected via a whole body “toothbrush method”. Mycological cure was defined as two consecutive negative fungal cultures.

Data was available for 570 cats, of whom 371 had complete data available and met the above description. Based on the response to therapy and mycological cure, these cats were divided into four groups.

90.3% of cats had all subsequent fungal cultures test negative after the first negative (ie the first negative culture indicated mycological cure. These cats subdivided into two groups; 70.7% of the group were otherwise healthy but with simple, obvious, lesional infection (mean time to cure was 4.3 weeks), while 29.3% were otherwise healthy and had only suspected lesions or had Wood’s lamp positive hairs (mean time to cure was 2 weeks).

In 9.7% of cats the first negative culture was not indicative of mycological cure. This population also subdivided into two groups; the first consisted of 53% of cats who had a negative culture early in treatment, but then tested positive after this followed by eventual cure later in the course of therapy (mean time to cure 7.3 weeks); while the other group of 47% of cats had concurrent illness and experienced multiple negative followed by positive cultures, with resolution of disease only occurring with treatment of the other systemic illness (mean time to cure 11.9 weeks).

This data suggests that in cats with “simple infections”, a single negative culture is likely sufficient to demonstrate mycological cure. In fact, in 95.4% of cats with “simple infections”, the single negative culture was as accurate as two cultures. When calculating the agreement between 1 vs two cultures, there was very good agreement with a kappa=0.903.

The authors suggest that the population of 19 simply infected cats where a single negative culture was followed by a positive was likely due to human error, with failure to properly inoculate the brush.

The authors suggest that in otherwise healthy cats, a single negative culture after resolution of clinical signs is likely sufficient to demonstrate mycological cure, however in animals with concurrent systemic disease (“complicated infection”), the systemic condition should be managed prior to attempting to demonstrate cure.

A limitation to this study was that all animals were treated in a shelter setting by well-trained individuals, and cultures were likewise collected by experienced staff. In a real-world situation compliance may be lower (ie owners may not give oral medication or bathe cats as directed) and staff may be less trained at collecting samples for fungal culture. This may lead to a higher rate of false-negative culutres.

See Also

Moriello KA, Coyner K, Paterson S, et al. Diagnosis and treatment of dermatophytosis in dogs and cats: Clinical Consensus Guidelines of the World Association for Veterinary Dermatology. Vet Dermatol 2017; 28: 266–e68.

Kaufmann R, Blum SE, Elad D, et al. Comparison between point-of-care dermatophyte test medium and mycology laboratory culture for diagnosis of dermatophytosis in dogs and cats. Vet Dermatol 2016; 27: 284–e68.

Mawby DI, Whittemore JC, Fowler LE, et al. Comparison of absorption characteristics of oral reference and compounded itraconazole formulations in healthy cats. J Am Vet Med Assoc 2018; 252: 195–200.