Discovery and Preliminary Diagnosis and Treatment of a Case of “Milk Disease” in River Crabs

I. Disease Onset Details  
Local river crab farmers purchased a total of 1,200 jin of crab seedlings from Jiangsu. One week after stocking, the crabs in the ponds suddenly developed a disease—diagnosed as “milk disease”—which continued to spread and persisted for an extended period. Post‑mortem examination of affected crabs revealed the hallmark symptoms: upon prying open the cephalothorax, a large quantity of characteristic milky‑white fluid was found within the body cavity. Mortality was high, with 70–80 dead crabs being removed daily, resulting in substantial losses for the farmers. As the local farming area had no prior history of this disease nor established control measures, the farmers were unable to manage the outbreak on their own and sought professional diagnostic and treatment assistance at a specialized store.

II. Overview of Milk-Related Illnesses  
“Milk disease” in Chinese mitten crabs is a newly emerging aquatic disease that has been spreading rapidly in recent years. Its hallmark feature is the accumulation of milky‑white, milk‑like fluid within the body cavity and tissues of affected crabs, often accompanied by a marked decline in physiological functions. Clinically, diseased crabs exhibit a sudden loss of appetite and a significant reduction in food intake, along with diminished activity and sluggish movement. Muscle tissue becomes abnormal, appearing pale and opaque, while the circulatory system is disrupted and metabolic processes are impaired. Upon dissection, a large volume of turbid, milky‑white fluid can be clearly observed accumulating in the cephalothoracic cavity—this is the most direct and definitive diagnostic sign of the disease.
The disease first broke out in early 2019, spreading rapidly across multiple river crab farming areas in Liaoning Province. It subsequently expanded to key river crab‑farming regions such as Jiangsu and Tianjin, and has now become a major pathogen of widespread concern in many localities. Testing has identified Mycoderma yeasts as the causative agent of “milk disease” in river crabs, and they constitute the sole core pathogen responsible for this condition.
Meyerozyma guilliermondii belongs to the genus Meyerozyma and grows normally on Rose Bengal medium, with colonies that are milky white in color, slightly convex in the center, and have smooth, well-defined edges. Microscopic examination reveals morphological diversity, with no uniform or fixed form. This pathogen exhibits exceptional environmental adaptability and a broad tolerance range, capable of normal growth and reproduction across temperatures of 5–37°C, salinities of 0–60%, and pH values of 2–10. Relevant research indicates that its optimal conditions for growth and reproduction are 28°C, a water salinity of 5, and a pH of 4–7; under these conditions, the strain displays the fastest proliferation rate and the highest pathogenicity.

III. Prevention and Treatment of Milk-Related Illnesses  
Based on the clinical signs of this crab disease, the characteristics of the causative pathogen, and the actual conditions of pond aquaculture, a targeted integrated control regimen has been developed for farmers. The approach employs a combined strategy of external application via spraying and internal administration for systemic conditioning, with specific medications and operational procedures as follows:
1. Early stage of the disease (Days 1–2): Apply allicin by broadcasting it throughout the entire pond at a dosage of one bottle per 2 mu of water, continuing for two consecutive days. This rapidly suppresses the activity of pathogenic bacteria, prevents their ongoing infection of healthy river crabs, and helps contain the spread of the disease.
2. Mid‑stage of the disease (Day 3): Apply the Sanhuang San formulation. Soak it in hot water for 30 minutes beforehand to fully activate its active ingredients, then evenly distribute it throughout the pond. The recommended dosage is one packet per 2.5 mu of water, which helps clear heat and detoxify, repair damage to the river crab’s body, inhibit pathogenic bacteria, and enhance disease resistance.
3. Comprehensive supportive conditioning: Throughout the entire treatment cycle, apply Shengmingsu by evenly dispersing it across the pond at a dosage of one packet per two mu of water. This continuously regulates water quality, replenishes essential nutrients for both the water and the river crabs, improves the aquaculture environment, and enhances the crabs’ innate immunity. To thoroughly eradicate the condition and prevent recurrence, after completing the first full treatment cycle, farmers should repeat the complete regimen described above as a consolidation phase to further strengthen preventive and therapeutic outcomes.

IV. Efficacy Follow-Up  
Following two rounds of systematic treatment, the aquaculture ponds were continuously monitored over several days, with follow-up assessments of treatment efficacy. The results showed a substantial reduction in the number of diseased crabs, with no further outbreaks of mass morbidity; affected Chinese mitten crabs gradually resumed feeding, with feed intake steadily increasing and physiological functions progressively returning to normal; daily mortality in the ponds declined, with no new deaths reported, indicating that the disease has been brought under control.

V. Disease Analysis  
Based on an integrated analysis of the disease’s clinical course, pathogen characteristics, source of postlarvae, and aquaculture environment, three key factors contributed to the outbreak of “milk disease” in Chinese mitten crabs: First, crab postlarvae purchased from high‑incidence areas in Jiangsu were latent carriers of *Metschnikowia agnata*; after being subjected to stress during long‑distance transport, their immune defenses weakened, allowing the pathogen to proliferate rapidly and trigger a mass infection. Second, favorable environmental conditions—such as water temperature, salinity, and pH—promoted rapid pathogen multiplication, while inadequate pre‑emptive quarantine of postlarvae and insufficient disinfection of the culture water facilitated widespread dissemination throughout the pond. Third, the local area lacked experience in controlling this disease; insufficient early‑stage quarantine, disinfection, water‑quality management, and postlarval care, coupled with delayed response after onset, exacerbated the severity of the outbreak. Moreover, *Metschnikowia agnata* exhibits strong stress tolerance and is difficult to suppress under natural conditions; without targeted pharmacological intervention, the disease course is prolonged and mortality remains high.