Fungal Pathogens in Milk: Aflatoxins and Their Impact

When dairy farmers focus on milk quality, most attention goes to bacterial contamination and hygiene at the milking shed. But there’s a quieter, more stubborn threat hiding in the feed bunk – one that doesn’t die during pasteurization, doesn’t smell off, and can end up in a glass of milk without any visible sign of a problem. That threat is aflatoxins, toxic compounds produced by certain molds, and understanding how they get into milk is critical for anyone working in dairy production or food safety.

What are aflatoxins?
How does aflatoxin get into milk?
How quickly does it appear in milk?
Why aflatoxin M1 in milk is a serious public health concern
Aflatoxicosis: acute vs chronic effects
The pasteurization problem: AFM1 survives heat treatment
Regulatory limits for AFM1 in milk
Prevention and control: starting at the feed
Good agricultural and storage practices
Feed testing and monitoring
Aflatoxin binders (sequestering agents)
Biological approaches
Climate change and the growing risk

What are aflatoxins?

Aflatoxins are poisonous chemical compounds – technically classified as mycotoxins (toxins produced by fungi). They are primarily generated by two mold species: Aspergillus flavus and Aspergillus parasiticus. These fungi are naturally present in soil and decaying organic matter. Under the right conditions – warmth, humidity, drought stress, and poor post-harvest handling – they colonize crops such as maize (corn), peanuts, cottonseed, and their processing by-products, all of which commonly form the basis of dairy cattle feed.

There are several types of aflatoxins (B1, B2, G1, G2), but Aflatoxin B1 (AFB1) is the most important to dairy safety. AFB1 is the most potent of the group, and when a dairy cow consumes feed contaminated with it, the toxin is metabolized in the liver and then excreted in milk as a derivative called Aflatoxin M1 (AFM1). AFM1 is a hydroxylated metabolite of AFB1 – chemically similar, and still biologically active.

How does aflatoxin get into milk?

The route is straightforward. Cows eat contaminated feed. AFB1 is absorbed through the intestinal wall, processed by the liver, and a fraction of it is converted to AFM1 and secreted into milk. The European Food Safety Authority (EFSA) has estimated that, on average, 1-2% of the AFB1 in feed is transferred to milk as AFM1, though in high-productivity dairy cattle this transfer rate can reach up to 6%. The actual amount that ends up in milk depends on how much contaminated feed the cow consumes, the duration of exposure, the cow’s metabolism, and the type of feed.

Feed sources most commonly associated with aflatoxin contamination include corn grain, corn silage, and cottonseed, particularly from crops grown in warm, humid climates or from harvests affected by drought and insect damage. Importantly, contamination doesn’t only occur in the field – poor storage conditions, where grain is exposed to moisture, heat, and inadequate ventilation, create ideal conditions for Aspergillus mold to grow and produce toxins even after harvest.

How quickly does it appear in milk?

AFM1 appears in milk fairly rapidly after exposure begins, and the dietary threshold at which cows begin excreting detectable AFM1 in milk is approximately 15 ppb in the diet. The good news is that once contaminated feed is removed, AFM1 levels in milk decline and typically become undetectable within 2-4 days. The bad news is that the problem can go unnoticed for weeks if regular testing is not in place.

Why aflatoxin M1 in milk is a serious public health concern

AFM1 is not just a passing contaminant – it presents real risks to human health. Both AFB1 and AFM1 have been classified as human carcinogens by the International Agency for Research on Cancer (IARC) – AFB1 as Group 1 (known carcinogen) and AFM1 as Group 2B (possibly carcinogenic to humans). The liver is the primary target organ.

Chronic exposure to aflatoxins – even at low levels – is linked to hepatocellular carcinoma (liver cancer), immune suppression, and growth impairment. In developing countries, the combination of chronic AFB1/AFM1 exposure with hepatitis B virus (HBV) infection significantly amplifies the risk of hepatocellular carcinoma, often presenting at a much younger age than in populations without such dual exposure. The risk is not just about single high-dose exposure – cumulative, long-term ingestion of even low levels carries a measurable cancer risk.

Infants, children, and pregnant women are particularly vulnerable groups, given their higher milk consumption relative to body weight and lower capacity to detoxify these compounds. A meta-analysis referenced in recent research found that over half of breast milk samples from lactating women in parts of Eastern Africa contained detectable AFM1, suggesting that dietary exposure can even reach nursing infants indirectly.

Aflatoxicosis: acute vs chronic effects

Human exposure to aflatoxins can result in two patterns of illness. Acute aflatoxicosis, caused by a sudden high-dose exposure, presents with symptoms including nausea, vomiting, abdominal pain, and acute liver failure. High-level exposure produces acute hepatic necrosis, which may progress to cirrhosis or hepatocellular carcinoma. Outbreaks have been documented – a 2004 incident in Kenya, linked to contaminated maize, resulted in over 100 deaths. In dairy animals, symptoms of acute aflatoxicosis include lethargy, ataxia, pale and enlarged fatty livers, and in severe cases, death.

Chronic aflatoxicosis, the more common and insidious form in both humans and animals, develops from prolonged low-level exposure. In dairy cows, chronic aflatoxin exposure leads to reduced feed efficiency, decreased milk production, liver toxicity, immune compromise, and reproductive disorders. For the farmer, this often manifests as unexplained drops in productivity or an unusual increase in disease incidence in the herd, making it easy to miss the underlying cause.

The pasteurization problem: AFM1 survives heat treatment

One of the most important – and troubling – facts about aflatoxin M1 is that it is heat-stable. Unlike most bacterial pathogens in milk, AFM1 is not eliminated by standard pasteurization. Studies have shown that AFM1 concentration remains relatively stable even after pasteurization and ultra-high-temperature (UHT) processing, meaning that standard heat treatments cannot be relied upon to make contaminated milk safe.

This thermal stability also means AFM1 persists through further dairy processing. AFM1 remains detectable in cheese, yogurt, milk powder, and other dairy products made from contaminated milk, sometimes even at concentrations higher per unit weight than in the original milk (particularly in hard cheeses, where concentration occurs during whey removal). The contamination is upstream in the food chain – it must be prevented at the feed level, not corrected at the processing stage.

Regulatory limits for AFM1 in milk

Because of its health implications, AFM1 in milk is regulated in more than 80 countries, though limits vary considerably. In the European Union, AFM1 in raw milk and heat-treated milk must not exceed 0.05 µg/kg (50 ng/kg) for general consumption, and a stricter limit of 0.025 µg/kg applies to products intended for infants and young children. In contrast, the U.S. Food and Drug Administration (FDA) sets an action level of 0.5 ppb (500 ng/kg) for AFM1 in milk – a tenfold higher limit than the EU threshold.

On the feed side, the FDA limits AFB1 in lactating dairy cow rations to no more than 20 ppb, above which milk contamination risk increases significantly. Any load of bulk milk testing above the regulatory threshold will be rejected by processors, making aflatoxin contamination a direct economic risk for dairy producers in addition to the public health concern.

The large gap between EU and US limits reflects ongoing international debate about how to balance trade practicality with precautionary health standards. This tenfold variation in adopted limits underscores the need for a comprehensive global reevaluation and harmonization of regulations to ensure consistent food safety standards.

Prevention and control: starting at the feed

Since AFM1 cannot be removed from milk by heat processing, prevention must happen before the cow is ever fed. This means managing aflatoxin contamination in animal feed through a combination of agronomic, storage, and nutritional strategies.

Good agricultural and storage practices

Aflatoxin contamination begins in the field. Crops under drought stress or insect attack are most susceptible to Aspergillus colonization. Preventive practices include selecting resistant hybrid varieties, proper tillage and crop rotation, timely harvesting, and storing grain in clean, dry, ventilated spaces that are protected from moisture and microbial contamination. Weather conditions during the growing season – particularly heat and moisture extremes – should be monitored as early warning signals.

Feed testing and monitoring

Regular testing of incoming and stored feed commodities is non-negotiable in a well-managed dairy. Veterinarians and nutritionists should evaluate multiple sources of aflatoxins in rations and assess commodity storage conditions on-farm. It is also important to account for the fact that multiple mycotoxins may be present simultaneously in a contaminated sample, which can produce additive or synergistic toxic effects.

Aflatoxin binders (sequestering agents)

When contaminated feed cannot be entirely avoided, the most widely used intervention is the addition of aflatoxin-sequestering agents – typically clay-based or yeast-based adsorbent products – to the ration. These adsorbents work by binding directly to aflatoxins in the gastrointestinal tract, minimizing the amount absorbed into the bloodstream and subsequently excreted in milk. Products based on calcium montmorillonite clay and hydrated sodium calcium aluminosilicate (HSCAS) have shown measurable reductions in milk AFM1 levels in several studies, though their efficacy can vary depending on dose, toxin load, and product composition.

Biological approaches

Emerging research points to biological control strategies as another layer of defense. Certain lactic acid bacteria, including species of Lactobacillus, Streptococcus thermophilus, and Bifidobacteria, have demonstrated the ability to bind and reduce free AFM1 levels in fermented dairy environments, offering a potential avenue for further reducing contamination risk during yogurt and cheese production. Non-toxigenic strains of Aspergillus are also being studied as competitive biocontrol agents that can outcompete toxin-producing strains in the field.

Climate change and the growing risk

Aflatoxin contamination is not a static problem. Rising global temperatures and increasingly frequent drought events are expected to elevate the prevalence of aflatoxin-producing fungi in dairy cattle feed, increasing the occurrence of AFM1 in milk and dairy products – including in regions that have historically had low contamination levels. Some projections suggest that by the 2030s, a large portion of the U.S. corn belt could see meaningfully higher aflatoxin risk than today. This makes routine monitoring and proactive feed management not just good practice for the present, but an essential part of a dairy farm’s long-term food safety strategy.

What do you think? Given that aflatoxin M1 survives pasteurization and standard dairy processing, does the current food safety framework in your region do enough to prevent contaminated milk from reaching consumers? And with climate change expected to increase aflatoxin risk in new geographies, how should dairy producers and regulators adapt their monitoring systems to keep pace?

References

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Table of Contents

What are aflatoxins? 🔗

How does aflatoxin get into milk? 🔗

How quickly does it appear in milk? 🔗

Why aflatoxin M1 in milk is a serious public health concern 🔗

Aflatoxicosis: acute vs chronic effects 🔗

The pasteurization problem: AFM1 survives heat treatment 🔗

Regulatory limits for AFM1 in milk 🔗

Prevention and control: starting at the feed 🔗

Good agricultural and storage practices 🔗

Feed testing and monitoring 🔗

Aflatoxin binders (sequestering agents) 🔗

Biological approaches 🔗

Climate change and the growing risk 🔗