How raw milk and red meat drive brucellosis transmission in humans

Brucellosis is a widespread zoonotic disease affecting humans, livestock and wildlife. According to the World Health Organisation (WHO), about half a million people are infected each year, with prevalence in some regions reaching up to 10 per cent of the population.

The disease remains a major public health concern in many developing countries, especially where livestock play a central role in daily life and livelihoods.

The WHO explains that brucellosis is caused by bacteria of the genus Brucella. Several species infect animals and humans, with most human cases linked to Brucella melitensis, Brucella abortus and Brucella suis. These bacteria primarily affect cattle, goats, sheep, pigs and dogs.

Humans usually acquire the infection through direct contact with infected animals, consumption of contaminated animal products, or inhalation of contaminated airborne particles. Worldwide, most cases are associated with the consumption of unpasteurised milk and dairy products.

Experts say detecting and controlling the disease remains challenging.

Dr Emmanuel Okunga, head of the Zoonotic Disease Unit (ZDU), notes that brucellosis is frequently mistaken for other febrile illnesses. 

What distinguishes brucellosis from more familiar illnesses, such as malaria is its chronic nature. “Unlike malaria, which usually resolves within a few days after treatment, brucellosis can be long-lasting, persisting for weeks or even months. Patients may experience weight loss and severe joint pain for prolonged periods,” he explains.

Dr Khadija Chepkorir, a veterinary epidemiologist at the ZDU, describes how the disease spreads among livestock. “Animals can become infected by ingesting contaminated pasture or water,” she says, adding that sexual transmission between animals is also possible. 

“Infected animals may not always show obvious signs,” Dr Chepkorir adds. “However, common indicators include abortion in late pregnancy, infertility problems and swelling of male reproductive organs.” Young animals born to infected mothers may also carry the infection.

In humans, the WHO notes that brucellosis typically begins with flu-like symptoms, such as fever, weakness, malaise, headache and weight loss, usually appearing two to four weeks after exposure. Without treatment, many patients develop recurrent or persistent fever, body aches and fatigue.

Treatment requires a prolonged course of antibiotics. Standard therapy recommended by the WHO involves doxycycline combined with streptomycin or rifampicin for at least 45 days. Inadequate treatment allows the bacteria to persist, leading to chronic disease and complications such as arthritis and recurrent fever.

Dr Khadija Chepkorir notes that treating brucellosis in humans is often more expensive than preventing it in livestock. Controlling infection in animals lowers overall disease burden, a principle that also applies to other zoonotic diseases such as rabies and Rift Valley fever.

Humans are mainly infected through consuming raw or undercooked meat, unboiled milk, or handling infectious animal materials such as blood or birth tissues without protective gear. “An example would be nyama choma that is still red,” Dr Chepkorir says, highlighting the risk posed by undercooking.

Studies in Kenya show animal prevalence ranging from 3.4 per cent to 22.3 per cent in cattle, goats and camels, while human seroprevalence in some pastoralist communities exceeds 14 per cent.

Prevention centres on controlling the disease in animals and limiting human exposure. Proper cooking of meat, boiling or pasteurising milk, use of protective clothing and vaccination of livestock in high-risk areas are effective measures. “Vaccination and removal of infected animals from herds can significantly interrupt transmission,” Dr Chepkorir says.

Although underreported due to non-specific symptoms and limited diagnostic capacity, brucellosis remains a serious threat to rural livelihoods and public health. According to the Centers for Disease Control and Prevention, human seroprevalence in parts of Africa ranges from under one per cent to much higher levels among frequently exposed populations. Sustained prevention, improved diagnosis and better data collection are essential to reduce transmission and protect livestock-dependent economies.