Pontiac fever: the mild form of legionellosis

Pontiac fever is the mild, self-limiting form of legionellosis, the family of illnesses caused by breathing in water droplets that carry Legionella bacteria. Unlike Legionnaires' disease it does not cause pneumonia, and it usually clears within a week without antibiotics.

What is Pontiac fever?

Pontiac fever is one of two illnesses grouped together as legionellosis. The other is Legionnaires' disease. Both are caused by the same bacteria, Legionella, most often the species Legionella pneumophila, and both are caught the same way: by breathing in tiny airborne water droplets that carry the organism. You can read more about the bacterium itself on our what is legionella page.

What sets Pontiac fever apart is how the body reacts. Instead of the lung infection seen in Legionnaires' disease, Pontiac fever produces a short, flu-like illness with no pneumonia. It is not spread from person to person, and most people recover on their own within a week.

Because it is mild and clears quickly, Pontiac fever is very often not tested for or reported. That under-reporting matters, because it means the water system that caused it can keep running unnoticed while the same aerosol is still being produced.

Why is it called Pontiac fever?

The name comes from Pontiac, Michigan. In 1968 a group of workers and visitors at the county health department building fell ill with a sudden flu-like illness. Almost everyone exposed became unwell, yet nobody developed pneumonia and nobody died, which puzzled investigators at the time.

The cause stayed a mystery until 1976, when a large outbreak of pneumonia at an American Legion convention in Philadelphia led scientists to identify a new bacterium, Legionella pneumophila. Researchers went back to stored samples from the 1968 Pontiac episode, retested them, and found the same organism. The milder 1968 illness was named Pontiac fever after the town where it was first recorded.

So the two illnesses were discovered in the wrong order. The severe form gave the bacterium and Legionnaires' disease their names, while the mild form kept the name of the place it first appeared.

Figures reflect UK Health Security Agency, HSE and international public health guidance.

Pontiac fever vs Legionnaires' disease

It helps to think of Pontiac fever and Legionnaires' disease not as two separate diseases but as two possible outcomes of the same exposure. The organism is the same, the contaminated aerosol is the same, and the water-system failure behind them is the same. What differs is the severity of the illness that follows.

Legionnaires' disease is a serious pneumonia. It can require hospital admission, needs antibiotics, and is fatal in roughly one in ten diagnosed cases, with a higher toll among older people and those with weakened immune systems. Pontiac fever, by contrast, has a very high attack rate but a very low severity: most of those exposed feel unwell, but the illness passes in a few days. The table below sets out the practical differences.

Pontiac fever symptoms and incubation period

Pontiac fever symptoms are non-specific and resemble a heavy dose of flu. They commonly include:

• fever and chills
• headache
• muscle and joint aches (myalgia)
• tiredness, malaise and a general lack of energy

The incubation period is short. Symptoms usually appear within a few hours to about three days of exposure, which is quicker than Legionnaires' disease, where onset is typically two to ten days. Pontiac fever also passes quickly, often within two to five days and almost always within a week, without antibiotics.

Because the symptoms overlap with flu and with the early stages of Legionnaires' disease, Pontiac fever cannot be told apart from more serious illness by symptoms alone. This page explains the condition; it is not medical advice. Anyone who is unwell after possible exposure should seek medical attention, partly so that Legionnaires' disease, which does need prompt treatment, can be ruled out.

Why a mild illness still matters for water safety

The most common and most dangerous misconception is that, because Pontiac fever is mild, it does not matter to whoever runs the building or the water system. That is wrong.

The exposure that causes Pontiac fever is identical to the one that causes Legionnaires' disease. The same aerosol, from the same contaminated cooling tower, spa pool, shower or tap, can give one person a few days of flu-like illness and give the next person, who happens to be older or immunocompromised, a life-threatening pneumonia. A cluster of Pontiac fever is therefore a warning sign that a water system is releasing infectious aerosol.

Under the Health and Safety at Work etc. Act and the Approved Code of Practice ACOP L8, a duty holder must control the risk of exposure to Legionella regardless of which illness a given exposure happens to produce. The law targets the exposure, not the diagnosis. A mild outcome in one person does not reduce the legal duty or the underlying risk.

How Pontiac fever is prevented

Preventing Pontiac fever is the same job as preventing Legionnaires' disease, because they share a cause. The core of it is stopping Legionella from multiplying and stopping contaminated water from being turned into a breathable aerosol.

Temperature control does most of the work. Legionella grows fastest between 20°C and 45°C, so guidance in HSG274 is built around keeping water out of that range. Cold water should be stored and distributed below 20°C. Hot water should be stored at 60°C and reach the tap at 50°C, or 55°C in healthcare settings. Our legionella temperature control page covers these figures in detail.

Alongside temperature, control means removing stagnation and dead legs, managing aerosol-producing equipment such as cooling towers and spa pools, and, where sampling is used, acting on the results. Action levels sit at 100 CFU per litre and 1,000 CFU per litre for hot and cold water systems, above which further investigation or remedial action is expected.

All of this should be written down and managed through a written scheme of control that flows from a suitable and sufficient risk assessment. Recording your controls does not by itself make a system safe; it is the tool that lets you show what you are doing and spot where you are falling short.