Spring parsley is a species of perennial shrub that is native to the western United States. Some gardeners have cultivated it in a decorative capacity, as it is hardy and well suited to arid, rocky environments. However, spring parsley is commonly regarded as a pest by landowners, due to its extremely toxic properties which cause significant losses amongst livestock. The chemicals contained within the plant are toxic to animals and humans alike, with cats being no exception.

Exposure to spring parsley can produce symptoms that are extremely dangerous to the cat's health. For this reason, owners should seek veterinary help as soon as possible after they notice the problem.

A short while after they have ingested the spring parsley material, the cat will begin to exhibit signs of heightened visual photosensitivity. This will usually present in the form of the cat avoiding bright lights and confining itself to darkened areas of the house due to the discomfort caused by looking into direct sunlight. This photosensitivity is the most dangerous consequence of spring parsley poisoning, as exposure to sunlight can cause blistering, irritation and open sores to develop on exposed tissues. If this affects the eyes, then the resultant burns can cause permanent scarring to easily appear, which will have a massive impact on the cat's quality of vision for the rest of its life. 

In some cases, the cat will experience a degree irritation of the tissues in its mouth after eating the spring parsley. As the cat attempts to wash the plant toxins out of its mouth it will produce large amounts of saliva, causing it to drool or seem to be foaming at the mouth. The chemicals contained within the spring parsley can also cause a degree of inflammation of the nose and throat. Another sign of irritation that owners can observe is redness and a small amount of swelling around the lips and nose. The volume of observable irritation is normally dependent on the quantity of spring parsley that the cat has eaten. It should be noted that in extreme cases, the irritants present in the plant can cause swelling in the throat, giving the cat noticeably labored breathing.

One of the hallmarks of spring parsley poisoning is a painful rash that appears on skin that has been exposed to direct sunlight. This skin reaction is characterized by extremely visible burns, blisters, and sores on the affected animal's skin. Cats affected by the photosensitivity will normally attempt to keep away from sunlight and may become aggressive if their owner attempts to touch an affected area. Unfortunately, the cat does not even have to eat the spring parsley in order to have this skin reaction, as simple physical contact with the plant is often enough to cause an outbreak. When transporting the cat (such as to the veterinary clinic), their cage should be kept covered at all times so as to avoid worsening their symptoms by exposure to sunlight.

The spring parsley plant contains large amounts of substances known as 'furanocoumarins'. These chemicals are produced by a variety of plants to fulfill a number of different roles, chief among which is to act as a deterrent against would-be predators. The specific member of the furanocoumarins family responsible for the aforementioned symptoms is called 'psoralen'. Once absorbed into the body, psoralen merges with cells and binds with their DNA. At this stage, the psoralen is relatively benign. However, once the cell is exposed to ultraviolet radiation, the psoralen undergoes a reaction that damages the structure of the DNA, preventing it from replicating and killing the cell. Some researchers also blame this process for creating cancers via altered cell DNA. This cell death results in the burn-like reaction to sunlight that an animal suffering from spring parsley poisoning will experience. In addition to this psoralen, the plant also contains various irritants that can cause oral irritation.

Once it has been presented at the clinic, the vet will perform a physical examination of the cat in order to verify the progression and severity of its symptoms and to check the animal's reactions, heart rate, breathing and other vitals. The vet may also decide to take a sample of the cat's blood for laboratory analysis so that the specific toxins present in the body can be identified. In addition, the vet will have a number of questions for the owner regarding the chronology of events surrounding the poisoning, as well as the cat's medical history and living environment. It is important for owners to have this information readily available, as it can help immensely with diagnosing the condition.

To prevent further damage due to photosensitivity from taking place, the vet may choose to administer a dosage of hydrogen peroxide to the cat in order to induce vomiting. By causing the animal to expel the contents of its stomach, no further psoralen should be absorbed. The vet may follow this up with activated charcoal, which will absorb any remnants of the spring parsley toxins that linger in the gut. The vet may then administer a mild sedative and place the cat in a darkened enclosure to rest and let the photosensitivity wear off. If the resultant dermatitis is particularly bad, then additional treatment with steroids and antiseptics may be required.

Following treatment, the majority of cats will recover in roughly a week, though those badly affected by exposure to sunlight may need ongoing aftercare to ensure their wounds properly heal. Once home from the vet, the cat should be kept in a cool, darkened room and given time to let the effects of the psoralen wear off. The vet may also provide owners with a cream to apply to irritated areas of the cat's skin in order to speed up their recovery time.