Vet explains how the invasive plant causes in dairy cows a problem known as photosensitisation
As I examined the grazing paddock, I noticed the hedge contained lots of lantana camara, commonly known as the Tickberry plant.
About three weeks ago, I received a call from farmer David of Ruiru. He told me one of his high-yielding Friesian cows was having a strange infection on the lower back.
“Initially the skin was red, then it reverted to normal without treatment but over the last few weeks, it has started peeling off and bleeding,” David said.
His workers told me they thought the cow could have been bewitched. “Why then would the skin only be affected on the white areas?” asked one of them. Many times in the course of my work, I get concerned about the ubiquity of myths that still exist regarding cow diseases and events that occur with animals. But that is a story for another day.
All health indicators for David’s prized cow were normal except for the unusual skin reaction he had reported. The cow was still producing 25 litres of milk a day. However, David had noticed that it would sometimes rub its lower back against a pole for some time.
The skin reaction on the back of the cow was limited to the white hair patches. The reaction’s epicentre had a hard horn-like skin protruding as multiple cones on the body surface. The recently affected areas were red with some parts oozing blood or tissue fluid. The edges of the wound showed inflamed skin indicating that the reaction was still expanding on the white- haired areas.
I asked David what he fed his cattle and he said ordinary grass, napier grass, some lucerne hay, dairy meal and vitamin mineral supplements. He also had a small paddock where he occasionally released the animals for grazing.
As I examined the grazing paddock, I noticed the hedge contained lots of lantana camara, commonly known as the Tickberry plant. Further scrutiny revealed the plant had been well-trimmed on the inside of the hedge by animals.
David’s workers told me the sick animal and two others were fond of feeding on the plant. The others had no interest even when hungry.
“Your cow has photosensitisation most probably resulting from consumption of the lantana,” I told him. He was surprised and told me he had even been considering propagating the plant to feed the cows since three of the animals appeared to like foraging on it.
His intended action would have been disastrous as the plant had chronic toxicity to cattle. Lantana camara is a ubiquitous colonising weed in Kenya and is a native of Latin America. It was distributed worldwide by Dutch explorers when they took it to Europe and people started planting it for its beautiful yellow, red and pink flowers. It is so common in many areas in Kenya that most people think it is an indigenous species.
Goats love the plant and it does not cause photosensitisation in them. This confuses farmers when I tell them the plant is toxic to cattle.
Photosensitisation refers to damage on the skin, especially in poorly pigmented areas, caused by the chemical reaction of substances in the tissues with the ultraviolet rays of the sun. Such chemicals are called photodynamic substances because they are activated by sunlight to form chemical products that kill skin cells.
If the chemicals are available in the skin in large quantities and for a long period, then the cell destruction and healing process forms the characteristic reaction as was seen on David’s cow.
Sometimes the reaction may even affect pigmented areas of the skin or cause sloughing off of large portions of the skin. This occurs when the photodynamic substances are available in large quantities on the skin or if intake of the offending chemicals is sustained.
Photosensitisation is not the same as sunburn and photodermatitis because these two conditions occur in the absence of photodynamic substances.
Animals will be affected if they have no pigmentation or they are exposed to intense sunlight for prolonged periods of time. Any species of animals may be affected depending on its sensitivity to the causative agent.
So, how do the photodynamic chemicals end up on the skin? The most common cause of photosensitisation in cattle is consumption of plants like lantana that contain chemicals that are broken down in the liver to generate photodynamic molecules. The molecules are transported in the blood and plasma to the skin tissues where they make contact with ultraviolet light from the sun. Once the chemicals interact with the ultraviolet light, they produce other chemicals that kill skin cells.
In Kenya, lantana camara is a common cause of photosensitisation in cattle. The condition is mainly seen in the white patches of the skin.
The reaction mainly occurs on the back of the animal because this is the position that the sun strikes directly between 10 and 4pm when it is most intense. Some plants in the cabbage and lucerne family as well as some moulds may also cause photosensitisation in cattle.
When in small quantities, the photodynamic agents are removed through the bile. However, with liver damage or when the chemicals are produced in excess, they are transported to the skin through the blood.
To treat photosensitisation, identify the causative material and prevent the cattle from eating it. Affected cattle should be kept in a sheltered place to stop contact with direct sunlight. Animals in the early to medium stages of photosensitisation before the formation of the horned skin protrusions may recover fully once the causative agent is removed.
The cattle with extreme growth of skin protrusions do not recover normal skin but further reaction is arrested once the cows are removed from direct sunlight and further consumption of the offending material.
Left unattended, photosensitisation may lead to skin bacterial infection, sloughing of large portions of skin, poor health, reduced production and death from bacterial infection.
I treated David’s cow with antibiotics to control opportunistic bacterial infection and anti- inflammation medicine to stop the itching and reddening of the skin. I also advised him to get rid of the lantana hedges and keep the cattle sheltered until the affected one had recovered.
The cow is recovering well and fortunately, the other two lantana eaters have not shown a similar skin reaction. With the time passed, withdrawal of lantana and provision of shelter it is unlikely the cows will show signs of toxicity.