By Dr Mackie Hobson BSc(Agric),BVSc

Wednesday, 25th January 2023

Most farmers will be familiar with ‘Geeldikkop’ caused by the wilted Tribulis terrestris. In most cases Tribulis terrestris will be the cause of the photosensitivity in Angora goats in the Karoo region but it is important to know what the other potential causes that farmers may encounter in the Karoo and adjacent areas.


What is photosensitivity?

Phototoxins (light reactive plant components) are toxins that can cause allergic reactions in particularly susceptible individuals when exposed to UV light. Light coloured, thin-skinned or hairless areas such as around the eyes, nose or teats are usually effected. This reaction causes the release of free radicals that in turn result in increased permeability of outer cell and lysosomal membranes. Lysosomal membrane damage releases lytic enzymes into the cell. This can lead to skin ulceration, necrosis, and oedema.


Clinical signs of Photosensitivity

  • Photophobic (seek shade) immediately when exposed to sunlight
  • Goats appear agitated and uncomfortable.
  • Goats may scratch or rub lightly pigmented, exposed areas of skin (ears, eyelids, muzzle).
  • Lesions initially appear in white-haired, non-pigmented, or hairless areas. Swelling of head (dikkop)
  • Initial erythema (reddening of skin) develops rapidly and is soon followed by oedema (swelling).
  • Lameness (coronitis) – hooves as well as base horn.
  • When exposure is prolonged, lesions may progress with serum exudation, ulceration, scab formation, and skin necrosis.
  • The final stage involves skin sloughing.
  • Depending on the initial cause of the accumulation of the photosensitizing agent, other clinical signs such as icterus (geelsig) for example may occur
  • Deaths may occur


Photosensitivity is classified according to the source of the photoxin

1. Primary photosensitization occurs when the photodynamic agent is ingested. The agent enters the systemic circulation in its native form, where it results in skin cell membrane damage after the animal is exposed to ultraviolet light.

  • Clovers and Alfalfa (Lucerne), Other plants have been suspected, but the toxins responsible have not been identified (eg, Cynodon ‘Kweek’)
  • Antibiotics: Tetracyclines, and some Sulfonamides have been reported to cause primary photosensitization.

2. Secondary photosensitization is the most frequent type of photosensitivity. The photosensitizing agent, phylloerythrin (a porphyrin), accumulates in the blood plasma because of impaired hepatobiliary (liver- excreted as bile) excretion. When it reaches the skin, it can absorb and release light energy, initiating a phototoxic reaction.


  • ‘Geeldikkop’ Tribulis terrestris
  • ‘Springbokbossie’ ‘Malkophaarpuisbos’ Hertia pallens
  • Stellenbosch photosensitivity’ Pithomyces chartarum
  • ‘Witbossie’, ‘Scholtz-bossie’, Pteronia pallens
  • ‘Vuursiektebossie’ Asaemia axillaris
  • ‘Ganskweek’ Lasiospermum bipinnatum Lantana camara,
  • ‘Dikoor’ Panicum spp
  • ‘Kweek’ Cynodon dactylon,
  • Blue-green Algae, Microcystis aeruginosa


Unknown Photosensitivity

Photosensitivity presumed to be caused by hepatogenous factors.

  • Water-damaged alfalfa hay (Lucerne), mouldy straw. These cases were suspected to be hepatogenous in origin.  Many of these plants are believed to be type I.
  • Oats, barley, wheat, and clover have been suspected in cases of photosensitizationand may be associated with specific environmental conditions.

In sheep photosensitivity cases on lucerne occurred when the lucerne had high numbers of aphids and ladybirds (NZ).


  • MSD Veterinary manual Photosensitization in Animals.
  • Botha, C.J. (Christoffel Jacobus); Venter, Elna; University of Pretoria. Faculty of Veterinary science. Dept. of Paraclinical Sciences. Section Pharmacology and Toxicology
  • Kleinvee-siektes : De Wet and Bath
  • Photosenisitation of sheep grazing alfalfa infested with aphids and ladybirds. Ferrer, Ortin, Loste, Fenandez, Verde, Ramoos

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